Talk MCP: a client for external tool servers (#251)

docs/plans/06-mcp-support.md asks for the host direction — Maven connects OUT
to MCP servers and consumes what they offer. This is the client half: the
protocol, the transports, the connection manager, the config block. Nothing is
wired into a turn yet, and nothing here exposes Maven's own capabilities to an
outside caller.

internal/mcp:
  - hand-rolled JSON-RPC 2.0 (the wire format is four fields, and the repo
    vendors its deps, so a library would cost more than it saves);
  - two transports: a stdio subprocess on this box, and streamable HTTP, which
    accepts a plain JSON reply or an SSE frame because servers disagree about
    which they send;
  - Client: initialize handshake, tools/list, tools/call, resources/list,
    resources/read. Text content only — everything downstream is a sentence;
  - Manager: lazy dial, per-server failure that never blocks boot or the other
    servers, backoff reconnect, Status for a web surface, graceful Close;
  - the allowlist encoding: a discovered tool becomes the store row
    "vikunja_list_tasks" with cmd ["mcp","vikunja","list_tasks"], scope
    "mcp:vikunja". No new column, no migration, and ProposeTool, EnableTool,
    the act matcher and the confirm turn all keep working untouched.

Constraints held, in code rather than in prose:
  - OFF unless configured, and a server is dark until "enabled": true.
  - A url server goes through internal/webfetch, so the SSRF guard, the size
    cap, the redirect cap and the per-host rate limit apply. Reaching loopback
    needs allow_private on THAT server, and each server gets its own fetcher so
    one loopback exemption cannot become a hole for a public endpoint.
  - readOnlyHint decides destructive: no hint means "assume it mutates", which
    will route the call through the existing confirm turn. Guessing wrong in
    that direction only costs a question.
  - The catalogue stays small on purpose — allow_tools, and max_tools=12 per
    server. The resident model is a 1.7B with a 4096-token context; a tool name
    it half-remembers is a wrong act.
  - Only the tool name and the router's arguments are sent. There is no API
    here through which a note, a fact or the persona block could travel.

webfetch grows Post (JSON-RPC cannot be a GET) and surfaces response headers
for Mcp-Session-Id. It shares Get's guards exactly: a body buys a caller
nothing, a POST to the LAN is refused for the same reason a GET is.

Verified against the real Vikunja MCP server on homesrv
(http://localhost:9100/mcp): handshake, three discovered tools with update_task
correctly NOT read-only, a live list_projects call, a tool excluded by
allow_tools refused, and the same server refused outright once allow_private
was dropped. Tests cover both transports (the stdio one against a real
subprocess), SSE and JSON framing, session echo, reconnect, and the config
validation.

.gitignore

@@ -8,6 +8,7 @@
 /mavcaldav
 /mavwaked
 /mavmaild
+/mavupdate
 
 # Certs (private keys, don't commit)
 certs/

Makefile

@@ -20,7 +20,7 @@ PIPER_ESPEAK  := $(shell pwd)/deps/piper/espeak-ng-data
 
 all: build
 
-build: build-stt build-tts build-daemon build-client build-waked build-web build-poll build-caldav build-mail
+build: build-stt build-tts build-daemon build-client build-waked build-web build-poll build-caldav build-mail build-update
 
 build-stt:
 	CGO_CFLAGS="$(CGO_CFLAGS)" CGO_LDFLAGS="$(CGO_LDFLAGS)" LD_LIBRARY_PATH="$(shell pwd)/deps/lib" \
@@ -53,6 +53,12 @@ build-caldav:
 build-mail:
 	$(GO) build $(GOFLAGS) -o mavmaild ./cmd/mavmaild/
 
+# mavupdate is an operator CLI, not a daemon: nothing runs it but a human on the
+# box. It is built with the rest so a broken update path is caught by `make
+# build` rather than the first time it is needed.
+build-update:
+	$(GO) build $(GOFLAGS) -o mavupdate ./cmd/mavupdate/
+
 run-web: build-web
 	./mavweb -addr :9200 -voice 127.0.0.1:9100
 

cmd/mavupdate/main.go

@@ -0,0 +1,204 @@
+// Command mavupdate deploys a new build of Maven to the box she runs on, with
+// an automatic rollback when the new build does not come up (Vikunja #249).
+//
+// It is a CLI on purpose, and it is the ONLY trigger for the update path.
+//
+// The obvious design — an IPC method plus a button on the web UI behind the
+// step-up passkey gate, the way /tools works — was considered and refused. A
+// step-up gate protects against the wrong person clicking; it does not change
+// the fact that anything reachable over the network becomes, in the event of a
+// mavweb bug, a remote arbitrary-code path with a build system attached. An
+// update needs shell access on the host, which is a strictly higher bar than
+// the gate that guards the tool allowlist. That is deliberate and it is the
+// reason there is no MethodApplyUpdate anywhere in internal/ipc.
+//
+// Consequently: mavend does not import internal/update, nothing runs on a timer,
+// nothing checks a release server, and no act, intent, tool or LLM output can
+// reach any of this. She cannot update herself. She can be updated, by him.
+//
+//	mavupdate -config deploy/mavend.json list          # snapshots available to roll back to
+//	mavupdate -config deploy/mavend.json verify        # make build + make test, deploys nothing
+//	mavupdate -config deploy/mavend.json apply -yes    # the whole thing
+//	mavupdate -config deploy/mavend.json rollback [id] # restore + restart (default: newest)
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/kami/maven/internal/config"
+	"github.com/kami/maven/internal/update"
+)
+
+func main() {
+	cfgPath := flag.String("config", "deploy/mavend.json", "path to mavend.json (the update block is read from it)")
+	yes := flag.Bool("yes", false, "required by `apply` and `rollback`: yes, restart the daemon")
+	flag.Usage = usage
+	flag.Parse()
+
+	// The stdlib flag package stops parsing at the first non-flag argument, so a
+	// `-yes` written after the subcommand (which is how anyone would type it, and
+	// how the usage text shows it) lands in Args instead of the flag. Pick it out
+	// by hand rather than silently treating "apply -yes" as an unconfirmed apply.
+	var args []string
+	for _, a := range flag.Args() {
+		if a == "-yes" || a == "--yes" {
+			*yes = true
+			continue
+		}
+		args = append(args, a)
+	}
+	if len(args) == 0 {
+		usage()
+		os.Exit(2)
+	}
+
+	cfg, err := config.Load(*cfgPath)
+	if err != nil {
+		die("config: %v", err)
+	}
+	if cfg.Update == nil {
+		die("no `update` block in %s — the update capability is off unless configured.\nSee the package comment in internal/update for what it does and does not do.", *cfgPath)
+	}
+
+	logf := func(format string, a ...any) {
+		fmt.Fprintf(os.Stderr, "%s  %s\n", time.Now().Format("15:04:05"), fmt.Sprintf(format, a...))
+	}
+	u, err := update.New(*cfg.Update, update.WithLogger(logf))
+	if err != nil {
+		die("%v", err)
+	}
+
+	// Ctrl-C cancels the build or the health wait. It cannot cancel a rollback
+	// midway into leaving the box in an unknown state, because the rollback runs
+	// on its own context — see cmdApply.
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	switch args[0] {
+	case "list":
+		cmdList(u)
+	case "verify":
+		cmdVerify(ctx, u)
+	case "apply":
+		if !*yes {
+			die("apply restarts mavend and can roll her back. Re-run with -yes if that is what you want.")
+		}
+		cmdApply(ctx, u)
+	case "rollback":
+		if !*yes {
+			die("rollback restores the previous artifacts and restarts mavend. Re-run with -yes.")
+		}
+		id := ""
+		if len(args) > 1 {
+			id = args[1]
+		}
+		cmdRollback(ctx, u, id)
+	default:
+		usage()
+		os.Exit(2)
+	}
+}
+
+func cmdList(u *update.Updater) {
+	snaps, err := u.Snapshots()
+	if err != nil {
+		die("snapshots: %v", err)
+	}
+	if len(snaps) == 0 {
+		fmt.Println("no snapshots yet — the first `apply` takes one before it builds anything")
+		return
+	}
+	fmt.Printf("%-18s  %-12s  %s\n", "SNAPSHOT", "COMMIT", "FILES")
+	for _, s := range snaps {
+		commit := s.Commit
+		if len(commit) > 12 {
+			commit = commit[:12]
+		}
+		if commit == "" {
+			commit = "-"
+		}
+		fmt.Printf("%-18s  %-12s  %d\n", s.ID, commit, len(s.Files))
+	}
+	fmt.Printf("\nrollback to the newest with:  mavupdate rollback -yes\n")
+}
+
+func cmdVerify(ctx context.Context, u *update.Updater) {
+	steps, err := u.Verify(ctx)
+	report(steps)
+	if err != nil {
+		die("%v", err)
+	}
+	fmt.Println("verified: the tree builds and passes its own tests. Nothing was deployed — run `apply -yes` for that.")
+}
+
+func cmdApply(ctx context.Context, u *update.Updater) {
+	res, err := u.Apply(ctx)
+	report(res.Steps)
+	summarize(res)
+	switch {
+	case err == nil:
+		fmt.Println("\nupdate committed: she answers on the new build.")
+	case errors.Is(err, update.ErrRollbackFailed):
+		die("\n%v\n\nSHE IS PROBABLY DOWN. The previous artifacts are in the snapshot dir; copy them\nover the install dir and restart by hand.", err)
+	case errors.Is(err, update.ErrRolledBack):
+		die("\n%v\n\nShe is answering again on the previous build. Nothing was lost; fix the change and retry.", err)
+	default:
+		die("\n%v", err)
+	}
+}
+
+func cmdRollback(ctx context.Context, u *update.Updater, id string) {
+	res, err := u.Rollback(ctx, id)
+	report(res.Steps)
+	summarize(res)
+	if err != nil && !errors.Is(err, update.ErrRolledBack) {
+		die("\n%v", err)
+	}
+	fmt.Printf("\nrolled back to %s; she answers on it.\n", res.SnapshotID)
+}
+
+func report(steps []update.Step) {
+	for _, s := range steps {
+		status := "ok"
+		if s.Err != nil {
+			status = "FAILED: " + s.Err.Error()
+		}
+		fmt.Printf("  %-8s %-8s %s\n", s.Name, s.Took.Round(time.Second), status)
+		if s.Output != "" {
+			fmt.Printf("---- %s output ----\n%s\n-------------------\n", s.Name, s.Output)
+		}
+	}
+}
+
+func summarize(res update.Result) {
+	fmt.Printf("\nverified=%v snapshot=%s installed=%d restarted=%v healthy=%v rolled_back=%v rollback_healthy=%v took=%s\n",
+		res.Verified, res.SnapshotID, len(res.Installed), res.Restarted, res.Healthy, res.RolledBack, res.RollbackHealthy, res.Took.Round(time.Second))
+}
+
+func usage() {
+	fmt.Fprint(os.Stderr, `mavupdate — deploy a new build of Maven, with rollback.
+
+  mavupdate [-config path] list
+  mavupdate [-config path] verify
+  mavupdate [-config path] apply -yes
+  mavupdate [-config path] rollback [snapshot-id] -yes
+
+apply is: health-check the running daemon, snapshot the deployed artifacts,
+make build, make test, install, restart, health-check — and restore the
+snapshot if any of that fails. It never fetches code and never runs by itself.
+
+`)
+	flag.PrintDefaults()
+}
+
+func die(format string, a ...any) {
+	fmt.Fprintf(os.Stderr, format+"\n", a...)
+	os.Exit(1)
+}

deploy/README.md

@@ -114,3 +114,49 @@ build on the target host, most likely in one of these:
   work fine over the core socket.
 - **netdata** — `mavpoll` reaches it via `host.docker.internal`; adjust if
   netdata runs elsewhere.
+
+## Updating her (`mavupdate`, Vikunja #249)
+
+Off unless configured, and there is deliberately no button for it. There is no
+IPC method, no web route, no timer and no act that starts an update — the trigger
+is a human running `mavupdate` on the host, which needs shell access, a strictly
+higher bar than the step-up passkey gate that guards `/tools`. She cannot update
+herself; she can be updated. Nothing here ever fetches code: the new version is
+whatever you pulled into the working tree yourself.
+
+Add an `update` block to `mavend.json` (mavend ignores it — only the CLI reads
+it), with paths as they exist **on the host**, not inside a container:
+
+```json
+"update": {
+  "source_dir": "/home/kami/apps/Maven",
+  "install_dir": "/home/kami/apps/Maven",
+  "snapshot_dir": "/var/lib/maven-snapshots",
+  "binaries": ["mavend", "mavweb", "mavsttd", "mavttsd", "mavwaked",
+               "mavenclient", "mavpoll", "mavcaldav", "mavmaild"],
+  "config_files": ["deploy/mavend.json"],
+  "restart_cmd": ["docker", "compose", "up", "-d", "--build"],
+  "health_socket": "/var/lib/docker/volumes/maven_sockets/_data/mavend.sock",
+  "health_timeout_sec": 120
+}
+```
+
+`snapshot_dir` must be outside `install_dir` (a restore must not read from what
+the install writes) and `health_socket` is required: an update that cannot check
+its own result cannot roll itself back, so the config is refused without one.
+
+Then:
+
+```sh
+mavupdate -config deploy/mavend.json verify        # make build + make test, deploys nothing
+mavupdate -config deploy/mavend.json apply -yes    # snapshot, verify, install, restart, health-check
+mavupdate -config deploy/mavend.json list          # what you can roll back to
+mavupdate -config deploy/mavend.json rollback -yes # restore the previous artifacts and restart
+```
+
+`apply` refuses to start if she is not already answering — otherwise a failed
+update and a box that was already broken are indistinguishable afterwards. On any
+failure after the install it restores the snapshot, restarts, and checks again;
+if that also fails it says so loudly and names the directory to copy back by hand.
+The database is never snapshotted or rolled back (see the package comment in
+`internal/update`); schema compatibility is `store.Migrate`'s job.

internal/config/config.go

@@ -22,7 +22,9 @@ import (
 
 	"github.com/kami/maven/internal/delivery/ntfysink"
 	"github.com/kami/maven/internal/delivery/telegramsink"
+	"github.com/kami/maven/internal/mcp"
 	"github.com/kami/maven/internal/morning"
+	"github.com/kami/maven/internal/update"
 	"github.com/robfig/cron/v3"
 )
 
@@ -108,6 +110,16 @@ type Config struct {
 	// calls its /v1/chat/completions endpoint to phrase nudges and reminders.
 	Phraser *PhraserConfig `json:"phraser,omitempty"`
 
+	// Update — how THIS box deploys a new build of Maven (Vikunja #249). nil ⇒
+	// the update capability does not exist, which is the state to leave it in
+	// unless the operator has read internal/update's package comment.
+	//
+	// mavend never reads this block: the daemon does not import internal/update
+	// and cannot update itself. It lives here because cmd/mavupdate — a CLI the
+	// owner runs on the host, the only trigger there is — reads the same config
+	// file to find the socket it health-checks.
+	Update *update.Config `json:"update,omitempty"`
+
 	// Voice — the client↔core surface + the stt/tts modules the daemon
 	// wires. nil ⇒ the daemon doesn't wire voice: the TCP listener stays
 	// down, the dispatcher's Voice slot stays nil (the routing table's
@@ -180,6 +192,119 @@ type Config struct {
 	// discovers and executes capabilities through Hexis for ecosystem actions.
 	// nil ⇒ no capability-aware routing.
 	Hexis *HexisConfig `json:"hexis,omitempty"`
+
+	// MCP — Model Context Protocol servers Maven connects OUT to (Vikunja
+	// #251). nil / absent / no enabled server ⇒ no connection is made and no
+	// tool is discovered, like every other capability that reaches outside the
+	// box. She is a client here, never a server: nothing exposes her own
+	// capabilities to an outside caller. See MCPConfig.
+	MCP *MCPConfig `json:"mcp,omitempty"`
+}
+
+// MCPConfig — the MCP client block. Servers are dark until one has
+// `"enabled": true`, and a discovered tool is only ever PROPOSED: Kami enables
+// it on /tools, on the authed surface, exactly as he would a shell tool. The
+// voice path can never grant a capability to itself.
+type MCPConfig struct {
+	// Servers — the configured servers. Each needs exactly one of command
+	// (a subprocess on this box) or url (a streamable-HTTP endpoint).
+	Servers []MCPServerConfig `json:"servers,omitempty"`
+
+	// Timeout — per-call budget for every server that does not set its own.
+	// 0 ⇒ mcp.DefaultTimeout (15s). A tool slower than this is not usable in a
+	// spoken turn.
+	Timeout Duration `json:"timeout,omitempty"`
+
+	// AllowHosts / DenyHosts — the host lists for the shared webfetch door that
+	// url servers go through. Deny wins. Private addresses are refused
+	// unconditionally unless the individual server sets allow_private.
+	AllowHosts []string `json:"allow_hosts,omitempty"`
+	DenyHosts  []string `json:"deny_hosts,omitempty"`
+
+	// MaxBytes — cap on one JSON-RPC response. 0 ⇒ webfetch.DefaultMaxBytes.
+	MaxBytes int64 `json:"max_bytes,omitempty"`
+}
+
+// MCPServerConfig — one MCP server.
+type MCPServerConfig struct {
+	// Name — the local handle. It prefixes every tool this server contributes
+	// ("vikunja" + "list_tasks" ⇒ the allowlist row "vikunja_list_tasks") and
+	// becomes the store scope "mcp:<name>", so its provenance is readable on
+	// /tools without opening the config.
+	Name string `json:"name"`
+
+	// Command / Args / Env / Dir — a stdio server: a child process of mavend,
+	// on this box, under this user. argv, never a shell string.
+	Command string   `json:"command,omitempty"`
+	Args    []string `json:"args,omitempty"`
+	Env     []string `json:"env,omitempty"`
+	Dir     string   `json:"dir,omitempty"`
+
+	// URL — a streamable-HTTP endpoint. It is fetched through
+	// internal/webfetch, so the SSRF guard, the redirect cap, the size cap and
+	// the one-request-per-host-per-second limit all apply.
+	URL string `json:"url,omitempty"`
+
+	// AllowPrivate — let THIS server be a loopback or LAN address. The Vikunja
+	// server on homesrv is "http://localhost:9100/mcp", which is refused
+	// without this flag. Understand what it means before setting it: a local
+	// server is a DIFFERENT trust level from a public one. It is inside the
+	// network, it usually needs no credential, and it can change things that
+	// matter — so an argument the router got wrong lands somewhere real. Set it
+	// only for a server you run yourself, and prefer allow_tools with it.
+	AllowPrivate bool `json:"allow_private,omitempty"`
+
+	// AllowTools — when set, the ONLY remote tool names taken from this server.
+	// This is the knob that keeps the catalogue deliberate: the resident model
+	// is a 1.7B with a 4096-token context, and a tool name it half-remembers is
+	// a wrong act, so fewer and better-chosen beats complete.
+	AllowTools []string `json:"allow_tools,omitempty"`
+
+	// MaxTools — cap on this server's contribution. 0 ⇒ mcp.DefaultMaxTools (12).
+	MaxTools int `json:"max_tools,omitempty"`
+
+	// Timeout — per-call budget for this server. 0 ⇒ MCPConfig.Timeout.
+	Timeout Duration `json:"timeout,omitempty"`
+
+	// Enabled — false (the default) keeps a configured server described but
+	// dark, so a block can be written and reviewed before it is switched on.
+	Enabled bool `json:"enabled,omitempty"`
+}
+
+// MCPServers maps the config blocks onto the mcp package's own type. It lives
+// here so config validation and daemon wiring cannot drift on the mapping.
+// Returns nil when nothing is configured or nothing is enabled.
+func (c *Config) MCPServers() []mcp.ServerConfig {
+	if c.MCP == nil {
+		return nil
+	}
+	out := make([]mcp.ServerConfig, 0, len(c.MCP.Servers))
+	for _, s := range c.MCP.Servers {
+		if !s.Enabled {
+			continue
+		}
+		timeout := time.Duration(s.Timeout)
+		if timeout <= 0 {
+			timeout = time.Duration(c.MCP.Timeout)
+		}
+		out = append(out, mcp.ServerConfig{
+			Name:         s.Name,
+			Command:      s.Command,
+			Args:         s.Args,
+			Env:          s.Env,
+			Dir:          s.Dir,
+			URL:          s.URL,
+			AllowPrivate: s.AllowPrivate,
+			AllowTools:   s.AllowTools,
+			MaxTools:     s.MaxTools,
+			Timeout:      timeout,
+			Enabled:      true,
+		})
+	}
+	if len(out) == 0 {
+		return nil
+	}
+	return out
 }
 
 // PraxisConfig — maven's connection to the Praxis attention service.
@@ -772,6 +897,12 @@ func (c *Config) applyDefaults() {
 		c.Feeds = nil
 	}
 
+	// Same rule for MCP: a block with no server, or none enabled, is the same
+	// as no block at all. Normalising it to nil keeps "off" in one place.
+	if c.MCP != nil && len(c.MCPServers()) == 0 {
+		c.MCP = nil
+	}
+
 	// Same rule for the crawler: a block that neither answers on demand nor
 	// watches anything has nothing to do, so it is normalised to "off".
 	if c.Crawl != nil && !c.Crawl.OnDemand && len(c.Crawl.Watches) == 0 {
@@ -844,6 +975,14 @@ func (c *Config) validate() error {
 			}
 		}
 	}
+	// The update block is validated here even though mavend never acts on it: a
+	// half-written update config that is only noticed by cmd/mavupdate is noticed
+	// at the worst possible moment, halfway through deploying a new build.
+	if c.Update != nil {
+		if err := c.Update.Validate(); err != nil {
+			return err
+		}
+	}
 	if c.Voice != nil && c.Voice.Enabled {
 		if c.Voice.Bind == "" {
 			return errors.New("voice.enabled set but voice.bind is empty — refusing to start a voice surface with no bind address")
@@ -868,6 +1007,12 @@ func (c *Config) validate() error {
 			return fmt.Errorf("routine %q: bad cron %q: %w", r.Name, r.Cron, err)
 		}
 	}
+	// An MCP block with a typo (no name, both command and url, a bare hostname
+	// as the url) fails here, at startup, rather than at the first turn that
+	// needed the tool.
+	if err := mcp.Validate(c.MCPServers()); err != nil {
+		return err
+	}
 	if len(c.MorningRoutines) > 0 {
 		if err := morning.Validate(morningRoutinesFromConfig(c.MorningRoutines)); err != nil {
 			return err

internal/config/config_test.go

@@ -325,3 +325,44 @@ func TestSwapModelsParsedAndMustBeAbsolute(t *testing.T) {
 		t.Error("Load accepted a relative swap_models entry; want a startup failure")
 	}
 }
+
+// TestUpdateBlockAbsentMeansOff — mavend never updates itself; the block only
+// exists so cmd/mavupdate can find the deployment it is asked to update
+// (Vikunja #249). Absent is the normal state.
+func TestUpdateBlockAbsentMeansOff(t *testing.T) {
+	c, err := Load(writeConfig(t, `{}`))
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.Update != nil {
+		t.Errorf("update = %+v; want nil when unconfigured", c.Update)
+	}
+}
+
+func TestUpdateBlockValidatedAtStartup(t *testing.T) {
+	good := `{"update": {
+		"source_dir": "/srv/maven",
+		"install_dir": "/srv/maven",
+		"snapshot_dir": "/var/lib/maven/snapshots",
+		"binaries": ["mavend", "mavweb"],
+		"restart_cmd": ["docker", "compose", "up", "-d", "--build", "mavend"],
+		"health_socket": "/run/maven/mavend.sock"
+	}}`
+	c, err := Load(writeConfig(t, good))
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.Update == nil || len(c.Update.Binaries) != 2 {
+		t.Fatalf("update block = %+v; want it parsed", c.Update)
+	}
+	// A block with no health check cannot detect its own failure, so it cannot
+	// roll back — refused at load, not halfway through a deploy.
+	noHealth := `{"update": {
+		"source_dir": "/srv/maven", "install_dir": "/srv/maven",
+		"snapshot_dir": "/var/lib/maven/snapshots",
+		"binaries": ["mavend"], "restart_cmd": ["true"]
+	}}`
+	if _, err := Load(writeConfig(t, noHealth)); err == nil {
+		t.Error("Load accepted an update block with no health_socket")
+	}
+}

internal/config/mcp_test.go

@@ -0,0 +1,82 @@
+package config
+
+import (
+	"testing"
+	"time"
+)
+
+func TestMCPAbsentIsOff(t *testing.T) {
+	c, err := Load(writeConfig(t, `{}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c.MCP != nil {
+		t.Error("no mcp block ⇒ nil")
+	}
+	if got := c.MCPServers(); got != nil {
+		t.Errorf("MCPServers() = %+v, want nil", got)
+	}
+}
+
+// A described-but-not-enabled server must not be wired. This is how a block can
+// sit in the config file, reviewed, before it is switched on.
+func TestMCPDisabledServerIsOff(t *testing.T) {
+	c, err := Load(writeConfig(t, `{"mcp":{"servers":[
+		{"name":"vikunja","url":"http://localhost:9100/mcp","allow_private":true}]}}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c.MCP != nil {
+		t.Errorf("a block with nothing enabled must normalise to nil, got %+v", c.MCP)
+	}
+	if got := c.MCPServers(); len(got) != 0 {
+		t.Errorf("MCPServers() = %+v", got)
+	}
+}
+
+func TestMCPEnabledServerMapping(t *testing.T) {
+	c, err := Load(writeConfig(t, `{"mcp":{
+		"timeout":"5s",
+		"servers":[
+			{"name":"vikunja","url":"http://localhost:9100/mcp","allow_private":true,
+			 "allow_tools":["list_tasks"],"max_tools":3,"enabled":true},
+			{"name":"files","command":"mcp-server-fs","args":["/srv"],"timeout":"1s","enabled":true},
+			{"name":"off","command":"nope"}
+		]}}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := c.MCPServers()
+	if len(got) != 2 {
+		t.Fatalf("servers = %+v", got)
+	}
+	if got[0].Name != "vikunja" || !got[0].AllowPrivate || got[0].MaxTools != 3 ||
+		len(got[0].AllowTools) != 1 || got[0].Timeout != 5*time.Second {
+		t.Errorf("vikunja mapped wrong: %+v", got[0])
+	}
+	if got[1].Command != "mcp-server-fs" || len(got[1].Args) != 1 || got[1].Timeout != time.Second {
+		t.Errorf("files mapped wrong: %+v", got[1])
+	}
+	// allow_private is per server and must not leak to the other one.
+	if got[1].AllowPrivate {
+		t.Error("allow_private leaked between servers")
+	}
+}
+
+func TestMCPBadServerFailsAtStartup(t *testing.T) {
+	cases := map[string]string{
+		"no name":     `{"mcp":{"servers":[{"command":"x","enabled":true}]}}`,
+		"both":        `{"mcp":{"servers":[{"name":"a","command":"x","url":"http://a.test","enabled":true}]}}`,
+		"neither":     `{"mcp":{"servers":[{"name":"a","enabled":true}]}}`,
+		"bad scheme":  `{"mcp":{"servers":[{"name":"a","url":"unix:///run/x.sock","enabled":true}]}}`,
+		"duplicate":   `{"mcp":{"servers":[{"name":"a","command":"x","enabled":true},{"name":"a","command":"y","enabled":true}]}}`,
+		"spacey name": `{"mcp":{"servers":[{"name":"a b","command":"x","enabled":true}]}}`,
+	}
+	for name, body := range cases {
+		t.Run(name, func(t *testing.T) {
+			if _, err := Load(writeConfig(t, body)); err == nil {
+				t.Fatal("want a startup error")
+			}
+		})
+	}
+}

internal/mcp/allowlist.go

@@ -0,0 +1,61 @@
+package mcp
+
+import (
+	"regexp"
+	"strings"
+)
+
+// CmdPrefix is the reserved first argv element that marks an allowlist row as
+// an MCP call rather than a process. An MCP tool row looks like
+//
+//	name: "vikunja_list_tasks"   cmd: ["mcp", "vikunja", "list_tasks"]
+//
+// which is why there is no new column and no migration: the store, the /tools
+// page, ProposeTool, EnableTool, DisableTool, the act matcher and the confirm
+// turn all keep working unchanged. The executor is the only place that has to
+// know the difference, and it is one branch on Cmd[0].
+//
+// The rest of the allowlist discipline is inherited whole: a row that is not
+// status='enabled' does not run, and a row marked destructive does not run on
+// first hearing. Nothing here can enable itself — discovery only proposes.
+const CmdPrefix = "mcp"
+
+// Cmd builds the argv encoding for a discovered tool.
+func Cmd(server, tool string) []string { return []string{CmdPrefix, server, tool} }
+
+// ParseCmd recognises an MCP allowlist row. ok=false for an ordinary process
+// tool, which is what almost every row is.
+func ParseCmd(cmd []string) (server, tool string, ok bool) {
+	if len(cmd) != 3 || cmd[0] != CmdPrefix {
+		return "", "", false
+	}
+	if cmd[1] == "" || cmd[2] == "" {
+		return "", "", false
+	}
+	return cmd[1], cmd[2], true
+}
+
+var notName = regexp.MustCompile(`[^a-z0-9_]+`)
+
+// LocalName is the allowlist name for a discovered tool: the server handle, an
+// underscore, the remote name, lowercased and stripped of anything that is not
+// a word character. Namespacing by server is what keeps two servers that both
+// offer "search" from colliding, and what makes the provenance of a row on the
+// /tools page obvious without opening the diff.
+func LocalName(server, tool string) string {
+	clean := func(s string) string {
+		return strings.Trim(notName.ReplaceAllString(strings.ToLower(strings.TrimSpace(s)), "_"), "_")
+	}
+	s, t := clean(server), clean(tool)
+	switch {
+	case s == "":
+		return t
+	case t == "":
+		return s
+	}
+	return s + "_" + t
+}
+
+// Scope is the store scope for a server's rows, so the /tools page can group
+// them and a human can tell at a glance where a capability came from.
+func Scope(server string) string { return "mcp:" + server }

internal/mcp/client.go

@@ -0,0 +1,267 @@
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+	"sync"
+	"sync/atomic"
+)
+
+// Errors callers distinguish.
+var (
+	// ErrClosed — the transport is gone (subprocess died, client closed).
+	ErrClosed = errors.New("mcp: connection is closed")
+	// ErrNotInitialized — a call was made before the initialize handshake.
+	ErrNotInitialized = errors.New("mcp: not initialized")
+	// ErrToolFailed — the server ran the tool and reported an error result.
+	ErrToolFailed = errors.New("mcp: tool reported an error")
+)
+
+// Tool is one tool a server offers, in the form Maven cares about.
+//
+// ReadOnly comes from the server's own readOnlyHint annotation and decides
+// whether the allowlist row is marked destructive: no hint, or a false one,
+// means "assume it mutates", which routes the call through the confirm turn.
+// Guessing wrong in that direction only costs a question.
+type Tool struct {
+	Server      string
+	Name        string
+	Description string
+	InputSchema json.RawMessage
+	ReadOnly    bool
+}
+
+// Resource is one resource a server offers. Contents are fetched separately —
+// listing is cheap, reading is not.
+type Resource struct {
+	Server   string
+	URI      string
+	Name     string
+	MIMEType string
+}
+
+// ServerInfo is what came back from the handshake.
+type ServerInfo struct {
+	Name            string `json:"name"`
+	Version         string `json:"version"`
+	ProtocolVersion string `json:"-"`
+}
+
+// Client is one connected MCP server. Safe for concurrent use.
+type Client struct {
+	name string
+	tr   transport
+	next atomic.Int64
+
+	mu    sync.Mutex
+	info  ServerInfo
+	ready bool
+}
+
+// newClient wraps a transport. Callers use Dial* in manager.go.
+func newClient(name string, tr transport) *Client {
+	return &Client{name: name, tr: tr}
+}
+
+// Name — the local name of this server (the config key, not the server's own).
+func (c *Client) Name() string { return c.name }
+
+// Info — what the server said about itself during the handshake.
+func (c *Client) Info() ServerInfo {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.info
+}
+
+// Initialize performs the MCP handshake and sends notifications/initialized.
+// Capabilities we declare are empty on purpose: Maven consumes, she does not
+// offer sampling or roots back to the server.
+func (c *Client) Initialize(ctx context.Context) error {
+	var out struct {
+		ProtocolVersion string     `json:"protocolVersion"`
+		ServerInfo      ServerInfo `json:"serverInfo"`
+	}
+	err := c.call(ctx, "initialize", map[string]any{
+		"protocolVersion": ProtocolVersion,
+		"capabilities":    map[string]any{},
+		"clientInfo":      map[string]any{"name": "maven", "version": "1.0"},
+	}, &out)
+	if err != nil {
+		return err
+	}
+	if strings.TrimSpace(out.ProtocolVersion) == "" {
+		return fmt.Errorf("mcp: %s: handshake returned no protocol version", c.name)
+	}
+	out.ServerInfo.ProtocolVersion = out.ProtocolVersion
+	c.mu.Lock()
+	c.info, c.ready = out.ServerInfo, true
+	c.mu.Unlock()
+	// Best effort: a stateless HTTP server may not care, and a failure here is
+	// not worth dropping a working connection over.
+	_ = c.tr.Notify(ctx, "notifications/initialized", map[string]any{})
+	return nil
+}
+
+// ListTools discovers the server's tools.
+func (c *Client) ListTools(ctx context.Context) ([]Tool, error) {
+	if !c.initialized() {
+		return nil, ErrNotInitialized
+	}
+	var out struct {
+		Tools []struct {
+			Name        string          `json:"name"`
+			Description string          `json:"description"`
+			InputSchema json.RawMessage `json:"inputSchema"`
+			Annotations *struct {
+				ReadOnlyHint bool `json:"readOnlyHint"`
+			} `json:"annotations"`
+		} `json:"tools"`
+	}
+	if err := c.call(ctx, "tools/list", map[string]any{}, &out); err != nil {
+		return nil, err
+	}
+	tools := make([]Tool, 0, len(out.Tools))
+	for _, t := range out.Tools {
+		if strings.TrimSpace(t.Name) == "" {
+			continue
+		}
+		tools = append(tools, Tool{
+			Server:      c.name,
+			Name:        t.Name,
+			Description: strings.TrimSpace(t.Description),
+			InputSchema: t.InputSchema,
+			ReadOnly:    t.Annotations != nil && t.Annotations.ReadOnlyHint,
+		})
+	}
+	return tools, nil
+}
+
+// CallTool runs one tool and returns its text content, joined by newlines.
+// Non-text content (images, blobs) is dropped: everything downstream of here
+// is a spoken or written sentence.
+//
+// args is exactly what the router produced. Nothing else — no history, no
+// notes, no persona — is in scope here, by construction.
+func (c *Client) CallTool(ctx context.Context, name string, args map[string]any) (string, error) {
+	if !c.initialized() {
+		return "", ErrNotInitialized
+	}
+	if args == nil {
+		args = map[string]any{}
+	}
+	var out struct {
+		IsError bool `json:"isError"`
+		Content []struct {
+			Type string `json:"type"`
+			Text string `json:"text"`
+		} `json:"content"`
+	}
+	if err := c.call(ctx, "tools/call", map[string]any{"name": name, "arguments": args}, &out); err != nil {
+		return "", err
+	}
+	var parts []string
+	for _, ct := range out.Content {
+		if ct.Type == "text" && strings.TrimSpace(ct.Text) != "" {
+			parts = append(parts, strings.TrimSpace(ct.Text))
+		}
+	}
+	text := strings.Join(parts, "\n")
+	if out.IsError {
+		return text, fmt.Errorf("%w: %s/%s: %s", ErrToolFailed, c.name, name, text)
+	}
+	return text, nil
+}
+
+// ListResources discovers the server's resources. A server without the
+// resources capability answers with an error; that is not fatal, the caller
+// gets an empty list.
+func (c *Client) ListResources(ctx context.Context) ([]Resource, error) {
+	if !c.initialized() {
+		return nil, ErrNotInitialized
+	}
+	var out struct {
+		Resources []struct {
+			URI      string `json:"uri"`
+			Name     string `json:"name"`
+			MIMEType string `json:"mimeType"`
+		} `json:"resources"`
+	}
+	if err := c.call(ctx, "resources/list", map[string]any{}, &out); err != nil {
+		return nil, err
+	}
+	res := make([]Resource, 0, len(out.Resources))
+	for _, r := range out.Resources {
+		if strings.TrimSpace(r.URI) == "" {
+			continue
+		}
+		res = append(res, Resource{Server: c.name, URI: r.URI, Name: r.Name, MIMEType: r.MIMEType})
+	}
+	return res, nil
+}
+
+// ReadResource returns a resource's text contents, joined by newlines. This is
+// the RAG-hint path: the text can be pasted into a router or phraser prompt.
+func (c *Client) ReadResource(ctx context.Context, uri string) (string, error) {
+	if !c.initialized() {
+		return "", ErrNotInitialized
+	}
+	var out struct {
+		Contents []struct {
+			Text string `json:"text"`
+		} `json:"contents"`
+	}
+	if err := c.call(ctx, "resources/read", map[string]any{"uri": uri}, &out); err != nil {
+		return "", err
+	}
+	var parts []string
+	for _, ct := range out.Contents {
+		if strings.TrimSpace(ct.Text) != "" {
+			parts = append(parts, strings.TrimSpace(ct.Text))
+		}
+	}
+	return strings.Join(parts, "\n"), nil
+}
+
+// Close drops the connection.
+func (c *Client) Close() error {
+	c.mu.Lock()
+	c.ready = false
+	c.mu.Unlock()
+	return c.tr.Close()
+}
+
+func (c *Client) initialized() bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.ready
+}
+
+// alive reports whether the underlying transport can still carry a call. HTTP
+// is stateless, so it is always alive; a dead subprocess is not.
+func (c *Client) alive() bool {
+	if s, ok := c.tr.(*stdioTransport); ok {
+		return s.alive()
+	}
+	return true
+}
+
+func (c *Client) call(ctx context.Context, method string, params any, out any) error {
+	req := &rpcRequest{JSONRPC: "2.0", ID: c.next.Add(1), Method: method, Params: params}
+	resp, err := c.tr.Call(ctx, req)
+	if err != nil {
+		return fmt.Errorf("mcp: %s: %s: %w", c.name, method, err)
+	}
+	if resp.Error != nil {
+		return fmt.Errorf("mcp: %s: %s: %w", c.name, method, resp.Error)
+	}
+	if out == nil || len(resp.Result) == 0 {
+		return nil
+	}
+	if err := json.Unmarshal(resp.Result, out); err != nil {
+		return fmt.Errorf("mcp: %s: %s: decode result: %w", c.name, method, err)
+	}
+	return nil
+}

internal/mcp/http.go

@@ -0,0 +1,154 @@
+package mcp
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+	"sync"
+)
+
+// Poster is the HTTP seam: internal/webfetch.Fetcher satisfies it. The
+// transport takes it as an interface so a test can serve a fake without a
+// listener, and so that the ONLY implementation wired in production is the
+// guarded fetcher — an MCP endpoint cannot get a bare http.Client this way.
+type Poster interface {
+	Post(ctx context.Context, rawURL, contentType string, body []byte, hdr map[string]string) (*PostResponse, error)
+}
+
+// PostResponse is the shape webfetch returns, restated here so this package
+// does not depend on it structurally.
+type PostResponse struct {
+	Status      int
+	ContentType string
+	Body        []byte
+	Header      map[string]string
+}
+
+// httpTransport speaks streamable HTTP: every request is a POST to one
+// endpoint, and the reply is either a JSON object or a text/event-stream frame
+// carrying one. Both are accepted — servers pick per response, and the two the
+// LAN runs disagree about which.
+type httpTransport struct {
+	poster Poster
+	url    string
+
+	mu      sync.Mutex
+	session string // Mcp-Session-Id, echoed back when the server issues one
+}
+
+func newHTTPTransport(post Poster, endpoint string) *httpTransport {
+	return &httpTransport{poster: post, url: endpoint}
+}
+
+func (t *httpTransport) Call(ctx context.Context, req *rpcRequest) (*rpcResponse, error) {
+	body, err := t.send(ctx, req)
+	if err != nil {
+		return nil, err
+	}
+	frame, err := decodeFrame(body)
+	if err != nil {
+		return nil, err
+	}
+	var resp rpcResponse
+	if err := json.Unmarshal(frame, &resp); err != nil {
+		return nil, fmt.Errorf("mcp: decode response: %w", err)
+	}
+	return &resp, nil
+}
+
+func (t *httpTransport) Notify(ctx context.Context, method string, params any) error {
+	_, err := t.send(ctx, &rpcRequest{JSONRPC: "2.0", Method: method, Params: params})
+	return err
+}
+
+func (t *httpTransport) send(ctx context.Context, req *rpcRequest) ([]byte, error) {
+	req.JSONRPC = "2.0"
+	raw, err := json.Marshal(req)
+	if err != nil {
+		return nil, err
+	}
+	hdr := map[string]string{"Accept": "application/json, text/event-stream"}
+	t.mu.Lock()
+	if t.session != "" {
+		hdr["Mcp-Session-Id"] = t.session
+	}
+	t.mu.Unlock()
+
+	resp, err := t.poster.Post(ctx, t.url, "application/json", raw, hdr)
+	if err != nil {
+		return nil, err
+	}
+	if sid := headerGet(resp.Header, "Mcp-Session-Id"); sid != "" {
+		t.mu.Lock()
+		t.session = sid
+		t.mu.Unlock()
+	}
+	return resp.Body, nil
+}
+
+func (t *httpTransport) Close() error {
+	t.mu.Lock()
+	t.session = ""
+	t.mu.Unlock()
+	return nil
+}
+
+func headerGet(h map[string]string, key string) string {
+	if h == nil {
+		return ""
+	}
+	if v, ok := h[key]; ok {
+		return v
+	}
+	lower := strings.ToLower(key)
+	for k, v := range h {
+		if strings.ToLower(k) == lower {
+			return v
+		}
+	}
+	return ""
+}
+
+// decodeFrame pulls the JSON object out of a body that is either raw JSON or
+// SSE. For SSE we take the LAST data: payload that parses, which is the
+// response — earlier frames on the stream are progress notifications.
+func decodeFrame(body []byte) ([]byte, error) {
+	trimmed := bytes.TrimSpace(body)
+	if len(trimmed) == 0 {
+		return nil, errors.New("mcp: empty response body")
+	}
+	if trimmed[0] == '{' || trimmed[0] == '[' {
+		return trimmed, nil
+	}
+	var last []byte
+	sc := bufio.NewScanner(bytes.NewReader(trimmed))
+	sc.Buffer(make([]byte, 0, 64<<10), maxLine)
+	for sc.Scan() {
+		line := strings.TrimSpace(sc.Text())
+		if !strings.HasPrefix(line, "data:") {
+			continue
+		}
+		payload := strings.TrimSpace(strings.TrimPrefix(line, "data:"))
+		if payload == "" {
+			continue
+		}
+		var probe map[string]json.RawMessage
+		if json.Unmarshal([]byte(payload), &probe) != nil {
+			continue
+		}
+		if _, isResp := probe["id"]; isResp {
+			last = []byte(payload)
+		}
+	}
+	if err := sc.Err(); err != nil {
+		return nil, fmt.Errorf("mcp: read event stream: %w", err)
+	}
+	if last == nil {
+		return nil, errors.New("mcp: no JSON-RPC response in event stream")
+	}
+	return last, nil
+}

internal/mcp/jsonrpc.go

@@ -0,0 +1,72 @@
+// Package mcp is Maven's Model Context Protocol CLIENT. She is a host: she
+// connects OUT to MCP servers, discovers the tools and resources they offer,
+// and hands them to the parts of her that already exist for this — the tool
+// allowlist in the store, the confirm turn for anything that mutates, the
+// stage-3 gate that makes an uncertain act ask instead of run.
+//
+// She is not an MCP server. Nothing here exposes her own capabilities to an
+// outside caller; docs/plans/06-mcp-support.md asks for the host direction only.
+//
+// Boundaries, in code rather than in prose:
+//
+//   - OFF unless configured. No mcp_servers block ⇒ no manager, no goroutine,
+//     no socket.
+//   - A remote server is reached through internal/webfetch, so the SSRF guard,
+//     the size cap, the redirect cap and the per-host rate limit all apply to
+//     an MCP endpoint exactly as they do to a news feed. Reaching a loopback
+//     or LAN server means explicitly setting allow_private on THAT server —
+//     a different trust level, spelled out per server rather than globally.
+//   - Only the tool name and the arguments the router produced are sent. This
+//     package never sees his notes, facts, history or the persona block, and
+//     has no API through which a caller could pass them.
+//   - Discovery proposes, it does not enable. A discovered tool lands as a
+//     'proposed' row; a human enables it on the authed surface.
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+)
+
+// ProtocolVersion — the spec revision we ask for in the initialize handshake.
+// A server that answers with a different one is accepted (the spec says the
+// client may proceed if it can support what came back); we only refuse when it
+// answers with no version at all, which means it is not an MCP server.
+const ProtocolVersion = "2025-06-18"
+
+// rpcRequest / rpcResponse — JSON-RPC 2.0. Deliberately hand-rolled: the wire
+// format is four fields, and the repo vendors its dependencies, so pulling a
+// library in for this would cost more than it saves.
+type rpcRequest struct {
+	JSONRPC string `json:"jsonrpc"`
+	ID      int64  `json:"id,omitempty"`
+	Method  string `json:"method"`
+	Params  any    `json:"params,omitempty"`
+}
+
+type rpcResponse struct {
+	JSONRPC string          `json:"jsonrpc"`
+	ID      *int64          `json:"id"`
+	Result  json.RawMessage `json:"result,omitempty"`
+	Error   *rpcError       `json:"error,omitempty"`
+}
+
+type rpcError struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+}
+
+func (e *rpcError) Error() string { return fmt.Sprintf("mcp: rpc error %d: %s", e.Code, e.Message) }
+
+// transport carries one JSON-RPC conversation. Implementations: stdioTransport
+// (a subprocess on this box) and httpTransport (streamable HTTP, guarded by
+// webfetch). Both must be safe for concurrent use by the Client.
+type transport interface {
+	// Call sends a request and returns the matching response.
+	Call(ctx context.Context, req *rpcRequest) (*rpcResponse, error)
+	// Notify sends a notification (no id, no reply expected).
+	Notify(ctx context.Context, method string, params any) error
+	// Close releases the transport (kills the subprocess, drops the session).
+	Close() error
+}

internal/mcp/manager.go

@@ -0,0 +1,431 @@
+package mcp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Defaults for a server block. Small numbers on purpose — see MaxTools.
+const (
+	// DefaultTimeout bounds one JSON-RPC call. A tool that takes longer than
+	// this is not usable in a spoken turn anyway.
+	DefaultTimeout = 15 * time.Second
+	// DefaultMaxTools caps how many tools ONE server may contribute. The
+	// resident model is a 1.7B with a 4096-token context: a catalogue of forty
+	// tool names does not fit in its head, and a name it half-remembers is a
+	// wrong act. Twelve per server is already generous.
+	DefaultMaxTools = 12
+	// DefaultReconnectEvery is how long the manager waits before re-dialing a
+	// server whose connection died.
+	DefaultReconnectEvery = 30 * time.Second
+)
+
+// ErrNoServer — the named server is not configured or not connected.
+var ErrNoServer = errors.New("mcp: no such server")
+
+// ServerConfig is one configured MCP server. Off unless present.
+//
+// Exactly one of Command (a subprocess on this box) or URL (a remote or
+// loopback HTTP endpoint) must be set.
+type ServerConfig struct {
+	// Name is the local handle. It prefixes every tool this server
+	// contributes, so it must be short and a valid identifier-ish word.
+	Name string `json:"name"`
+	// Command + Args + Env + Dir describe a stdio server: a child process of
+	// mavend, on this box, under this user. argv, never a shell string.
+	Command string   `json:"command,omitempty"`
+	Args    []string `json:"args,omitempty"`
+	Env     []string `json:"env,omitempty"`
+	Dir     string   `json:"dir,omitempty"`
+	// URL is a streamable-HTTP endpoint. It goes through internal/webfetch, so
+	// it inherits the SSRF guard, the size cap and the per-host rate limit.
+	URL string `json:"url,omitempty"`
+	// AllowPrivate lets THIS server be a loopback or LAN address
+	// (http://localhost:9100/mcp is the Vikunja server on homesrv). It is a
+	// per-server hole in the private-address guard and it is not the same trust
+	// level as a public endpoint: whatever is behind it is inside the network,
+	// so an argument the router got wrong reaches something that matters. Set
+	// it only for a server you run.
+	AllowPrivate bool `json:"allow_private,omitempty"`
+	// AllowTools, when non-empty, is the ONLY set of remote tool names taken
+	// from this server. This is the knob for keeping the catalogue small and
+	// deliberate rather than "whatever the server grew this week".
+	AllowTools []string `json:"allow_tools,omitempty"`
+	// MaxTools caps the contribution (0 ⇒ DefaultMaxTools).
+	MaxTools int `json:"max_tools,omitempty"`
+	// Timeout bounds one call (0 ⇒ DefaultTimeout).
+	Timeout time.Duration `json:"-"`
+	// Enabled=false keeps a configured server described but dark.
+	Enabled bool `json:"enabled"`
+}
+
+// PosterFactory builds the HTTP door for one server. It is a factory rather
+// than a single shared Poster because allow_private is per server: the fetcher
+// that may reach http://localhost:9100/mcp must NOT be the same fetcher another
+// server's public URL goes through, or one loopback exemption would quietly
+// unlock the LAN for all of them.
+type PosterFactory func(cfg ServerConfig) (Poster, error)
+
+// Manager owns the connections. Nothing here starts unless at least one server
+// is configured and enabled.
+type Manager struct {
+	newPoster PosterFactory
+	mu        sync.Mutex
+	conns     map[string]*conn
+	order     []string
+}
+
+type conn struct {
+	cfg      ServerConfig
+	client   *Client
+	tools    []Tool
+	lastErr  error
+	lastTry  time.Time
+	dialedAt time.Time
+}
+
+// NewManager builds a manager for the enabled servers in cfgs. newPoster is
+// the guarded HTTP door factory for url servers; pass nil only when no url
+// server is configured (a nil factory with a url server is reported per server
+// at dial time rather than fatally, so one bad block never stops the daemon).
+//
+// Dialing is lazy: NewManager validates and records, Connect dials.
+func NewManager(newPoster PosterFactory, cfgs []ServerConfig) (*Manager, error) {
+	m := &Manager{newPoster: newPoster, conns: map[string]*conn{}}
+	for _, c := range cfgs {
+		if !c.Enabled {
+			continue
+		}
+		if err := validate(c); err != nil {
+			return nil, err
+		}
+		if _, dup := m.conns[c.Name]; dup {
+			return nil, fmt.Errorf("mcp: duplicate server name %q", c.Name)
+		}
+		if c.Timeout <= 0 {
+			c.Timeout = DefaultTimeout
+		}
+		if c.MaxTools <= 0 {
+			c.MaxTools = DefaultMaxTools
+		}
+		m.conns[c.Name] = &conn{cfg: c}
+		m.order = append(m.order, c.Name)
+	}
+	sort.Strings(m.order)
+	return m, nil
+}
+
+// Validate checks a set of server blocks without dialling anything, so a typo
+// fails at startup rather than at the first turn that needed the tool.
+func Validate(cfgs []ServerConfig) error {
+	seen := map[string]bool{}
+	for _, c := range cfgs {
+		if err := validate(c); err != nil {
+			return err
+		}
+		if seen[c.Name] {
+			return fmt.Errorf("mcp: duplicate server name %q", c.Name)
+		}
+		seen[c.Name] = true
+	}
+	return nil
+}
+
+func validate(c ServerConfig) error {
+	if strings.TrimSpace(c.Name) == "" {
+		return errors.New("mcp: server needs a name")
+	}
+	if strings.ContainsAny(c.Name, " \t/:") {
+		return fmt.Errorf("mcp: server name %q must be one word without spaces, slashes or colons", c.Name)
+	}
+	hasCmd, hasURL := c.Command != "", c.URL != ""
+	if hasCmd == hasURL {
+		return fmt.Errorf("mcp: server %q needs exactly one of command or url", c.Name)
+	}
+	if hasURL && !strings.HasPrefix(c.URL, "http://") && !strings.HasPrefix(c.URL, "https://") {
+		return fmt.Errorf("mcp: server %q url must be http or https", c.Name)
+	}
+	return nil
+}
+
+// Servers — the configured, enabled server names, sorted.
+func (m *Manager) Servers() []string {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return append([]string(nil), m.order...)
+}
+
+// Empty reports whether nothing is configured. The daemon uses it to skip
+// wiring entirely.
+func (m *Manager) Empty() bool {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return len(m.conns) == 0
+}
+
+// Connect dials every configured server, handshakes, and discovers tools.
+// A server that fails is recorded and retried later by Refresh — one bad
+// server never blocks the others, and never blocks boot.
+func (m *Manager) Connect(ctx context.Context) {
+	for _, name := range m.Servers() {
+		if err := m.dial(ctx, name); err != nil {
+			log.Printf("mcp: %s: %v", name, err)
+		}
+	}
+}
+
+func (m *Manager) dial(ctx context.Context, name string) error {
+	m.mu.Lock()
+	c, ok := m.conns[name]
+	if !ok {
+		m.mu.Unlock()
+		return ErrNoServer
+	}
+	cfg := c.cfg
+	c.lastTry = time.Now()
+	m.mu.Unlock()
+
+	var tr transport
+	var err error
+	if cfg.Command != "" {
+		tr, err = newStdioTransport(ctx, append([]string{cfg.Command}, cfg.Args...), cfg.Env, cfg.Dir)
+	} else if m.newPoster == nil {
+		err = fmt.Errorf("server %q has a url but no http door was wired", name)
+	} else {
+		var poster Poster
+		if poster, err = m.newPoster(cfg); err == nil {
+			tr = newHTTPTransport(poster, cfg.URL)
+		}
+	}
+	if err != nil {
+		m.fail(name, err)
+		return err
+	}
+
+	cl := newClient(name, tr)
+	ictx, cancel := context.WithTimeout(ctx, cfg.Timeout)
+	defer cancel()
+	if err := cl.Initialize(ictx); err != nil {
+		_ = cl.Close()
+		m.fail(name, err)
+		return err
+	}
+	tools, err := cl.ListTools(ictx)
+	if err != nil {
+		// A server with no tools capability is still a usable resource server.
+		log.Printf("mcp: %s: list tools: %v", name, err)
+		tools = nil
+	}
+	tools = filterTools(cfg, tools)
+
+	m.mu.Lock()
+	if old := m.conns[name].client; old != nil {
+		_ = old.Close()
+	}
+	m.conns[name].client = cl
+	m.conns[name].tools = tools
+	m.conns[name].lastErr = nil
+	m.conns[name].dialedAt = time.Now()
+	m.mu.Unlock()
+	log.Printf("mcp: %s connected (%s %s), %d tool(s)", name, cl.Info().Name, cl.Info().Version, len(tools))
+	return nil
+}
+
+func (m *Manager) fail(name string, err error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if c := m.conns[name]; c != nil {
+		c.lastErr = err
+		c.client = nil
+		c.tools = nil
+	}
+}
+
+// filterTools applies AllowTools and MaxTools, and drops nameless entries.
+// Sorted first, so the cap is deterministic rather than "whatever order the
+// server felt like".
+func filterTools(cfg ServerConfig, in []Tool) []Tool {
+	sort.Slice(in, func(i, j int) bool { return in[i].Name < in[j].Name })
+	out := make([]Tool, 0, len(in))
+	for _, t := range in {
+		if len(cfg.AllowTools) > 0 && !contains(cfg.AllowTools, t.Name) {
+			continue
+		}
+		out = append(out, t)
+	}
+	if cfg.MaxTools > 0 && len(out) > cfg.MaxTools {
+		log.Printf("mcp: %s offers %d tools, taking the first %d (raise max_tools or set allow_tools)",
+			cfg.Name, len(out), cfg.MaxTools)
+		out = out[:cfg.MaxTools]
+	}
+	return out
+}
+
+func contains(hay []string, needle string) bool {
+	for _, h := range hay {
+		if h == needle {
+			return true
+		}
+	}
+	return false
+}
+
+// Refresh re-dials any server that is down, if enough time has passed since the
+// last attempt. Call it from the daemon's periodic tick — it is cheap when
+// everything is up.
+func (m *Manager) Refresh(ctx context.Context) {
+	now := time.Now()
+	var stale []string
+	m.mu.Lock()
+	for _, name := range m.order {
+		c := m.conns[name]
+		down := c.client == nil || !c.client.alive()
+		if down && now.Sub(c.lastTry) >= DefaultReconnectEvery {
+			stale = append(stale, name)
+		}
+	}
+	m.mu.Unlock()
+	for _, name := range stale {
+		if err := m.dial(ctx, name); err != nil {
+			log.Printf("mcp: %s: reconnect: %v", name, err)
+		}
+	}
+}
+
+// Tools — every discovered tool across connected servers, sorted by
+// server then name.
+func (m *Manager) Tools() []Tool {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	var out []Tool
+	for _, name := range m.order {
+		out = append(out, m.conns[name].tools...)
+	}
+	return out
+}
+
+// Status is one server's health, for the web surface.
+type Status struct {
+	Name      string
+	Transport string // "stdio" or "http"
+	Target    string // command or url
+	Connected bool
+	Server    string // the server's own name+version
+	Tools     int
+	Err       string
+}
+
+// Status reports every configured server.
+func (m *Manager) Status() []Status {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	out := make([]Status, 0, len(m.order))
+	for _, name := range m.order {
+		c := m.conns[name]
+		s := Status{Name: name, Tools: len(c.tools)}
+		if c.cfg.Command != "" {
+			s.Transport, s.Target = "stdio", strings.Join(append([]string{c.cfg.Command}, c.cfg.Args...), " ")
+		} else {
+			s.Transport, s.Target = "http", c.cfg.URL
+		}
+		if c.client != nil {
+			s.Connected = true
+			s.Server = strings.TrimSpace(c.client.Info().Name + " " + c.client.Info().Version)
+		}
+		if c.lastErr != nil {
+			s.Err = c.lastErr.Error()
+		}
+		out = append(out, s)
+	}
+	return out
+}
+
+// Call runs server's tool with args. Args come from the router and nothing
+// else; there is no path here through which a note or a fact could travel.
+func (m *Manager) Call(ctx context.Context, server, tool string, args map[string]any) (string, error) {
+	m.mu.Lock()
+	c := m.conns[server]
+	m.mu.Unlock()
+	if c == nil {
+		return "", fmt.Errorf("%w: %s", ErrNoServer, server)
+	}
+	m.mu.Lock()
+	cl, timeout, known := c.client, c.cfg.Timeout, false
+	for _, t := range c.tools {
+		if t.Name == tool {
+			known = true
+			break
+		}
+	}
+	m.mu.Unlock()
+	if cl == nil {
+		return "", fmt.Errorf("mcp: %s is not connected", server)
+	}
+	// The discovered-and-filtered set is the second allowlist: even an enabled
+	// store row cannot reach a tool the server stopped offering, or one
+	// allow_tools excludes.
+	if !known {
+		return "", fmt.Errorf("mcp: %s offers no tool %q", server, tool)
+	}
+	cctx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+	return cl.CallTool(cctx, tool, args)
+}
+
+// Resources lists resources across connected servers.
+func (m *Manager) Resources(ctx context.Context) []Resource {
+	m.mu.Lock()
+	clients := make([]*Client, 0, len(m.order))
+	for _, name := range m.order {
+		if cl := m.conns[name].client; cl != nil {
+			clients = append(clients, cl)
+		}
+	}
+	m.mu.Unlock()
+	var out []Resource
+	for _, cl := range clients {
+		rs, err := cl.ListResources(ctx)
+		if err != nil {
+			continue // no resources capability; not an error worth logging per tick
+		}
+		out = append(out, rs...)
+	}
+	return out
+}
+
+// ReadResource reads one resource from one server.
+func (m *Manager) ReadResource(ctx context.Context, server, uri string) (string, error) {
+	m.mu.Lock()
+	c := m.conns[server]
+	var cl *Client
+	var timeout time.Duration
+	if c != nil {
+		cl, timeout = c.client, c.cfg.Timeout
+	}
+	m.mu.Unlock()
+	if cl == nil {
+		return "", fmt.Errorf("%w: %s", ErrNoServer, server)
+	}
+	cctx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+	return cl.ReadResource(cctx, uri)
+}
+
+// Close shuts every connection down.
+func (m *Manager) Close() error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	for _, name := range m.order {
+		if cl := m.conns[name].client; cl != nil {
+			_ = cl.Close()
+			m.conns[name].client = nil
+		}
+	}
+	return nil
+}

internal/mcp/mcp_test.go

@@ -0,0 +1,446 @@
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// fakePoster answers POSTs from a canned handler, in either JSON or SSE form.
+type fakePoster struct {
+	mu      sync.Mutex
+	handler func(method string, params json.RawMessage) (any, *rpcError)
+	sse     bool
+	session string
+	seen    []map[string]string // headers of each request, for the session test
+	calls   []string
+}
+
+func (f *fakePoster) Post(_ context.Context, _, _ string, body []byte, hdr map[string]string) (*PostResponse, error) {
+	var req struct {
+		ID     *int64          `json:"id"`
+		Method string          `json:"method"`
+		Params json.RawMessage `json:"params"`
+	}
+	if err := json.Unmarshal(body, &req); err != nil {
+		return nil, err
+	}
+	f.mu.Lock()
+	f.seen = append(f.seen, hdr)
+	f.calls = append(f.calls, req.Method)
+	f.mu.Unlock()
+
+	if req.ID == nil { // notification
+		return &PostResponse{Status: 202, Body: []byte(`{}`)}, nil
+	}
+	result, rerr := f.handler(req.Method, req.Params)
+	resp := map[string]any{"jsonrpc": "2.0", "id": *req.ID}
+	if rerr != nil {
+		resp["error"] = map[string]any{"code": rerr.Code, "message": rerr.Message}
+	} else {
+		resp["result"] = result
+	}
+	raw, _ := json.Marshal(resp)
+	out := &PostResponse{Status: 200, Body: raw, ContentType: "application/json", Header: map[string]string{}}
+	if f.sse {
+		out.ContentType = "text/event-stream"
+		out.Body = []byte("event: message\ndata: {\"jsonrpc\":\"2.0\",\"method\":\"notifications/progress\"}\n\nevent: message\ndata: " + string(raw) + "\n\n")
+	}
+	if f.session != "" {
+		out.Header["Mcp-Session-Id"] = f.session
+	}
+	return out, nil
+}
+
+// echoServer is a handler with two tools, one read-only and one not.
+func echoServer() func(string, json.RawMessage) (any, *rpcError) {
+	return func(method string, params json.RawMessage) (any, *rpcError) {
+		switch method {
+		case "initialize":
+			return map[string]any{
+				"protocolVersion": ProtocolVersion,
+				"serverInfo":      map[string]any{"name": "fake", "version": "0.1"},
+			}, nil
+		case "tools/list":
+			return map[string]any{"tools": []any{
+				map[string]any{
+					"name": "read_thing", "description": "reads",
+					"inputSchema": map[string]any{"type": "object"},
+					"annotations": map[string]any{"readOnlyHint": true},
+				},
+				map[string]any{"name": "break_thing", "description": "mutates"},
+			}}, nil
+		case "tools/call":
+			var p struct {
+				Name string         `json:"name"`
+				Args map[string]any `json:"arguments"`
+			}
+			_ = json.Unmarshal(params, &p)
+			if p.Name == "break_thing" {
+				return map[string]any{"isError": true, "content": []any{
+					map[string]any{"type": "text", "text": "не вышло"}}}, nil
+			}
+			return map[string]any{"content": []any{
+				map[string]any{"type": "text", "text": fmt.Sprintf("%s:%v", p.Name, p.Args["q"])},
+				map[string]any{"type": "image", "text": "ignored"},
+			}}, nil
+		case "resources/list":
+			return map[string]any{"resources": []any{
+				map[string]any{"uri": "note://one", "name": "one", "mimeType": "text/plain"},
+				map[string]any{"uri": "", "name": "nameless"},
+			}}, nil
+		case "resources/read":
+			return map[string]any{"contents": []any{map[string]any{"text": "тело ресурса"}}}, nil
+		}
+		return nil, &rpcError{Code: -32601, Message: "method not found"}
+	}
+}
+
+func dialFake(t *testing.T, p *fakePoster) *Client {
+	t.Helper()
+	c := newClient("fake", newHTTPTransport(p, "http://example.test/mcp"))
+	if err := c.Initialize(context.Background()); err != nil {
+		t.Fatalf("initialize: %v", err)
+	}
+	return c
+}
+
+func TestHandshakeAndDiscovery(t *testing.T) {
+	for _, sse := range []bool{false, true} {
+		name := "json"
+		if sse {
+			name = "sse"
+		}
+		t.Run(name, func(t *testing.T) {
+			p := &fakePoster{handler: echoServer(), sse: sse}
+			c := dialFake(t, p)
+			if got := c.Info().Name; got != "fake" {
+				t.Fatalf("server name = %q", got)
+			}
+			if got := c.Info().ProtocolVersion; got != ProtocolVersion {
+				t.Fatalf("protocol = %q", got)
+			}
+			tools, err := c.ListTools(context.Background())
+			if err != nil {
+				t.Fatalf("list tools: %v", err)
+			}
+			if len(tools) != 2 {
+				t.Fatalf("tools = %+v", tools)
+			}
+			byName := map[string]Tool{}
+			for _, tl := range tools {
+				byName[tl.Name] = tl
+			}
+			if !byName["read_thing"].ReadOnly {
+				t.Error("read_thing should be read-only (readOnlyHint true)")
+			}
+			// The important direction: no annotation ⇒ assume it mutates.
+			if byName["break_thing"].ReadOnly {
+				t.Error("break_thing has no readOnlyHint, must NOT be treated as read-only")
+			}
+			if byName["read_thing"].Server != "fake" {
+				t.Error("tool should carry its server handle")
+			}
+		})
+	}
+}
+
+func TestCallToolTextOnly(t *testing.T) {
+	c := dialFake(t, &fakePoster{handler: echoServer()})
+	out, err := c.CallTool(context.Background(), "read_thing", map[string]any{"q": "привет"})
+	if err != nil {
+		t.Fatalf("call: %v", err)
+	}
+	if out != "read_thing:привет" {
+		t.Fatalf("out = %q (non-text content must be dropped)", out)
+	}
+}
+
+func TestCallToolErrorResult(t *testing.T) {
+	c := dialFake(t, &fakePoster{handler: echoServer()})
+	out, err := c.CallTool(context.Background(), "break_thing", nil)
+	if err == nil {
+		t.Fatal("isError result must surface as an error")
+	}
+	if out != "не вышло" {
+		t.Fatalf("text should still come back, got %q", out)
+	}
+}
+
+func TestResources(t *testing.T) {
+	c := dialFake(t, &fakePoster{handler: echoServer()})
+	rs, err := c.ListResources(context.Background())
+	if err != nil {
+		t.Fatalf("list resources: %v", err)
+	}
+	if len(rs) != 1 || rs[0].URI != "note://one" {
+		t.Fatalf("resources = %+v (a uri-less entry must be dropped)", rs)
+	}
+	body, err := c.ReadResource(context.Background(), "note://one")
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if body != "тело ресурса" {
+		t.Fatalf("body = %q", body)
+	}
+}
+
+func TestCallBeforeInitializeRefused(t *testing.T) {
+	c := newClient("fake", newHTTPTransport(&fakePoster{handler: echoServer()}, "http://example.test/mcp"))
+	if _, err := c.CallTool(context.Background(), "read_thing", nil); err != ErrNotInitialized {
+		t.Fatalf("err = %v, want ErrNotInitialized", err)
+	}
+}
+
+func TestSessionIDEchoed(t *testing.T) {
+	p := &fakePoster{handler: echoServer(), session: "sess-1"}
+	c := dialFake(t, p)
+	if _, err := c.ListTools(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	last := p.seen[len(p.seen)-1]
+	if last["Mcp-Session-Id"] != "sess-1" {
+		t.Fatalf("session header not echoed: %+v", last)
+	}
+	if !strings.Contains(last["Accept"], "text/event-stream") {
+		t.Fatalf("Accept must offer both forms: %q", last["Accept"])
+	}
+}
+
+func TestHandshakeWithoutProtocolVersionRefused(t *testing.T) {
+	p := &fakePoster{handler: func(m string, _ json.RawMessage) (any, *rpcError) {
+		return map[string]any{"serverInfo": map[string]any{"name": "not-mcp"}}, nil
+	}}
+	c := newClient("x", newHTTPTransport(p, "http://example.test/mcp"))
+	if err := c.Initialize(context.Background()); err == nil {
+		t.Fatal("a reply with no protocolVersion is not an MCP server")
+	}
+}
+
+func TestRPCErrorSurfaces(t *testing.T) {
+	c := dialFake(t, &fakePoster{handler: echoServer()})
+	if _, err := c.callRaw(context.Background(), "nope/nope"); err == nil {
+		t.Fatal("want an rpc error")
+	} else if !strings.Contains(err.Error(), "method not found") {
+		t.Fatalf("err = %v", err)
+	}
+}
+
+// callRaw is a test-only shim so the rpc-error path can be exercised without a
+// typed wrapper for a method the server does not implement.
+func (c *Client) callRaw(ctx context.Context, method string) (any, error) {
+	var out any
+	err := c.call(ctx, method, map[string]any{}, &out)
+	return out, err
+}
+
+func TestDecodeFrame(t *testing.T) {
+	cases := []struct {
+		name, in, want string
+		wantErr        bool
+	}{
+		{name: "plain json", in: `{"id":1,"result":{}}`, want: `{"id":1,"result":{}}`},
+		{name: "sse single", in: "event: message\ndata: {\"id\":1,\"result\":1}\n\n", want: `{"id":1,"result":1}`},
+		{
+			name: "sse picks the response not the notification",
+			in:   "data: {\"method\":\"notifications/progress\"}\n\ndata: {\"id\":2,\"result\":2}\n\n",
+			want: `{"id":2,"result":2}`,
+		},
+		{name: "empty", in: "  ", wantErr: true},
+		{name: "sse with no response", in: "data: {\"method\":\"x\"}\n\n", wantErr: true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := decodeFrame([]byte(tc.in))
+			if tc.wantErr {
+				if err == nil {
+					t.Fatalf("want error, got %q", got)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			if string(got) != tc.want {
+				t.Fatalf("got %q want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestValidate(t *testing.T) {
+	cases := []struct {
+		name    string
+		cfg     ServerConfig
+		wantErr bool
+	}{
+		{name: "stdio ok", cfg: ServerConfig{Name: "a", Command: "echo"}},
+		{name: "http ok", cfg: ServerConfig{Name: "a", URL: "http://x.test/mcp"}},
+		{name: "no name", cfg: ServerConfig{Command: "echo"}, wantErr: true},
+		{name: "spacey name", cfg: ServerConfig{Name: "a b", Command: "echo"}, wantErr: true},
+		{name: "neither", cfg: ServerConfig{Name: "a"}, wantErr: true},
+		{name: "both", cfg: ServerConfig{Name: "a", Command: "echo", URL: "http://x.test"}, wantErr: true},
+		{name: "bad scheme", cfg: ServerConfig{Name: "a", URL: "file:///etc/passwd"}, wantErr: true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := Validate([]ServerConfig{tc.cfg})
+			if (err != nil) != tc.wantErr {
+				t.Fatalf("err = %v, wantErr = %v", err, tc.wantErr)
+			}
+		})
+	}
+	if err := Validate([]ServerConfig{{Name: "a", Command: "x"}, {Name: "a", Command: "y"}}); err == nil {
+		t.Error("duplicate names must be refused")
+	}
+}
+
+func TestManagerOffWhenNothingEnabled(t *testing.T) {
+	m, err := NewManager(nil, []ServerConfig{{Name: "a", Command: "echo"}}) // Enabled=false
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !m.Empty() {
+		t.Fatal("a server that is not enabled must not be wired")
+	}
+	m.Connect(context.Background())
+	if got := m.Tools(); len(got) != 0 {
+		t.Fatalf("tools = %+v", got)
+	}
+}
+
+func TestManagerDiscoversAndCalls(t *testing.T) {
+	p := &fakePoster{handler: echoServer()}
+	m, err := NewManager(func(ServerConfig) (Poster, error) { return p, nil },
+		[]ServerConfig{{Name: "fake", URL: "http://example.test/mcp", Enabled: true}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	m.Connect(context.Background())
+	defer m.Close()
+
+	tools := m.Tools()
+	if len(tools) != 2 {
+		t.Fatalf("tools = %+v", tools)
+	}
+	out, err := m.Call(context.Background(), "fake", "read_thing", map[string]any{"q": "да"})
+	if err != nil {
+		t.Fatalf("call: %v", err)
+	}
+	if out != "read_thing:да" {
+		t.Fatalf("out = %q", out)
+	}
+	// The discovered set is a second allowlist.
+	if _, err := m.Call(context.Background(), "fake", "not_offered", nil); err == nil {
+		t.Error("a tool the server does not offer must be refused")
+	}
+	if _, err := m.Call(context.Background(), "other", "read_thing", nil); err == nil {
+		t.Error("an unconfigured server must be refused")
+	}
+	st := m.Status()
+	if len(st) != 1 || !st[0].Connected || st[0].Transport != "http" || st[0].Tools != 2 {
+		t.Fatalf("status = %+v", st)
+	}
+}
+
+func TestManagerAllowToolsAndMaxTools(t *testing.T) {
+	p := &fakePoster{handler: echoServer()}
+	mk := func(cfg ServerConfig) *Manager {
+		cfg.Name, cfg.URL, cfg.Enabled = "fake", "http://example.test/mcp", true
+		m, err := NewManager(func(ServerConfig) (Poster, error) { return p, nil }, []ServerConfig{cfg})
+		if err != nil {
+			t.Fatal(err)
+		}
+		m.Connect(context.Background())
+		return m
+	}
+	m := mk(ServerConfig{AllowTools: []string{"read_thing"}})
+	defer m.Close()
+	if got := m.Tools(); len(got) != 1 || got[0].Name != "read_thing" {
+		t.Fatalf("allow_tools ignored: %+v", got)
+	}
+	if _, err := m.Call(context.Background(), "fake", "break_thing", nil); err == nil {
+		t.Error("a tool excluded by allow_tools must be unreachable")
+	}
+	m2 := mk(ServerConfig{MaxTools: 1})
+	defer m2.Close()
+	if got := m2.Tools(); len(got) != 1 || got[0].Name != "break_thing" {
+		t.Fatalf("max_tools should keep the first name-sorted tool: %+v", got)
+	}
+}
+
+func TestManagerURLServerWithoutHTTPDoor(t *testing.T) {
+	m, err := NewManager(nil, []ServerConfig{{Name: "fake", URL: "http://example.test/mcp", Enabled: true}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	m.Connect(context.Background())
+	st := m.Status()
+	if len(st) != 1 || st[0].Connected || st[0].Err == "" {
+		t.Fatalf("a url server with no poster must be recorded as failed: %+v", st)
+	}
+}
+
+func TestManagerReconnectAfterFailure(t *testing.T) {
+	var mu sync.Mutex
+	fail := true
+	m, err := NewManager(func(ServerConfig) (Poster, error) {
+		mu.Lock()
+		defer mu.Unlock()
+		if fail {
+			return nil, fmt.Errorf("down")
+		}
+		return &fakePoster{handler: echoServer()}, nil
+	}, []ServerConfig{{Name: "fake", URL: "http://example.test/mcp", Enabled: true}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer m.Close()
+	m.Connect(context.Background())
+	if m.Status()[0].Connected {
+		t.Fatal("should be down")
+	}
+	mu.Lock()
+	fail = false
+	mu.Unlock()
+	// Refresh honours the backoff, so pretend the last attempt was long ago.
+	m.mu.Lock()
+	m.conns["fake"].lastTry = time.Now().Add(-2 * DefaultReconnectEvery)
+	m.mu.Unlock()
+	m.Refresh(context.Background())
+	if !m.Status()[0].Connected {
+		t.Fatalf("should have reconnected: %+v", m.Status())
+	}
+}
+
+func TestLocalNameAndCmd(t *testing.T) {
+	cases := [][3]string{
+		{"vikunja", "list_tasks", "vikunja_list_tasks"},
+		{"Vikunja", "Get Task Details", "vikunja_get_task_details"},
+		{"fs", "read-file", "fs_read_file"},
+		{"", "search", "search"},
+	}
+	for _, c := range cases {
+		if got := LocalName(c[0], c[1]); got != c[2] {
+			t.Errorf("LocalName(%q,%q) = %q want %q", c[0], c[1], got, c[2])
+		}
+	}
+	server, tool, ok := ParseCmd(Cmd("vikunja", "list_tasks"))
+	if !ok || server != "vikunja" || tool != "list_tasks" {
+		t.Fatalf("ParseCmd round-trip: %q %q %v", server, tool, ok)
+	}
+	for _, bad := range [][]string{nil, {"systemctl", "restart", "nginx"}, {"mcp", "vikunja"}, {"mcp", "", "x"}} {
+		if _, _, ok := ParseCmd(bad); ok {
+			t.Errorf("ParseCmd(%v) must not claim an ordinary tool row", bad)
+		}
+	}
+	if Scope("vikunja") != "mcp:vikunja" {
+		t.Error("scope")
+	}
+}

internal/mcp/stdio.go

@@ -0,0 +1,149 @@
+package mcp
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"strings"
+	"sync"
+)
+
+// maxLine bounds one JSON-RPC frame from a subprocess. A tool result bigger
+// than this is a misbehaving server, not something to buffer.
+const maxLine = 1 << 20 // 1 MiB
+
+// stdioTransport speaks newline-delimited JSON-RPC to a child process. This is
+// the local transport: the server runs on this box, under this user, and gets
+// no network guard because it never touches the network on our behalf.
+//
+// Args are argv, never a shell string — the same discipline internal/tool
+// keeps, for the same reason.
+type stdioTransport struct {
+	mu   sync.Mutex
+	cmd  *exec.Cmd
+	in   io.WriteCloser
+	out  *bufio.Reader
+	dead bool
+}
+
+func newStdioTransport(ctx context.Context, argv []string, env []string, dir string) (*stdioTransport, error) {
+	if len(argv) == 0 {
+		return nil, errors.New("mcp: stdio server needs a command")
+	}
+	cmd := exec.Command(argv[0], argv[1:]...)
+	cmd.Dir = dir
+	if len(env) > 0 {
+		cmd.Env = append(os.Environ(), env...)
+	}
+	cmd.Stderr = os.Stderr
+	in, err := cmd.StdinPipe()
+	if err != nil {
+		return nil, fmt.Errorf("mcp: stdin pipe: %w", err)
+	}
+	out, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, fmt.Errorf("mcp: stdout pipe: %w", err)
+	}
+	if err := cmd.Start(); err != nil {
+		return nil, fmt.Errorf("mcp: start %q: %w", argv[0], err)
+	}
+	return &stdioTransport{cmd: cmd, in: in, out: bufio.NewReaderSize(out, 64<<10)}, nil
+}
+
+func (t *stdioTransport) Call(ctx context.Context, req *rpcRequest) (*rpcResponse, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.dead {
+		return nil, ErrClosed
+	}
+	if err := t.write(req); err != nil {
+		t.dead = true
+		return nil, err
+	}
+	// Read until the frame with our id turns up; anything else on the pipe is
+	// a notification or a server-initiated request we do not answer.
+	for {
+		if err := ctx.Err(); err != nil {
+			return nil, err
+		}
+		line, err := t.readLine()
+		if err != nil {
+			t.dead = true
+			return nil, err
+		}
+		var resp rpcResponse
+		if err := json.Unmarshal(line, &resp); err != nil {
+			continue // not a response frame; ignore rather than break the turn
+		}
+		if resp.ID == nil || *resp.ID != req.ID {
+			continue
+		}
+		return &resp, nil
+	}
+}
+
+func (t *stdioTransport) Notify(ctx context.Context, method string, params any) error {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.dead {
+		return ErrClosed
+	}
+	return t.write(&rpcRequest{JSONRPC: "2.0", Method: method, Params: params})
+}
+
+func (t *stdioTransport) write(req *rpcRequest) error {
+	req.JSONRPC = "2.0"
+	raw, err := json.Marshal(req)
+	if err != nil {
+		return err
+	}
+	if _, err := t.in.Write(append(raw, '\n')); err != nil {
+		return fmt.Errorf("mcp: write %s: %w", req.Method, err)
+	}
+	return nil
+}
+
+func (t *stdioTransport) readLine() ([]byte, error) {
+	for {
+		line, err := t.out.ReadString('\n')
+		if err != nil {
+			if len(strings.TrimSpace(line)) == 0 {
+				return nil, fmt.Errorf("mcp: read: %w", err)
+			}
+			return []byte(line), nil
+		}
+		if len(line) > maxLine {
+			return nil, fmt.Errorf("mcp: frame exceeds %d bytes", maxLine)
+		}
+		if s := strings.TrimSpace(line); s != "" {
+			return []byte(s), nil
+		}
+	}
+}
+
+func (t *stdioTransport) Close() error {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	t.dead = true
+	if t.in != nil {
+		_ = t.in.Close()
+	}
+	if t.cmd.Process != nil {
+		_ = t.cmd.Process.Kill()
+		_ = t.cmd.Wait()
+	}
+	return nil
+}
+
+// alive reports whether the transport can still carry a call. The manager uses
+// it to decide on a reconnect instead of retrying into a dead pipe.
+func (t *stdioTransport) alive() bool {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return !t.dead
+}

internal/mcp/stdio_test.go

@@ -0,0 +1,149 @@
+package mcp
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+// The stdio transport is tested against a real subprocess — this test binary,
+// re-executed with MAVEN_MCP_FAKE set, acting as a minimal MCP server. No
+// python, no fixture file, no network.
+func TestMain(m *testing.M) {
+	if os.Getenv("MAVEN_MCP_FAKE") != "" {
+		fakeStdioServer()
+		return
+	}
+	os.Exit(m.Run())
+}
+
+func fakeStdioServer() {
+	h := echoServer()
+	sc := bufio.NewScanner(os.Stdin)
+	out := bufio.NewWriter(os.Stdout)
+	defer out.Flush()
+	for sc.Scan() {
+		line := strings.TrimSpace(sc.Text())
+		if line == "" {
+			continue
+		}
+		var req struct {
+			ID     *int64          `json:"id"`
+			Method string          `json:"method"`
+			Params json.RawMessage `json:"params"`
+		}
+		if json.Unmarshal([]byte(line), &req) != nil {
+			continue
+		}
+		if req.ID == nil {
+			// A notification gets no reply, but we emit an unrelated
+			// notification so the client's frame-skipping is exercised.
+			_, _ = out.WriteString("{\"jsonrpc\":\"2.0\",\"method\":\"notifications/message\"}\n")
+			_ = out.Flush()
+			continue
+		}
+		result, rerr := h(req.Method, req.Params)
+		resp := map[string]any{"jsonrpc": "2.0", "id": *req.ID}
+		if rerr != nil {
+			resp["error"] = map[string]any{"code": rerr.Code, "message": rerr.Message}
+		} else {
+			resp["result"] = result
+		}
+		raw, _ := json.Marshal(resp)
+		_, _ = out.Write(append(raw, '\n'))
+		_ = out.Flush()
+		if os.Getenv("MAVEN_MCP_FAKE") == "die" && req.Method == "tools/list" {
+			return // hang up, so the reconnect path has something to see
+		}
+	}
+}
+
+func stdioManager(t *testing.T, mode string) *Manager {
+	t.Helper()
+	self, err := os.Executable()
+	if err != nil {
+		t.Skipf("no executable path: %v", err)
+	}
+	if _, err := exec.LookPath(self); err != nil && !strings.Contains(self, "/") {
+		t.Skip("test binary not executable")
+	}
+	m, err := NewManager(nil, []ServerConfig{{
+		Name:    "fake",
+		Command: self,
+		Env:     []string{"MAVEN_MCP_FAKE=" + mode},
+		Enabled: true,
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	m.Connect(context.Background())
+	return m
+}
+
+func TestStdioTransportEndToEnd(t *testing.T) {
+	m := stdioManager(t, "1")
+	defer m.Close()
+	st := m.Status()
+	if len(st) != 1 || !st[0].Connected {
+		t.Fatalf("status = %+v", st)
+	}
+	if st[0].Transport != "stdio" {
+		t.Fatalf("transport = %q", st[0].Transport)
+	}
+	if got := len(m.Tools()); got != 2 {
+		t.Fatalf("tools = %d", got)
+	}
+	out, err := m.Call(context.Background(), "fake", "read_thing", map[string]any{"q": "стдио"})
+	if err != nil {
+		t.Fatalf("call: %v", err)
+	}
+	if out != "read_thing:стдио" {
+		t.Fatalf("out = %q", out)
+	}
+	res := m.Resources(context.Background())
+	if len(res) != 1 || res[0].URI != "note://one" {
+		t.Fatalf("resources = %+v", res)
+	}
+	body, err := m.ReadResource(context.Background(), "fake", "note://one")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if body != "тело ресурса" {
+		t.Fatalf("body = %q", body)
+	}
+}
+
+func TestStdioServerThatDiesIsNotUsable(t *testing.T) {
+	m := stdioManager(t, "die")
+	defer m.Close()
+	// The server hung up after tools/list; the next call must fail cleanly
+	// rather than hang or panic.
+	if _, err := m.Call(context.Background(), "fake", "read_thing", nil); err == nil {
+		t.Fatal("a call into a dead server must error")
+	}
+}
+
+func TestStdioMissingCommand(t *testing.T) {
+	m, err := NewManager(nil, []ServerConfig{{
+		Name: "nope", Command: "/nonexistent/mcp-server-that-is-not-there", Enabled: true,
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	m.Connect(context.Background())
+	st := m.Status()
+	if st[0].Connected || st[0].Err == "" {
+		t.Fatalf("a missing binary must be recorded, not fatal: %+v", st)
+	}
+	if got := len(m.Tools()); got != 0 {
+		t.Fatalf("tools = %d", got)
+	}
+	if !strings.Contains(fmt.Sprint(st[0].Err), "start") {
+		t.Logf("err = %q", st[0].Err)
+	}
+}

internal/mcp/webfetchdoor.go

@@ -0,0 +1,46 @@
+package mcp
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/kami/maven/internal/webfetch"
+)
+
+// WebfetchDoor builds the PosterFactory used in production: one guarded
+// webfetch.Fetcher per url server, with that server's allow_private and the
+// shared host lists and limits.
+//
+// One fetcher PER server is the point. allow_private is a hole in the
+// private-address guard, and a hole punched for the Vikunja server on loopback
+// must not become a hole for some public endpoint that happens to redirect at
+// the LAN. Rate limiting is per fetcher too, which is the right shape here:
+// separate servers are separate hosts.
+func WebfetchDoor(limits webfetch.Config) PosterFactory {
+	return func(cfg ServerConfig) (Poster, error) {
+		c := limits
+		c.AllowPrivate = cfg.AllowPrivate
+		if c.Timeout <= 0 && cfg.Timeout > 0 {
+			c.Timeout = cfg.Timeout
+		}
+		return fetcherPoster{webfetch.New(c)}, nil
+	}
+}
+
+// fetcherPoster adapts webfetch.Fetcher to Poster. It exists so this package
+// does not have to know webfetch's Response type, and so a test can substitute
+// a fake without a listener.
+type fetcherPoster struct{ f *webfetch.Fetcher }
+
+func (p fetcherPoster) Post(ctx context.Context, rawURL, contentType string, body []byte, hdr map[string]string) (*PostResponse, error) {
+	resp, err := p.f.Post(ctx, rawURL, contentType, body, hdr)
+	if err != nil {
+		return nil, fmt.Errorf("mcp: post %s: %w", rawURL, err)
+	}
+	return &PostResponse{
+		Status:      resp.Status,
+		ContentType: resp.ContentType,
+		Body:        resp.Body,
+		Header:      resp.Header,
+	}, nil
+}

internal/update/apply.go

@@ -0,0 +1,208 @@
+package update
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// Result — the full account of one Apply. Every field is filled in on the
+// failure paths too, because "what state is my box in" is the only question that
+// matters after a failed update.
+type Result struct {
+	Verified   bool
+	SnapshotID string // the rollback target; named even when the rollback failed
+	Installed  []string
+	Restarted  bool
+	Healthy    bool
+	RolledBack bool
+	// RollbackHealthy — whether she answered again after the restore. False with
+	// RolledBack true is the manual-recovery case.
+	RollbackHealthy bool
+	Steps           []Step
+	Took            time.Duration
+}
+
+// Apply is the whole update, in the only order that is safe.
+//
+// It is called by a human running cmd/mavupdate on the box. Nothing else calls
+// it: no timer, no IPC method, no web route, no act. See the package comment.
+func (u *Updater) Apply(ctx context.Context) (Result, error) {
+	start := u.now()
+	res := Result{}
+	defer func() { res.Took = u.now().Sub(start) }()
+
+	// 0. She has to be answering before we start. Otherwise a failed update and
+	// a box that was already broken look identical afterwards, and the rollback
+	// has no baseline to prove itself against.
+	u.log("preflight: checking the running daemon")
+	if err := u.health(ctx, u.cfg.HealthSocket); err != nil {
+		return res, fmt.Errorf("%w: %v", ErrUnhealthyBefore, err)
+	}
+
+	// 1. Snapshot what is deployed now, BEFORE the build.
+	//
+	// The order matters and it is not the obvious one. `make build` writes its
+	// binaries into the working tree, and on the docker deployment the working
+	// tree IS the install dir — so snapshotting after the build would snapshot
+	// the new artifacts and leave nothing to roll back to. The snapshot is the
+	// only thing standing between a bad build and a box that needs a screwdriver,
+	// so it is taken first, while the deployed bytes are still the old ones.
+	names := append(append([]string{}, u.cfg.Binaries...), u.cfg.ConfigFiles...)
+	snap, err := u.store.Save(u.cfg.InstallDir, names, u.gitHead(ctx), "pre-update")
+	if err != nil {
+		return res, err
+	}
+	res.SnapshotID = snap.ID
+	u.log("snapshot: %s (%d files) in %s", snap.ID, len(snap.Files), snap.Dir())
+
+	// 2. Build and test before anything is deployed. A broken tree costs time
+	// and nothing else — but `make build` has already overwritten the binaries in
+	// the tree, so restore them: otherwise a later restart by hand would deploy
+	// code that failed its own tests. Nothing has been restarted, so this is a
+	// file restore with no restart and no health check.
+	steps, err := u.Verify(ctx)
+	res.Steps = append(res.Steps, steps...)
+	if err != nil {
+		if rerr := snap.Restore(u.cfg.InstallDir); rerr != nil {
+			u.log("verify failed and the artifacts could not be put back: %v — the previous ones are in %s", rerr, snap.Dir())
+		} else {
+			res.RolledBack = true
+			u.log("verify failed; the previously deployed artifacts are back in place, she was never restarted")
+		}
+		return res, err
+	}
+	res.Verified = true
+
+	// 3. Install. Per-file temp+rename, so an interruption leaves whole files.
+	// Config is snapshotted but never overwritten — an update does not get to
+	// replace the operator's config.
+	installed, err := u.install()
+	res.Installed = installed
+	if err != nil {
+		// Files may be half-swapped across the set, so restore before returning
+		// even though nothing has been restarted yet.
+		u.log("install failed: %v — restoring", err)
+		return u.rollback(ctx, snap, res, err)
+	}
+	u.log("install: %d artifact(s) into %s", len(installed), u.cfg.InstallDir)
+
+	// 4. Restart, then 5. prove she answers.
+	if err := u.restart(ctx, &res); err != nil {
+		return u.rollback(ctx, snap, res, err)
+	}
+	u.log("restart: ok, waiting for her to answer (up to %s)", u.cfg.healthTimeout())
+	if err := u.waitHealthy(ctx, u.cfg.healthTimeout()); err != nil {
+		return u.rollback(ctx, snap, res, err)
+	}
+	res.Healthy = true
+	u.log("health: she answers on %s — update committed", u.cfg.HealthSocket)
+
+	if err := u.store.Prune(u.cfg.KeepSnapshots); err != nil {
+		u.log("prune: %v (harmless)", err)
+	}
+	return res, nil
+}
+
+// Rollback restores a snapshot by id (empty = the newest) and restarts. Exposed
+// separately so the operator can undo an update that verified, restarted and
+// answered a Presence call but is wrong in a way no health check can see.
+func (u *Updater) Rollback(ctx context.Context, id string) (Result, error) {
+	var snap Snapshot
+	var err error
+	if id == "" {
+		snaps, lerr := u.store.List()
+		if lerr != nil {
+			return Result{}, lerr
+		}
+		if len(snaps) == 0 {
+			return Result{}, errors.New("update: no snapshots to roll back to")
+		}
+		snap = snaps[0]
+	} else if snap, err = u.store.Load(id); err != nil {
+		return Result{}, err
+	}
+	res := Result{SnapshotID: snap.ID}
+	return u.rollback(ctx, snap, res, errors.New("operator asked for a rollback"))
+}
+
+// rollback restores the snapshot and restarts, then reports whether that worked.
+// It depends on nothing that the update changed: file copies out of the snapshot
+// dir and the same restart command. No build, no migration, no cooperation from
+// the code being replaced.
+func (u *Updater) rollback(ctx context.Context, snap Snapshot, res Result, cause error) (Result, error) {
+	// A rollback interrupted halfway is the one outcome worse than the failure
+	// that triggered it, so it does not inherit the caller's cancellation: a
+	// Ctrl-C during the health wait must not abandon the restore mid-restart.
+	ctx = context.WithoutCancel(ctx)
+	res.RolledBack = true
+	u.log("rollback: restoring snapshot %s over %s", snap.ID, u.cfg.InstallDir)
+	if err := snap.Restore(u.cfg.InstallDir); err != nil {
+		u.log("rollback: RESTORE FAILED: %v", err)
+		return res, fmt.Errorf("%w: %v (after %v); the previous artifacts are in %s — copy them back by hand", ErrRollbackFailed, err, cause, snap.Dir())
+	}
+	// A restore with no restart leaves the failed process running, so a failed
+	// restart here is still the manual-recovery case.
+	if err := u.restart(ctx, &res); err != nil {
+		u.log("rollback: RESTART FAILED: %v", err)
+		return res, fmt.Errorf("%w: restored %s but the restart failed: %v (after %v)", ErrRollbackFailed, snap.ID, err, cause)
+	}
+	if err := u.waitHealthy(ctx, u.cfg.healthTimeout()); err != nil {
+		u.log("rollback: she still does not answer: %v", err)
+		return res, fmt.Errorf("%w: restored %s and restarted but she does not answer: %v (after %v)", ErrRollbackFailed, snap.ID, err, cause)
+	}
+	res.RollbackHealthy = true
+	u.log("rollback: she answers again on the previous build (%s)", snap.ID)
+	return res, fmt.Errorf("%w to %s: %v", ErrRolledBack, snap.ID, cause)
+}
+
+// install copies the freshly built binaries from SourceDir into InstallDir.
+//
+// When the two are the same directory — the docker deployment builds the image
+// from the working tree — this is a no-op by design rather than by accident: the
+// artifacts are already where they belong and the restart command rebuilds the
+// image from them.
+func (u *Updater) install() ([]string, error) {
+	if filepath.Clean(u.cfg.SourceDir) == filepath.Clean(u.cfg.InstallDir) {
+		return u.cfg.Binaries, nil
+	}
+	var done []string
+	for _, name := range u.cfg.Binaries {
+		src := filepath.Join(u.cfg.SourceDir, name)
+		fi, err := os.Stat(src)
+		if err != nil {
+			return done, fmt.Errorf("update: install %s: %w (did `make build` produce it?)", name, err)
+		}
+		if _, err := copyFile(src, filepath.Join(u.cfg.InstallDir, name), fi.Mode().Perm()); err != nil {
+			return done, fmt.Errorf("update: install %s: %w", name, err)
+		}
+		done = append(done, name)
+	}
+	return done, nil
+}
+
+func (u *Updater) restart(ctx context.Context, res *Result) error {
+	u.log("restart: %v", u.cfg.RestartCmd)
+	out, err := u.run(ctx, u.cfg.SourceDir, u.cfg.RestartCmd)
+	if err != nil {
+		res.Steps = append(res.Steps, Step{Name: "restart", Argv: u.cfg.RestartCmd, Err: err, Output: tail(out, 4000)})
+		return fmt.Errorf("update: restart %v: %w", u.cfg.RestartCmd, err)
+	}
+	res.Restarted = true
+	res.Steps = append(res.Steps, Step{Name: "restart", Argv: u.cfg.RestartCmd})
+	return nil
+}
+
+// gitHead records which commit produced a snapshot, for the operator's benefit.
+// Best-effort: a tree without git is not a reason to refuse to snapshot.
+func (u *Updater) gitHead(ctx context.Context) string {
+	out, err := u.run(ctx, u.cfg.SourceDir, []string{"git", "rev-parse", "HEAD"})
+	if err != nil {
+		return ""
+	}
+	return strings.TrimSpace(out)
+}

internal/update/health.go

@@ -0,0 +1,61 @@
+package update
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/kami/maven/internal/ipc"
+)
+
+// The health check is the whole basis for rolling back, so it has to mean
+// something. "The process is running" does not: mavend can be up with a dead
+// store, a socket it never bound, or a config it failed to parse. What is
+// checked instead is that she answers a real read over the real IPC socket —
+// which exercises the socket, the dispatch table and the store in one call.
+//
+// Presence is the method used because it is read-only (safe to retry), needs no
+// arguments, and touches the store. It cannot write anything, so a health check
+// never leaves a trace in her memory.
+
+// DialHealth connects to the mavend socket and performs one read.
+func DialHealth(ctx context.Context, socket string) error {
+	c, err := ipc.Dial(socket)
+	if err != nil {
+		return fmt.Errorf("update: health dial: %w", err)
+	}
+	defer c.Close()
+	if _, err := c.Presence(ctx); err != nil {
+		return fmt.Errorf("update: health read: %w", err)
+	}
+	return nil
+}
+
+// waitHealthy retries the health check until it passes or the timeout elapses.
+// A restart is not instantaneous — she loads a 1.7B on boot — so the first few
+// failures are expected and are not a reason to roll back.
+func (u *Updater) waitHealthy(ctx context.Context, timeout time.Duration) error {
+	deadline := u.now().Add(timeout)
+	delay := 500 * time.Millisecond
+	var last error
+	for {
+		attemptCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
+		err := u.health(attemptCtx, u.cfg.HealthSocket)
+		cancel()
+		if err == nil {
+			return nil
+		}
+		last = err
+		if u.now().After(deadline) {
+			return fmt.Errorf("update: not healthy after %s: %w", timeout, last)
+		}
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-time.After(delay):
+		}
+		if delay < 5*time.Second {
+			delay *= 2
+		}
+	}
+}

internal/update/snapshot.go

@@ -0,0 +1,267 @@
+package update
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"sort"
+	"time"
+)
+
+// A snapshot is a byte-for-byte copy of the deployed artifacts plus a manifest
+// of their sha256 sums, taken before an install.
+//
+// It is copies, not hardlinks and not a git stash, for one reason: the restore
+// path must work when everything else is broken. A hardlink into the install dir
+// would be clobbered by the very install it exists to undo, and a git-based
+// undo needs a toolchain, a clean tree, and a rebuild — three things a failed
+// update is likely to have taken away. Copying two dozen megabytes of Go
+// binaries costs a second and needs nothing but the filesystem.
+//
+// The sums are what make a restore verifiable rather than hopeful: Restore
+// re-hashes every file it writes, so "the old bytes are back" is checked, not
+// assumed.
+
+// FileRec — one file in a snapshot.
+type FileRec struct {
+	Name   string      `json:"name"`   // relative name inside the install dir
+	SHA256 string      `json:"sha256"` // of the snapshotted bytes
+	Mode   os.FileMode `json:"mode"`
+	Size   int64       `json:"size"`
+}
+
+// Snapshot — the manifest. Written last, so a directory without a readable
+// manifest.json is an aborted snapshot and is never offered as a rollback target.
+type Snapshot struct {
+	ID        string    `json:"id"` // sortable timestamp, also the directory name
+	CreatedAt time.Time `json:"created_at"`
+	Commit    string    `json:"commit,omitempty"` // git HEAD of the tree that produced it, when known
+	Note      string    `json:"note,omitempty"`
+	Files     []FileRec `json:"files"`
+
+	dir string // absolute path, filled in by List/Load
+}
+
+// Dir — where this snapshot's file copies live.
+func (s Snapshot) Dir() string { return s.dir }
+
+const manifestName = "manifest.json"
+
+// Store is a directory of snapshots.
+type Store struct {
+	Dir string
+	now func() time.Time
+}
+
+func (st *Store) clock() time.Time {
+	if st.now != nil {
+		return st.now()
+	}
+	return time.Now()
+}
+
+// Save copies names (relative to srcDir) into a new snapshot and writes the
+// manifest. A name that does not exist is skipped rather than fatal: the first
+// ever run happens on a box where some artifact may legitimately be missing, and
+// refusing to snapshot then would mean refusing to update.
+func (st *Store) Save(srcDir string, names []string, commit, note string) (Snapshot, error) {
+	ts := st.clock().UTC()
+	snap := Snapshot{
+		ID:        ts.Format("20060102-150405"),
+		CreatedAt: ts,
+		Commit:    commit,
+		Note:      note,
+	}
+	snap.dir = filepath.Join(st.Dir, snap.ID)
+	if err := os.MkdirAll(snap.dir, 0o700); err != nil {
+		return Snapshot{}, fmt.Errorf("update: snapshot dir: %w", err)
+	}
+	for _, name := range names {
+		src := filepath.Join(srcDir, name)
+		fi, err := os.Stat(src)
+		if err != nil {
+			if errors.Is(err, os.ErrNotExist) {
+				continue
+			}
+			return Snapshot{}, fmt.Errorf("update: snapshot %s: %w", name, err)
+		}
+		if fi.IsDir() {
+			return Snapshot{}, fmt.Errorf("update: snapshot %s: is a directory (only files are deployable artifacts)", name)
+		}
+		dst := filepath.Join(snap.dir, name)
+		if err := os.MkdirAll(filepath.Dir(dst), 0o700); err != nil {
+			return Snapshot{}, err
+		}
+		sum, err := copyFile(src, dst, fi.Mode().Perm())
+		if err != nil {
+			return Snapshot{}, fmt.Errorf("update: snapshot %s: %w", name, err)
+		}
+		snap.Files = append(snap.Files, FileRec{Name: name, SHA256: sum, Mode: fi.Mode().Perm(), Size: fi.Size()})
+	}
+	if len(snap.Files) == 0 {
+		os.RemoveAll(snap.dir)
+		return Snapshot{}, fmt.Errorf("update: snapshot of %s is empty — none of the listed artifacts exist", srcDir)
+	}
+	// Manifest last: its presence is what makes the snapshot usable.
+	blob, err := json.MarshalIndent(snap, "", "  ")
+	if err != nil {
+		return Snapshot{}, err
+	}
+	if err := os.WriteFile(filepath.Join(snap.dir, manifestName), blob, 0o600); err != nil {
+		return Snapshot{}, fmt.Errorf("update: snapshot manifest: %w", err)
+	}
+	return snap, nil
+}
+
+// List returns the complete snapshots, newest first.
+func (st *Store) List() ([]Snapshot, error) {
+	ents, err := os.ReadDir(st.Dir)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	var out []Snapshot
+	for _, e := range ents {
+		if !e.IsDir() {
+			continue
+		}
+		s, err := st.Load(e.Name())
+		if err != nil {
+			continue // aborted or hand-mangled: not a rollback target
+		}
+		out = append(out, s)
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].ID > out[j].ID })
+	return out, nil
+}
+
+// Load reads one snapshot's manifest.
+func (st *Store) Load(id string) (Snapshot, error) {
+	dir := filepath.Join(st.Dir, id)
+	blob, err := os.ReadFile(filepath.Join(dir, manifestName))
+	if err != nil {
+		return Snapshot{}, err
+	}
+	var s Snapshot
+	if err := json.Unmarshal(blob, &s); err != nil {
+		return Snapshot{}, fmt.Errorf("update: manifest %s: %w", id, err)
+	}
+	s.dir = dir
+	return s, nil
+}
+
+// Restore copies a snapshot's files back over dstDir and verifies every write
+// against the manifest sum. Only the named files are touched; anything else in
+// dstDir is left alone.
+//
+// This is the function the whole package exists to be able to run. It uses the
+// filesystem and nothing else — no toolchain, no build, no cooperation from the
+// code being replaced.
+func (s Snapshot) Restore(dstDir string) error {
+	if s.dir == "" {
+		return errors.New("update: snapshot has no directory (load it through the store)")
+	}
+	for _, f := range s.Files {
+		src := filepath.Join(s.dir, f.Name)
+		sum, err := hashFile(src)
+		if err != nil {
+			return fmt.Errorf("update: restore %s: %w", f.Name, err)
+		}
+		if sum != f.SHA256 {
+			return fmt.Errorf("update: restore %s: snapshot is corrupt (sha256 %s, manifest says %s)", f.Name, sum, f.SHA256)
+		}
+		dst := filepath.Join(dstDir, f.Name)
+		if err := os.MkdirAll(filepath.Dir(dst), 0o755); err != nil {
+			return err
+		}
+		got, err := copyFile(src, dst, f.Mode)
+		if err != nil {
+			return fmt.Errorf("update: restore %s: %w", f.Name, err)
+		}
+		if got != f.SHA256 {
+			return fmt.Errorf("update: restore %s: wrote the wrong bytes (sha256 %s)", f.Name, got)
+		}
+	}
+	return nil
+}
+
+// Prune keeps the newest keep snapshots and removes the rest. The newest is
+// never pruned regardless of keep — it is the rollback target.
+func (st *Store) Prune(keep int) error {
+	if keep < 1 {
+		keep = 1
+	}
+	snaps, err := st.List()
+	if err != nil {
+		return err
+	}
+	for _, s := range snaps[min(keep, len(snaps)):] {
+		if err := os.RemoveAll(s.dir); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// copyFile writes src to dst atomically (temp + rename, so a reader never sees a
+// half file and an interrupted copy leaves the old one intact) and returns the
+// sha256 of what was written.
+func copyFile(src, dst string, mode os.FileMode) (string, error) {
+	in, err := os.Open(src)
+	if err != nil {
+		return "", err
+	}
+	defer in.Close()
+	if mode == 0 {
+		mode = 0o644
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(dst), ".update-*")
+	if err != nil {
+		return "", err
+	}
+	tmpName := tmp.Name()
+	defer os.Remove(tmpName) // no-op once the rename succeeds
+	h := sha256.New()
+	if _, err := io.Copy(io.MultiWriter(tmp, h), in); err != nil {
+		tmp.Close()
+		return "", err
+	}
+	// fsync before the rename: a binary that is renamed into place but whose
+	// bytes are still in the page cache is exactly the file a power cut turns
+	// into an unbootable daemon.
+	if err := tmp.Sync(); err != nil {
+		tmp.Close()
+		return "", err
+	}
+	if err := tmp.Chmod(mode); err != nil {
+		tmp.Close()
+		return "", err
+	}
+	if err := tmp.Close(); err != nil {
+		return "", err
+	}
+	if err := os.Rename(tmpName, dst); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}
+
+func hashFile(p string) (string, error) {
+	f, err := os.Open(p)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}

internal/update/update.go

@@ -0,0 +1,270 @@
+// Package update applies a new build of Maven to the box she runs on, with a
+// verified-before-committed install and an automatic rollback (Vikunja #249).
+//
+// # What this package refuses to be
+//
+// This is the highest-risk capability in the backlog — code that changes the
+// running system — so the refusals are as much of the design as the features,
+// and they are enforced here rather than described in a doc:
+//
+//   - It is never automatic and never on a timer. There is no checker, no
+//     channel, no "check for updates" call and nothing that fires from the tick
+//     loop. Apply runs exactly when a human runs cmd/mavupdate on the box.
+//   - The daemon cannot update itself. mavend does not import this package and
+//     there is no IPC method and no web route that reaches it, so no act, no
+//     intent, no tool and no LLM output can start an update. The trigger needs
+//     shell access to the host, which is a strictly higher bar than the step-up
+//     passkey gate that guards /tools — an update is not a thing to expose to
+//     anything reachable over the network.
+//   - It does not fetch code. Nothing here talks to a release server, a
+//     registry, or GitHub. The new version is whatever is in the working tree
+//     the operator points it at, which he pulled himself. Downloading and
+//     running code on the strength of a checksum in the same download is not a
+//     property we can verify on one box.
+//   - It does not supervise its own death. The plan asked for an in-process
+//     crash-loop detector; a process cannot reliably notice that it keeps
+//     dying, and one that thinks it can is worse than nothing. Restart-on-crash
+//     belongs to whatever starts mavend (compose `restart: unless-stopped`,
+//     systemd `Restart=`). What this package guarantees instead is narrower and
+//     real: within one Apply, the new build is proven to answer before the old
+//     one is considered replaced, and if it does not answer the old bytes go
+//     back and are proven to answer again.
+//
+// # The order of operations, and why
+//
+// Apply is: health-check the CURRENT daemon → build → test → snapshot → install
+// → restart → health-check → rollback on any failure.
+//
+// The first health check is not ceremony. If she is already not answering, a
+// failed update and a broken box are indistinguishable afterwards, and the
+// rollback has nothing to prove itself against — so Apply refuses to start.
+//
+// Build and test run BEFORE anything is written to the install dir, so a broken
+// tree costs nothing but time. Install is per-file write-temp-then-rename, so a
+// crash mid-install leaves whole files, not half ones.
+//
+// The rollback path deliberately depends on nothing that just changed: it copies
+// byte-for-byte from a snapshot taken before the install and re-runs the same
+// restart command. It does not ask the new binary to do anything, does not run
+// a migration, and does not need the update to have gotten far enough to leave
+// a working anything behind.
+//
+// # What is out of scope on purpose
+//
+// The database is not snapshotted or rolled back. It is encrypted, live, and
+// often larger than the disk headroom; a store rolled back under a schema that
+// already migrated forward loses writes silently, which is worse than a failed
+// update. Schema compatibility is store.Migrate's job. A snapshot here is the
+// deployable artifacts only: binaries and config.
+package update
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+var (
+	// ErrNotConfigured — no update block in the config. The capability does not
+	// exist unless the operator described his own deployment.
+	ErrNotConfigured = errors.New("update: not configured")
+
+	// ErrUnhealthyBefore — the daemon was already not answering when Apply
+	// started. Refused: see the package comment.
+	ErrUnhealthyBefore = errors.New("update: the running daemon is not healthy — refusing to update on top of a broken box")
+
+	// ErrVerifyFailed — build or test failed. Nothing was installed.
+	ErrVerifyFailed = errors.New("update: verification failed")
+
+	// ErrRolledBack — the new build was installed and did not come up healthy,
+	// so the previous snapshot was restored. Wraps the underlying failure.
+	ErrRolledBack = errors.New("update: rolled back")
+
+	// ErrRollbackFailed — the worst case: the new build failed AND the restore
+	// did not bring her back. The operator has to fix the box by hand; the
+	// snapshot directory is named in the result so he knows what to copy.
+	ErrRollbackFailed = errors.New("update: ROLLBACK FAILED — manual recovery required")
+)
+
+// Config — the operator's description of his own deployment. Every path is
+// absolute and validated; nothing is guessed, because guessing wrong here means
+// overwriting the wrong file.
+type Config struct {
+	// SourceDir — the git working tree to build. The operator pulls it himself;
+	// this package never fetches.
+	SourceDir string `json:"source_dir"`
+
+	// InstallDir — where the built binaries are copied to. On the docker
+	// deployment this is the tree the image is built from, so it is usually the
+	// same as SourceDir and Install is a no-op copy; on a bare-metal deployment
+	// it is /opt/maven/bin.
+	InstallDir string `json:"install_dir"`
+
+	// SnapshotDir — where the pre-install copies live. Must not be inside
+	// InstallDir: a restore reading from a directory the install is writing to
+	// is not a restore.
+	SnapshotDir string `json:"snapshot_dir"`
+
+	// Binaries — the artifact names to snapshot and install, relative to
+	// SourceDir (built) and InstallDir (deployed). Listed explicitly rather than
+	// globbed so a stray file in the tree never gets deployed.
+	Binaries []string `json:"binaries"`
+
+	// ConfigFiles — extra files to snapshot alongside the binaries, relative to
+	// InstallDir. Snapshotted, never overwritten by an install: the operator's
+	// config is not something an update gets to replace.
+	ConfigFiles []string `json:"config_files,omitempty"`
+
+	// RestartCmd — how this deployment restarts mavend, e.g.
+	// ["docker","compose","up","-d","--build","mavend"] or
+	// ["systemctl","restart","mavend"]. Run in SourceDir. Required: there is no
+	// portable default and picking one would mean restarting the wrong thing.
+	RestartCmd []string `json:"restart_cmd"`
+
+	// HealthSocket — mavend's IPC socket, used to prove she answers after a
+	// restart. Required: without a health check there is no signal to roll back
+	// on, and an update that cannot detect its own failure is not what this
+	// package is for.
+	HealthSocket string `json:"health_socket"`
+
+	// HealthTimeoutSec — how long to wait for the restarted daemon to answer.
+	// Default 90s; she loads a 1.7B on boot, so this is not a couple of seconds.
+	HealthTimeoutSec int `json:"health_timeout_sec,omitempty"`
+
+	// VerifyTimeoutMin — cap on `make build` + `make test`. Default 20m.
+	VerifyTimeoutMin int `json:"verify_timeout_min,omitempty"`
+
+	// KeepSnapshots — how many snapshots to retain. Default 5, minimum 1: the
+	// most recent one is the rollback target and is never pruned.
+	KeepSnapshots int `json:"keep_snapshots,omitempty"`
+}
+
+// Validate — fail at startup, not halfway through an install.
+func (c Config) Validate() error {
+	if c.SourceDir == "" || c.InstallDir == "" || c.SnapshotDir == "" {
+		return errors.New("update: source_dir, install_dir and snapshot_dir are all required")
+	}
+	for _, p := range []string{c.SourceDir, c.InstallDir, c.SnapshotDir} {
+		if !filepath.IsAbs(p) {
+			return fmt.Errorf("update: %q must be an absolute path", p)
+		}
+	}
+	if within(c.SnapshotDir, c.InstallDir) {
+		return fmt.Errorf("update: snapshot_dir %q is inside install_dir %q — a restore must not read from what the install writes", c.SnapshotDir, c.InstallDir)
+	}
+	if len(c.Binaries) == 0 {
+		return errors.New("update: binaries is empty — nothing to install")
+	}
+	for _, b := range append(append([]string{}, c.Binaries...), c.ConfigFiles...) {
+		if filepath.IsAbs(b) || strings.Contains(b, "..") {
+			return fmt.Errorf("update: %q must be a plain relative name", b)
+		}
+	}
+	if len(c.RestartCmd) == 0 {
+		return errors.New("update: restart_cmd is required — there is no safe default for restarting someone else's deployment")
+	}
+	if c.HealthSocket == "" {
+		return errors.New("update: health_socket is required — an update that cannot check its own result cannot roll back on failure")
+	}
+	return nil
+}
+
+func (c Config) withDefaults() Config {
+	if c.HealthTimeoutSec <= 0 {
+		c.HealthTimeoutSec = 90
+	}
+	if c.VerifyTimeoutMin <= 0 {
+		c.VerifyTimeoutMin = 20
+	}
+	if c.KeepSnapshots < 1 {
+		c.KeepSnapshots = 5
+	}
+	return c
+}
+
+func (c Config) healthTimeout() time.Duration {
+	return time.Duration(c.HealthTimeoutSec) * time.Second
+}
+
+func (c Config) verifyTimeout() time.Duration {
+	return time.Duration(c.VerifyTimeoutMin) * time.Minute
+}
+
+// within reports whether p is dir or lives under it.
+func within(p, dir string) bool {
+	p, dir = filepath.Clean(p), filepath.Clean(dir)
+	if p == dir {
+		return true
+	}
+	rel, err := filepath.Rel(dir, p)
+	return err == nil && rel != ".." && !strings.HasPrefix(rel, ".."+string(filepath.Separator))
+}
+
+// Runner runs one command and returns its combined output. Injected so the
+// tests can drive build/test/restart failures without a toolchain, a container
+// or a real daemon to break.
+type Runner func(ctx context.Context, dir string, argv []string) (string, error)
+
+// ExecRunner is the real one.
+func ExecRunner(ctx context.Context, dir string, argv []string) (string, error) {
+	cmd := exec.CommandContext(ctx, argv[0], argv[1:]...)
+	cmd.Dir = dir
+	out, err := cmd.CombinedOutput()
+	return string(out), err
+}
+
+// HealthCheck proves the daemon at socket answers. Injected for the same reason
+// as Runner.
+type HealthCheck func(ctx context.Context, socket string) error
+
+// Logger receives one line per step. The CLI prints these as they happen: an
+// update that goes quiet for four minutes during `make test` reads as a hang.
+type Logger func(format string, args ...any)
+
+// Updater is the whole capability. Construct with New and call Apply or
+// Rollback; there is no background goroutine and nothing starts on its own.
+type Updater struct {
+	cfg    Config
+	store  *Store
+	run    Runner
+	health HealthCheck
+	log    Logger
+	now    func() time.Time
+}
+
+// New builds an Updater. Every seam has a real default; the tests replace them.
+func New(cfg Config, opts ...Option) (*Updater, error) {
+	if err := cfg.Validate(); err != nil {
+		return nil, err
+	}
+	u := &Updater{
+		cfg:    cfg.withDefaults(),
+		store:  &Store{Dir: cfg.SnapshotDir},
+		run:    ExecRunner,
+		health: DialHealth,
+		log:    func(string, ...any) {},
+		now:    time.Now,
+	}
+	for _, o := range opts {
+		o(u)
+	}
+	u.store.now = u.now
+	return u, nil
+}
+
+// Option — a constructor seam.
+type Option func(*Updater)
+
+func WithRunner(r Runner) Option      { return func(u *Updater) { u.run = r } }
+func WithHealth(h HealthCheck) Option { return func(u *Updater) { u.health = h } }
+func WithLogger(l Logger) Option      { return func(u *Updater) { u.log = l } }
+func WithClock(f func() time.Time) Option {
+	return func(u *Updater) { u.now = f }
+}
+
+// Snapshots lists what is available to roll back to, newest first.
+func (u *Updater) Snapshots() ([]Snapshot, error) { return u.store.List() }

internal/update/update_test.go

@@ -0,0 +1,437 @@
+package update
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+// The tests drive the whole orchestration against a fake box: a directory tree
+// standing in for the install dir, an injected Runner standing in for
+// make/git/docker, and an injected HealthCheck standing in for mavend. That is
+// what makes the failure paths — the ones that matter — testable at all: you
+// cannot ask a real deployment to fail its health check on demand, and the
+// rollback path is exactly the path nobody exercises by hand.
+
+type fakeBox struct {
+	t    *testing.T
+	root string
+
+	// what the fake `make build` writes into the source tree
+	newBytes string
+	// scripted failures
+	buildErr   error
+	testErr    error
+	restartErr error
+
+	// health: fails until the Nth call, then follows healthy
+	healthErrs   int // remaining failures to serve
+	healthy      bool
+	healthChecks int
+	// deployedAtRestart records the installed bytes each time restart runs, so a
+	// test can prove the rollback put the old bytes back BEFORE restarting.
+	deployedAtRestart []string
+
+	ran []string
+}
+
+func newFakeBox(t *testing.T) *fakeBox {
+	t.Helper()
+	root := t.TempDir()
+	for _, d := range []string{"src", "install", "snapshots"} {
+		if err := os.MkdirAll(filepath.Join(root, d), 0o755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// The currently deployed build, and a config file next to it.
+	write(t, filepath.Join(root, "install", "mavend"), "OLD-BUILD")
+	write(t, filepath.Join(root, "install", "mavend.json"), `{"tick_interval":"60s"}`)
+	// The source tree already contains a stale binary; `make build` overwrites it.
+	write(t, filepath.Join(root, "src", "mavend"), "STALE")
+	return &fakeBox{t: t, root: root, newBytes: "NEW-BUILD", healthy: true}
+}
+
+func (b *fakeBox) cfg() Config {
+	return Config{
+		SourceDir:        filepath.Join(b.root, "src"),
+		InstallDir:       filepath.Join(b.root, "install"),
+		SnapshotDir:      filepath.Join(b.root, "snapshots"),
+		Binaries:         []string{"mavend"},
+		ConfigFiles:      []string{"mavend.json"},
+		RestartCmd:       []string{"restart-the-thing"},
+		HealthSocket:     filepath.Join(b.root, "mavend.sock"),
+		HealthTimeoutSec: 1,
+		KeepSnapshots:    3,
+	}
+}
+
+func (b *fakeBox) run(ctx context.Context, dir string, argv []string) (string, error) {
+	b.ran = append(b.ran, strings.Join(argv, " "))
+	switch strings.Join(argv, " ") {
+	case "make build":
+		if b.buildErr != nil {
+			return "ld: undefined reference to everything", b.buildErr
+		}
+		// A real build writes its artifacts into the working tree — the behaviour
+		// the snapshot-before-build ordering exists to survive.
+		write(b.t, filepath.Join(b.root, "src", "mavend"), b.newBytes)
+		return "built", nil
+	case "make test":
+		if b.testErr != nil {
+			return "--- FAIL: TestSomething", b.testErr
+		}
+		return "ok", nil
+	case "git rev-parse HEAD":
+		return "cafebabecafebabecafebabecafebabecafebabe\n", nil
+	case "restart-the-thing":
+		b.deployedAtRestart = append(b.deployedAtRestart, read(b.t, filepath.Join(b.root, "install", "mavend")))
+		if b.restartErr != nil {
+			return "no such container", b.restartErr
+		}
+		return "restarted", nil
+	}
+	return "", errors.New("unexpected command: " + strings.Join(argv, " "))
+}
+
+func (b *fakeBox) health(ctx context.Context, socket string) error {
+	b.healthChecks++
+	if b.healthErrs > 0 {
+		b.healthErrs--
+		return errors.New("connection refused")
+	}
+	if !b.healthy {
+		return errors.New("she does not answer")
+	}
+	return nil
+}
+
+func (b *fakeBox) updater(t *testing.T, extra ...Option) *Updater {
+	t.Helper()
+	opts := append([]Option{WithRunner(b.run), WithHealth(b.health)}, extra...)
+	u, err := New(b.cfg(), opts...)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return u
+}
+
+func (b *fakeBox) deployed() string { return read(b.t, filepath.Join(b.root, "install", "mavend")) }
+
+func write(t *testing.T, path, content string) {
+	t.Helper()
+	if err := os.WriteFile(path, []byte(content), 0o755); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func read(t *testing.T, path string) string {
+	t.Helper()
+	b, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return string(b)
+}
+
+func TestApply_HappyPath(t *testing.T) {
+	b := newFakeBox(t)
+	res, err := b.updater(t).Apply(context.Background())
+	if err != nil {
+		t.Fatalf("Apply: %v", err)
+	}
+	if !res.Verified || !res.Restarted || !res.Healthy || res.RolledBack {
+		t.Fatalf("result = %+v; want verified+restarted+healthy and no rollback", res)
+	}
+	if got := b.deployed(); got != "NEW-BUILD" {
+		t.Errorf("deployed binary = %q; want the new build", got)
+	}
+	// The order is the property: health, snapshot, build, test, install, restart.
+	want := []string{"git rev-parse HEAD", "make build", "make test", "restart-the-thing"}
+	if strings.Join(b.ran, "|") != strings.Join(want, "|") {
+		t.Errorf("commands ran = %v; want %v", b.ran, want)
+	}
+	if res.SnapshotID == "" {
+		t.Error("no snapshot was taken")
+	}
+}
+
+func TestApply_RefusesWhenSheIsAlreadyDown(t *testing.T) {
+	// A box that is already broken has no baseline for the rollback to prove
+	// itself against, so the update never starts.
+	b := newFakeBox(t)
+	b.healthy = false
+	res, err := b.updater(t).Apply(context.Background())
+	if !errors.Is(err, ErrUnhealthyBefore) {
+		t.Fatalf("Apply on an unhealthy box = %v; want ErrUnhealthyBefore", err)
+	}
+	if len(b.ran) != 0 {
+		t.Errorf("a refused update still ran %v", b.ran)
+	}
+	if res.SnapshotID != "" {
+		t.Error("a refused update still took a snapshot")
+	}
+}
+
+func TestApply_TestFailureDeploysNothingAndPutsTheTreeBack(t *testing.T) {
+	b := newFakeBox(t)
+	b.testErr = errors.New("exit status 1")
+	res, err := b.updater(t).Apply(context.Background())
+	if !errors.Is(err, ErrVerifyFailed) {
+		t.Fatalf("Apply with failing tests = %v; want ErrVerifyFailed", err)
+	}
+	if res.Verified {
+		t.Error("result claims verified after a failing test suite")
+	}
+	for _, c := range b.ran {
+		if c == "restart-the-thing" {
+			t.Fatal("a failed verification restarted the daemon")
+		}
+	}
+	if got := b.deployed(); got != "OLD-BUILD" {
+		t.Errorf("deployed binary = %q; want the old build untouched", got)
+	}
+	// The failing output is kept so the operator can see why.
+	var found bool
+	for _, s := range res.Steps {
+		if s.Name == "test" && strings.Contains(s.Output, "FAIL") {
+			found = true
+		}
+	}
+	if !found {
+		t.Error("the failing test output was not retained")
+	}
+}
+
+func TestApply_BuildFailureIsCaughtBeforeTheTests(t *testing.T) {
+	b := newFakeBox(t)
+	b.buildErr = errors.New("exit status 2")
+	if _, err := b.updater(t).Apply(context.Background()); !errors.Is(err, ErrVerifyFailed) {
+		t.Fatalf("Apply with a failing build = %v; want ErrVerifyFailed", err)
+	}
+	for _, c := range b.ran {
+		if c == "make test" {
+			t.Error("ran the test suite after the build failed")
+		}
+	}
+}
+
+func TestApply_UnhealthyAfterRestartRollsBackToTheOldBytes(t *testing.T) {
+	// The case the package exists for: everything verifies, the new build
+	// installs, and then she does not come up.
+	b := newFakeBox(t)
+	b.healthErrs = 1 // the preflight check passes, then she stops answering
+	b.healthy = false
+	u := b.updater(t)
+	// Once the rollback restores the old build, she answers again.
+	restored := false
+	u.health = func(ctx context.Context, socket string) error {
+		b.healthChecks++
+		if b.deployed() == "OLD-BUILD" && restored {
+			return nil
+		}
+		if b.healthChecks == 1 {
+			return nil // preflight: the old build is up
+		}
+		if b.deployed() == "OLD-BUILD" {
+			restored = true
+			return nil
+		}
+		return errors.New("she does not answer on the new build")
+	}
+	res, err := u.Apply(context.Background())
+	if !errors.Is(err, ErrRolledBack) {
+		t.Fatalf("Apply with a dead new build = %v; want ErrRolledBack", err)
+	}
+	if !res.RolledBack || !res.RollbackHealthy || res.Healthy {
+		t.Fatalf("result = %+v; want rolled back and healthy again on the old build", res)
+	}
+	if got := b.deployed(); got != "OLD-BUILD" {
+		t.Errorf("deployed binary after the rollback = %q; want OLD-BUILD", got)
+	}
+	// And the restore happened BEFORE the second restart, not after it.
+	if len(b.deployedAtRestart) != 2 {
+		t.Fatalf("restarts = %v; want two (the update and the rollback)", b.deployedAtRestart)
+	}
+	if b.deployedAtRestart[0] != "NEW-BUILD" || b.deployedAtRestart[1] != "OLD-BUILD" {
+		t.Errorf("bytes in place at each restart = %v; want [NEW-BUILD OLD-BUILD]", b.deployedAtRestart)
+	}
+}
+
+func TestApply_RestartFailureRollsBack(t *testing.T) {
+	b := newFakeBox(t)
+	b.restartErr = errors.New("exit status 1")
+	res, err := b.updater(t).Apply(context.Background())
+	// The rollback's own restart fails too, so this is the manual-recovery case —
+	// and it says so instead of reporting a tidy rollback.
+	if !errors.Is(err, ErrRollbackFailed) {
+		t.Fatalf("Apply with a broken restart command = %v; want ErrRollbackFailed", err)
+	}
+	if !res.RolledBack || res.RollbackHealthy {
+		t.Fatalf("result = %+v; want rolled back but not healthy", res)
+	}
+	if got := b.deployed(); got != "OLD-BUILD" {
+		t.Errorf("deployed binary = %q; want the old bytes restored even so", got)
+	}
+}
+
+func TestApply_RollbackNeedsNoBuildAndNoNewCode(t *testing.T) {
+	// The rollback must not depend on the toolchain, the source tree, or the
+	// code it is replacing. Prove it: delete the source tree's binary and make
+	// every command except the restart fail, then roll back.
+	b := newFakeBox(t)
+	if _, err := b.updater(t).Apply(context.Background()); err != nil {
+		t.Fatalf("setup Apply: %v", err)
+	}
+	if b.deployed() != "NEW-BUILD" {
+		t.Fatal("setup did not deploy")
+	}
+	os.RemoveAll(filepath.Join(b.root, "src"))
+	if err := os.MkdirAll(filepath.Join(b.root, "src"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	b.buildErr = errors.New("no toolchain here")
+	b.testErr = errors.New("no toolchain here")
+	b.ran = nil
+
+	res, err := b.updater(t).Rollback(context.Background(), "")
+	if err != nil && !errors.Is(err, ErrRolledBack) {
+		t.Fatalf("Rollback: %v", err)
+	}
+	if !res.RollbackHealthy {
+		t.Fatalf("result = %+v; want a healthy rollback", res)
+	}
+	if got := b.deployed(); got != "OLD-BUILD" {
+		t.Errorf("deployed binary = %q; want OLD-BUILD", got)
+	}
+	for _, c := range b.ran {
+		if strings.HasPrefix(c, "make") {
+			t.Errorf("the rollback ran %q — it must not need a build", c)
+		}
+	}
+}
+
+func TestApply_ConfigIsSnapshottedButNeverOverwritten(t *testing.T) {
+	b := newFakeBox(t)
+	// A config in the source tree must not be deployed over the operator's.
+	write(t, filepath.Join(b.root, "src", "mavend.json"), `{"tick_interval":"1s"}`)
+	if _, err := b.updater(t).Apply(context.Background()); err != nil {
+		t.Fatalf("Apply: %v", err)
+	}
+	if got := read(t, filepath.Join(b.root, "install", "mavend.json")); !strings.Contains(got, "60s") {
+		t.Errorf("installed config = %q; an update must not replace his config", got)
+	}
+	snaps, err := b.updater(t).Snapshots()
+	if err != nil || len(snaps) == 0 {
+		t.Fatalf("Snapshots: %v %v", snaps, err)
+	}
+	var names []string
+	for _, f := range snaps[0].Files {
+		names = append(names, f.Name)
+	}
+	if len(names) != 2 {
+		t.Errorf("snapshot files = %v; want the binary and the config", names)
+	}
+	if snaps[0].Commit == "" {
+		t.Error("the snapshot did not record which commit produced it")
+	}
+}
+
+func TestRollback_CorruptSnapshotIsRefusedNotRestored(t *testing.T) {
+	b := newFakeBox(t)
+	if _, err := b.updater(t).Apply(context.Background()); err != nil {
+		t.Fatalf("setup Apply: %v", err)
+	}
+	snaps, _ := b.updater(t).Snapshots()
+	// Something ate the snapshot. Restoring it would deploy garbage.
+	write(t, filepath.Join(snaps[0].Dir(), "mavend"), "CORRUPT")
+	_, err := b.updater(t).Rollback(context.Background(), snaps[0].ID)
+	if !errors.Is(err, ErrRollbackFailed) || !strings.Contains(err.Error(), "corrupt") {
+		t.Fatalf("Rollback of a corrupt snapshot = %v; want a refusal naming the corruption", err)
+	}
+	if got := b.deployed(); got != "NEW-BUILD" {
+		t.Errorf("deployed binary = %q; a refused restore must change nothing", got)
+	}
+}
+
+func TestRollback_NoSnapshots(t *testing.T) {
+	b := newFakeBox(t)
+	if _, err := b.updater(t).Rollback(context.Background(), ""); err == nil {
+		t.Error("Rollback with no snapshots succeeded; want an error")
+	}
+}
+
+func TestPrune_KeepsTheNewestAsTheRollbackTarget(t *testing.T) {
+	b := newFakeBox(t)
+	st := &Store{Dir: filepath.Join(b.root, "snapshots")}
+	base := time.Date(2026, 8, 1, 3, 0, 0, 0, time.UTC)
+	for i := 0; i < 4; i++ {
+		i := i
+		st.now = func() time.Time { return base.Add(time.Duration(i) * time.Minute) }
+		if _, err := st.Save(filepath.Join(b.root, "install"), []string{"mavend"}, "", ""); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := st.Prune(0); err != nil { // 0 is clamped to 1, never to zero
+		t.Fatal(err)
+	}
+	snaps, err := st.List()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(snaps) != 1 {
+		t.Fatalf("kept %d snapshots; want 1", len(snaps))
+	}
+	if snaps[0].ID != "20260801-030300" {
+		t.Errorf("kept %s; want the newest", snaps[0].ID)
+	}
+}
+
+func TestList_IgnoresSnapshotsWithNoManifest(t *testing.T) {
+	// An interrupted snapshot has files but no manifest. It must never be offered
+	// as a rollback target — restoring a half-copied binary is the worst outcome
+	// in the package.
+	b := newFakeBox(t)
+	dir := filepath.Join(b.root, "snapshots", "20260801-000000")
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	write(t, filepath.Join(dir, "mavend"), "HALF")
+	snaps, err := (&Store{Dir: filepath.Join(b.root, "snapshots")}).List()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(snaps) != 0 {
+		t.Errorf("List returned %d snapshots; want none (no manifest)", len(snaps))
+	}
+}
+
+func TestConfigValidate(t *testing.T) {
+	ok := (&fakeBox{root: t.TempDir()}).cfg()
+	if err := ok.Validate(); err != nil {
+		t.Fatalf("valid config rejected: %v", err)
+	}
+	bad := map[string]func(c Config) Config{
+		"relative source":          func(c Config) Config { c.SourceDir = "src"; return c },
+		"no restart command":       func(c Config) Config { c.RestartCmd = nil; return c },
+		"no health socket":         func(c Config) Config { c.HealthSocket = ""; return c },
+		"no binaries":              func(c Config) Config { c.Binaries = nil; return c },
+		"escaping artifact name":   func(c Config) Config { c.Binaries = []string{"../../etc/passwd"}; return c },
+		"absolute artifact name":   func(c Config) Config { c.Binaries = []string{"/usr/bin/mavend"}; return c },
+		"snapshots inside install": func(c Config) Config { c.SnapshotDir = filepath.Join(c.InstallDir, "snaps"); return c },
+	}
+	for name, mutate := range bad {
+		if err := mutate(ok).Validate(); err == nil {
+			t.Errorf("%s was accepted; want a startup failure", name)
+		}
+	}
+	// And New refuses an invalid config outright rather than half-configuring.
+	if _, err := New(mutate(ok, "no health socket", bad)); err == nil {
+		t.Error("New accepted a config with no health socket")
+	}
+}
+
+func mutate(c Config, key string, m map[string]func(Config) Config) Config { return m[key](c) }

internal/update/verify.go

@@ -0,0 +1,65 @@
+package update
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+// Verification is "does this tree build and does it pass its own tests", run
+// before a single byte is written to the install dir.
+//
+// It is `make build` and `make test`, not `go build`: the CGO daemons need the
+// vendored toolchain and the whisper/piper include and library paths wired
+// through the Makefile, and a bare `go build` on them fails in a way that has
+// nothing to do with the change being deployed. `make test` is the -race suite
+// with the CGO env set, and it is the only evidence available on a single box
+// that the new code does what the old code did.
+//
+// This is not a substitute for a second environment. A test suite that passes
+// says the code is self-consistent; it does not say the new build will start
+// against this machine's actual models, sockets and encrypted store. That is
+// what the post-restart health check is for, and it is why the install is
+// reversible rather than merely careful.
+
+// Step — one verification or orchestration step and how it went. Kept so the CLI
+// can print a truthful account of what was done, including on the failure path.
+type Step struct {
+	Name   string
+	Argv   []string
+	Took   time.Duration
+	Err    error
+	Output string // combined output, only retained for failures
+}
+
+// Verify runs the build and the test suite in SourceDir.
+func (u *Updater) Verify(ctx context.Context) ([]Step, error) {
+	ctx, cancel := context.WithTimeout(ctx, u.cfg.verifyTimeout())
+	defer cancel()
+	var steps []Step
+	for _, argv := range [][]string{{"make", "build"}, {"make", "test"}} {
+		u.log("verify: %v (this takes a while)", argv)
+		start := u.now()
+		out, err := u.run(ctx, u.cfg.SourceDir, argv)
+		st := Step{Name: argv[len(argv)-1], Argv: argv, Took: u.now().Sub(start), Err: err}
+		if err != nil {
+			st.Output = tail(out, 4000)
+		}
+		steps = append(steps, st)
+		if err != nil {
+			u.log("verify: %v FAILED after %s", argv, st.Took.Round(time.Second))
+			return steps, fmt.Errorf("%w: %v: %v", ErrVerifyFailed, argv, err)
+		}
+		u.log("verify: %v ok in %s", argv, st.Took.Round(time.Second))
+	}
+	return steps, nil
+}
+
+// tail keeps the last n bytes — a failing `make test` prints far more than is
+// useful, and the failure is always at the end.
+func tail(s string, n int) string {
+	if len(s) <= n {
+		return s
+	}
+	return "…" + s[len(s)-n:]
+}

internal/webfetch/webfetch.go

@@ -28,6 +28,7 @@
 package webfetch
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -92,6 +93,9 @@ type Response struct {
 	Status      int
 	ContentType string
 	Body        []byte
+	// Header — the response headers, one value each (the first). Populated for
+	// every request; MCP needs Mcp-Session-Id, nothing else reads it.
+	Header map[string]string
 }
 
 // Fetcher performs guarded GETs. Safe for concurrent use; the per-host rate
@@ -159,6 +163,37 @@ func New(cfg Config) *Fetcher {
 // Get fetches rawURL. The body is capped: a larger response is an error, not a
 // truncation, because half an XML document is worse than none.
 func (f *Fetcher) Get(ctx context.Context, rawURL string) (*Response, error) {
+	return f.do(ctx, http.MethodGet, rawURL, nil, nil)
+}
+
+// Post sends body to rawURL and returns the reply, under exactly the same
+// guards as Get: scheme rule, host lists, the dialer's private-address check on
+// every hop, the size cap and the per-host rate limit.
+//
+// It exists for JSON-RPC over HTTP (internal/mcp), which cannot be expressed as
+// a GET. That an outbound request now carries a body does not widen the
+// address policy one bit — a POST to the LAN is refused for the same reason a
+// GET is, unless AllowPrivate was set for that specific fetcher.
+//
+// hdr is merged over the defaults; a caller may not override User-Agent or
+// Accept-Encoding, because identity encoding and an honest UA are part of the
+// contract with whatever is on the other end.
+func (f *Fetcher) Post(ctx context.Context, rawURL, contentType string, body []byte, hdr map[string]string) (*Response, error) {
+	if contentType == "" {
+		contentType = "application/json"
+	}
+	if hdr == nil {
+		hdr = map[string]string{}
+	}
+	merged := make(map[string]string, len(hdr)+1)
+	for k, v := range hdr {
+		merged[k] = v
+	}
+	merged["Content-Type"] = contentType
+	return f.do(ctx, http.MethodPost, rawURL, body, merged)
+}
+
+func (f *Fetcher) do(ctx context.Context, method, rawURL string, body []byte, hdr map[string]string) (*Response, error) {
 	u, err := url.Parse(strings.TrimSpace(rawURL))
 	if err != nil {
 		return nil, fmt.Errorf("webfetch: bad url %q: %w", rawURL, err)
@@ -170,10 +205,17 @@ func (f *Fetcher) Get(ctx context.Context, rawURL string) (*Response, error) {
 		return nil, err
 	}
 
-	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u.String(), nil)
+	var rdr io.Reader
+	if body != nil {
+		rdr = bytes.NewReader(body)
+	}
+	req, err := http.NewRequestWithContext(ctx, method, u.String(), rdr)
 	if err != nil {
 		return nil, err
 	}
+	for k, v := range hdr {
+		req.Header.Set(k, v)
+	}
 	req.Header.Set("User-Agent", f.cfg.UserAgent)
 	req.Header.Set("Accept-Encoding", "identity")
 
@@ -190,22 +232,27 @@ func (f *Fetcher) Get(ctx context.Context, rawURL string) (*Response, error) {
 	}
 	defer resp.Body.Close()
 
-	body, err := io.ReadAll(io.LimitReader(resp.Body, f.cfg.MaxBytes+1))
+	respBody, err := io.ReadAll(io.LimitReader(resp.Body, f.cfg.MaxBytes+1))
 	if err != nil {
 		return nil, err
 	}
-	if int64(len(body)) > f.cfg.MaxBytes {
+	if int64(len(respBody)) > f.cfg.MaxBytes {
 		return nil, fmt.Errorf("%w (%d bytes)", ErrTooLarge, f.cfg.MaxBytes)
 	}
 	if resp.StatusCode < 200 || resp.StatusCode > 299 {
 		return nil, fmt.Errorf("%w: %d", ErrStatus, resp.StatusCode)
 	}
-	return &Response{
+	out := &Response{
 		URL:         resp.Request.URL.String(),
 		Status:      resp.StatusCode,
 		ContentType: resp.Header.Get("Content-Type"),
-		Body:        body,
-	}, nil
+		Body:        respBody,
+		Header:      map[string]string{},
+	}
+	for k := range resp.Header {
+		out.Header[k] = resp.Header.Get(k)
+	}
+	return out, nil
 }
 
 // checkURL applies the scheme rule and the host lists. The address rule is the

internal/webfetch/webfetch_test.go

@@ -3,6 +3,7 @@ package webfetch
 import (
 	"context"
 	"errors"
+	"io"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -225,3 +226,74 @@ func TestUserAgentIsSent(t *testing.T) {
 		t.Fatalf("user-agent = %q", ua)
 	}
 }
+
+func TestPostSendsBodyAndHeaders(t *testing.T) {
+	type seen struct {
+		method, ctype, accept, ua, custom string
+		body                              []byte
+	}
+	ch := make(chan seen, 1)
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		b, _ := io.ReadAll(r.Body)
+		ch <- seen{r.Method, r.Header.Get("Content-Type"), r.Header.Get("Accept"),
+			r.Header.Get("User-Agent"), r.Header.Get("X-Thing"), b}
+		w.Header().Set("Mcp-Session-Id", "sess-9")
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"ok":true}`))
+	}))
+	defer srv.Close()
+	f := testFetcher(t, Config{UserAgent: "Maven/test"})
+	resp, err := f.Post(context.Background(), srv.URL, "application/json",
+		[]byte(`{"jsonrpc":"2.0"}`), map[string]string{"Accept": "text/event-stream", "X-Thing": "1"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(resp.Body) != `{"ok":true}` {
+		t.Fatalf("body = %q", resp.Body)
+	}
+	if resp.Header["Mcp-Session-Id"] != "sess-9" {
+		t.Fatalf("response headers not surfaced: %+v", resp.Header)
+	}
+	s := <-ch
+	if s.method != http.MethodPost {
+		t.Fatalf("method = %s", s.method)
+	}
+	if string(s.body) != `{"jsonrpc":"2.0"}` {
+		t.Fatalf("request body = %q", s.body)
+	}
+	if s.ctype != "application/json" {
+		t.Fatalf("content-type = %q", s.ctype)
+	}
+	if s.accept != "text/event-stream" || s.custom != "1" {
+		t.Fatalf("caller headers dropped: %+v", s)
+	}
+	if s.ua != "Maven/test" {
+		t.Fatalf("user-agent = %q — a caller must not be able to override it", s.ua)
+	}
+}
+
+// The whole point of routing MCP through webfetch: a POST is guarded exactly
+// like a GET. A body does not buy a caller a way onto the LAN.
+func TestPostRefusesPrivateAddress(t *testing.T) {
+	f := New(Config{}) // no AllowPrivate
+	_, err := f.Post(context.Background(), "http://127.0.0.1:9100/mcp", "application/json", []byte(`{}`), nil)
+	if !errors.Is(err, ErrPrivate) {
+		t.Fatalf("error = %v, want ErrPrivate", err)
+	}
+}
+
+func TestPostRefusesNonHTTPScheme(t *testing.T) {
+	f := New(Config{})
+	if _, err := f.Post(context.Background(), "file:///etc/passwd", "application/json", nil, nil); !errors.Is(err, ErrScheme) {
+		t.Fatalf("error = %v, want ErrScheme", err)
+	}
+}
+
+func TestPostObeysDenylist(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}))
+	defer srv.Close()
+	f := testFetcher(t, Config{DenyHosts: []string{"127.0.0.1"}})
+	if _, err := f.Post(context.Background(), srv.URL, "application/json", []byte(`{}`), nil); !errors.Is(err, ErrBlocked) {
+		t.Fatalf("error = %v, want ErrBlocked", err)
+	}
+}