an mcp answer with no result is not a success (V-613)

Two defects in internal/mcp, both about a call that reports done when
nothing happened.

Client.call accepted a frame carrying our id and neither result nor
error. The HTTP transport already refuses one; the stdio transport does
not, so the refusal depended on which door the server was behind. Down
that path tools/call returns an empty string and a nil error, and the
act is recorded as run.

Manager.ReadResource reported ErrNoServer for a server that is
configured but down. Call keeps those two apart on purpose — one says
the tool can never exist, the other says not right now.

internal/mcp/client.go

@@ -257,7 +257,16 @@ func (c *Client) call(ctx context.Context, method string, params any, out any) e
 	if resp.Error != nil {
 		return fmt.Errorf("mcp: %s: %s: %w", c.name, method, resp.Error)
 	}
-	if out == nil || len(resp.Result) == 0 {
+	// A frame carrying our id and neither result nor error is not an answer.
+	// The HTTP transport already refuses one; the stdio transport does not, and
+	// without this check the refusal depended on which door the server was
+	// behind. Letting it through is the one failure that lies: tools/call
+	// returns an empty string and a nil error, so the act is recorded as done
+	// and the tool never ran.
+	if len(resp.Result) == 0 {
+		return fmt.Errorf("mcp: %s: %s: response carries neither result nor error", c.name, method)
+	}
+	if out == nil {
 		return nil
 	}
 	if err := json.Unmarshal(resp.Result, out); err != nil {

internal/mcp/manager.go

@@ -525,10 +525,16 @@ func (m *Manager) Resources(ctx context.Context) []Resource {
 
 // ReadResource reads one resource from one server.
 func (m *Manager) ReadResource(ctx context.Context, server, uri string) (string, error) {
-	cl, cfg, _, _, _ := m.lookup(server, "")
-	if cl == nil {
+	cl, cfg, _, configured, _ := m.lookup(server, "")
+	if !configured {
 		return "", fmt.Errorf("%w: %s", ErrNoServer, server)
 	}
+	// A configured server that is merely down is not an unknown server. Call
+	// already keeps the two apart; reporting ErrNoServer here tells a caller
+	// the resource can never exist, when the truth is "not right now".
+	if cl == nil {
+		return "", fmt.Errorf("%w: %s", ErrNotConnected, server)
+	}
 	cctx, cancel := context.WithTimeout(ctx, cfg.Timeout)
 	defer cancel()
 	return cl.ReadResource(cctx, uri)

internal/mcp/mcp_test.go

@@ -641,3 +641,59 @@ func TestReconnectBackoffGrows(t *testing.T) {
 		t.Fatalf("first retry = %v, want %v", c.backoff(), DefaultReconnectEvery)
 	}
 }
+
+// emptyFrameTransport answers the handshake normally and then replies to every
+// later call with a well-formed frame carrying our id and nothing else — no
+// result, no error. That is the answer a partially-implemented server gives,
+// and it is the one that lies: without a check it reads as success.
+type emptyFrameTransport struct{ handshaken bool }
+
+func (t *emptyFrameTransport) Call(ctx context.Context, req *rpcRequest) (*rpcResponse, error) {
+	id := req.ID
+	if !t.handshaken {
+		t.handshaken = true
+		raw, _ := json.Marshal(map[string]any{
+			"protocolVersion": ProtocolVersion,
+			"serverInfo":      map[string]any{"name": "empty", "version": "0"},
+		})
+		return &rpcResponse{JSONRPC: "2.0", ID: &id, Result: raw}, nil
+	}
+	return &rpcResponse{JSONRPC: "2.0", ID: &id}, nil
+}
+
+func (t *emptyFrameTransport) Notify(context.Context, string, any) error { return nil }
+func (t *emptyFrameTransport) Close() error                             { return nil }
+
+func TestResultlessResponseIsNotSuccess(t *testing.T) {
+	c := newClient("empty", &emptyFrameTransport{})
+	if err := c.Initialize(context.Background()); err != nil {
+		t.Fatalf("initialize: %v", err)
+	}
+	out, err := c.CallTool(context.Background(), "break_thing", map[string]any{"q": "x"})
+	if err == nil {
+		t.Fatalf("a frame with neither result nor error must not read as success (got %q)", out)
+	}
+	if out != "" {
+		t.Fatalf("out = %q", out)
+	}
+	if _, err := c.ListTools(context.Background()); err == nil {
+		t.Fatal("tools/list with no result must be an error, not an empty catalogue")
+	}
+}
+
+func TestReadResourceOnDownServerIsNotConnected(t *testing.T) {
+	// A url server with no poster factory: configured, validated, never dialed.
+	m, err := NewManager(nil, []ServerConfig{{
+		Name: "down", URL: "http://example.test/mcp", Enabled: true,
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	m.Connect(context.Background())
+	if _, err := m.ReadResource(context.Background(), "down", "note://one"); !errors.Is(err, ErrNotConnected) {
+		t.Fatalf("err = %v, want ErrNotConnected", err)
+	}
+	if _, err := m.ReadResource(context.Background(), "nosuch", "note://one"); !errors.Is(err, ErrNoServer) {
+		t.Fatalf("err = %v, want ErrNoServer", err)
+	}
+}