feat(server,tui): session stats pane — metrics over the WS bus + Bubble Tea overlay

Surfaces the observability metrics (observability-spec §3-tier-2) in the Go TUI:
a session-summary pane showing duration, token throughput per provider, tool
time, approval latency per tier, failure counts, and the session wall-time
accounting split. Press `S` (or palette → "session stats").

Mirrors the artifacts/config fetch pattern exactly — a WS request/response, not a
new transport. Server-side reuses the already-tested MetricsInspectionService
(one MetricsReport definition, two wires: REST `correx stats` + WS).

Server:
- ClientMessage.GetSessionStats + ServerMessage.SessionStats(@SerialName
  session.stats), reusing MetricsReport as the nested payload.
- StreamQueries.sessionStats replays via MetricsInspectionService (pure read,
  replay-neutral); routed in GlobalStreamHandler.
- ServerMessageSerializationTest golden pins the session.stats wire format.

TUI (Go/Bubble Tea):
- protocol: TypeSessionStats + StatsDto/nested structs + GetSessionStats encoder
  + golden decode test (cross-language contract).
- OverlayStats pane (statsModal), `S` key + palette entry + footer hint,
  loading/cache-by-session state, bounded breakdown rows.
- demo `stats` preview case for serverless visual verification.

apps/server/src/main/kotlin/com/correx/apps/server/protocol/ClientMessage.kt

@@ -42,6 +42,10 @@ sealed class ClientMessage {
     @Serializable
     data class ListArtifacts(val sessionId: SessionId) : ClientMessage()
 
+    /** Operator request for a session's derived metrics (replied to with a SessionStats). */
+    @Serializable
+    data class GetSessionStats(val sessionId: SessionId) : ClientMessage()
+
     /** Operator request for the current editable config (replied to with a ConfigSnapshot). */
     @Serializable
     data object GetConfig : ClientMessage()

apps/server/src/main/kotlin/com/correx/apps/server/protocol/ServerMessage.kt

@@ -1,5 +1,6 @@
 package com.correx.apps.server.protocol
 
+import com.correx.apps.server.metrics.MetricsReport
 import com.correx.core.approvals.Tier
 import com.correx.core.events.types.ApprovalRequestId
 import com.correx.core.events.types.ArtifactId
@@ -425,6 +426,20 @@ sealed interface ServerMessage {
         override val sessionSequence: Long? = null,
     ) : ServerMessage, NonEventMessage
 
+    /**
+     * A session's derived metrics, returned in response to a GetSessionStats request. Not
+     * event-derived (it is a projection-replay snapshot), so cursors are null. [stats] is the same
+     * [MetricsReport] the REST `/sessions/{id}/metrics` endpoint serves — one definition, two wires.
+     */
+    @Serializable
+    @SerialName("session.stats")
+    data class SessionStats(
+        val sessionId: SessionId,
+        val stats: MetricsReport,
+        override val sequence: Long? = null,
+        override val sessionSequence: Long? = null,
+    ) : ServerMessage, NonEventMessage
+
     /**
      * The current editable config (reply to GetConfig / UpdateConfig). [restartRequired] lists the
      * keys in a just-applied patch that need a server restart to take effect; [error] is set (and

apps/server/src/main/kotlin/com/correx/apps/server/ws/GlobalStreamHandler.kt

@@ -229,6 +229,7 @@ class GlobalStreamHandler(private val module: ServerModule) {
                     .onFailure { log.error("cancel failed for session={}: {}", msg.sessionId.value, it.message, it) }
             }
             is ClientMessage.ListArtifacts -> sendFrame(queries.listArtifacts(msg.sessionId))
+            is ClientMessage.GetSessionStats -> sendFrame(queries.sessionStats(msg.sessionId))
             is ClientMessage.GetConfig -> sendFrame(queries.configSnapshot(error = null, restartRequired = emptyList()))
             is ClientMessage.UpdateConfig -> sendFrame(queries.updateConfig(msg.patch))
             is ClientMessage.ResumeSession -> session.send(encodeError("ResumeSession not supported"))

apps/server/src/main/kotlin/com/correx/apps/server/ws/StreamQueries.kt

@@ -2,6 +2,7 @@ package com.correx.apps.server.ws
 
 import com.correx.apps.server.ServerModule
 import com.correx.apps.server.config.ConfigUpdateResult
+import com.correx.apps.server.metrics.MetricsInspectionService
 import com.correx.apps.server.protocol.ArtifactSummaryDto
 import com.correx.apps.server.protocol.ConfigFieldDto
 import com.correx.apps.server.protocol.ServerMessage
@@ -33,6 +34,18 @@ private val lenientJson = Json { ignoreUnknownKeys = true; isLenient = true }
  */
 class StreamQueries(private val module: ServerModule) {
 
+    private val metricsService = MetricsInspectionService(module.eventStore)
+
+    /**
+     * A session's derived metrics as a snapshot frame. Replays the event log through
+     * [MetricsInspectionService] (the same projection the REST metrics endpoint uses) — pure read,
+     * no events appended, so it is replay-neutral (invariant #1/#8).
+     */
+    suspend fun sessionStats(sessionId: SessionId): ServerMessage.SessionStats {
+        val report = withContext(Dispatchers.IO) { metricsService.inspect(sessionId) }
+        return ServerMessage.SessionStats(sessionId = sessionId, stats = report)
+    }
+
     /**
      * Reads a session's events to assemble its artifact listing (creation order preserved),
      * resolving each artifact's stored bytes via the CAS artifact store. Pure snapshot read —

apps/server/src/test/kotlin/com/correx/apps/server/protocol/ServerMessageSerializationTest.kt

@@ -1,5 +1,10 @@
 package com.correx.apps.server.protocol
 
+import com.correx.apps.server.metrics.FailureMetrics
+import com.correx.apps.server.metrics.MetricsReport
+import com.correx.apps.server.metrics.ProviderStatsDto
+import com.correx.apps.server.metrics.TierApprovalStatsDto
+import com.correx.apps.server.metrics.ToolStatsDto
 import com.correx.core.events.types.SessionId
 import kotlinx.serialization.json.Json
 import org.junit.jupiter.api.Assertions.assertEquals
@@ -146,6 +151,48 @@ class ServerMessageSerializationTest {
         assertEquals(listOf("server.port"), decoded.restartRequired)
     }
 
+    @Test
+    fun `SessionStats encodes type, sessionId and nested metrics`() {
+        val msg = ServerMessage.SessionStats(
+            sessionId = SessionId("sess-stats"),
+            stats = MetricsReport(
+                sessionId = "sess-stats",
+                eventCount = 12,
+                sessionDurationMs = 60_000,
+                inferenceCount = 3,
+                inferenceMs = 6000,
+                promptTokens = 310,
+                completionTokens = 170,
+                tokensPerSecond = 28.3,
+                perProvider = listOf(ProviderStatsDto("lfm:a", 2, 5000, 300, 150, 30.0)),
+                toolCount = 3,
+                toolMs = 1500,
+                perTool = listOf(ToolStatsDto("shell.read", 2, 1200)),
+                approvalsRequested = 2,
+                approvalsResolved = 1,
+                approvalsPending = 1,
+                approvalWaitMs = 30_000,
+                avgApprovalWaitMs = 30_000,
+                perTier = listOf(TierApprovalStatsDto("T2", 2, 1, 30_000, 30_000)),
+                failures = FailureMetrics(inferenceFailures = 1, toolFailures = 1, workflowFailures = 1),
+                inferencePct = 10.0,
+                toolPct = 2.5,
+                approvalWaitPct = 50.0,
+            ),
+        )
+        val jsonStr = ProtocolSerializer.encodeServerMessage(msg)
+        assert(jsonStr.contains("\"type\":\"session.stats\"")) { "expected type=session.stats" }
+        assert(jsonStr.contains("\"sessionId\":\"sess-stats\"")) { "expected sessionId" }
+        assert(jsonStr.contains("\"completionTokens\":170")) { "expected nested metric field" }
+        assert(jsonStr.contains("\"provider\":\"lfm:a\"")) { "expected per-provider row" }
+
+        val decoded = json.decodeFromString<ServerMessage.SessionStats>(jsonStr)
+        assertEquals("sess-stats", decoded.stats.sessionId)
+        assertEquals(3, decoded.stats.inferenceCount)
+        assertEquals(1, decoded.stats.perProvider.size)
+        assertEquals(50.0, decoded.stats.approvalWaitPct)
+    }
+
     @Test
     fun `SessionAnnounced round-trips through JSON`() {
         val original = ServerMessage.SessionAnnounced(

apps/tui-go/internal/app/demo.go

@@ -1,5 +1,7 @@
 package app
 
+import "github.com/correx/tui-go/internal/protocol"
+
 // PreviewFrame renders a single static frame for a named UI state at the given
 // terminal size. Used by cmd/preview to screenshot the look without a live
 // server. Not part of the runtime path.
@@ -138,10 +140,57 @@ func PreviewFrame(kind string, w, h int) string {
 		m.selectedID = "04a546aa"
 		m.overlay = OverlayPalette
 		m.paletteFilter = "to"
+
+	case "stats":
+		m.connected = true
+		m.currentModel = "llama-cpp:default"
+		m.sessions = sampleSessions()
+		m.selectedID = "04a546aa"
+		m.sessionEntered = true
+		m.overlay = OverlayStats
+		m.statsFor = "04a546aa"
+		m.stats = sampleStats()
 	}
 	return m.View()
 }
 
+func sampleStats() *protocol.StatsDto {
+	return &protocol.StatsDto{
+		SessionID:         "04a546aa",
+		EventCount:        28,
+		SessionDurationMs: 204_000,
+		InferenceCount:    5,
+		InferenceMs:       38_400,
+		PromptTokens:      4120,
+		CompletionTokens:  1860,
+		TokensPerSecond:   48.4,
+		PerProvider: []protocol.ProviderStatsDto{
+			{Provider: "llama-cpp:qwen2.5-coder-14b", CompletedCount: 4, TotalLatencyMs: 33_200, PromptTokens: 3800, CompletionTokens: 1700, TokensPerSecond: 51.2},
+			{Provider: "llama-cpp:default", CompletedCount: 1, TotalLatencyMs: 5200, PromptTokens: 320, CompletionTokens: 160, TokensPerSecond: 30.8},
+		},
+		ToolCount: 6,
+		ToolMs:    4300,
+		PerTool: []protocol.ToolStatsDto{
+			{ToolName: "file_write", CompletedCount: 2, TotalDurationMs: 2600},
+			{ToolName: "read_file", CompletedCount: 3, TotalDurationMs: 1200},
+			{ToolName: "shell_exec", CompletedCount: 1, TotalDurationMs: 500},
+		},
+		ApprovalsRequested: 2,
+		ApprovalsResolved:  2,
+		ApprovalsPending:   0,
+		ApprovalWaitMs:     45_000,
+		AvgApprovalWaitMs:  22_500,
+		PerTier: []protocol.TierApprovalStatsDto{
+			{Tier: "T2", RequestedCount: 1, ResolvedCount: 1, TotalWaitMs: 15_000, AvgWaitMs: 15_000},
+			{Tier: "T3", RequestedCount: 1, ResolvedCount: 1, TotalWaitMs: 30_000, AvgWaitMs: 30_000},
+		},
+		Failures:        protocol.FailureMetricsDto{ToolFailures: 1},
+		InferencePct:    18.8,
+		ToolPct:         2.1,
+		ApprovalWaitPct: 22.1,
+	}
+}
+
 func sampleWorkflows() []Workflow {
 	return []Workflow{
 		{ID: "healthcheck", Description: "ping endpoints and report status"},

apps/tui-go/internal/app/model.go

@@ -49,6 +49,7 @@ const (
 	OverlayModels
 	OverlayArtifacts
 	OverlayConfig
+	OverlayStats
 )
 
 // RouterEntry is one line in a session's conversation transcript.
@@ -209,6 +210,11 @@ type Model struct {
 	configRestart []string // keys from the last save that need a restart
 	configLoading bool
 
+	// session stats (OverlayStats) — populated by the session.stats reply
+	stats        *protocol.StatsDto
+	statsFor     string // sessionId the current stats belong to
+	statsLoading bool
+
 	// command palette
 	paletteFilter string
 	paletteIndex  int

apps/tui-go/internal/app/overlays.go

@@ -1,6 +1,7 @@
 package app
 
 import (
+	"fmt"
 	"sort"
 	"strings"
 
@@ -43,6 +44,8 @@ func (m Model) renderOverlay(base string) string {
 		return m.center(m.artifactsModal())
 	case OverlayConfig:
 		return m.center(m.configModal())
+	case OverlayStats:
+		return m.center(m.statsModal())
 	}
 	return base
 }
@@ -545,6 +548,116 @@ func (m Model) artifactsModal() string {
 	return t.Overlay.Width(w).Render(b.String())
 }
 
+// statsBreakdownRows caps how many per-provider / per-tool / per-tier rows the pane
+// shows; the server already sorts each list by descending cost, so the cap keeps the
+// heaviest contributors and the modal bounded.
+const statsBreakdownRows = 6
+
+// humanDurMs renders a millisecond span as a compact "1h 2m 3s" string.
+func humanDurMs(ms int64) string {
+	if ms <= 0 {
+		return "0s"
+	}
+	total := ms / 1000
+	h := total / 3600
+	mn := (total / 60) % 60
+	s := total % 60
+	out := ""
+	if h > 0 {
+		out += itoa(int(h)) + "h "
+	}
+	if h > 0 || mn > 0 {
+		out += itoa(int(mn)) + "m "
+	}
+	return out + itoa(int(s)) + "s"
+}
+
+func (m Model) statsModal() string {
+	t := m.theme
+	w := m.modalWidth()
+
+	var b strings.Builder
+	b.WriteString(m.titleLine("session stats"))
+	if m.selectedID != "" {
+		b.WriteString(mbg(t, "   ("+shortID(m.selectedID)+")", t.P.Faint))
+	}
+	b.WriteString("\n\n")
+
+	if m.statsLoading || m.stats == nil || m.statsFor != m.selectedID {
+		msg := "loading…"
+		if !m.statsLoading && m.stats == nil {
+			msg = "no stats for this session"
+		}
+		b.WriteString(mbg(t, "  "+msg, t.P.Faint) + "\n")
+		b.WriteString("\n" + modalHints(t, [][2]string{{"S/esc", "close"}}))
+		return t.Overlay.Width(w).Render(b.String())
+	}
+
+	s := m.stats
+	section := func(label string) string {
+		return lipgloss.NewStyle().Foreground(t.P.Accent2).Background(t.P.BgPanel).Bold(true).Render(label)
+	}
+	line := func(s string) string { return mbg(t, s, t.P.Fg) }
+	faint := func(s string) string { return mbg(t, s, t.P.Faint) }
+
+	// header row: duration + event count
+	b.WriteString(faint("  duration ") + line(humanDurMs(s.SessionDurationMs)) +
+		faint("    events ") + line(itoa(int(s.EventCount))) + "\n\n")
+
+	// inference
+	b.WriteString(section("Inference") + "\n")
+	b.WriteString(line(fmt.Sprintf("  %d calls · %d prompt + %d completion = %d tok · %.1f tok/s",
+		s.InferenceCount, s.PromptTokens, s.CompletionTokens, s.PromptTokens+s.CompletionTokens, s.TokensPerSecond)) + "\n")
+	for i, p := range s.PerProvider {
+		if i >= statsBreakdownRows {
+			b.WriteString(faint(fmt.Sprintf("    … +%d more", len(s.PerProvider)-statsBreakdownRows)) + "\n")
+			break
+		}
+		b.WriteString(faint(fmt.Sprintf("    %-20s %3d calls  %6d tok  %7d ms  %.1f tok/s",
+			padRaw(p.Provider, 20), p.CompletedCount, p.PromptTokens+p.CompletionTokens, p.TotalLatencyMs, p.TokensPerSecond)) + "\n")
+	}
+	b.WriteString("\n")
+
+	// tools
+	b.WriteString(section("Tools") + "\n")
+	b.WriteString(line(fmt.Sprintf("  %d calls · %d ms", s.ToolCount, s.ToolMs)) + "\n")
+	for i, tl := range s.PerTool {
+		if i >= statsBreakdownRows {
+			b.WriteString(faint(fmt.Sprintf("    … +%d more", len(s.PerTool)-statsBreakdownRows)) + "\n")
+			break
+		}
+		b.WriteString(faint(fmt.Sprintf("    %-24s %3d  %7d ms", padRaw(tl.ToolName, 24), tl.CompletedCount, tl.TotalDurationMs)) + "\n")
+	}
+	b.WriteString("\n")
+
+	// approvals
+	b.WriteString(section("Approvals") + "\n")
+	b.WriteString(line(fmt.Sprintf("  %d req · %d resolved · %d pending · avg wait %d ms",
+		s.ApprovalsRequested, s.ApprovalsResolved, s.ApprovalsPending, s.AvgApprovalWaitMs)) + "\n")
+	for _, tr := range s.PerTier {
+		b.WriteString(faint(fmt.Sprintf("    %-4s req %d  res %d  avg %d ms", tr.Tier, tr.RequestedCount, tr.ResolvedCount, tr.AvgWaitMs)) + "\n")
+	}
+	b.WriteString("\n")
+
+	// failures
+	f := s.Failures
+	b.WriteString(section("Failures") + "\n")
+	b.WriteString(line(fmt.Sprintf("  inference %d · timeouts %d · tool %d · rejected %d · stage %d · workflow %d",
+		f.InferenceFailures, f.InferenceTimeouts, f.ToolFailures, f.ToolRejections, f.StageFailures, f.WorkflowFailures)) + "\n\n")
+
+	// time accounting
+	other := 100.0 - s.InferencePct - s.ToolPct - s.ApprovalWaitPct
+	if other < 0 {
+		other = 0
+	}
+	b.WriteString(section("Time") + " " +
+		line(fmt.Sprintf("inference %.1f%% · tools %.1f%% · approval-wait %.1f%% · other %.1f%%",
+			s.InferencePct, s.ToolPct, s.ApprovalWaitPct, other)) + "\n")
+
+	b.WriteString("\n" + modalHints(t, [][2]string{{"S/esc", "close"}}))
+	return t.Overlay.Width(w).Render(b.String())
+}
+
 // shortID trims a long artifact id for the list column while staying identifiable.
 func shortID(id string) string {
 	if len(id) <= 14 {

apps/tui-go/internal/app/server.go

@@ -304,6 +304,10 @@ func (m *Model) applyServer(msg protocol.ServerMessage) {
 		if m.artifactsIndex >= len(m.artifacts) {
 			m.artifactsIndex = 0
 		}
+	case protocol.TypeSessionStats:
+		m.stats = msg.Stats
+		m.statsFor = msg.SessionID
+		m.statsLoading = false
 	case protocol.TypeConfigSnapshot:
 		m.configFields = msg.ConfigFields
 		m.configRestart = msg.ConfigRestartRequired
@@ -332,7 +336,7 @@ func sessionIDOf(msg protocol.ServerMessage) string {
 		protocol.TypeProtocolError, protocol.TypeProviderStatus,
 		protocol.TypeWorkflowList, protocol.TypeRouterResponse,
 		protocol.TypeModelChanged, protocol.TypeModelList, protocol.TypeResourceStatus,
-		protocol.TypeArtifactList, protocol.TypeConfigSnapshot:
+		protocol.TypeArtifactList, protocol.TypeConfigSnapshot, protocol.TypeSessionStats:
 		return ""
 	default:
 		return msg.SessionID

apps/tui-go/internal/app/update.go

@@ -169,6 +169,8 @@ func (m Model) handleNormalKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 		m.overlay = OverlayToolPalette
 	case "v":
 		m.openArtifacts()
+	case "S":
+		m.openStats()
 	case "g":
 		m.openConfig()
 	case "m":
@@ -429,6 +431,10 @@ func (m Model) handleOverlayKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 		case runeIs(k, "m"):
 			m.overlay = OverlayNone
 		}
+	case OverlayStats:
+		if runeIs(k, "S") {
+			m.overlay = OverlayNone
+		}
 	}
 	return m, nil
 }
@@ -450,6 +456,21 @@ func (m *Model) openArtifacts() {
 	m.client.Send(protocol.ListArtifacts(m.selectedID))
 }
 
+// openStats opens the session-stats pane for the selected session and requests its
+// metrics from the server. No-op when no session is selected.
+func (m *Model) openStats() {
+	if m.selectedID == "" {
+		return
+	}
+	m.overlay = OverlayStats
+	// Reuse a cached report only if it's for this session; otherwise show a loading state.
+	if m.statsFor != m.selectedID {
+		m.stats = nil
+		m.statsLoading = true
+	}
+	m.client.Send(protocol.GetSessionStats(m.selectedID))
+}
+
 // openModelsOverlay opens the model picker, pre-selecting the resident model.
 func (m *Model) openModelsOverlay() {
 	m.overlay = OverlayModels
@@ -502,6 +523,7 @@ func paletteCommands() []paletteCmd {
 		{"models", "swap model", "pick / pin the local model"},
 		{"events", "event inspector", "browse the event stream"},
 		{"artifacts", "view artifacts", "browse this session's artifacts"},
+		{"stats", "session stats", "metrics for the selected session"},
 		{"config", "edit config", "view / change correx settings"},
 		{"mode", "toggle mode", "switch chat / steering"},
 		{"cancel", "cancel session", "stop the selected session"},
@@ -539,6 +561,8 @@ func (m Model) execPalette(id string) (tea.Model, tea.Cmd) {
 		m.overlayEventIdx = 0
 	case "artifacts":
 		m.openArtifacts()
+	case "stats":
+		m.openStats()
 	case "config":
 		m.openConfig()
 	case "mode":

apps/tui-go/internal/app/view.go

@@ -155,7 +155,7 @@ func (m Model) renderFooter() string {
 	case m.displayState() == StateIdle:
 		hints = []string{hint("i", "name"), hint("/", "filter"), hint("enter", "open"), hint("jk", "move"), hint("w", "workflows"), hint("p", "cmds"), hint("q", "quit")}
 	default: // StateInSession
-		hints = []string{hint("i", "message"), hint("e", "events"), hint("t", "tools"), hint("m", "model"), hint("s", "mode"), hint("l", "back"), hint("p", "cmds"), hint("q", "quit")}
+		hints = []string{hint("i", "message"), hint("e", "events"), hint("t", "tools"), hint("S", "stats"), hint("m", "model"), hint("s", "mode"), hint("l", "back"), hint("p", "cmds"), hint("q", "quit")}
 		if s := m.session(m.selectedID); s != nil && s.Pending != nil {
 			hints = append(hints, hint("a", "approval (pending)"))
 		}

apps/tui-go/internal/protocol/golden_test.go

@@ -116,6 +116,39 @@ func TestDecodeGoldenServerFrames(t *testing.T) {
 			wantType:  TypeSnapshotComplete,
 			eventBear: false,
 		},
+		{
+			name: "session.stats carries nested derived metrics",
+			json: `{"type":"session.stats","sessionId":"s1","stats":{"sessionId":"s1","eventCount":12,` +
+				`"sessionDurationMs":60000,"inferenceCount":3,"inferenceMs":6000,"promptTokens":310,` +
+				`"completionTokens":170,"tokensPerSecond":28.3,` +
+				`"perProvider":[{"provider":"lfm:a","completedCount":2,"totalLatencyMs":5000,` +
+				`"promptTokens":300,"completionTokens":150,"tokensPerSecond":30.0}],` +
+				`"toolCount":3,"toolMs":1500,"perTool":[{"toolName":"shell.read","completedCount":2,"totalDurationMs":1200}],` +
+				`"approvalsRequested":2,"approvalsResolved":1,"approvalsPending":1,"approvalWaitMs":30000,` +
+				`"avgApprovalWaitMs":30000,"perTier":[{"tier":"T2","requestedCount":2,"resolvedCount":1,` +
+				`"totalWaitMs":30000,"avgWaitMs":30000}],` +
+				`"failures":{"inferenceFailures":1,"inferenceTimeouts":0,"toolFailures":1,"toolRejections":0,` +
+				`"stageFailures":0,"workflowFailures":1},"inferencePct":10.0,"toolPct":2.5,"approvalWaitPct":50.0}}`,
+			wantType:  TypeSessionStats,
+			eventBear: false,
+			check: func(t *testing.T, m ServerMessage) {
+				if m.SessionID != "s1" {
+					t.Fatalf("session.stats sessionId: %+v", m)
+				}
+				if m.Stats == nil {
+					t.Fatalf("session.stats: Stats is nil")
+				}
+				if m.Stats.InferenceCount != 3 || m.Stats.CompletionTokens != 170 {
+					t.Fatalf("session.stats inference totals: %+v", m.Stats)
+				}
+				if len(m.Stats.PerProvider) != 1 || m.Stats.PerProvider[0].Provider != "lfm:a" {
+					t.Fatalf("session.stats perProvider: %+v", m.Stats.PerProvider)
+				}
+				if m.Stats.Failures.WorkflowFailures != 1 || m.Stats.ApprovalWaitPct != 50.0 {
+					t.Fatalf("session.stats failures/pct: %+v", m.Stats)
+				}
+			},
+		},
 	}
 
 	for _, c := range cases {

apps/tui-go/internal/protocol/protocol.go

@@ -51,6 +51,7 @@ const (
 	TypeRouterNarration  = "router.narration"
 	TypeArtifactList     = "artifact.list"
 	TypeConfigSnapshot   = "config.snapshot"
+	TypeSessionStats     = "session.stats"
 )
 
 // ServerMessage is a flat decode of every server->client variant. Field names
@@ -131,6 +132,68 @@ type ServerMessage struct {
 	ConfigFields          []ConfigFieldDto `json:"fields"`
 	ConfigRestartRequired []string         `json:"restartRequired"`
 	ConfigError           *string          `json:"error"`
+
+	// session.stats — derived metrics for a session (reply to GetSessionStats)
+	Stats *StatsDto `json:"stats"`
+}
+
+// StatsDto mirrors the server's MetricsReport: a session's derived metrics
+// (observability-spec §2/§3). Raw sums plus the derived ratios the pane renders.
+type StatsDto struct {
+	SessionID          string                 `json:"sessionId"`
+	EventCount         int64                  `json:"eventCount"`
+	SessionDurationMs  int64                  `json:"sessionDurationMs"`
+	InferenceCount     int64                  `json:"inferenceCount"`
+	InferenceMs        int64                  `json:"inferenceMs"`
+	PromptTokens       int64                  `json:"promptTokens"`
+	CompletionTokens   int64                  `json:"completionTokens"`
+	TokensPerSecond    float64                `json:"tokensPerSecond"`
+	PerProvider        []ProviderStatsDto     `json:"perProvider"`
+	ToolCount          int64                  `json:"toolCount"`
+	ToolMs             int64                  `json:"toolMs"`
+	PerTool            []ToolStatsDto         `json:"perTool"`
+	ApprovalsRequested int64                  `json:"approvalsRequested"`
+	ApprovalsResolved  int64                  `json:"approvalsResolved"`
+	ApprovalsPending   int64                  `json:"approvalsPending"`
+	ApprovalWaitMs     int64                  `json:"approvalWaitMs"`
+	AvgApprovalWaitMs  int64                  `json:"avgApprovalWaitMs"`
+	PerTier            []TierApprovalStatsDto `json:"perTier"`
+	Failures           FailureMetricsDto      `json:"failures"`
+	InferencePct       float64                `json:"inferencePct"`
+	ToolPct            float64                `json:"toolPct"`
+	ApprovalWaitPct    float64                `json:"approvalWaitPct"`
+}
+
+type ProviderStatsDto struct {
+	Provider         string  `json:"provider"`
+	CompletedCount   int64   `json:"completedCount"`
+	TotalLatencyMs   int64   `json:"totalLatencyMs"`
+	PromptTokens     int64   `json:"promptTokens"`
+	CompletionTokens int64   `json:"completionTokens"`
+	TokensPerSecond  float64 `json:"tokensPerSecond"`
+}
+
+type ToolStatsDto struct {
+	ToolName        string `json:"toolName"`
+	CompletedCount  int64  `json:"completedCount"`
+	TotalDurationMs int64  `json:"totalDurationMs"`
+}
+
+type TierApprovalStatsDto struct {
+	Tier           string `json:"tier"`
+	RequestedCount int64  `json:"requestedCount"`
+	ResolvedCount  int64  `json:"resolvedCount"`
+	TotalWaitMs    int64  `json:"totalWaitMs"`
+	AvgWaitMs      int64  `json:"avgWaitMs"`
+}
+
+type FailureMetricsDto struct {
+	InferenceFailures int64 `json:"inferenceFailures"`
+	InferenceTimeouts int64 `json:"inferenceTimeouts"`
+	ToolFailures      int64 `json:"toolFailures"`
+	ToolRejections    int64 `json:"toolRejections"`
+	StageFailures     int64 `json:"stageFailures"`
+	WorkflowFailures  int64 `json:"workflowFailures"`
 }
 
 type ConfigFieldDto struct {
@@ -275,6 +338,11 @@ func ListArtifacts(sessionID string) []byte {
 	return encode("ListArtifacts", map[string]any{"sessionId": sessionID})
 }
 
+// GetSessionStats asks the server for a session's derived metrics (replied to with session.stats).
+func GetSessionStats(sessionID string) []byte {
+	return encode("GetSessionStats", map[string]any{"sessionId": sessionID})
+}
+
 // GetConfig requests the current editable config (replied to with a config.snapshot).
 func GetConfig() []byte {
 	return encode("GetConfig", map[string]any{})