diff --git a/src/logging/diagnostic-session-state.ts b/src/logging/diagnostic-session-state.ts
new file mode 100644
index 000000000..c060866ff
--- /dev/null
+++ b/src/logging/diagnostic-session-state.ts
@@ -0,0 +1,91 @@
+export type SessionStateValue = "idle" | "processing" | "waiting";
+
+export type SessionState = {
+  sessionId?: string;
+  sessionKey?: string;
+  lastActivity: number;
+  state: SessionStateValue;
+  queueDepth: number;
+};
+
+export type SessionRef = {
+  sessionId?: string;
+  sessionKey?: string;
+};
+
+export const diagnosticSessionStates = new Map<string, SessionState>();
+
+const SESSION_STATE_TTL_MS = 30 * 60 * 1000;
+const SESSION_STATE_PRUNE_INTERVAL_MS = 60 * 1000;
+const SESSION_STATE_MAX_ENTRIES = 2000;
+
+let lastSessionPruneAt = 0;
+
+export function pruneDiagnosticSessionStates(now = Date.now(), force = false): void {
+  const shouldPruneForSize = diagnosticSessionStates.size > SESSION_STATE_MAX_ENTRIES;
+  if (!force && !shouldPruneForSize && now - lastSessionPruneAt < SESSION_STATE_PRUNE_INTERVAL_MS) {
+    return;
+  }
+  lastSessionPruneAt = now;
+
+  for (const [key, state] of diagnosticSessionStates.entries()) {
+    const ageMs = now - state.lastActivity;
+    const isIdle = state.state === "idle";
+    if (isIdle && state.queueDepth <= 0 && ageMs > SESSION_STATE_TTL_MS) {
+      diagnosticSessionStates.delete(key);
+    }
+  }
+
+  if (diagnosticSessionStates.size <= SESSION_STATE_MAX_ENTRIES) {
+    return;
+  }
+  const excess = diagnosticSessionStates.size - SESSION_STATE_MAX_ENTRIES;
+  const ordered = Array.from(diagnosticSessionStates.entries()).toSorted(
+    (a, b) => a[1].lastActivity - b[1].lastActivity,
+  );
+  for (let i = 0; i < excess; i += 1) {
+    const key = ordered[i]?.[0];
+    if (!key) {
+      break;
+    }
+    diagnosticSessionStates.delete(key);
+  }
+}
+
+function resolveSessionKey({ sessionKey, sessionId }: SessionRef) {
+  return sessionKey ?? sessionId ?? "unknown";
+}
+
+export function getDiagnosticSessionState(ref: SessionRef): SessionState {
+  pruneDiagnosticSessionStates();
+  const key = resolveSessionKey(ref);
+  const existing = diagnosticSessionStates.get(key);
+  if (existing) {
+    if (ref.sessionId) {
+      existing.sessionId = ref.sessionId;
+    }
+    if (ref.sessionKey) {
+      existing.sessionKey = ref.sessionKey;
+    }
+    return existing;
+  }
+  const created: SessionState = {
+    sessionId: ref.sessionId,
+    sessionKey: ref.sessionKey,
+    lastActivity: Date.now(),
+    state: "idle",
+    queueDepth: 0,
+  };
+  diagnosticSessionStates.set(key, created);
+  pruneDiagnosticSessionStates(Date.now(), true);
+  return created;
+}
+
+export function getDiagnosticSessionStateCountForTest(): number {
+  return diagnosticSessionStates.size;
+}
+
+export function resetDiagnosticSessionStateForTest(): void {
+  diagnosticSessionStates.clear();
+  lastSessionPruneAt = 0;
+}
diff --git a/src/logging/diagnostic.test.ts b/src/logging/diagnostic.test.ts
index 75f6d942e..c7cd72f5c 100644
--- a/src/logging/diagnostic.test.ts
+++ b/src/logging/diagnostic.test.ts
@@ -1,34 +1,34 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import {
   getDiagnosticSessionStateCountForTest,
-  logSessionStateChange,
-  resetDiagnosticStateForTest,
-} from "./diagnostic.js";
+  getDiagnosticSessionState,
+  resetDiagnosticSessionStateForTest,
+} from "./diagnostic-session-state.js";
 
 describe("diagnostic session state pruning", () => {
   beforeEach(() => {
     vi.useFakeTimers();
-    resetDiagnosticStateForTest();
+    resetDiagnosticSessionStateForTest();
   });
 
   afterEach(() => {
-    resetDiagnosticStateForTest();
+    resetDiagnosticSessionStateForTest();
     vi.useRealTimers();
   });
 
   it("evicts stale idle session states", () => {
-    logSessionStateChange({ sessionId: "stale-1", state: "idle" });
+    getDiagnosticSessionState({ sessionId: "stale-1" });
     expect(getDiagnosticSessionStateCountForTest()).toBe(1);
 
     vi.advanceTimersByTime(31 * 60 * 1000);
-    logSessionStateChange({ sessionId: "fresh-1", state: "idle" });
+    getDiagnosticSessionState({ sessionId: "fresh-1" });
 
     expect(getDiagnosticSessionStateCountForTest()).toBe(1);
   });
 
   it("caps tracked session states to a bounded max", () => {
     for (let i = 0; i < 2001; i += 1) {
-      logSessionStateChange({ sessionId: `session-${i}`, state: "idle" });
+      getDiagnosticSessionState({ sessionId: `session-${i}` });
     }
 
     expect(getDiagnosticSessionStateCountForTest()).toBe(2000);
diff --git a/src/logging/diagnostic.ts b/src/logging/diagnostic.ts
index e5e8de402..73a3ac893 100644
--- a/src/logging/diagnostic.ts
+++ b/src/logging/diagnostic.ts
@@ -1,28 +1,17 @@
 import { emitDiagnosticEvent } from "../infra/diagnostic-events.js";
+import {
+  diagnosticSessionStates,
+  getDiagnosticSessionState,
+  getDiagnosticSessionStateCountForTest as getDiagnosticSessionStateCountForTestImpl,
+  pruneDiagnosticSessionStates,
+  resetDiagnosticSessionStateForTest,
+  type SessionRef,
+  type SessionStateValue,
+} from "./diagnostic-session-state.js";
 import { createSubsystemLogger } from "./subsystem.js";
 
 const diag = createSubsystemLogger("diagnostic");
 
-type SessionStateValue = "idle" | "processing" | "waiting";
-
-type SessionState = {
-  sessionId?: string;
-  sessionKey?: string;
-  lastActivity: number;
-  state: SessionStateValue;
-  queueDepth: number;
-};
-
-type SessionRef = {
-  sessionId?: string;
-  sessionKey?: string;
-};
-
-const sessionStates = new Map<string, SessionState>();
-const SESSION_STATE_TTL_MS = 30 * 60 * 1000;
-const SESSION_STATE_PRUNE_INTERVAL_MS = 60 * 1000;
-const SESSION_STATE_MAX_ENTRIES = 2000;
-
 const webhookStats = {
   received: 0,
   processed: 0,
@@ -31,72 +20,11 @@ const webhookStats = {
 };
 
 let lastActivityAt = 0;
-let lastSessionPruneAt = 0;
 
 function markActivity() {
   lastActivityAt = Date.now();
 }
 
-function pruneSessionStates(now = Date.now(), force = false): void {
-  const shouldPruneForSize = sessionStates.size > SESSION_STATE_MAX_ENTRIES;
-  if (!force && !shouldPruneForSize && now - lastSessionPruneAt < SESSION_STATE_PRUNE_INTERVAL_MS) {
-    return;
-  }
-  lastSessionPruneAt = now;
-
-  for (const [key, state] of sessionStates.entries()) {
-    const ageMs = now - state.lastActivity;
-    const isIdle = state.state === "idle";
-    if (isIdle && state.queueDepth <= 0 && ageMs > SESSION_STATE_TTL_MS) {
-      sessionStates.delete(key);
-    }
-  }
-
-  if (sessionStates.size <= SESSION_STATE_MAX_ENTRIES) {
-    return;
-  }
-  const excess = sessionStates.size - SESSION_STATE_MAX_ENTRIES;
-  const ordered = Array.from(sessionStates.entries()).toSorted(
-    (a, b) => a[1].lastActivity - b[1].lastActivity,
-  );
-  for (let i = 0; i < excess; i += 1) {
-    const key = ordered[i]?.[0];
-    if (!key) {
-      break;
-    }
-    sessionStates.delete(key);
-  }
-}
-
-function resolveSessionKey({ sessionKey, sessionId }: SessionRef) {
-  return sessionKey ?? sessionId ?? "unknown";
-}
-
-function getSessionState(ref: SessionRef): SessionState {
-  pruneSessionStates();
-  const key = resolveSessionKey(ref);
-  const existing = sessionStates.get(key);
-  if (existing) {
-    if (ref.sessionId) {
-      existing.sessionId = ref.sessionId;
-    }
-    if (ref.sessionKey) {
-      existing.sessionKey = ref.sessionKey;
-    }
-    return existing;
-  }
-  const created: SessionState = {
-    sessionId: ref.sessionId,
-    sessionKey: ref.sessionKey,
-    lastActivity: Date.now(),
-    state: "idle",
-    queueDepth: 0,
-  };
-  sessionStates.set(key, created);
-  pruneSessionStates(Date.now(), true);
-  return created;
-}
-
 export function logWebhookReceived(params: {
   channel: string;
   updateType?: string;
@@ -174,7 +102,7 @@ export function logMessageQueued(params: {
   channel?: string;
   source: string;
 }) {
-  const state = getSessionState(params);
+  const state = getDiagnosticSessionState(params);
   state.queueDepth += 1;
   state.lastActivity = Date.now();
   if (diag.isEnabled("debug")) {
@@ -244,7 +172,7 @@ export function logSessionStateChange(
     reason?: string;
   },
 ) {
-  const state = getSessionState(params);
+  const state = getDiagnosticSessionState(params);
   const isProbeSession = state.sessionId?.startsWith("probe-") ?? false;
   const prevState = state.state;
   state.state = params.state;
@@ -274,7 +202,7 @@ export function logSessionStateChange(
 }
 
 export function logSessionStuck(params: SessionRef & { state: SessionStateValue; ageMs: number }) {
-  const state = getSessionState(params);
+  const state = getDiagnosticSessionState(params);
   diag.warn(
     `stuck session: sessionId=${state.sessionId ?? "unknown"} sessionKey=${
       state.sessionKey ?? "unknown"
@@ -329,7 +257,7 @@ export function logRunAttempt(params: SessionRef & { runId: string; attempt: num
 }
 
 export function logActiveRuns() {
-  const activeSessions = Array.from(sessionStates.entries())
+  const activeSessions = Array.from(diagnosticSessionStates.entries())
     .filter(([, s]) => s.state === "processing")
     .map(
       ([id, s]) =>
@@ -347,14 +275,14 @@ export function startDiagnosticHeartbeat() {
   }
   heartbeatInterval = setInterval(() => {
     const now = Date.now();
-    pruneSessionStates(now, true);
-    const activeCount = Array.from(sessionStates.values()).filter(
+    pruneDiagnosticSessionStates(now, true);
+    const activeCount = Array.from(diagnosticSessionStates.values()).filter(
       (s) => s.state === "processing",
     ).length;
-    const waitingCount = Array.from(sessionStates.values()).filter(
+    const waitingCount = Array.from(diagnosticSessionStates.values()).filter(
       (s) => s.state === "waiting",
     ).length;
-    const totalQueued = Array.from(sessionStates.values()).reduce(
+    const totalQueued = Array.from(diagnosticSessionStates.values()).reduce(
       (sum, s) => sum + s.queueDepth,
       0,
     );
@@ -386,7 +314,7 @@ export function startDiagnosticHeartbeat() {
       queued: totalQueued,
     });
 
-    for (const [, state] of sessionStates) {
+    for (const [, state] of diagnosticSessionStates) {
       const ageMs = now - state.lastActivity;
       if (state.state === "processing" && ageMs > 120_000) {
         logSessionStuck({
@@ -409,17 +337,16 @@ export function stopDiagnosticHeartbeat() {
 }
 
 export function getDiagnosticSessionStateCountForTest(): number {
-  return sessionStates.size;
+  return getDiagnosticSessionStateCountForTestImpl();
 }
 
 export function resetDiagnosticStateForTest(): void {
-  sessionStates.clear();
+  resetDiagnosticSessionStateForTest();
   webhookStats.received = 0;
   webhookStats.processed = 0;
   webhookStats.errors = 0;
   webhookStats.lastReceived = 0;
   lastActivityAt = 0;
-  lastSessionPruneAt = 0;
   stopDiagnosticHeartbeat();
 }