diff --git a/src/logging/diagnostic-session-attention.test.ts b/src/logging/diagnostic-session-attention.test.ts
new file mode 100644
index 00000000000..a19d0c64650
--- /dev/null
+++ b/src/logging/diagnostic-session-attention.test.ts
@@ -0,0 +1,98 @@
+import { describe, expect, it } from "vitest";
+import { classifySessionAttention } from "./diagnostic-session-attention.js";
+
+describe("classifySessionAttention", () => {
+  it.each([
+    {
+      name: "stale state without queued work",
+      queueDepth: 0,
+      activity: {},
+      expected: {
+        eventType: "session.stuck",
+        reason: "stale_session_state",
+        classification: "stale_session_state",
+        recoveryEligible: true,
+      },
+    },
+    {
+      name: "queued stale state without active work",
+      queueDepth: 1,
+      activity: {},
+      expected: {
+        eventType: "session.stuck",
+        reason: "queued_work_without_active_run",
+        classification: "stale_session_state",
+        recoveryEligible: true,
+      },
+    },
+    {
+      name: "active embedded run making progress",
+      queueDepth: 0,
+      activity: {
+        activeWorkKind: "embedded_run" as const,
+        lastProgressAgeMs: 10_000,
+      },
+      expected: {
+        eventType: "session.long_running",
+        reason: "active_work",
+        classification: "long_running",
+        activeWorkKind: "embedded_run",
+        recoveryEligible: false,
+      },
+    },
+    {
+      name: "queued behind active work",
+      queueDepth: 1,
+      activity: {
+        activeWorkKind: "embedded_run" as const,
+        lastProgressAgeMs: 10_000,
+      },
+      expected: {
+        eventType: "session.long_running",
+        reason: "queued_behind_active_work",
+        classification: "long_running",
+        activeWorkKind: "embedded_run",
+        recoveryEligible: false,
+      },
+    },
+    {
+      name: "active work without progress",
+      queueDepth: 0,
+      activity: {
+        activeWorkKind: "model_call" as const,
+        lastProgressAgeMs: 31_000,
+      },
+      expected: {
+        eventType: "session.stalled",
+        reason: "active_work_without_progress",
+        classification: "stalled_agent_run",
+        activeWorkKind: "model_call",
+        recoveryEligible: false,
+      },
+    },
+    {
+      name: "blocked tool call",
+      queueDepth: 0,
+      activity: {
+        activeWorkKind: "tool_call" as const,
+        activeToolAgeMs: 31_000,
+        lastProgressAgeMs: 31_000,
+      },
+      expected: {
+        eventType: "session.stalled",
+        reason: "blocked_tool_call",
+        classification: "blocked_tool_call",
+        activeWorkKind: "tool_call",
+        recoveryEligible: false,
+      },
+    },
+  ])("$name", ({ activity, expected, queueDepth }) => {
+    expect(
+      classifySessionAttention({
+        queueDepth,
+        activity,
+        staleMs: 30_000,
+      }),
+    ).toEqual(expected);
+  });
+});
diff --git a/src/logging/diagnostic-session-attention.ts b/src/logging/diagnostic-session-attention.ts
new file mode 100644
index 00000000000..31a9b53a969
--- /dev/null
+++ b/src/logging/diagnostic-session-attention.ts
@@ -0,0 +1,70 @@
+import type { DiagnosticSessionActiveWorkKind } from "../infra/diagnostic-events.js";
+import type { DiagnosticSessionActivitySnapshot } from "./diagnostic-run-activity.js";
+
+export type SessionAttentionClassification =
+  | {
+      eventType: "session.long_running";
+      reason: string;
+      classification: "long_running";
+      activeWorkKind?: DiagnosticSessionActiveWorkKind;
+      recoveryEligible: false;
+    }
+  | {
+      eventType: "session.stalled";
+      reason: string;
+      classification: "blocked_tool_call" | "stalled_agent_run";
+      activeWorkKind?: DiagnosticSessionActiveWorkKind;
+      recoveryEligible: false;
+    }
+  | {
+      eventType: "session.stuck";
+      reason: string;
+      classification: "stale_session_state";
+      activeWorkKind?: undefined;
+      recoveryEligible: true;
+    };
+
+export function classifySessionAttention(params: {
+  queueDepth: number;
+  activity: DiagnosticSessionActivitySnapshot;
+  staleMs: number;
+}): SessionAttentionClassification {
+  if (params.activity.activeWorkKind) {
+    if (
+      params.activity.activeWorkKind === "tool_call" &&
+      (params.activity.activeToolAgeMs ?? 0) > params.staleMs &&
+      (params.activity.lastProgressAgeMs ?? 0) > params.staleMs
+    ) {
+      return {
+        eventType: "session.stalled",
+        reason: "blocked_tool_call",
+        classification: "blocked_tool_call",
+        activeWorkKind: params.activity.activeWorkKind,
+        recoveryEligible: false,
+      };
+    }
+    if ((params.activity.lastProgressAgeMs ?? 0) > params.staleMs) {
+      return {
+        eventType: "session.stalled",
+        reason: "active_work_without_progress",
+        classification: "stalled_agent_run",
+        activeWorkKind: params.activity.activeWorkKind,
+        recoveryEligible: false,
+      };
+    }
+    return {
+      eventType: "session.long_running",
+      reason: params.queueDepth > 0 ? "queued_behind_active_work" : "active_work",
+      classification: "long_running",
+      activeWorkKind: params.activity.activeWorkKind,
+      recoveryEligible: false,
+    };
+  }
+
+  return {
+    eventType: "session.stuck",
+    reason: params.queueDepth > 0 ? "queued_work_without_active_run" : "stale_session_state",
+    classification: "stale_session_state",
+    recoveryEligible: true,
+  };
+}
diff --git a/src/logging/diagnostic.ts b/src/logging/diagnostic.ts
index df563810223..23e5d913e7f 100644
--- a/src/logging/diagnostic.ts
+++ b/src/logging/diagnostic.ts
@@ -5,7 +5,6 @@ import {
   areDiagnosticsEnabledForProcess,
   emitDiagnosticEvent,
   isDiagnosticsEnabled,
-  type DiagnosticSessionActiveWorkKind,
   type DiagnosticLivenessWarningReason,
 } from "../infra/diagnostic-events.js";
 import { emitDiagnosticMemorySample, resetDiagnosticMemoryForTest } from "./diagnostic-memory.js";
@@ -20,6 +19,10 @@ import {
   markDiagnosticActivity as markActivity,
   resetDiagnosticActivityForTest,
 } from "./diagnostic-runtime.js";
+import {
+  classifySessionAttention,
+  type SessionAttentionClassification,
+} from "./diagnostic-session-attention.js";
 import {
   diagnosticSessionStates,
   getDiagnosticSessionState,
@@ -156,74 +159,6 @@ function hasRecentDiagnosticActivity(now: number): boolean {
   return lastActivityAt > 0 && now - lastActivityAt <= RECENT_DIAGNOSTIC_ACTIVITY_MS;
 }
 
-type SessionAttentionClassification =
-  | {
-      eventType: "session.long_running";
-      reason: string;
-      classification: "long_running";
-      activeWorkKind?: DiagnosticSessionActiveWorkKind;
-      recoveryEligible: false;
-    }
-  | {
-      eventType: "session.stalled";
-      reason: string;
-      classification: "blocked_tool_call" | "stalled_agent_run";
-      activeWorkKind?: DiagnosticSessionActiveWorkKind;
-      recoveryEligible: false;
-    }
-  | {
-      eventType: "session.stuck";
-      reason: string;
-      classification: "stale_session_state";
-      activeWorkKind?: undefined;
-      recoveryEligible: true;
-    };
-
-function classifySessionAttention(params: {
-  queueDepth: number;
-  activity: DiagnosticSessionActivitySnapshot;
-  staleMs: number;
-}): SessionAttentionClassification {
-  if (params.activity.activeWorkKind) {
-    if (
-      params.activity.activeWorkKind === "tool_call" &&
-      (params.activity.activeToolAgeMs ?? 0) > params.staleMs &&
-      (params.activity.lastProgressAgeMs ?? 0) > params.staleMs
-    ) {
-      return {
-        eventType: "session.stalled",
-        reason: "blocked_tool_call",
-        classification: "blocked_tool_call",
-        activeWorkKind: params.activity.activeWorkKind,
-        recoveryEligible: false,
-      };
-    }
-    if ((params.activity.lastProgressAgeMs ?? 0) > params.staleMs) {
-      return {
-        eventType: "session.stalled",
-        reason: "active_work_without_progress",
-        classification: "stalled_agent_run",
-        activeWorkKind: params.activity.activeWorkKind,
-        recoveryEligible: false,
-      };
-    }
-    return {
-      eventType: "session.long_running",
-      reason: params.queueDepth > 0 ? "queued_behind_active_work" : "active_work",
-      classification: "long_running",
-      activeWorkKind: params.activity.activeWorkKind,
-      recoveryEligible: false,
-    };
-  }
-
-  return {
-    eventType: "session.stuck",
-    reason: params.queueDepth > 0 ? "queued_work_without_active_run" : "stale_session_state",
-    classification: "stale_session_state",
-    recoveryEligible: true,
-  };
-}
-
 function roundDiagnosticMetric(value: number, digits = 3): number {
   if (!Number.isFinite(value)) {
     return 0;