diff --git a/scripts/deadcode-exports.baseline.mjs b/scripts/deadcode-exports.baseline.mjs
index 5ec04485afd2..f57bb1b6f67d 100644
--- a/scripts/deadcode-exports.baseline.mjs
+++ b/scripts/deadcode-exports.baseline.mjs
@@ -666,12 +666,9 @@ export const KNIP_UNUSED_EXPORT_BASELINE = [
   "src/llm/utils/oauth/github-copilot.ts: testing",
   "src/llm/utils/oauth/openai-chatgpt.ts: loginOpenAICodex",
   "src/llm/utils/oauth/openai-chatgpt.ts: refreshOpenAICodexToken",
-  "src/logging/diagnostic-phase.ts: recordDiagnosticPhase",
   "src/logging/diagnostic-run-activity.ts: markDiagnosticModelStartedForTest",
   "src/logging/diagnostic-run-activity.ts: markDiagnosticRunProgressForTest",
   "src/logging/diagnostic-run-activity.ts: markDiagnosticToolStartedForTest",
-  "src/logging/diagnostic-session-context.ts: parseCronRunSessionKey",
-  "src/logging/diagnostic-session-context.ts: readLastAssistantFromSessionFile",
   "src/logging/redact-internal.ts: withFullContextToolPayloadRedaction",
   "src/logging/secret-redaction-registry.ts: resetSecretRedactionRegistryForTest",
   "src/mcp/channel-bridge.ts: shouldRetryInitialMcpGatewayConnect",
diff --git a/scripts/repro/limit-edge-case-live-proof.mjs b/scripts/repro/limit-edge-case-live-proof.mjs
index 743238fa2135..f286537cf5f1 100644
--- a/scripts/repro/limit-edge-case-live-proof.mjs
+++ b/scripts/repro/limit-edge-case-live-proof.mjs
@@ -12,8 +12,8 @@ import { testing as voiceCallCliTesting } from "../../extensions/voice-call/src/
 import { loadSessionLogs, loadSessionUsageTimeSeries } from "../../src/infra/session-cost-usage.ts";
 import {
   getRecentDiagnosticPhases,
-  recordDiagnosticPhase,
   resetDiagnosticPhasesForTest,
+  withDiagnosticPhase,
 } from "../../src/logging/diagnostic-phase.ts";
 
 /**
@@ -30,26 +30,8 @@ export async function withProofTempRoot(callback) {
 
 async function main() {
   resetDiagnosticPhasesForTest();
-  recordDiagnosticPhase({
-    name: "phase-a",
-    startedAt: 1,
-    endedAt: 2,
-    durationMs: 1,
-    cpuUserMs: 0,
-    cpuSystemMs: 0,
-    cpuTotalMs: 0,
-    cpuCoreRatio: 0,
-  });
-  recordDiagnosticPhase({
-    name: "phase-b",
-    startedAt: 3,
-    endedAt: 4,
-    durationMs: 1,
-    cpuUserMs: 0,
-    cpuSystemMs: 0,
-    cpuTotalMs: 0,
-    cpuCoreRatio: 0,
-  });
+  await withDiagnosticPhase("phase-a", () => undefined);
+  await withDiagnosticPhase("phase-b", () => undefined);
   const zeroPhases = getRecentDiagnosticPhases(0);
   assert.equal(zeroPhases.length, 0);
   console.log("getRecentDiagnosticPhases(0).length =", zeroPhases.length);
diff --git a/src/logging/diagnostic-phase.test.ts b/src/logging/diagnostic-phase.test.ts
index e9d48b411966..5c1d09d67d31 100644
--- a/src/logging/diagnostic-phase.test.ts
+++ b/src/logging/diagnostic-phase.test.ts
@@ -2,33 +2,15 @@
 import { describe, expect, it } from "vitest";
 import {
   getRecentDiagnosticPhases,
-  recordDiagnosticPhase,
   resetDiagnosticPhasesForTest,
+  withDiagnosticPhase,
 } from "./diagnostic-phase.js";
 
 describe("getRecentDiagnosticPhases", () => {
-  it("returns an empty list for zero, negative, and non-finite limits", () => {
+  it("returns an empty list for zero, negative, and non-finite limits", async () => {
     resetDiagnosticPhasesForTest();
-    recordDiagnosticPhase({
-      name: "phase-a",
-      startedAt: 1,
-      endedAt: 2,
-      durationMs: 1,
-      cpuUserMs: 0,
-      cpuSystemMs: 0,
-      cpuTotalMs: 0,
-      cpuCoreRatio: 0,
-    });
-    recordDiagnosticPhase({
-      name: "phase-b",
-      startedAt: 3,
-      endedAt: 4,
-      durationMs: 1,
-      cpuUserMs: 0,
-      cpuSystemMs: 0,
-      cpuTotalMs: 0,
-      cpuCoreRatio: 0,
-    });
+    await withDiagnosticPhase("phase-a", () => undefined);
+    await withDiagnosticPhase("phase-b", () => undefined);
 
     expect(getRecentDiagnosticPhases(0)).toEqual([]);
     expect(getRecentDiagnosticPhases(-1)).toEqual([]);
@@ -36,28 +18,10 @@ describe("getRecentDiagnosticPhases", () => {
     expect(getRecentDiagnosticPhases(Number.POSITIVE_INFINITY)).toEqual([]);
   });
 
-  it("returns the most recent phases for positive limits", () => {
+  it("returns the most recent phases for positive limits", async () => {
     resetDiagnosticPhasesForTest();
-    recordDiagnosticPhase({
-      name: "phase-a",
-      startedAt: 1,
-      endedAt: 2,
-      durationMs: 1,
-      cpuUserMs: 0,
-      cpuSystemMs: 0,
-      cpuTotalMs: 0,
-      cpuCoreRatio: 0,
-    });
-    recordDiagnosticPhase({
-      name: "phase-b",
-      startedAt: 3,
-      endedAt: 4,
-      durationMs: 1,
-      cpuUserMs: 0,
-      cpuSystemMs: 0,
-      cpuTotalMs: 0,
-      cpuCoreRatio: 0,
-    });
+    await withDiagnosticPhase("phase-a", () => undefined);
+    await withDiagnosticPhase("phase-b", () => undefined);
 
     const recent = getRecentDiagnosticPhases(1);
     expect(recent).toHaveLength(1);
diff --git a/src/logging/diagnostic-phase.ts b/src/logging/diagnostic-phase.ts
index c326cb56943e..c4c7b3a32b72 100644
--- a/src/logging/diagnostic-phase.ts
+++ b/src/logging/diagnostic-phase.ts
@@ -56,7 +56,7 @@ export function getRecentDiagnosticPhases(limit = 8): DiagnosticPhaseSnapshot[]
 }
 
 /** Records a completed phase in memory and emits it when diagnostics are enabled. */
-export function recordDiagnosticPhase(snapshot: DiagnosticPhaseSnapshot): void {
+function recordDiagnosticPhase(snapshot: DiagnosticPhaseSnapshot): void {
   pushRecentPhase(snapshot);
   if (!areDiagnosticsEnabledForProcess()) {
     return;
diff --git a/src/logging/diagnostic-session-context.test.ts b/src/logging/diagnostic-session-context.test.ts
index b1fa59c55811..11734b443c62 100644
--- a/src/logging/diagnostic-session-context.test.ts
+++ b/src/logging/diagnostic-session-context.test.ts
@@ -10,8 +10,6 @@ import {
 import {
   formatCronSessionDiagnosticFields,
   formatStoppedCronSessionDiagnosticFields,
-  parseCronRunSessionKey,
-  readLastAssistantFromSessionFile,
   resolveCronSessionDiagnosticContext,
 } from "./diagnostic-session-context.js";
 
@@ -39,11 +37,11 @@ describe("diagnostic session context", () => {
   });
 
   it("parses cron run session keys", () => {
-    expect(parseCronRunSessionKey("agent:clawblocker:cron:job-123:run:run-456")).toEqual({
-      agentId: "clawblocker",
-      cronJobId: "job-123",
-      cronRunId: "run-456",
-    });
+    expect(
+      resolveCronSessionDiagnosticContext({
+        sessionKey: "agent:clawblocker:cron:job-123:run:run-456",
+      }),
+    ).toMatchObject({ agentId: "clawblocker", cronJobId: "job-123", cronRunId: "run-456" });
   });
 
   it("formats cron job and last assistant context for stalled session logs", async () => {
@@ -93,14 +91,18 @@ describe("diagnostic session context", () => {
   });
 
   it("reads the latest assistant message from a transcript tail", () => {
-    const filePath = path.join(tempDir!, "session.jsonl");
+    const filePath = path.join(tempDir!, "agents", "clawblocker", "sessions", "run-456.jsonl");
     writeJsonl(filePath, [
       { message: { role: "assistant", content: "older" } },
       { message: { role: "user", content: "later user" } },
       { message: { role: "assistant", content: "newer" } },
     ]);
 
-    expect(readLastAssistantFromSessionFile(filePath)).toBe("newer");
+    expect(
+      resolveCronSessionDiagnosticContext({
+        sessionKey: "agent:clawblocker:cron:job-123:run:run-456",
+      }).lastAssistant,
+    ).toBe("newer");
   });
 
   it("keeps bounded quoted fields UTF-16 safe", () => {
@@ -112,6 +114,11 @@ describe("diagnostic session context", () => {
   });
 
   it("ignores missing transcript tail files", () => {
-    expect(readLastAssistantFromSessionFile(path.join(tempDir!, "missing.jsonl"))).toBeUndefined();
+    expect(
+      resolveCronSessionDiagnosticContext({
+        sessionKey: "agent:clawblocker:cron:job-123:run:run-456",
+        activeSessionId: "missing",
+      }).lastAssistant,
+    ).toBeUndefined();
   });
 });
diff --git a/src/logging/diagnostic-session-context.ts b/src/logging/diagnostic-session-context.ts
index 9b1577a68947..2d9c06c49c33 100644
--- a/src/logging/diagnostic-session-context.ts
+++ b/src/logging/diagnostic-session-context.ts
@@ -26,7 +26,7 @@ function quoteLogField(value: string): string {
   return `"${truncated.replace(/["\\]/g, "\\$&")}"`;
 }
 
-export function parseCronRunSessionKey(sessionKey?: string): {
+function parseCronRunSessionKey(sessionKey?: string): {
   agentId?: string;
   cronJobId?: string;
   cronRunId?: string;
@@ -105,7 +105,7 @@ function textFromContent(content: unknown): string | undefined {
   return texts.length ? texts.join(" ") : undefined;
 }
 
-export function readLastAssistantFromSessionFile(filePath: string | undefined): string | undefined {
+function readLastAssistantFromSessionFile(filePath: string | undefined): string | undefined {
   if (!filePath) {
     return undefined;
   }