diff --git a/packages/platform-apple/src/runner/__tests__/runner-toolchain-probe.test.ts b/packages/platform-apple/src/runner/__tests__/runner-toolchain-probe.test.ts
index fb3fd5d3c6..7df14c9580 100644
--- a/packages/platform-apple/src/runner/__tests__/runner-toolchain-probe.test.ts
+++ b/packages/platform-apple/src/runner/__tests__/runner-toolchain-probe.test.ts
@@ -5,6 +5,7 @@ import path from 'node:path';
 import { isRequestCanceledError } from '@agent-device/kernel/errors';
 import { resetAllProcessMemosForTests } from '@agent-device/kernel/ttl-memo';
 import type { ExecOptions, ExecResult } from '@agent-device/host-kit/command';
+import type { DiagnosticEventInput } from '@agent-device/host-kit/diagnostics';
 import { IOS_SIMULATOR } from './device-fixtures.ts';
 import { appleRunnerTestHost } from '../test-host.ts';
 import {
@@ -106,6 +107,103 @@ function blockResolutionForWholeTimeout(clock: { nowMs: number }): void {
   });
 }
 
+test('an exhausted fingerprint retains the failed probe and retry in diagnostics', () => {
+  const clock = installToolchainProbeClock();
+  vi.spyOn(Date, 'now').mockImplementation(() => clock.nowMs);
+  const events: DiagnosticEventInput[] = [];
+  appleRunnerTestHost.update({ emitDiagnostic: (event) => events.push(event) });
+  runCmdSync.mockImplementation((command: string, args: string[], options: ExecOptions) => {
+    if (command === 'xcode-select') return selectedDeveloperDirResult(XCODE_A);
+    throw blockForWholeTimeout(clock, command, args, options);
+  });
+
+  expect(readFingerprint).toThrow('The Apple runner budget ran out before this step began');
+
+  expect(
+    events
+      .filter(({ data }) => data?.cmd)
+      .map(({ phase, data }) => [phase, data?.event, data?.cmd, data?.timeoutMs]),
+  ).toEqual([
+    ['apple_toolchain_probe', 'started', 'xcodebuild', 30_000],
+    ['apple_toolchain_probe', 'failed', 'xcodebuild', 30_000],
+    ['apple_toolchain_probe', 'started', 'xcodebuild', 15_000],
+    ['apple_toolchain_probe', 'failed', 'xcodebuild', 15_000],
+  ]);
+  for (const event of events.filter(({ data }) => data?.event === 'failed')) {
+    expect(event).toMatchObject({
+      durationMs: event.data?.timeoutMs,
+      data: {
+        args: ['-version'],
+        error: { code: 'COMMAND_FAILED', details: { timeoutMs: event.data?.timeoutMs } },
+      },
+    });
+  }
+  expect(clock.nowMs).toBe(45_000);
+});
+
+test.each([
+  {
+    label: 'nonzero exit',
+    output: { exitCode: 70, stdout: '', stderr: 'SDK unavailable' },
+    reason: 'nonzero_exit',
+    detail: 'exit 70: SDK unavailable',
+  },
+  {
+    label: 'empty output',
+    output: { exitCode: 0, stdout: ' \n', stderr: '' },
+    reason: 'empty_output',
+    detail: 'no output',
+  },
+  {
+    label: 'unparsable output',
+    output: { exitCode: 0, stdout: 'unexpected version format', stderr: '' },
+    reason: 'unparsable_output',
+    detail: 'unrecognized output: unexpected version format',
+  },
+  {
+    label: 'exec failure',
+    output: new Error('version probe failed'),
+    reason: 'probe_error',
+    detail: 'version probe failed',
+  },
+])(
+  'the trace records the classified $label before a later probe exhausts the budget',
+  ({ output, reason, detail }) => {
+    const clock = installToolchainProbeClock();
+    const events: DiagnosticEventInput[] = [];
+    appleRunnerTestHost.update({ emitDiagnostic: (event) => events.push(event) });
+    runCmdSync.mockImplementation((command: string, args: string[], options: ExecOptions) => {
+      if (command === 'xcode-select') return selectedDeveloperDirResult(XCODE_A);
+      if (command === 'xcodebuild') {
+        if (output instanceof Error) throw output;
+        return output;
+      }
+      throw blockForWholeTimeout(clock, command, args, options);
+    });
+
+    expect(readFingerprint).toThrow('The Apple runner budget ran out before this step began');
+    expect(
+      events.filter(
+        ({ data }) => data?.event === 'rejected' && data?.probe === 'xcodebuild -version',
+      ),
+    ).toEqual([
+      {
+        level: 'error',
+        phase: 'apple_toolchain_probe',
+        data: { event: 'rejected', probe: 'xcodebuild -version', reason, detail },
+      },
+    ]);
+  },
+);
+
+test('the trace does not reject a readable toolchain fingerprint', () => {
+  const events: DiagnosticEventInput[] = [];
+  appleRunnerTestHost.update({ emitDiagnostic: (event) => events.push(event) });
+
+  expect(readFingerprint().xcodeBuildVersion).toBe(STUBBED_APPLE_TOOLCHAIN.xcodeBuildVersion);
+  expect(events.filter(({ data }) => data?.event === 'rejected')).toEqual([]);
+});
+
 describe('the developer dir a toolchain fingerprint answers for', () => {
   test('a default selection that moved re-reads instead of answering the Xcode it replaced', () => {
     scriptSelectableXcodes();
diff --git a/packages/platform-apple/src/runner/runner-toolchain-probe.ts b/packages/platform-apple/src/runner/runner-toolchain-probe.ts
index 1f10433211..0eb4cc9cf5 100644
--- a/packages/platform-apple/src/runner/runner-toolchain-probe.ts
+++ b/packages/platform-apple/src/runner/runner-toolchain-probe.ts
@@ -2,6 +2,7 @@ import {
   AppError,
   createRequestCanceledError,
   isRequestCanceledError,
+  normalizeError,
 } from '@agent-device/kernel/errors';
 import type { DeviceInfo } from '@agent-device/kernel/device';
 import type { TtlMemo } from '@agent-device/kernel/ttl-memo';
@@ -9,6 +10,7 @@ import {
   commandDeveloperDir,
   createTtlMemo,
   Deadline,
+  emitDiagnostic,
   isCommandTimeoutError,
   runCmdSync,
 } from './host.ts';
@@ -439,11 +441,35 @@ function attemptToolchainProbe(
   clock.throwIfCanceled();
   const timeoutMs = clock.attemptTimeoutMs();
   if (timeoutMs <= 0) throw runnerPhaseBudgetExhaustedError('apple_toolchain_probe');
-  return runCmdSync(cmd, args, {
-    allowFailure: true,
-    timeoutMs,
-    maxBuffer: TOOLCHAIN_PROBE_MAX_BUFFER,
-  });
+  const startedAtMs = Date.now();
+  const data = { cmd, args, timeoutMs };
+  emitDiagnostic({ phase: 'apple_toolchain_probe', data: { ...data, event: 'started' } });
+  try {
+    const result = runCmdSync(cmd, args, {
+      allowFailure: true,
+      timeoutMs,
+      maxBuffer: TOOLCHAIN_PROBE_MAX_BUFFER,
+    });
+    emitDiagnostic({
+      phase: 'apple_toolchain_probe',
+      durationMs: Date.now() - startedAtMs,
+      data: {
+        ...data,
+        event: 'completed',
+        exitCode: result.exitCode,
+        stderr: result.stderr.slice(0, TOOLCHAIN_PROBE_DETAIL_MAX_LENGTH),
+      },
+    });
+    return result;
+  } catch (error) {
+    emitDiagnostic({
+      level: 'error',
+      phase: 'apple_toolchain_probe',
+      durationMs: Date.now() - startedAtMs,
+      data: { ...data, event: 'failed', error: normalizeError(error) },
+    });
+    throw error;
+  }
 }
 
 function parseXcodeVersionOutput(
@@ -473,5 +499,11 @@ function probeFailure(
     detail.length > TOOLCHAIN_PROBE_DETAIL_MAX_LENGTH
       ? `${detail.slice(0, TOOLCHAIN_PROBE_DETAIL_MAX_LENGTH)}…`
       : detail;
-  return { ok: false, failure: { probe, reason, detail: bounded } };
+  const failure = { probe, reason, detail: bounded };
+  emitDiagnostic({
+    level: 'error',
+    phase: 'apple_toolchain_probe',
+    data: { event: 'rejected', ...failure },
+  });
+  return { ok: false, failure };
 }
diff --git a/scripts/write-xcuitest-cache-metadata.ts b/scripts/write-xcuitest-cache-metadata.ts
index b0c4caa70a..b2385a11e0 100644
--- a/scripts/write-xcuitest-cache-metadata.ts
+++ b/scripts/write-xcuitest-cache-metadata.ts
@@ -3,6 +3,8 @@ import path from 'node:path';
 import { pathToFileURL } from 'node:url';
 import { withCommandHost } from '@agent-device/host-kit/command';
 import { captureHostEnvironment } from '@agent-device/host-kit/process';
+import { withDiagnosticsScope } from '@agent-device/host-kit/diagnostics';
+import { normalizeError } from '@agent-device/kernel/errors';
 import type { RunnerXctestrunCacheMetadata } from '@agent-device/platform-apple/runner/operations';
 import {
   findRunnerXctestrun,
@@ -80,10 +82,14 @@ if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href)
     setAppleRunnerHostEnvironment(host);
     await withCommandHost(
       { host, requestEnv: undefined },
-      async () => await writeXcuitestCacheMetadata(process.argv.slice(2), host.cwd),
+      async () =>
+        await withDiagnosticsScope(
+          { debug: true },
+          async () => await writeXcuitestCacheMetadata(process.argv.slice(2), host.cwd),
+        ),
     );
   } catch (error) {
-    console.error(error instanceof Error ? error.message : String(error));
+    console.error(JSON.stringify(normalizeError(error)));
     process.exit(1);
   }
 }