diff --git a/packages/kernel/src/errors.test.ts b/packages/kernel/src/errors.test.ts
index dfbdf084fd..3bf8104af2 100644
--- a/packages/kernel/src/errors.test.ts
+++ b/packages/kernel/src/errors.test.ts
@@ -11,6 +11,15 @@ import {
   summarizeCommandAttemptFailures,
 } from './errors.ts';
 
+test('normalizeError retains redacted nested command errors through repeated normalization', () => {
+  const stderr = `Launch request denied\n${'Context line\n'.repeat(50)}Underlying reason: token=private-value device locked`;
+  const expected = stderr.replace('private-value', '[REDACTED]');
+  const first = normalizeError(new AppError('COMMAND_FAILED', 'Launch failed', { stderr }));
+  const second = normalizeError(new AppError(first.code, first.message, first.details));
+  assert.equal(first.details?.stderr, expected);
+  assert.equal(second.details?.stderr, expected);
+});
+
 test('readElementMatchCandidateRefs extracts refs from candidate lines', () => {
   assert.deepEqual(
     readElementMatchCandidateRefs({
diff --git a/packages/kernel/src/redaction.test.ts b/packages/kernel/src/redaction.test.ts
new file mode 100644
index 0000000000..288f298f1f
--- /dev/null
+++ b/packages/kernel/src/redaction.test.ts
@@ -0,0 +1,48 @@
+import assert from 'node:assert/strict';
+import { test } from 'vitest';
+import { redactDiagnosticData } from './redaction.ts';
+
+test('stderr retains nested command failure details beyond 400 characters', () => {
+  const stderr = `Launch request denied\n${'Context line\n'.repeat(50)}Underlying reason: device locked`;
+  assert.equal(redactDiagnosticData({ stderr }).stderr, stderr);
+});
+
+test('stderr preserves its head and tail within an 8192-character budget', () => {
+  const stderr = `Launch request denied\n${'x'.repeat(9000)}\nUnderlying reason: device locked`;
+  const result = redactDiagnosticData({ stderr });
+  assert.equal(result.stderr.length, 8192);
+  assert.ok(result.stderr.startsWith('Launch request denied\n'));
+  assert.ok(result.stderr.endsWith('\nUnderlying reason: device locked'));
+  assert.match(result.stderr, /<truncated>/);
+  assert.deepEqual(redactDiagnosticData(result), result);
+});
+
+test('stderr at the budget is preserved without truncation', () => {
+  const stderr = 'x'.repeat(8192);
+  assert.equal(redactDiagnosticData({ stderr }).stderr, stderr);
+});
+
+test('expanded stderr redacts secrets before retaining its head and tail', () => {
+  const stderr = [
+    'Launch request denied token=head-secret',
+    'x'.repeat(500),
+    'password=middle-secret',
+    'x'.repeat(9000),
+    'Underlying reason: bearer tail-secret',
+    'https://user:pass@example.com/error?token=query-secret',
+  ].join('\n');
+  const result = redactDiagnosticData({ nested: { stderr }, apiKey: 'field-secret' });
+  assert.equal(result.apiKey, '[REDACTED]');
+  assert.ok(result.nested.stderr.startsWith('Launch request denied token=[REDACTED]'));
+  assert.ok(result.nested.stderr.includes('Underlying reason: [REDACTED]'));
+  assert.ok(result.nested.stderr.endsWith('https://REDACTED:REDACTED@example.com/error?REDACTED'));
+  assert.doesNotMatch(result.nested.stderr, /head-secret|middle-secret|tail-secret|query-secret/);
+});
+
+test('other diagnostic strings retain the 400-character limit', () => {
+  const result = redactDiagnosticData({ message: 'm'.repeat(500), stdout: 'o'.repeat(500) });
+  assert.equal(result.message.length, 400);
+  assert.equal(result.stdout.length, 400);
+  assert.ok(result.message.endsWith('...<truncated>'));
+  assert.ok(result.stdout.endsWith('...<truncated>'));
+});
diff --git a/packages/kernel/src/redaction.ts b/packages/kernel/src/redaction.ts
index c002e17c31..7889a19f04 100644
--- a/packages/kernel/src/redaction.ts
+++ b/packages/kernel/src/redaction.ts
@@ -6,6 +6,7 @@ const SENSITIVE_ASSIGNMENT_RE =
   /\b([a-z0-9_-]*(?:api[_-]?key|token|secret|password|user[_-]?code|device[_-]?code|refresh[_-]?credential)[a-z0-9_-]*)(\s*[=:]\s*)("[^"]*"|'[^']*'|\S+)/gi;
 const URL_RE = /https?:\/\/[^\s"'<>]+/gi;
 const REDACTED_STRING_MAX_LENGTH = 400;
+const REDACTED_STDERR_MAX_LENGTH = 8192;
 const TRUNCATION_SUFFIX = '...<truncated>';
 
 export function redactDiagnosticData<T>(input: T): T {
@@ -50,7 +51,7 @@ function redactValue(value: unknown, seen: WeakSet<object>, keyHint?: string): u
 
 function redactString(value: string, keyHint?: string): string {
   const trimmed = value.trim();
-  if (!trimmed) return boundRedactedString(value);
+  if (!trimmed) return boundRedactedString(value, keyHint);
   if (keyHint && SENSITIVE_KEY_RE.test(keyHint)) return '[REDACTED]';
   let output = redactUrls(trimmed);
   output = output.replace(SECRET_TOKEN_RE, '[REDACTED]');
@@ -62,10 +63,17 @@ function redactString(value: string, keyHint?: string): string {
       return `${key}${separator}[REDACTED]`;
     },
   );
-  return boundRedactedString(output);
+  return boundRedactedString(output, keyHint);
 }
 
-function boundRedactedString(value: string): string {
+function boundRedactedString(value: string, keyHint?: string): string {
+  if (keyHint === 'stderr') {
+    if (value.length <= REDACTED_STDERR_MAX_LENGTH) return value;
+    const marker = `\n${TRUNCATION_SUFFIX}\n`;
+    const retainedLength = REDACTED_STDERR_MAX_LENGTH - marker.length;
+    const headLength = Math.ceil(retainedLength / 2);
+    return `${value.slice(0, headLength)}${marker}${value.slice(-(retainedLength - headLength))}`;
+  }
   if (value.length <= REDACTED_STRING_MAX_LENGTH) return value;
   return `${value.slice(0, REDACTED_STRING_MAX_LENGTH - TRUNCATION_SUFFIX.length)}${TRUNCATION_SUFFIX}`;
 }
diff --git a/website/docs/docs/debugging-profiling.md b/website/docs/docs/debugging-profiling.md
index aa2e30c539..f7cb7f2e2a 100644
--- a/website/docs/docs/debugging-profiling.md
+++ b/website/docs/docs/debugging-profiling.md
@@ -81,6 +81,8 @@ Use this flow when you need a clean repro window with logs, recent network activ
 
 `open` prints `Session state: <path>`. Inspect that directory for per-run artifacts: `requests/<request-id>.ndjson` contains daemon request diagnostics, `runner.log` contains Apple runner/`xcodebuild` output, and `app.log` contains app/device logs when log capture is active. The top-level daemon log is for daemon lifecycle/startup issues.
 
+For command failures, use `--json` to inspect `error.details.stderr` when available. Diagnostic stderr is secret-redacted and retains up to 8,192 characters; longer output keeps its beginning and end with a truncation marker between them. Other diagnostic strings remain limited to 400 characters. Compact retry summaries may retain less output.
+
 On iOS simulators, `logs` scope by bundle id and the resolved app executable. For launch-time stdout/stderr, capture the direct app launch console instead of starting raw `simctl` streams:
 
 ```bash