diff --git a/docs/spikes/claude-code-hook-mutation.md b/docs/spikes/claude-code-hook-mutation.md
index 70a4ae18a..9d91e16cd 100644
--- a/docs/spikes/claude-code-hook-mutation.md
+++ b/docs/spikes/claude-code-hook-mutation.md
@@ -127,6 +127,38 @@ required for our hook to fire there.
 (PostToolUse hooks can also set `additionalContext` to append to the tool
 result; we don't need this for v1 capture.)
 
+## PostToolUse on tool error (AUQ-failure fallback, OV3:B) — UNVERIFIED
+
+The AUQ-failure prose fallback adds a defensive PostToolUse hook
+(`hosts/claude/hooks/auq-error-fallback-hook.ts`) that, when an AskUserQuestion
+call returns an error / missing result, injects `additionalContext` reminding the
+model to run the prose fallback per `SESSION_KIND`. It uses the same
+`additionalContext` mechanism documented above.
+
+**Open question we could NOT settle in a harness:** does Claude Code invoke
+PostToolUse hooks when an MCP tool call returns a transport/missing-result error
+(the Conductor bug surfaces `[Tool result missing due to internal error]`)? The
+docs above cover PostToolUse on *success*. We could not force the Conductor
+internal MCP failure on demand to observe it.
+
+**Decision (OV3:B = A):** build the hook defensively anyway.
+- It is **inert on success** (only fires when `isErrorResponse(tool_response)` is
+  true) and **inert if the platform never invokes it** on the error path.
+- The prompt-level fallback in `generate-ask-user-format.ts` covers the case
+  regardless — the hook is a reliability *layer*, not the mechanism.
+- Its decision logic is unit-tested deterministically
+  (`test/auq-error-fallback-hook.test.ts`): given a synthetic error `tool_response`
+  + each `SESSION_KIND`, it emits the correct directive; given a real answer it
+  defers.
+
+**Recommended manual / partial spike (to close the gap later):** register a
+throwaway PostToolUse hook that logs on fire, then (a) trigger a normal tool
+error (e.g. a failing `Bash` call) to confirm PostToolUse fires on tool errors at
+all, and (b) reproduce the Conductor MCP AUQ failure and check the log. If (b)
+confirms a fire, promote the hook from "defensive/inert" to "verified". Until
+then, treat the runtime layer as best-effort and the prompt-level fallback as the
+guaranteed path.
+
 ## Settings.json snippet for T8 hook installer
 
 ```json
diff --git a/hosts/claude/hooks/auq-error-fallback-hook b/hosts/claude/hooks/auq-error-fallback-hook
new file mode 100755
index 000000000..063d7fce9
--- /dev/null
+++ b/hosts/claude/hooks/auq-error-fallback-hook
@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+# Bash shim — Claude Code hooks run `command` strings via /bin/sh, so this
+# wrapper makes the TypeScript hook executable via bun. Settings.json
+# references this file directly.
+set -e
+HERE="$(cd "$(dirname "$0")" && pwd)"
+exec bun "$HERE/auq-error-fallback-hook.ts"
diff --git a/hosts/claude/hooks/auq-error-fallback-hook.ts b/hosts/claude/hooks/auq-error-fallback-hook.ts
new file mode 100755
index 000000000..be0900830
--- /dev/null
+++ b/hosts/claude/hooks/auq-error-fallback-hook.ts
@@ -0,0 +1,197 @@
+#!/usr/bin/env bun
+/**
+ * PostToolUse hook for AskUserQuestion — runtime reliability layer for the
+ * AUQ-failure prose fallback (OV3:B).
+ *
+ * When an AskUserQuestion call comes back as an ERROR / missing result (the
+ * Conductor MCP bug returns `[Tool result missing due to internal error]`), this
+ * hook injects `additionalContext` reminding the model of the failure-fallback
+ * rule, tailored to the session kind. It does NOT render the prose itself — the
+ * model still emits it. The hook only guarantees the reminder fires at the moment
+ * of failure, instead of relying on the model noticing the error result and
+ * recalling the echoed SESSION_KIND.
+ *
+ * DEFENSIVE / INERT-IF-UNSUPPORTED: it is unverified whether Claude Code invokes
+ * PostToolUse hooks when an MCP tool returns a transport/missing-result error (we
+ * could not force that Conductor-internal failure in a harness — see
+ * docs/spikes/claude-code-hook-mutation.md §"PostToolUse on tool error"). If the
+ * platform does NOT fire the hook on that path, this is simply never invoked — no
+ * harm; the prompt-level fallback in generate-ask-user-format.ts still covers it.
+ * On a SUCCESSFUL AskUserQuestion (a real answer), the hook defers (no output).
+ *
+ * Triggered by ~/.claude/settings.json (registered by `setup` next to
+ * question-log-hook):
+ *   PostToolUse matcher "(AskUserQuestion|mcp__.*__AskUserQuestion)"
+ *
+ * Invariants:
+ *   - Always exits 0. A failing hook MUST NOT block the user's session.
+ *   - Never triggers on a successful answer (would corrupt a normal AUQ).
+ *   - Errors land in ~/.gstack/hook-errors.log.
+ */
+import * as fs from 'fs';
+import * as path from 'path';
+import * as os from 'os';
+import { spawnSync } from 'child_process';
+
+interface HookStdin {
+  tool_name?: string;
+  tool_response?: unknown;
+  cwd?: string;
+}
+
+function stateRoot(): string {
+  return (
+    process.env.GSTACK_STATE_ROOT ||
+    process.env.GSTACK_HOME ||
+    path.join(os.homedir(), '.gstack')
+  );
+}
+
+function logHookError(msg: string): void {
+  try {
+    const sr = stateRoot();
+    fs.mkdirSync(sr, { recursive: true });
+    fs.appendFileSync(
+      path.join(sr, 'hook-errors.log'),
+      `${new Date().toISOString()} auq-error-fallback-hook: ${msg}\n`,
+    );
+  } catch {
+    // last-resort swallow
+  }
+}
+
+function readStdin(): Promise<string> {
+  return new Promise((resolve) => {
+    let buf = '';
+    process.stdin.setEncoding('utf-8');
+    process.stdin.on('data', (chunk) => (buf += chunk));
+    process.stdin.on('end', () => resolve(buf));
+    process.stdin.on('error', () => resolve(buf));
+    setTimeout(() => resolve(buf), 2000);
+  });
+}
+
+/** No-op output — let the tool result stand untouched. */
+function defer(): void {
+  process.stdout.write(
+    JSON.stringify({ hookSpecificOutput: { hookEventName: 'PostToolUse' } }),
+  );
+  process.exit(0);
+}
+
+function inject(additionalContext: string): void {
+  process.stdout.write(
+    JSON.stringify({
+      hookSpecificOutput: { hookEventName: 'PostToolUse', additionalContext },
+    }),
+  );
+  process.exit(0);
+}
+
+/**
+ * Decide whether the tool_response is an ERROR / missing result rather than a
+ * real answer. Conservative: only flag clear failure shapes, so a successful
+ * AskUserQuestion (which carries the user's choice) is never misread as failure.
+ */
+export function isErrorResponse(response: unknown): boolean {
+  if (response === null || response === undefined) return true;
+  if (typeof response === 'string') {
+    const s = response.trim();
+    if (s === '') return true;
+    return /tool result missing|internal error|\bis_error\b/i.test(s);
+  }
+  if (typeof response === 'object') {
+    const rec = response as Record<string, unknown>;
+    if (rec.is_error === true || rec.isError === true || rec.error) return true;
+    // Some hosts wrap the payload as { content: "..." } or { content: [{text}] }.
+    const content = rec.content;
+    if (typeof content === 'string') return isErrorResponse(content);
+    if (Array.isArray(content)) {
+      const text = content
+        .map((c) => (typeof c === 'string' ? c : (c as Record<string, unknown>)?.text ?? ''))
+        .join(' ');
+      if (text.trim() === '') return false; // empty content array on success is ambiguous; don't trigger
+      return /tool result missing|internal error/i.test(text);
+    }
+  }
+  return false;
+}
+
+/** Resolve SESSION_KIND via the shared helper (same classification the preamble
+ *  echoes). Falls back to 'interactive' (degrade-safe) on any failure. */
+export function sessionKind(cwd?: string): 'spawned' | 'headless' | 'interactive' {
+  try {
+    const here = path.dirname(new URL(import.meta.url).pathname);
+    const bin = path.resolve(here, '..', '..', '..', 'bin', 'gstack-session-kind');
+    const res = spawnSync(bin, [], {
+      encoding: 'utf-8',
+      timeout: 3000,
+      cwd: cwd && fs.existsSync(cwd) ? cwd : undefined,
+    });
+    const out = (res.stdout || '').trim();
+    if (out === 'spawned' || out === 'headless' || out === 'interactive') return out;
+  } catch (e) {
+    logHookError(`sessionKind failed: ${(e as Error).message}`);
+  }
+  return 'interactive';
+}
+
+/** The directive injected per session kind. Exported for unit testing. */
+export function directiveFor(kind: 'spawned' | 'headless' | 'interactive'): string {
+  const lead =
+    'The AskUserQuestion call did not return a usable answer (error / missing result). ' +
+    'Per the AskUserQuestion failure-fallback rule: ';
+  switch (kind) {
+    case 'spawned':
+      return (
+        lead +
+        'SESSION_KIND=spawned — auto-choose the recommended option per the Spawned session block. ' +
+        'Do not emit prose, do not BLOCK.'
+      );
+    case 'headless':
+      return (
+        lead +
+        'SESSION_KIND=headless — report `BLOCKED — AskUserQuestion unavailable` and stop; no human can answer.'
+      );
+    case 'interactive':
+    default:
+      return (
+        lead +
+        'SESSION_KIND=interactive — render the decision as a PROSE message now: a clear ELI10 of the issue, ' +
+        'then a Recommendation line, then ONE paragraph per choice carrying its `(recommended)` marker, its ' +
+        '`Completeness: X/10`, and 2-4 sentences of reasoning. Tell the user to reply with a letter, then STOP. ' +
+        '(Retry the call once first only if no answer could have surfaced.)'
+      );
+  }
+}
+
+async function main(): Promise<void> {
+  const raw = await readStdin();
+  if (!raw.trim()) return defer();
+
+  let stdin: HookStdin;
+  try {
+    stdin = JSON.parse(raw);
+  } catch (e) {
+    logHookError(`stdin parse failed: ${(e as Error).message}`);
+    return defer();
+  }
+
+  const toolName = stdin.tool_name || '';
+  if (toolName !== 'AskUserQuestion' && !/^mcp__.+__AskUserQuestion$/.test(toolName)) {
+    return defer();
+  }
+
+  if (!isErrorResponse(stdin.tool_response)) return defer();
+
+  inject(directiveFor(sessionKind(stdin.cwd)));
+}
+
+// Only run the stdin→stdout pipeline when executed as a hook, not when imported
+// by the unit test (which exercises the exported pure functions).
+if (import.meta.main) {
+  main().catch((e) => {
+    logHookError(`main crash: ${(e as Error).message}`);
+    defer();
+  });
+}
diff --git a/setup b/setup
index 37991eda7..1fce574ce 100755
--- a/setup
+++ b/setup
@@ -1317,6 +1317,7 @@ fi
 # already registered under that tag, the install is a no-op (no prompt).
 PLAN_TUNE_LOG_HOOK="$SOURCE_GSTACK_DIR/hosts/claude/hooks/question-log-hook"
 PLAN_TUNE_PREF_HOOK="$SOURCE_GSTACK_DIR/hosts/claude/hooks/question-preference-hook"
+AUQ_ERROR_FALLBACK_HOOK="$SOURCE_GSTACK_DIR/hosts/claude/hooks/auq-error-fallback-hook"
 PLAN_TUNE_INSTALL_MARKER="$HOME/.gstack/.plan-tune-hooks-prompted"
 
 if [ "$NO_TEAM_MODE" -ne 1 ] \
@@ -1364,6 +1365,17 @@ if [ "$NO_TEAM_MODE" -ne 1 ] \
       --command "$PLAN_TUNE_PREF_HOOK" \
       --source plan-tune-cathedral \
       --timeout 5
+    # AUQ-failure prose-fallback reliability hook (OV3:B). Fires only when an
+    # AskUserQuestion call returns an error/missing result; inert on success and
+    # inert if the platform doesn't invoke PostToolUse on tool errors.
+    if [ -x "$AUQ_ERROR_FALLBACK_HOOK" ]; then
+      "$SETTINGS_HOOK" add-event \
+        --event PostToolUse \
+        --matcher '(AskUserQuestion|mcp__.*__AskUserQuestion)' \
+        --command "$AUQ_ERROR_FALLBACK_HOOK" \
+        --source plan-tune-cathedral \
+        --timeout 5
+    fi
   }
 
   if [ "$ALREADY_INSTALLED" -eq 1 ]; then
diff --git a/test/auq-error-fallback-hook.test.ts b/test/auq-error-fallback-hook.test.ts
new file mode 100644
index 000000000..48a996edb
--- /dev/null
+++ b/test/auq-error-fallback-hook.test.ts
@@ -0,0 +1,122 @@
+/**
+ * auq-error-fallback-hook — the OV3:B runtime reliability layer.
+ *
+ * Two layers of testing:
+ *  - PURE functions (isErrorResponse, directiveFor): deterministic, the core logic.
+ *  - INTEGRATION: spawn the hook as a PostToolUse process with synthetic stdin and
+ *    a controlled env, assert it injects the right directive on an error result and
+ *    stays inert on a real answer.
+ *
+ * NOTE: whether the Claude Code PLATFORM invokes PostToolUse on an MCP
+ * transport/missing-result error is unverified (could not force the Conductor
+ * bug in a harness — see docs/spikes/claude-code-hook-mutation.md). These tests
+ * pin the hook's BEHAVIOR given it is invoked; the platform trigger is the
+ * documented residual risk. The hook is inert if never invoked.
+ */
+import { describe, test, expect } from 'bun:test';
+import { spawnSync } from 'child_process';
+import * as path from 'path';
+import { isErrorResponse, directiveFor } from '../hosts/claude/hooks/auq-error-fallback-hook.ts';
+
+const HOOK = path.resolve(__dirname, '..', 'hosts', 'claude', 'hooks', 'auq-error-fallback-hook.ts');
+
+describe('isErrorResponse — only clear failures, never a real answer', () => {
+  test('null / undefined / empty string are failures', () => {
+    expect(isErrorResponse(null)).toBe(true);
+    expect(isErrorResponse(undefined)).toBe(true);
+    expect(isErrorResponse('')).toBe(true);
+    expect(isErrorResponse('   ')).toBe(true);
+  });
+
+  test('the Conductor missing-result string is a failure', () => {
+    expect(isErrorResponse('[Tool result missing due to internal error]')).toBe(true);
+  });
+
+  test('is_error / error object shapes are failures', () => {
+    expect(isErrorResponse({ is_error: true })).toBe(true);
+    expect(isErrorResponse({ isError: true })).toBe(true);
+    expect(isErrorResponse({ error: 'boom' })).toBe(true);
+    expect(isErrorResponse({ content: '...internal error...' })).toBe(true);
+  });
+
+  test('a real answer is NOT a failure (no false trigger)', () => {
+    expect(isErrorResponse({ answers: [{ option_label: 'A' }] })).toBe(false);
+    expect(isErrorResponse('A')).toBe(false);
+    // a choice that coincidentally contains the word "error" must not trip it
+    expect(isErrorResponse({ answers: [{ option_label: 'Fix the error' }] })).toBe(false);
+    expect(isErrorResponse('Investigate the login error')).toBe(false);
+  });
+});
+
+describe('directiveFor — per-session-kind instruction', () => {
+  test('interactive directive demands the prose triad', () => {
+    const d = directiveFor('interactive');
+    expect(d).toMatch(/ELI10/);
+    expect(d).toMatch(/Completeness: X\/10/);
+    expect(d).toMatch(/\(recommended\)/);
+    expect(d).toMatch(/reply with a letter/i);
+    expect(d).toMatch(/STOP/);
+  });
+
+  test('headless directive BLOCKs', () => {
+    expect(directiveFor('headless')).toMatch(/BLOCKED — AskUserQuestion unavailable/);
+  });
+
+  test('spawned directive auto-chooses', () => {
+    expect(directiveFor('spawned')).toMatch(/auto-choose/i);
+  });
+});
+
+/** Spawn the hook with synthetic stdin + controlled env; parse its JSON stdout. */
+function runHook(stdin: object, env: Record<string, string>): { additionalContext?: string } {
+  const res = spawnSync('bun', [HOOK], {
+    input: JSON.stringify(stdin),
+    encoding: 'utf-8',
+    env: { PATH: process.env.PATH ?? '/usr/bin:/bin', ...env },
+  });
+  const parsed = JSON.parse(res.stdout || '{}');
+  return parsed.hookSpecificOutput ?? {};
+}
+
+describe('hook integration — invoked as PostToolUse', () => {
+  test('error result + headless env → injects BLOCK directive', () => {
+    const out = runHook(
+      { tool_name: 'mcp__conductor__AskUserQuestion', tool_response: '[Tool result missing due to internal error]' },
+      { GSTACK_HEADLESS: '1' },
+    );
+    expect(out.additionalContext).toMatch(/BLOCKED — AskUserQuestion unavailable/);
+  });
+
+  test('error result + interactive env → injects prose-triad directive', () => {
+    const out = runHook(
+      { tool_name: 'AskUserQuestion', tool_response: null },
+      { CONDUCTOR_PORT: '55010' },
+    );
+    expect(out.additionalContext).toMatch(/render the decision as a PROSE message/i);
+    expect(out.additionalContext).toMatch(/Completeness: X\/10/);
+  });
+
+  test('error result + spawned env → injects auto-choose directive', () => {
+    const out = runHook(
+      { tool_name: 'AskUserQuestion', tool_response: { is_error: true } },
+      { OPENCLAW_SESSION: '1' },
+    );
+    expect(out.additionalContext).toMatch(/auto-choose/i);
+  });
+
+  test('SUCCESSFUL answer → no injection (inert on real answers)', () => {
+    const out = runHook(
+      { tool_name: 'AskUserQuestion', tool_response: { answers: [{ option_label: 'A' }] } },
+      { GSTACK_HEADLESS: '1' },
+    );
+    expect(out.additionalContext).toBeUndefined();
+  });
+
+  test('non-AUQ tool → defers (no injection)', () => {
+    const out = runHook(
+      { tool_name: 'Bash', tool_response: null },
+      { GSTACK_HEADLESS: '1' },
+    );
+    expect(out.additionalContext).toBeUndefined();
+  });
+});