diff --git a/docs/spikes/claude-code-hook-mutation.md b/docs/spikes/claude-code-hook-mutation.md
index 9d91e16cd4..b41b7a6ca0 100644
--- a/docs/spikes/claude-code-hook-mutation.md
+++ b/docs/spikes/claude-code-hook-mutation.md
@@ -107,15 +107,26 @@ required for our hook to fire there.
 ```
 
 **Pass-through (no preference, or one-way safety override):**
+
+Emit **no** `permissionDecision` — absence is "no opinion". Either write nothing to
+stdout, or emit only `additionalContext`:
 ```json
 {
   "hookSpecificOutput": {
     "hookEventName": "PreToolUse",
-    "permissionDecision": "defer"
+    "additionalContext": "…optional…"
   }
 }
 ```
 
+> ⚠️ **Do not use `permissionDecision: "defer"` here.** (This spike originally
+> recommended it; that was wrong and shipped a real bug — see the AskUserQuestion
+> note in `~/.claude/CLAUDE.md`.) The documented set is `allow | deny | ask`.
+> `defer` is an undocumented print-mode-only value: interactive Claude Code logs
+> "defer is print-mode only" and ignores it, but non-interactive sessions (Cowork,
+> Agent SDK, `claude -p`) honor it and leave the tool_use **unresolved with no
+> result**, which the agent sees as `[Tool result missing due to internal error]`.
+
 **PostToolUse capture (always):**
 ```json
 {
diff --git a/hosts/claude/hooks/question-preference-hook.ts b/hosts/claude/hooks/question-preference-hook.ts
index 12cbd5ea28..78085e3f99 100644
--- a/hosts/claude/hooks/question-preference-hook.ts
+++ b/hosts/claude/hooks/question-preference-hook.ts
@@ -92,13 +92,29 @@ function readStdin(): Promise<string> {
   });
 }
 
+/**
+ * Pass the tool through untouched. Emits NO `permissionDecision` — absence is
+ * the correct "no opinion" signal, and the only one that is safe in both modes.
+ *
+ * Do NOT emit `permissionDecision: 'defer'` here. It is an undocumented
+ * print-mode-only value (the documented set is allow | deny | ask): interactive
+ * Claude Code logs "defer is print-mode only" and ignores it, but a
+ * non-interactive / print session — Cowork, the Agent SDK, `claude -p` — honors
+ * it and leaves the tool_use unresolved with no result, which surfaces to the
+ * agent as "[Tool result missing due to internal error]". That made
+ * AskUserQuestion permanently dead in Cowork while looking fine in the terminal.
+ */
 function defer(additionalContext?: string): void {
-  const out: Record<string, unknown> = {
-    hookEventName: 'PreToolUse',
-    permissionDecision: 'defer',
-  };
-  if (additionalContext) out.additionalContext = additionalContext;
-  process.stdout.write(JSON.stringify({ hookSpecificOutput: out }));
+  if (additionalContext) {
+    process.stdout.write(
+      JSON.stringify({
+        hookSpecificOutput: {
+          hookEventName: 'PreToolUse',
+          additionalContext,
+        },
+      }),
+    );
+  }
   process.exit(0);
 }
 
diff --git a/test/memory-cache-injection.test.ts b/test/memory-cache-injection.test.ts
index 3ab6a2144a..4f8a3b68a7 100644
--- a/test/memory-cache-injection.test.ts
+++ b/test/memory-cache-injection.test.ts
@@ -91,7 +91,7 @@ describe('memory injection', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
     expect(r.parsed?.hookSpecificOutput?.additionalContext).toContain('verbose explanations');
   });
 
@@ -115,7 +115,7 @@ describe('memory injection', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
     expect(r.parsed?.hookSpecificOutput?.additionalContext).toBeUndefined();
   });
 
@@ -219,7 +219,7 @@ describe('per-session memory cache', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
     expect(r.parsed?.hookSpecificOutput?.additionalContext).toBeUndefined();
   });
 });
diff --git a/test/question-preference-hook.test.ts b/test/question-preference-hook.test.ts
index 39de02f4e8..c104f44940 100644
--- a/test/question-preference-hook.test.ts
+++ b/test/question-preference-hook.test.ts
@@ -126,7 +126,7 @@ describe('defers (no enforcement)', () => {
       },
     });
     expect(r.status).toBe(0);
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 
   test('marker missing → defer (D18)', () => {
@@ -141,7 +141,7 @@ describe('defers (no enforcement)', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 
   test('always-ask preference → defer', () => {
@@ -156,7 +156,7 @@ describe('defers (no enforcement)', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 
   test('empty stdin → defer (crash safety)', () => {
@@ -168,13 +168,13 @@ describe('defers (no enforcement)', () => {
     const res = spawnSync(HOOK, [], { env, input: '', encoding: 'utf-8' });
     expect(res.status).toBe(0);
     const parsed = JSON.parse(res.stdout || '{}');
-    expect(parsed.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(parsed.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 
   test('non-AUQ tool_name → defer (defensive)', () => {
     writeProjectPref('test-q', 'never-ask');
     const r = runHook({ session_id: 's4', tool_name: 'Bash', tool_use_id: 'tu-4', tool_input: {} });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 });
 
@@ -219,7 +219,7 @@ describe('enforces never-ask preferences', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 
   test('ambiguous recommendation (two labels) → defer (D2 refuse-on-ambiguous)', () => {
@@ -237,7 +237,7 @@ describe('enforces never-ask preferences', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 
   test('no recommendation marker AND no prose match → defer', () => {
@@ -255,7 +255,7 @@ describe('enforces never-ask preferences', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 });
 
@@ -317,7 +317,7 @@ describe('precedence: project wins over global (D8)', () => {
         ],
       },
     });
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 });
 
@@ -443,7 +443,7 @@ describe('Conductor prose redirect', () => {
       undefined,
       CONDUCTOR,
     );
-    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(r.parsed?.hookSpecificOutput?.permissionDecision).toBeUndefined();
   });
 });
 
diff --git a/test/skill-e2e-plan-tune-cathedral.test.ts b/test/skill-e2e-plan-tune-cathedral.test.ts
index f9c006914e..e07eb70fa1 100644
--- a/test/skill-e2e-plan-tune-cathedral.test.ts
+++ b/test/skill-e2e-plan-tune-cathedral.test.ts
@@ -296,7 +296,7 @@ describeIfSelected('PlanTune cathedral E2E: annotation', ['plan-tune-annotation'
     });
     expect(res.status).toBe(0);
     const parsed = JSON.parse(res.stdout || '{}');
-    expect(parsed.hookSpecificOutput?.permissionDecision).toBe('defer');
+    expect(parsed.hookSpecificOutput?.permissionDecision).toBeUndefined();
     expect(parsed.hookSpecificOutput?.additionalContext).toContain('verbose explanations');
   });
 });