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Spike: Claude Code hook mutation for plan-tune cathedral

Status: complete (2026-05-27) Surfaces: D10 (does PreToolUse allow mutating AUQ input?), D19/Codex (matcher must cover MCP variants) Downstream consumers: T3, T5, T6, T8

Question this spike answers

Can a PreToolUse hook on AskUserQuestion actually substitute the user's answer via updatedInput? If yes, what's the exact protocol?

Answer

Yes. updatedInput is the supported mechanism. Source: https://code.claude.com/docs/en/hooks (confirmed 2026-04 reference).

Hook stdin schema (PreToolUse + PostToolUse)

{
  "session_id": "abc123",
  "transcript_path": "/path/to/transcript.jsonl",
  "cwd": "/current/working/dir",
  "permission_mode": "default",
  "effort": { "level": "medium" },
  "hook_event_name": "PreToolUse",
  "tool_name": "AskUserQuestion",
  "tool_input": { /* tool-specific */ },
  "tool_use_id": "unique-id-12345"
}

Optional in subagent context: agent_id, agent_type.

PreToolUse hook stdout schema for `allow + updatedInput`

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "permissionDecisionReason": "auto-decided by plan-tune preference",
    "updatedInput": { /* shallow-merged into original tool_input */ },
    "additionalContext": "optional context for Claude"
  }
}

permissionDecision values:

"allow" — proceed, optionally with updatedInput
"deny" — block (feedback to Claude, NOT a synthetic answer per Codex correction in D-prefixed decisions)
"ask" — escalate to user
"defer" — let permission flow continue

updatedInput semantics: shallow merge of fields present in the returned object onto the original tool_input. Only valid with permissionDecision: "allow". This is what lets us substitute an auto-decided answer for never-ask preferences.

Matcher schema

The matcher field in ~/.claude/settings.json supports JS-regex syntax when it contains regex metacharacters. A matcher with only letters/ underscores is an exact match.

To cover both native + MCP AskUserQuestion:

"matcher": "(AskUserQuestion|mcp__.*__AskUserQuestion)"

Conductor disables native AskUserQuestion via --disallowedTools and routes through mcp__conductor__AskUserQuestion — the MCP suffix is required for our hook to fire there.

Multiple-hook concurrency caveat

All matching hooks run in parallel, and identical handlers are deduplicated automatically.

For our use case:

gstack registers exactly one PreToolUse hook and one PostToolUse hook on AUQ-shaped tool names.
If a user has THEIR own hook that also returns updatedInput on AskUserQuestion, the merge order is undefined.
Mitigation: document this constraint in bin/gstack-settings-hook install prompt. User can detect the conflict from the diff preview before accepting.

permissionDecision precedence (when multiple hooks decide): deny > ask > allow > defer — most restrictive wins.

Implementation hookSpecificOutput examples

Auto-decide (PreToolUse, never-ask preference + non-one-way):

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "permissionDecisionReason": "plan-tune: never-ask preference on ship-test-failure-triage",
    "updatedInput": {
      "questions": [{ /* same as input, but with auto-selected answer */ }]
    }
  }
}

Pass-through (no preference, or one-way safety override):

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "defer"
  }
}

PostToolUse capture (always):

{
  "hookSpecificOutput": {
    "hookEventName": "PostToolUse"
  }
}

(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

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "(AskUserQuestion|mcp__.*__AskUserQuestion)",
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/skills/gstack/hosts/claude/hooks/question-preference-hook",
            "timeout": 5
          }
        ]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "(AskUserQuestion|mcp__.*__AskUserQuestion)",
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/skills/gstack/hosts/claude/hooks/question-log-hook",
            "timeout": 5
          }
        ]
      }
    ]
  }
}

Hook commands take bun invocation under the hood; absolute paths (or $CLAUDE_PROJECT_DIR substitution) are required by Claude Code's hook runner. The hooks themselves are TypeScript files that the bash wrapper shells into bun.

Open questions deferred to implementation

Recommended-option parsing scope. D2 says parse (recommended) label first. The label is on the option's label field per AskUserQuestion Format. Implementation will need to walk tool_input. questions[*].options[*] looking for the label suffix. Worked examples: ship/SKILL.md.tmpl emits options like "A) Fix now" (recommended).
Auto-decided event tagging. When hook returns updatedInput, the PostToolUse hook will see the resolved input and log a normal event. Need an extra field on the PostToolUse payload (e.g., was_auto_decided: true) that the hook can set via session state tracking — write a marker file in ~/.gstack/sessions/<id>/.auto-decided-<tool_use_id> from PreToolUse, read it from PostToolUse, delete on read.
Timeout behavior. Default hook timeout is 60s but the docs are thin on what happens at timeout. Set explicit timeout: 5 so the user never waits >5s on a hook misfire. Falls back to pass-through.

References

https://code.claude.com/docs/en/hooks (canonical, latest as of 2026-04)
WebSearch results 2026-05-27
Existing bin/gstack-settings-hook (SessionStart-only impl, to be superseded by T3 schema-aware rewrite)

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