Claude Code Hooks

Reference for creating and configuring Claude Code hooks. When uncertain about syntax or features, use the Agent tool with subagent_type='claude-code-guide' to consult official docs.

Hook Types

Type	Trigger	Use Cases
PreToolUse	Before tool execution	Validate inputs, block operations, modify parameters
PostToolUse	After tool completes	Check results, run linters, provide feedback
UserPromptSubmit	When user sends message	Pre-process input, add context
PermissionRequest	Permission dialog appears	Return a decision, notify a remote approver
Stop	Agent finishes a turn	Cleanup, save state
StopFailure	Agent stops on failure	Report the failure
SubagentStart / SubagentStop	Subagent spawns / completes	Process results
SessionStart / SessionEnd	Session begins / ends	Inject context, export state
PreCompact / PostCompact	Around context compaction	Save important state
Notification	System notification	Log events
TeammateIdle	Teammate about to go idle	Hand off work

Configuration Files

• ~/.claude/settings.json - User-level (global)
• .claude/settings.json - Project-level
• .claude/settings.local.json - Local (not committed)
• Plugin hooks: plugins/<name>/hooks/hooks.json

Hook Structure

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "bun ./hooks/ox"
          }
        ]
      }
    ]
  }
}

Matcher Patterns

• Simple: "Write", "Edit"
• Multiple: "Edit|Write|MultiEdit"
• With args: "Bash(npm:*)", "Bash(osascript:*)|Bash(open:*)"
• MCP tools: "mcp__linear__create_issue"
• Plugin MCP tools: "mcp__plugin_<plugin>_<namespace>__<tool>"
• Claude AI MCP tools: "mcp__claude_ai_<DisplayName>__<tool>"
• All three patterns: "mcp__linear__create_issue|mcp__plugin_linear_linear__create_issue|mcp__claude_ai_Linear__save_issue"

Hook Input

Commands receive JSON on stdin:

{
  "tool_name": "Write",
  "tool_input": {
    "file_path": "/path/to/file.ts",
    "content": "..."
  },
  "cwd": "/project/root",
  "session_id": "...",
  "transcript_path": "..."
}

Stdin is external data, so decode it with a zod schema covering the fields the hook reads. A plugin hook keeps that schema local, since it cannot import a workspace package:

import { z } from "zod";

const HookInput = z.looseObject({
  cwd: z.string().catch(""),
  tool_input: z.looseObject({ file_path: z.string().optional().catch(undefined) }).catch({}),
});

let input: z.infer<typeof HookInput>;
try {
  input = HookInput.parse(JSON.parse(await Bun.stdin.text()));
} catch (error) {
  console.error(`[my-hook] undecodable stdin: ${error}`);
  process.exit(0);
}

looseObject keeps fields the harness adds later. A per-field .catch means one unexpected value costs that field rather than the whole payload.

On an undecodable payload, log the failure to stderr and exit 0 so the tool call proceeds. A gate can exit 1 instead. The call still goes through, and the harness surfaces the stderr line, so a gate that has stopped deciding does not read as a call that passed every rule (plugins/plan/hooks/gate.ts).

Hook Output

PreToolUse - Control execution:

{"hookSpecificOutput": {"hookEventName": "PreToolUse", "permissionDecision": "deny", "permissionDecisionReason": "Use gh cli instead"}}

{"hookSpecificOutput": {"hookEventName": "PreToolUse", "updatedInput": {"state": "Todo"}}}

On ExitPlanMode, ask is inert: the tool runs its own plan-approval prompt, and the harness drops both permissionDecisionReason and systemMessage. Use deny there to carry a reason back.

PostToolUse - Provide feedback:

{"hookSpecificOutput": {"hookEventName": "PostToolUse", "additionalContext": "Lint errors found..."}}

Exit with no output to allow without modification.

Async

"async": true on a command hook backgrounds it so the turn does not wait. Nothing above reaches the model: no permissionDecision, no updatedInput, no additionalContext, and exit code 2 does not block. Verified on 2.1.220 that stderr and a nonzero exit are both dropped. "asyncRewake": true backgrounds the hook but wakes the model on exit code 2, delivering stderr (or stdout when stderr is empty) as a system reminder. Neither field applies to prompt or agent hooks.

Use async only for a hook that is a pure side effect: a notifier, a status bridge, a terminal bell, a state export. Keep it off when the hook returns any of the output above, mutates state a later step reads, or gates a tool call.

Two events behave differently from the rest, measured on 2.1.220:

• Stop kills the backgrounded process almost immediately. Anything past a few milliseconds never finishes. A Stop notifier must stay synchronous.
• SessionEnd outlives the CLI. A backgrounded hook there runs to completion after the process exits, making it safe to background.

SubagentStop, SessionStart, UserPromptSubmit, and PostToolUse all run to completion when backgrounded.

Hooks on the same event already run concurrently with each other, so async only shortens the turn when the side-effect hook is the slowest one on its event. It is still worth setting on a hook that qualifies, because the payoff moves as sibling hooks change.

Script Storage

Store complex hooks in .claude/hooks/ or a project hooks/ directory, referenced with:

"command": "bun $CLAUDE_PROJECT_DIR/.claude/hooks/my-hook.ts"

Examples

See these repositories for hook implementations:

• Input modification: plugins/linear/hooks/
• Permission decisions: plugins/github/scripts/
• PostToolUse feedback: .claude/hooks/ox/

Debugging

For troubleshooting hook failures, see debugging.