Common input fields

Hook events receive these fields as JSON, in addition to event-specific fields documented in each hook event section. For command hooks, this JSON arrives via stdin. For HTTP hooks, it arrives as the POST request body.

Field	Description
session_id	Current session identifier
prompt_id	UUID identifying the user prompt currently being processed. Matches the prompt.id attribute on OpenTelemetry events, so you can correlate hook output with telemetry for a single prompt. Absent until the first user input. Requires Claude Code v2.1.196 or later
transcript_path	Path to conversation JSON. The transcript file is written asynchronously and may lag the in-memory conversation, so it may not yet include the current turn's most recent messages when a hook fires. Hooks that need the final assistant text of the current turn should use last_assistant_message on Stop and SubagentStop instead of reading the transcript
cwd	Current working directory when the hook is invoked
scratchpad_dir	Path to the session's scratchpad directory, where Claude keeps temporary working files. Absent when the session has no scratchpad or the temp directory is unavailable. Requires Claude Code v2.1.257 or later
permission_mode	Current permission mode: "default", "plan", "acceptEdits", "auto", "dontAsk", or "bypassPermissions". The mode labeled Manual arrives as "default", never as "manual", so scripts that match "default" keep working. Not all events receive this field. Check the JSON example in each hook event section
effort	Object with a level field holding the effort level in effect when the hook runs: "low", "medium", "high", "xhigh", or "max". If you set a level the active model doesn't support, level reports the level Claude Code ran instead; Adjust effort level says how it picks that level. Ultracode is not a distinct level and reports as "xhigh". The object matches the status line effort field. Present for events that fire within a tool-use context, such as PreToolUse, PostToolUse, Stop, and SubagentStop, when the current model supports the effort parameter. The level is also available to hook commands and the Bash tool as the $CLAUDE_EFFORT environment variable.
hook_event_name	Name of the event that fired

When running with --agent or inside a subagent, two additional fields are included:

Field	Description
agent_id	Unique identifier for the subagent. Present only when the hook fires inside a subagent call. Use this to distinguish subagent hook calls from main-thread calls.
agent_type	Agent name (for example, "Explore" or "security-reviewer"). Present when the session uses --agent or the hook fires inside a subagent. For subagents, the subagent's type takes precedence over the session's --agent value. See SubagentStart for the values custom and plugin subagents report and how to write a matcher against a plugin-scoped name.

Only SessionStart hooks can receive a model field, and Claude Code doesn't always include it. PreModelSwitch and PostModelSwitch hooks receive from_model and to_model instead, so use a PostModelSwitch hook to follow the model as it changes during a session.

There is no $CLAUDE_MODEL environment variable. The hook can read $ANTHROPIC_MODEL if you set it in your shell, but that value doesn't change when you switch models with /model during a session.

A hook process inherits the parent environment, apart from the OTEL_* exporter variables that Claude Code removes from every subprocess it spawns and, when CLAUDE_CODE_SUBPROCESS_ENV_SCRUB is set to 1, the variables it strips.

For example, a PreToolUse hook for a Bash command receives this on stdin:

{
  "session_id": "abc123",
  "prompt_id": "550e8400-e29b-41d4-a716-446655440000",
  "transcript_path": "/home/user/.claude/projects/.../transcript.jsonl",
  "cwd": "/home/user/my-project",
  "scratchpad_dir": "/tmp/claude-1000/-home-user-my-project/abc123/scratchpad",
  "permission_mode": "default",
  "hook_event_name": "PreToolUse",
  "tool_name": "Bash",
  "tool_input": {
    "command": "npm test",
    "description": "Run test suite",
    "timeout": 120000,
    "run_in_background": false
  },
  "tool_use_id": "toolu_01ABC123..."
}

The tool_name, tool_input, and tool_use_id fields are event-specific. Each hook event section documents the additional fields for that event.