The Tool Desk
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Claude Code sends command hooks a JSON object on standard input; HTTP hooks receive the same JSON as the POST request body. The payload combines context fields with fields for the event that fired, so branch on hook_event_name and check optional fields rather than treating every hook as one fixed schema. Anthropic’s Hooks reference is the authority for the exact, version-sensitive event inputs.
How Claude Code delivers hook input
The transport depends on the handler: command hooks read JSON from stdin, while HTTP hooks receive JSON in the request body. The event name, hook_event_name, tells your handler which event-specific fields to expect. Not every event contains every common field, and newer fields can require a minimum Claude Code version.
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Model the input as a shared context layer plus an event-specific layer, not as one universal payload. Your parser should first identify the event, then check for each field it needs.
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Common fields and their caveats
Anthropic documents these fields as common inputs, while noting that particular events may omit some of them:
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| Field | What it contains | Important caveat |
|---|---|---|
session_id |
The current session identifier. | Check for presence rather than assuming it is included in every event. |
prompt_id |
The UUID for the user prompt being processed; useful for correlating hook output with OpenTelemetry prompt events. | Absent until the first user input. |
transcript_path |
Path to the conversation JSON file. | The file is written asynchronously and may not yet contain the latest messages when the hook fires. |
cwd |
The working directory when the hook runs. | It describes the invocation context; do not assume it is a permanent project path. |
scratchpad_dir |
The session scratchpad directory, when available. | May be absent if there is no scratchpad or the temporary directory is unavailable; requires Claude Code v2.1.257 or later. |
permission_mode |
The current mode: default, plan, acceptEdits, auto, dontAsk, or bypassPermissions. |
Not present on every event. Manual mode is represented as default, not manual. |
effort |
An object with a level, such as low, medium, high, xhigh, or max. |
Appears in relevant tool-use contexts when the active model supports the effort parameter. |
hook_event_name |
The name of the event that fired. | Use it to select event-specific handling. |
agent_id |
Identifies the subagent when a hook runs inside a subagent call. | Distinguishes subagent hook calls from main-thread calls. |
agent_type |
The agent name when running with --agent or inside a subagent. |
For a subagent, its type takes precedence over the session’s --agent value. |
model is a special case: only SessionStart hooks can receive it, and it is not guaranteed to be present. PreModelSwitch and PostModelSwitch instead receive from_model and to_model.
What changes by event
The event catalog covers session setup, prompts, tool calls and permissions, subagents and tasks, stopping, workspace and configuration changes, compaction, model switches, MCP elicitation, and session termination. The following examples show why handling by event matters; they are not a complete substitute for the live schema.
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| Event or group | Documented event-specific fields | How to use them |
|---|---|---|
SessionStart |
source (for example startup, resume, clear, compact, or fork); may also include model, agent_type, and session_title. Qualifying resumed or forked sessions in newer versions can include elapsed-time, context-token, and prompt-cache estimates. |
Use source to distinguish how the session began. Check optional fields before reading them. |
Setup and InstructionsLoaded |
Setup includes trigger (init or maintenance). InstructionsLoaded includes instruction-file details such as file_path, memory_type, and load_reason; optional fields can describe path globs or the file that triggered a lazy load. |
Expect different structures even among events associated with session initialization. |
UserPromptSubmit and UserPromptExpansion |
UserPromptSubmit includes prompt and may include a custom session_title. UserPromptExpansion includes expansion_type, command_name, command_args, command_source, and the original prompt. |
Pasted content can arrive expanded in the submitted prompt. |
MessageDisplay |
turn_id, message_id, batch index, final, and new text in delta. |
Interactive sessions can invoke it for successive message batches; non-interactive runs invoke it once per assistant message. |
PreToolUse |
tool_name, tool_input, and tool_use_id; MCP calls can also include mcp_server. |
tool_input depends on the tool. The mcp_server field requires Claude Code v2.1.274 or later. |
PostToolUse |
Tool input and result. Some Bash executions can include tool_response.bashEditDiff. |
The Bash edit diff is a best-effort public beta feature requiring v2.1.269 or later; it may be incomplete and is for review, not policy enforcement. |
PostToolUseFailure and PostToolBatch |
PostToolUseFailure includes tool identity and input, top-level error, and optional is_interrupt and duration_ms. PostToolBatch includes a tool_calls array with resolved calls, including tool name, input, use ID, and response. |
Error string formats vary by tool. Parse batch entries as individual tool-call records. |
PermissionRequest and PermissionDenied |
PermissionDenied includes tool details and a reason; output can indicate whether retry is allowed in applicable cases. |
Use the exact event’s schema when handling permission decisions; do not infer it from tool-use input alone. |
Notification |
message, optional title, and notification_type. |
Check whether title is present. |
SubagentStart and SubagentStop |
SubagentStart includes agent_id and agent_type. SubagentStop includes stop_hook_active, agent identifiers/type, agent_transcript_path, and last_assistant_message. |
The ordinary transcript_path still points to the main session transcript. |
TaskCreated and TaskCompleted |
task_id, task_subject, and optional task description and team/teammate names. |
Do not assume optional descriptions or names are populated. |
Stop and StopFailure |
Stop includes stop_hook_active, last_assistant_message, background-task information, and session cron information. StopFailure includes an error type, optional error details, and optional last assistant message. |
Check the current event page for exact shapes. |
TeammateIdle |
teammate_name and team_name. |
These identify the idle teammate and team. |
ConfigChange, CwdChanged, DirectoryAdded, and FileChanged |
ConfigChange: configuration source and optional file_path. CwdChanged: old_cwd and new_cwd. DirectoryAdded: added directory and how it was added. FileChanged: file_path and change event. |
These describe different workspace or configuration changes; dispatch separately if the reaction differs. |
WorktreeCreate and WorktreeRemove |
WorktreeCreate includes name; WorktreeRemove includes worktree_path. |
Use the field appropriate to the lifecycle event. |
PreCompact and PostCompact |
Both include the compaction trigger; PreCompact can include custom instructions, while PostCompact includes the compacted summary. |
Before and after compaction have distinct data and timing. |
PreModelSwitch and PostModelSwitch |
Include the models involved; current versions can add context and cache estimates for pre-switch cost reporting. | Use from_model and to_model, not the SessionStart model field. |
Elicitation and ElicitationResult |
MCP server and request/response details such as message, action, and optional form content. | Check whether optional form content is supplied. |
SessionEnd |
A reason explaining why the session ended. |
Handle termination based on the reported reason. |
Parse defensively and choose the right source of truth
- Read and decode the JSON, then branch on
hook_event_name. - Check for a field before using it. Optional fields, version-gated fields, and fields omitted on particular events are normal.
- Do not assume
tool_inputhas the same shape for all tools. For example, a Bash input includes a command, while a Write input includesfile_pathandcontent. - For Windows path matching, normalize backslashes before applying path checks.
- Do not use the transcript file as a guaranteed up-to-the-moment record. Anthropic says it is written asynchronously and can lag behind the in-memory conversation. For final response text, use
last_assistant_messageonStoporSubagentStopwhen that is the data you need. - Confirm minimum Claude Code versions before depending on recently added fields. The live reference is version-sensitive.
Example: branch on the event name
This illustrative shell snippet reads the command-hook payload once and extracts fields only for the events it handles. It is not a tested script:
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#!/usr/bin/env bash
payload=$(cat)
event=$(jq -r '.hook_event_name // empty' <<<"$payload")
case "$event" in
PreToolUse)
tool=$(jq -r '.tool_name // empty' <<<"$payload")
;;
UserPromptSubmit)
prompt=$(jq -r '.prompt // empty' <<<"$payload")
;;
esacAnthropic’s official example reads tool_input.command from stdin for a PreToolUse Bash hook. That path is tool-specific: do not apply it to a different tool without checking that tool’s input structure.
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