If a Claude Code hook exits with status 1, that alone usually does not stop the action. To block a tool call with a command-based PreToolUse hook, exit with 2 or return valid, event-specific JSON containing a denial decision. First confirm the hook runs before the action and matches the tool you intend to block.
Why exit code 1 does not block the action
For most hook events, a nonzero exit code is not automatically a denial. When a hook produces no valid decision JSON, exit code 1 is treated as a non-blocking error and the action generally continues. Anthropic’s Claude Code hooks reference explains that, for most events, exit code 2 is the only exit code that blocks through the code alone.
That distinction matters: an error from your script can show that it ran, but it does not necessarily tell Claude Code to stop. Blocking behavior also depends on the event and, for structured responses, whether the output uses a decision field that event supports.
Block a tool call with PreToolUse
Use PreToolUse when the goal is to prevent a tool call before it executes. A command hook can block by exiting with code 2; write a useful explanation to stderr so the user can see why the call was stopped.
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#!/bin/sh
printf '%sn' 'Blocked: this tool call is not allowed.' >&2
exit 2For more control, return valid JSON on stdout using the denial decision supported by PreToolUse. Keep stdout limited to that JSON object: banners, debug messages, or other text can make the response unparsable. Send diagnostics to stderr or a log file instead. Check the current hooks reference for the required fields and accepted values.
Check that the hook is in the right place and matches
- Verify the event. The hook must be configured for
PreToolUseto stop a tool before it runs.PostToolUseruns after the tool has succeeded, so it cannot prevent or undo that call. - Verify the settings and matcher. Check that the hook is in the settings scope you intend, that its matcher targets the actual tool name, and that capitalization matches. The official hooks guide describes configuring events, matchers, and commands; the reference documents matcher behavior and hook input.
- Confirm what the hook receives. Tool-related hook events receive JSON input on stdin, including tool information. Inspect that input to ensure your condition is checking the tool or action that is actually being called.
- Choose one documented denial method. For a simple command hook, use exit code
2and an explanatory stderr message. For event-specific control, print valid JSON with the supported denial field. Exit code1alone is not a substitute for either.
Use the exit-code behavior that fits the event
| Hook response | General effect | What it means |
|---|---|---|
0, no decision JSON |
Normal flow continues | Success without a decision is not a denial. |
1, no valid decision JSON |
Non-blocking error for most events | The action generally proceeds. |
2 on a blockable event |
Blocking error | For PreToolUse, the tool call is blocked. |
| Valid event-specific JSON | The supported decision is applied | Use the event’s documented schema and keep stdout clean. |
This is a general guide, not a rule that overrides each event’s contract. For example, WorktreeCreate treats any nonzero command exit as failure, while PermissionRequest does not use exit code 2 as a denial and instead requires its decision object. Some events cannot block because the action has already happened or the event does not support blocking. Check the relevant event entry in the reference before relying on an exit code.
Diagnose why a specific hook is not blocking
Changing 1 to 2 only fixes the problem if the hook is running on a blockable event and its response is interpreted as intended. Use Claude Code’s hook or debug output and temporary script logging to capture the event, matched tool, exit status, stdout, and stderr. Also confirm that the configured command path exists and is executable.
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- The hook never matches: check the event, settings scope, tool name, and matcher capitalization.
- The hook runs but the action continues: check whether it exits
1without a supported JSON denial, or whether the event permits blocking. - Structured output is ignored: validate the event-specific JSON fields and remove all non-JSON text from stdout.
- The hook appears to do nothing: check command startup, path and executable permissions, plus hook/debug logs.
- A timeout occurs: a timed-out command hook generally does not block a
PreToolUsecall; the call proceeds through the normal permission flow. - You rely on a recently introduced field or behavior: verify the installed Claude Code version supports it.
Without the event name, relevant settings entry and matcher, script, installed version, and a captured debug log, the exact cause cannot be identified. A non-blocking error does not prove that the hook is absent: it may have run and returned an error without a blocking decision.
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