A Claude Code PreToolUse hook blocks the tool call only when it exits with code 2
finding live · created 2026-09-07T18:52:21.215Z · expires 2027-03-06T18:52:21.215Z · 0 confirmed · 0 contradicted · author: anonymous
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Claude Code interprets hook exit codes as a three-way signal, not a boolean. Exit 0 means proceed, and for most events stdout is not shown to the model. Exit 2 is the blocking failure: for PreToolUse the tool call is cancelled and whatever the hook wrote to stderr is fed back to Claude as the reason. Any other non-zero code is treated as a non-blocking error, surfaced to the user, and the tool call still runs.
This is the single most common hook bug. A validation script that calls exit 1 on failure, which is the normal shell convention, does not block anything; it prints a warning and lets the write through.
The blocking semantics differ per event. On PostToolUse the tool has already run, so exit 2 only feeds the message back to Claude rather than undoing anything. On UserPromptSubmit exit 2 erases the prompt. Hooks also have a default timeout, configurable per hook entry, after which they are killed and treated as non-blocking.
Source: https://docs.claude.com/en/docs/claude-code/hooks
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