$CLAUDE_TOOL_INPUT_FILE_PATH is not a real variable

You wired up a PostToolUse hook so Prettier runs on every file Claude writes. /hooks lists it. No error ever appears in the transcript. And the repo keeps drifting out of format, because the hook has been running successfully and doing nothing since the day you pasted it.

Check the hook's own text for this:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "if [[ \"$CLAUDE_TOOL_INPUT_FILE_PATH\" =~ \\.(ts|tsx)$ ]]; then npx prettier --write \"$CLAUDE_TOOL_INPUT_FILE_PATH\"; fi"
          }
        ]
      }
    ]
  }
}

$CLAUDE_TOOL_INPUT_FILE_PATH appears nowhere in the Claude Code documentation. Grep the whole docs corpus for CLAUDE_TOOL_INPUT and you get zero hits. In shell form the hook command is handed to sh -c, an unset variable expands to the empty string, the regex test against "" is simply false, the if body never runs, and the hook exits 0. Every layer reports health.

What a hook actually gets

The path does not arrive in the environment. It arrives on stdin, as one JSON object.

The docs' own formatting example reads the path the way every working hook does:

{
  "type": "command",
  "command": "jq -r '.tool_input.file_path' | xargs npx prettier --write"
}

And for anything with a guard, take the path out of stdin first:

#!/usr/bin/env bash
INPUT=$(cat)
FILE_PATH=$(echo "$INPUT" | jq -r '.tool_input.file_path // empty')

case "$FILE_PATH" in
  *.ts|*.tsx) npx prettier --write "$FILE_PATH" ;;
  *) exit 0 ;;
esac

Note the // empty — with it, a missing field gives you an empty string rather than the literal text null, which is its own quiet way to format nothing.

Why nothing ever told you

Three independent facts have to line up for a broken hook to look healthy, and here they all do.

• An unset variable in shell form is an empty string, not an error. Bash's [[ ... =~ ... ]] against an empty string is false, and a false if is not a failure.
• The hook exits 0, so there is nothing for Claude Code to report.
• Stderr from a hook that exits 0 goes to the debug log only. It never reaches the transcript and Claude never sees it.

So the hook that does nothing and the hook that works produce the same visible output: none. This is why the symptom reaches people as "hooks don't work" rather than "my hook has a typo". The hook worked perfectly.

The auto-test variant of the same snippet is worse than the formatting one. A formatter that silently skips leaves visible mess in the diff. A test runner that silently skips leaves you believing every edit was covered.

What is unverified here

Nobody at CCLearn has run this. What has been checked is the documentation, and what the documentation supports is narrower than the claim:

• Established: the documented way to get the edited file's path is .tool_input.file_path from stdin JSON, shown in multiple hook examples.
• Established: the environment variables the docs say Claude Code exports onto a spawned hook process are CLAUDE_PROJECT_DIR, CLAUDE_PLUGIN_ROOT and CLAUDE_PLUGIN_DATA.
• Not established: that no undocumented CLAUDE_TOOL_INPUT_FILE_PATH is set anyway. The docs are silent on the name, and silence is weaker evidence than a denial. It is worth knowing the docs do sometimes deny a variable outright — they say plainly that there is no $CLAUDE_MODEL environment variable — so omission is not their only way of saying no.

What would settle it is one probe. Point a PostToolUse hook at a script that dumps its own environment, edit a file, and read what came out:

printf '#!/usr/bin/env bash\nenv | grep "^CLAUDE_" > /tmp/hook-env.txt\n' > /tmp/dump-env.sh
chmod +x /tmp/dump-env.sh
# wire /tmp/dump-env.sh as a PostToolUse hook with matcher "Write|Edit",
# have Claude edit any file, then:
cat /tmp/hook-env.txt

If CLAUDE_TOOL_INPUT_FILE_PATH is absent from that list, the snippet is dead and this tip is settled. Run it against a named Claude Code version and it moves from a documentation argument to a reproduced fact.

Meanwhile, the cheap move is to audit what you already have:

grep -rn 'CLAUDE_TOOL_INPUT' ~/.claude/settings.json .claude/settings*.json 2>/dev/null