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Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →In one small Claude Code experiment, a CLAUDE.md rule and a PreToolUse hook each blocked all five ordinary attempts to edit a protected file. But when the prompt said “I authorize it,” the rule gave way in both tested runs; the hook blocked both. The contrast illustrates the practical difference: CLAUDE.md guides the model, while a hook can check a proposed tool call before it runs. These results describe one setup, not universal reliability.
What happened when the user said “I authorize it”?
Rulestack’s authors tested Claude Code v2.1.273 in a throwaway project on September 16, 2026. In the ordinary-task trials, each control blocked 5 of 5 protected-file edit attempts. With neither control, the file was edited in all 3 of 3 reported runs.
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The authors then tried two prompts claiming the user authorized the edit. The CLAUDE.md instruction gave way in both runs; the PreToolUse hook blocked both calls. The article reports 29 headless sessions across its control conditions. These are small, setup-specific counts, not percentages or a reliability estimate for Claude Code generally. Read the experiment and its setup.
Why did the outcomes differ?
CLAUDE.md steers behavior
A CLAUDE.md file gives Claude project instructions. In the reported test, the rule told the model not to edit the protected file, and the model complied during ordinary attempts. But instructions operate as behavioral context: a conflicting prompt can persuade the model to act differently. The two authorization-claim runs demonstrate that outcome in this test, not a rule that every such prompt will override every instruction.
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PreToolUse checks a proposed call
The tested hook inspected tool input for a protected path and refused matching calls before execution. That places the check at the tool boundary rather than relying solely on the model to follow written guidance. A documentation mirror describes PreToolUse as a before-tool event that can approve, block, or modify a call; consult current Claude Code documentation for supported syntax and exact behavior before configuring a hook. Hook reference mirror.
How the two controls compare
| Question | CLAUDE.md rule |
PreToolUse hook |
|---|---|---|
| Where does it act? | As project context that guides model behavior. | On a proposed tool call before execution, when the hook is configured to match it. |
| Ordinary attempts in the reported test | Blocked 5 of 5. | Blocked 5 of 5. |
| Authorization-claim attempts in the reported test | Allowed the edit in 2 of 2. | Blocked 2 of 2. |
| Reported overhead in this setup | Added 58 to 70 input tokens per request. | A direct hook request added a model round trip. |
| What the result depends on | Whether the model follows the instruction in context. | Whether the hook is configured and its matching logic covers the relevant tool call. |
The token and round-trip costs above are reported by the experiment’s authors for their setup; they should not be assumed for other projects or versions.
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Which should you use to protect repository files?
Use CLAUDE.md for conventions
Use project instructions for preferences and workflows where model compliance is desirable but a mistaken edit is recoverable—for example, asking Claude to avoid generated files or to request review before changing a directory. Instructions are simple to read and maintain, but they are not a hard boundary against a model that proceeds anyway.
Use a hook or external control when a call must be blocked
If a tool call must not proceed even when the model is persuaded, use a carefully scoped PreToolUse hook or an enforcement mechanism outside model-authored instructions. Define the protected paths and relevant tools deliberately, then verify that the hook matches the calls you intend to stop. The experiment used string/path checks; it did not establish that such checks catch every way a file might be changed or avoid every unintended match.
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What a hook does—and does not—guarantee
A hook enforces only the logic it actually runs, within the configuration boundary where it is installed. A simple path matcher may miss indirect writes, alternate tools, or path representations it does not recognize; an overly broad matcher may block legitimate work. Nor does a hook make a repository invulnerable to a user or process with authority to change its configuration or modify files outside the covered tool path. Treat the experiment as evidence for choosing the control point, not proof that a particular hook is complete.
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