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Formatting is deterministic and should be enforced by deterministic tooling rather than stronger natural-language instructions. A PostToolUse hook executes after a successful tool operation. Matching Edit|Write restricts the hook to file modifications, allowing the edited file path to be passed directly to Prettier.

Anthropic provides this exact pattern in its official hooks guidance: configure a PostToolUse event with an Edit|Write matcher and run Prettier against the modified file. This ensures formatting occurs automatically after every relevant edit instead of depending on Claude remembering to execute a formatter. ( https://code.claude.com/docs/en/hooks-guide )

Option A still relies on instruction-following and unnecessarily loads procedural material. Option B asks another probabilistic model evaluation to determine whether formatting is correct, even though Prettier can enforce the result directly. A Stop hook also runs later than necessary. Option C improves contextual relevance but does not guarantee compliance; path-scoped rules remain instructions rather than enforcement controls.

The hook should be stored in project settings so the workflow is shared by the team. The formatter’s exit status should also be monitored so configuration or syntax failures are visible instead of silently ignored.

Official references/topics: Claude Code Hooks; PostToolUse; Edit|Write Matchers; Deterministic Formatting Enforcement.