Always Read a file before Edit

rule Sat Jun 20 2026 00:00:00 GMT+0000 (Coordinated Universal Time) rulesagent-harnesseditclaude-codesafety

Always Read a file before Edit

Always call Read on a file in the current session before calling Edit (or Write to overwrite it). The agent harness already enforces this — Edit fails on a file the session hasn't Read — but agents still occasionally try to bypass by guessing context. Don't.

Why

The harness rule exists for two reasons; the rule file makes them explicit so agents stop trying to outsmart it.

1. Stale-match failures. Edit requires old_string to match exactly, character-for-character. A file the agent thinks it knows from training data, an earlier session, or a different branch is almost always slightly different — different whitespace, different import order, a recent commit. Reading first guarantees the old_string is taken from the file's current state.

2. Accidental clobbering. Write overwrites the entire file. If the agent hasn't Read it, there's no way to know whether the overwrite drops a function someone added since the agent's last look. Reading first surfaces those changes before they're lost.

A third reason, equally important in this codebase: many edits should match the surrounding style (match-surrounding-style.md). You can't match what you haven't seen.

What this means concretely

• Every Edit call must be preceded by a Read of the same file_path in the current session. Reads from a different session don't count — file contents change.
• For a multi-file edit, batch the Read calls in a single assistant turn (parallel function calls), then batch the Edit calls — don't serialise.
• For surgical edits, prefer Edit over Write. Only Write when creating a net-new file or fully replacing one already Read in this session.
• Don't Read the same file again just to "verify" the edit — the harness tracks file state and Edit would have errored if the match failed.

Exceptions

None. The harness will block you anyway.

See also

• match-surrounding-style.md
• agent edit-mode preferences in ~/.claude/CLAUDE.md