Claude Code's Auto Mode routes edits through sed, not Edit

A Claude Code user posted the Auto Mode system reminder verbatim. It tells the model to read files with cat, head and sed -n, search with grep and find, and edit with sed and heredocs. The built-in Read, Edit and Write tools are the fallback, used only when Bash can't do the job.

Anyone who built automation on the file tools pays first. PostToolUse hooks keyed on Edit or Write stop firing, since the write now happens through Bash. Permission rules that deny writes to specific paths get routed around too, because the tool policy targets a file-tool call that never happens.

An Anthropic employee replied with the reasoning, speaking personally rather than officially. The Edit tool makes the model emit the full original block plus the full replacement, then fails and retries when the match isn't unique. Shell commands cost fewer tokens and show up far more often in training data. Optimizing for tokens per successful edit, shell wins. None of this is in a release note.

A heredoc rewrite of a file produces a different diff shape than a targeted block replacement, which shows up as noisier pull requests. It's the same failure as a skill file the model reads and ignores, seen from a different angle. What you configured and what the agent does have diverged, and no release note explains why.

What to change

The fix showing up in a top r/ClaudeAI thread is to move deterministic rules out of CLAUDE.md and into hooks. Key those hooks on Bash calls, Edit and Write alike. A hook that only watches the file tools misses every edit Auto Mode routes through the shell. The top comment on that thread carries 146 upvotes. It argues linters, CI and static analysis are the undervalued layer, since they check the output regardless of which tool wrote it. One commenter shared a hook that blocks the model from using sed outright.