[BUG] Read tool returns "Unchanged since last read" after git checkout changes file content

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The Read tool's cache does not track file changes caused by git operations (git checkout, git reset --hard). After editing a file and switching branches with git checkout, if the file content happens to match what was previously cached (e.g., switching away and back to the same branch), Read returns "Unchanged since last read" instead of showing the file content.

This also causes the related Edit error "File has been modified since read" when the file content differs from the cache after a branch switch.

What Should Happen?

Read should always return the current file content. Git operations that modify files on disk should invalidate the Read cache.

Error Messages/Logs

● Read(/path/to/file.yml)
  ⎿  Unchanged since last read

Steps to Reproduce

1. On branch main, Read a file (e.g., a workflow YAML)
2. Create branch feat/x, checkout to it
3. Edit the file (e.g., add a line) — Edit succeeds, Read cache is updated
4. Run git checkout main && git reset --hard origin/main — file reverts to original content on disk
5. Run git checkout feat/x — file is back to the edited version (same content as step 3)
6. Try to Read the file → "Unchanged since last read" — file content is not shown

The cache compares step 3 (post-Edit) to step 5 (post-checkout) and sees them as identical, not realizing the file changed in between (step 4). The user never sees the file content and cannot proceed.

Claude Model

Opus

Is this a regression?

I don't know

Claude Code Version

2.1.96

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

iTerm2

Additional Information

Related: #42264 reports the same "Unchanged since last read" symptom triggered by conversation revert (/rewind). This issue documents a separate trigger path: git branch operations modifying files on disk without going through the Edit tool.

The root cause appears to be the same: the Read cache tracks content hashes but does not monitor filesystem changes from external processes (git, build tools, etc.).

✍️ Author: Claude Code with @carrotRakko (AI-written, human-approved)