[BUG] Edit tool fails with "File has been unexpectedly modified" error immediately after Read

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 Edit tool consistently fails with "File has been unexpectedly modified" error even when using it immediately after a Read tool call, with no actual file modifications occurring
between the Read and Edit operations.

Environment

• OS: Windows 11
• Claude Code Version: [latest as of 2025-11-20]
• Editor: VS Code
• File System: NTFS

What Should Happen?

### Expected Behavior

The Edit tool should successfully apply the edit
when:

1. The file content at edit time matches the

content that was read

1. Only the specified old_string is being

replaced with new_string

1. No other process has modified the actual file

content

The tool should use content-based verification
rather than timestamp-based verification. If the
file content matches what was read (or only has
the intended changes), the edit should proceed
regardless of mtime changes.

Error Messages/Logs

Steps to Reproduce

Reproduction Steps

1. Use Read tool to read a file (e.g., MaterialMixingPanel.cpp)
2. Immediately use Edit tool to modify the same file
3. Error occurs: "File has been unexpectedly modified"

This happens even when:

• No other tools are used between Read and Edit
• VS Code is not actively being used
• No other processes are accessing the file
• The file content hasn't actually changed

Expected Behavior

The Edit tool should successfully modify the file when the content matches what was read.

Actual Behavior

Edit tool fails with "File has been unexpectedly modified" error, preventing any edits.

Claude Model

Sonnet (default)

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.0.46

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_