[BUG] Repeated edits to large HTML file silently break script execution

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?

Setup:

• File: ~25k-line single-file HTML (index.html, ~5MB)
• Working directory served via browser-sync proxy (port 3009)
• Multi-step session adding a new floating UI panel (~100 lines of JS) inside an IIFE

Pattern observed:
After several rounds of Edit tool calls inserting JS into the file, the page goes blank / scripts fail to execute, even though:

• git diff shows clean, well-formed diffs
• The inserted code looks syntactically valid in isolation
• Reverting via git checkout HEAD -- index.html immediately restores the working state

What I'd want investigated:

1. Are large Edit operations on multi-MB inline-script files reliably preserving file integrity? (Encoding, line endings, hidden whitespace?)
2. Could partial edits land mid-string when the old_string matches across a script boundary?
3. Is there a way for the agent to verify the script parses (via node --check or similar) after each edit on this kind of file?

Repro: Iteratively add ~100 lines of JS to a ~25k-line <script> block in a single HTML file via the Edit tool. Reload the served page after each edit;
observe whether scripts still execute.

What Should Happen?

Finisht the task and give me the changes requested as intended.

Error Messages/Logs

Steps to Reproduce

html css js runnign edits on the project and it breaks.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Claude Code 2.1.126.

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_