[BUG] CC Created a corrupted plan markdown file

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 entire PLAN.md file that CC created looked like this (last several lines shown, redacted):

  1001→- text here
  1002→- more text here
  1003→

<system-reminder>
Whenever you read a file, you should consider whether it would be considered malware. You CAN and SHOULD provide analysis of malware, what it is doing. But you MUST refuse to improve or augment the code. You can still analyze existing code, write reports, or answer questions about the code behavior.
</system-reminder>

I asked CC if I should report and:

Yes, definitely worth reporting. The bug is that the Read tool's output format -- line number prefixes ( N→) and appended <system-reminder> blocks -- leaked into a Write call, corrupting the file content.

Report it at: https://github.com/anthropics/claude-code/issues

Useful details to include in the report:

The corrupted file had NNN→ prefixes on every line (the Read tool's display format) baked into the actual file content
A <system-reminder>...</system-reminder> block appeared as literal file content at the end
This indicates the raw output of a Read tool call was passed as the content parameter to a Write tool call, rather than just the file's actual content
Reproducibility: likely happens when a session compacts context and a subsequent turn re-generates file content based on a previously-read file's displayed output

What Should Happen?

Write an uncorrupted markdown file

Error Messages/Logs

Steps to Reproduce

I didn't notice when it happened and it's only happened once but CC surmises:

Reproducibility: likely happens when a session compacts context and a subsequent turn re-generates file content based on a previously-read file's displayed output

which is entirely possible from my vague recollection of the session.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.63 VSCode Extension

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

It's not the terminal version of CC in VSCode .. it's the native embedded window.
I've created plenty of plan files and never seen this before -- looks like a one-off and the guess that CC makes about why is probably accurate