[BUG] Claude Code should not refuse to edit ".a" files

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?

Opus 4.8 on Claude Code 2.1.176 says that the Read (and Write and Edit) tools are refused by the harness on ".a" files, apparently on the assumption that they are Unix object-file archives (i.e., static libraries). But ".a" can be used for assembly source.

In general, Claude Code should not assume that a file is or is not plaintext based on its filename.

What Should Happen?

Claude Code should determine whether a file is plaintext based on its contents.

Error Messages/Logs

Steps to Reproduce

1. Let there be a text file with a ".a" file extension
2. Ask Claude Code to edit it

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.176

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_