codex exec background task rewrote an existing pre-commit hook shebang while asked to "preserve" it

Summary

Asked a Codex background task (codex exec --json, invoked via the Claude Code codex-openai-codex plugin) to add a small delegated call to an existing git pre-commit hook, explicitly instructing it to insert the new block right after the shebang line and leave everything else untouched. In 2 of 7 repositories, the existing hook was a pre-commit (https://pre-commit.com) generated script starting with #!/usr/bin/env bash. After Codex's edit, the file's first line had become #!/bin/sh instead of the original #!/usr/bin/env bash — the shebang itself was altered even though the instruction was to insert after it and preserve the rest.

Reproduction sketch

1. Start from a file:

   #!/usr/bin/env bash
   # File generated by pre-commit: https://pre-commit.com
   ...
   

2. Prompt: "insert this block right after the shebang line, do not touch anything else":

   # MARKER:START
   ...
   # MARKER:END
   

3. Expected: line 1 stays #!/usr/bin/env bash, block inserted as line 2.
4. Observed: line 1 became #!/bin/sh.

I don't have the exact prompt/transcript to share publicly (it was proxied through another agent's orchestration), but wanted to flag the behavior in case it's a known class of issue with shebang-adjacent edits.

Secondary observation (same run)

In the same task, a loop over multiple repos calling attrib.exe (Windows) consumed the loop's stdin, causing only the first repo's target folder to be processed per iteration — the agent noticed this itself mid-task and redid the affected repos. Not filing this as a primary bug (looks like a known bash/Windows-tool stdin-consumption gotcha), just noting it happened in the same session in case it's useful context.

Environment

• Windows 11, Git Bash, codex exec --json --sandbox workspace-write via the Claude Code plugin codex-openai-codex

Start by reproducing the edit with codex exec --json --sandbox workspace-write in Git Bash, using the supplied pre-commit example and insertion instruction. Compare the original and edited shebangs across repositories; done means inserting the block after the shebang without changing that line or other existing content.