[BUG] Co-Authored-By capitalization: system prompt says 'Co-Authored-By' but git writes 'Co-authored-by'

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

The system prompt instructs:

End git commit messages with:
Co-Authored-By: Claude <noreply@anthropic.com>

But GitHub squash merge always writes the git-standard lowercase form:

Co-authored-by: Claude <noreply@anthropic.com>

Co-Authored-By (capital A, capital B) is what the CLI claims. Co-authored-by (lowercase a, lowercase b) is what actually lands in git log.

Reproduction Data

41/41 commits with the trailer in the last 60 commits of a real repo all show the lowercase form:

git log --format='%(trailers:key=Co-authored-by,valueonly)' -60 | grep -c 'Claude'
# 41 commits have the trailer

git log --format='%B' -60 | grep 'Co-authored-by' | wc -l
# 41 — all lowercase

git log --format='%B' -60 | grep 'Co-Authored-By' | wc -l
# 0 — zero uppercase

What Should Happen?

Either:

1. The system prompt should state Co-authored-by (matching git standard output), or
2. The commit tool should output Co-Authored-By (matching the system prompt claim)

Related Issues

• #58033 — intermittent missing Co-authored-by trailer
• #66506 — system prompt hardcodes model name in attribution
• #66602 — Co-Authored-By copyright concerns