attribution.commit: "" does not disable Co-Authored-By trailer

Bug

Setting attribution.commit to "" (empty string) in ~/.claude/settings.json does not disable the Co-Authored-By trailer on commits. The docs and the embedded help text both state:

Set commit or pr to empty string "" to hide that attribution.

But commits still get the trailer appended.

Repro

1. Set ~/.claude/settings.json:

{
  "attribution": {
    "commit": "",
    "pr": ""
  }
}

1. Start a new Claude Code session
2. Ask Claude to make a change and commit it (or use /commit)
3. The commit message includes Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Analysis

From the compiled binary (v2.1.85), the relevant logic:

function fVH() {
  // ...
  let K = `Co-Authored-By: ${q} <noreply@anthropic.com>`;
  let O = R8(); // reads settings
  
  if (O.attribution)
    return { commit: O.attribution.commit ?? K, pr: O.attribution.pr ?? $ };
  
  if (O.includeCoAuthoredBy === false)
    return { commit: "", pr: "" };
  
  return { commit: K, pr: $ };
}

The ?? (nullish coalescing) logic is correct — "" ?? K should return "". So fVH() likely returns { commit: "", pr: "" } as expected.

However, the caller (the built-in /commit skill prompt via Rs1()) appears to inject the trailer into the commit message regardless, possibly through the system prompt instructing the model to add it, or through a separate code path that doesn't check the return value properly.

Expected behavior

With attribution.commit: "", no Co-Authored-By trailer should appear in commit messages.

Workaround

A commit-msg git hook that strips the trailer:

#!/bin/sh
sed -i '' '/^Co-Authored-By:.*<noreply@anthropic.com>/d' "$1"

Environment

• Claude Code v2.1.85
• macOS (Darwin 24.6.0, arm64)
• Setting was in ~/.claude/settings.json (user scope), last modified before the affected commits
• New sessions still produce the trailer