From 1f6dc84f68b4631e77ebc11a452cb0b03eecde57 Mon Sep 17 00:00:00 2001
From: sommerfeld <sommerfeld@sommerfeld.dev>
Date: Tue, 19 May 2026 16:45:17 +0100
Subject: feat(git): commit-msg hook strips AI Co-authored-by trailers
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Various agentic tools (Copilot CLI, VS Code chat, etc.) auto-append
`Co-authored-by: Copilot <...>` / Claude / Codex trailers, which then
trip the pre-push hook's agent-coauthor check and force a manual
amend before the push goes through. Scrub at commit time instead.

Uses the same agent-substring list as executable_pre-push (kept in
sync by comment). Triggered as commit-msg (not pre-commit — pre-commit
runs before the message exists). Drops matching trailers in-place,
collapses trailing blanks, and is a no-op otherwise.

Also symlinks the new hook in the remote-dev home-manager config so
it deploys on the Ubuntu VM.

Bypass: `git commit --no-verify`.
---
 dot_config/git/hooks/executable_commit-msg | 60 ++++++++++++++++++++++++++++++
 remote-dev/home.nix                        |  1 +
 2 files changed, 61 insertions(+)
 create mode 100755 dot_config/git/hooks/executable_commit-msg

diff --git a/dot_config/git/hooks/executable_commit-msg b/dot_config/git/hooks/executable_commit-msg
new file mode 100755
index 0000000..78dba63
--- /dev/null
+++ b/dot_config/git/hooks/executable_commit-msg
@@ -0,0 +1,60 @@
+#!/bin/sh
+# Strip Co-authored-by trailers whose identity looks like a coding agent
+# (Copilot, Claude, Codex, ChatGPT, Cursor, Aider, Devin, ...). Various
+# tools — GitHub Copilot CLI, VS Code chat, etc. — append these
+# automatically and they then trip the pre-push hook's agent check.
+# Easier to scrub at commit time than to amend later.
+#
+# Activated via core.hooksPath in ~/.config/git/config so it applies to
+# every repo unless that repo overrides hooksPath itself.
+#
+# Bypass: git commit --no-verify
+
+set -eu
+
+msg_file=$1
+
+# Keep this list in sync with executable_pre-push.
+agent_subs='copilot claude codex chatgpt cursor aider devin [bot] @openai. @anthropic.'
+
+# Rewrite the message file, dropping any Co-authored-by line whose
+# lowercased content contains an agent substring. We deliberately
+# operate line-by-line rather than on trailer blocks because the file
+# already includes git's auto-generated comment block (`# ...`) and we
+# don't want to touch any of that.
+tmp=$(mktemp)
+trap 'rm -f "$tmp"' EXIT INT TERM
+
+awk -v agent_subs="$agent_subs" '
+  BEGIN { n = split(agent_subs, subs, " ") }
+  function is_agent(s,    i) {
+    for (i = 1; i <= n; i++) if (index(s, subs[i])) return 1
+    return 0
+  }
+  {
+    # Match the trailer case-insensitively but keep the original
+    # casing for everything we pass through.
+    lower = tolower($0)
+    if (lower ~ /^co-authored-by:[[:space:]]/ && is_agent(lower)) {
+      stripped = 1
+      next
+    }
+    print
+  }
+  END { exit stripped ? 10 : 0 }
+' "$msg_file" >"$tmp" && rc=0 || rc=$?
+
+# awk exits 10 only when we actually dropped a line; anything else is
+# treated as a real error.
+case $rc in
+  0) exit 0 ;;
+  10) ;;
+  *) printf 'commit-msg: awk failed (rc=%d); leaving message untouched.\n' "$rc" >&2; exit 0 ;;
+esac
+
+# Collapse any trailing blank lines produced by the removal so we don't
+# leave a dangling blank trailer block.
+sed -e :a -e '/^$/{$d;N;ba' -e '}' "$tmp" >"$msg_file"
+
+printf 'commit-msg: stripped AI Co-authored-by trailer(s).\n' >&2
+exit 0
diff --git a/remote-dev/home.nix b/remote-dev/home.nix
index 926aadb..5dc55d0 100644
--- a/remote-dev/home.nix
+++ b/remote-dev/home.nix
@@ -161,6 +161,7 @@ in
     # so map each hook to its stripped name explicitly. The executable bit
     # comes from the working-tree file mode (git resolves the symlink).
     "git/hooks/pre-push".source = link "dot_config/git/hooks/executable_pre-push";
+    "git/hooks/commit-msg".source = link "dot_config/git/hooks/executable_commit-msg";
   };
 
   # ── Rootless podman config ──────────────────────────────────────────────────
-- 
cgit v1.3.1