🔧 Forbid Co-authored-by trailer in commits

The Co-authored-by trailer is now forbidden. The
commit checker fails any message holding it, whatever
the case, and the docs record the ban.

AI-assisted-by: muse-spark-1.3-contributor
Signed-off-by: Andrey Antukh <niwi@niwi.nz>

.agents/skills/create-commit/SKILL.md

@@ -61,5 +61,6 @@ subject/body limits, and the `AI-assisted-by` trailer. Follow it exactly.
 - Do not pass `--author`. Author identity comes from the local git config.
 - Do not amend a commit you did not create in this session, unless explicitly asked.
 - Do not bypass pre-commit hooks (`--no-verify`) unless explicitly asked.
+- Never add a `Co-authored-by:` trailer. It is forbidden and fails `./scripts/check-commit`.
 - Do not add untracked files that were not created in this session.
 - Do not skip the `scripts/check-commit` verification step (Iron Rule 4).

.serena/memories/workflow/creating-commits.md

@@ -37,8 +37,9 @@ that breaks them is wrong, even if the code is right.
   - For another commit: `./scripts/check-commit -c <ref>`
 - **NEVER** hand-wave the body as "one long line". If a line exceeds 76,
   break it at a space.
+- **NEVER add a `Co-authored-by:` trailer.** It is forbidden; `scripts/check-commit` fails the commit when any line matches it (case-insensitive).
 - Exceptions inside the body (do not wrap these): `Signed-off-by:`,
-  `Co-authored-by:`, `AI-assisted-by:` trailers, and lines carrying a URL.
+  `AI-assisted-by:` trailers, and lines carrying a URL.
 
 **AI-assisted-by trailer rules:**
 - Use only the model name, e.g. `mimo-v2.5`, `deepseek-v4-flash`

CONTRIBUTING.md

@@ -223,6 +223,8 @@ Commit messages must follow this format:
 - Keep the subject to **70 characters** or fewer
 - **Wrap body lines at 76 characters or fewer** (trailers and URLs excepted)
 - Separate the subject from the body with a **blank line**
+- Never add a `Co-authored-by` trailer — it is forbidden and fails
+  `./scripts/check-commit`
 
 You can check a commit against these rules with `./scripts/check-commit`.
 

scripts/check-commit

@@ -47,10 +47,12 @@ MERGE_PATTERN = re.compile(r"^(Merge|Revert|Reapply).+[^.]$")
 MAX_BODY_LINE = 76
 
 TRAILER_PATTERN = re.compile(
-    r"^(Signed-off-by|Co-authored-by|Co-developed-by|Reviewed-by|"
+    r"^(Signed-off-by|Co-developed-by|Reviewed-by|"
     r"Acked-by|Tested-by|Reported-by|Suggested-by|AI-assisted-by):"
 )
 
+FORBIDDEN_TRAILER_PATTERN = re.compile(r"^\s*Co-authored-by\s*:", re.IGNORECASE)
+
 URL_PATTERN = re.compile(r"https?://\S+")
 
 # ═══════════════════════════════════════════════════════════════════════════════
@@ -209,6 +211,25 @@ def check_signed_off_by(message):
     return True, None
 
 
+def check_no_co_authored_by(message):
+    """Check that the forbidden Co-authored-by trailer is absent."""
+    offenders = [
+        (line_number, line)
+        for line_number, line in enumerate(message.split("\n"), start=1)
+        if FORBIDDEN_TRAILER_PATTERN.match(line)
+    ]
+    if not offenders:
+        return True, None
+    details = "\n".join(
+        f"    line {line_number}: {line!r}"
+        for line_number, line in offenders
+    )
+    return False, (
+        "The 'Co-authored-by:' trailer is forbidden and must not appear "
+        "in the commit message. Offending line(s):\n" + details
+    )
+
+
 # ═══════════════════════════════════════════════════════════════════════════════
 
 def main():
@@ -234,6 +255,7 @@ def main():
         ("Subject capitalized", check_subject_capitalized),
         ("Blank line after subject", check_body_blank_line),
         ("Body wrapped at 76 chars", check_body_line_length),
+        ("No Co-authored-by trailer", check_no_co_authored_by),
     ]
 
     all_ok = True

scripts/test_check_commit.py

@@ -106,6 +106,35 @@ class BodyLineLengthTests(unittest.TestCase):
         self.assertIn("line 4", error)
 
 
+class ForbiddenTrailerTests(unittest.TestCase):
+    def assert_ok(self, message):
+        ok, error = check_commit.check_no_co_authored_by(message)
+        self.assertTrue(ok, error)
+        self.assertIsNone(error)
+
+    def assert_fail(self, message):
+        ok, error = check_commit.check_no_co_authored_by(message)
+        self.assertFalse(ok)
+        self.assertIsNotNone(error)
+        return error
+
+    def test_body_without_trailer_passes(self):
+        self.assert_ok(":bug: Fix crash\n\nBody.\n\nSigned-off-by: Someone <someone@example.com>\n")
+
+    def test_co_authored_by_fails(self):
+        error = self.assert_fail(":bug: Fix crash\n\nBody.\n\nCo-authored-by: Someone <someone@example.com>\n")
+        self.assertIn("Co-authored-by", error)
+
+    def test_co_authored_by_case_insensitive_fails(self):
+        self.assert_fail(":bug: Fix crash\n\nBody.\n\nco-authored-by: Someone <someone@example.com>\n")
+
+    def test_co_authored_by_in_body_fails(self):
+        self.assert_fail(":bug: Fix crash\n\nCo-authored-by: Someone <someone@example.com>\nBody.\n")
+
+    def test_similar_trailer_passes(self):
+        self.assert_ok(":bug: Fix crash\n\nBody.\n\nCo-developed-by: Someone <someone@example.com>\n")
+
+
 class SubjectRulesRegressionTests(unittest.TestCase):
     """Guard the pre-existing validators against accidental breakage."""