diff --git a/README.md b/README.md
index 3f2e6b5..1ed5350 100644
--- a/README.md
+++ b/README.md
@@ -19,6 +19,30 @@ The commit will also be flagged if the commit message does not include a Jira
 ID (unless marked with NO_JIRA or a Copilot Autofix co-author line), or if the
 size of new or modified files exceeds a threshold.
 
+## Conventional Commits
+
+Conventional Commits validation is optional. Add a `.conventional-commits` file
+at the repository root to enable it. The supported header format is:
+
+```text
+<type>: <subject>
+<type>(<scope>): <subject>
+```
+
+Examples:
+
+```text
+fix: handle an empty search response
+feat(PLA-0001): add structure filtering
+break(NO_JIRA): remove the legacy search endpoint
+```
+
+Supported types are `break`, `feat`, `fix`, `refactor`, `build`, `chore`, `ci`,
+`docs`, `perf`, `revert`, `style`, and `test`. CCDC release configurations use
+`break` for a major version. A branch may contain more than one type.
+Releases should choose the highest required version bump,
+so `break` takes precedence over `feat`, which takes precedence over `fix`.
+
 
 # GitHub Actions
 
diff --git a/main/githooks.py b/main/githooks.py
index 035d75e..7059823 100644
--- a/main/githooks.py
+++ b/main/githooks.py
@@ -960,11 +960,6 @@ def check_commit_msg(message, files, repo):
         # Not checking for JIRA or large file in commit message generated by github
         return 0
 
-    if re.match(r'^ccdc-opensource/', repo):
-        # Do not check for JIRA in opensource repo as we don't want to require external contributors to do this
-        return 0
-    
-        
     # Check for Conventional Commits compliance.
     # Opt-in per repo: commit an empty marker file named
     # `.conventional-commits` at the repo root.
@@ -977,6 +972,8 @@ def check_commit_msg(message, files, repo):
             return 1
         
     if (
+        not re.match(r'^ccdc-opensource/', repo)
+        and
         NO_JIRA_MARKER not in message
         and copilot_autofix_coauthor_pattern.search(message) is None
         and jira_id_pattern.search(message) is None
@@ -1071,6 +1068,28 @@ def _test(input, is_good=True):
         _test('I forgot to add the jira marker!', False)
         _test('Close but no cigar abc-1234', False)
 
+    @patch('githooks._conventional_commits_enabled', return_value=True)
+    def test_opensource_repo_still_checks_conventional_commit(self, _enabled):
+        self.assertEqual(0, check_commit_msg('feat: valid change', [], 'ccdc-opensource/example'))
+        self.assertEqual(1, check_commit_msg('invalid: invalid change', [], 'ccdc-opensource/example'))
+
+
+class TestConventionalCommitPresent(unittest.TestCase):
+    def test_supported_types(self):
+        for commit_type in (
+            'break', 'feat', 'fix', 'refactor', 'build', 'chore', 'ci',
+            'docs', 'perf', 'revert', 'style', 'test'
+        ):
+            with self.subTest(commit_type=commit_type):
+                self.assertTrue(conventional_commit_present(f'{commit_type}: subject'))
+
+    def test_scope_and_multiline_description(self):
+        self.assertTrue(conventional_commit_present('feat(api): subject\n\nMore detail'))
+
+    def test_unsupported_type(self):
+        self.assertFalse(conventional_commit_present('invalid(PLA-3474): test'))
+        self.assertFalse(conventional_commit_present('breaking-change: test'))
+
 
 def run_licence_check(files):
     '''Check or fix complete CCDC licence headers.