diff --git a/.github/workflows/canon-marker.yml b/.github/workflows/canon-marker.yml
new file mode 100644
index 0000000..39600d4
--- /dev/null
+++ b/.github/workflows/canon-marker.yml
@@ -0,0 +1,17 @@
+name: Canon marker
+
+on:
+  pull_request:
+
+jobs:
+  check:
+    # The forge's runner carries the label `docker` (image node:22-bookworm);
+    # `ubuntu-latest` matches no runner there and the job is cancelled.
+    runs-on: docker
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - run: apt-get update -qq && apt-get install -y -qq python3-yaml
+      - run: python3 -m unittest scripts/tests/test_check_canon_marker.py
+      - run: python3 scripts/check_canon_marker.py "origin/${{ github.base_ref }}"
diff --git a/.gitignore b/.gitignore
index fea6634..046c00e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -32,3 +32,4 @@ Thumbs.db
 /quartz-site/node_modules/
 /quartz-site/public/
 /quartz-site/*.log
+__pycache__/
diff --git a/scripts/check_canon_marker.py b/scripts/check_canon_marker.py
new file mode 100644
index 0000000..2ad615f
--- /dev/null
+++ b/scripts/check_canon_marker.py
@@ -0,0 +1,117 @@
+#!/usr/bin/env python3
+"""Canon/proposal marker check. Usage: check_canon_marker.py <base-ref>
+
+Checks .md files under Aetheria/ added, modified or renamed against <base-ref>.
+Proves the form and presence of ruling ids, not that the ruling exists.
+"""
+import re, subprocess, sys
+
+import yaml
+
+ID = re.compile(r"^[a-z0-9][a-z0-9-]*:ruling:[a-z0-9][a-z0-9._-]*$")
+
+
+def git(*a):
+    return subprocess.run(["git", *a], check=True, capture_output=True, text=True).stdout
+
+
+class UniqueKeyLoader(yaml.SafeLoader):
+    """safe_load that refuses a repeated key: one page, one reading of `kind`."""
+
+    def construct_mapping(self, node, deep=False):
+        seen = set()
+        for key_node, _ in node.value:
+            key = self.construct_object(key_node, deep=True)
+            if key in seen:
+                raise yaml.constructor.ConstructorError(
+                    None, None, f"duplicate key {key!r}", key_node.start_mark)
+            seen.add(key)
+        return super().construct_mapping(node, deep)
+
+
+FRONT = re.compile(r"---\n(?:(.*?)\n)?---[ \t]*(?:\n|\Z)", re.S)
+
+
+def frontmatter(text):
+    """Return (kind, rulings or None, error). Frontmatter is real YAML."""
+    if not text.startswith("---\n"):
+        return None, None, None
+    m = FRONT.match(text)
+    if not m:
+        return None, None, "unterminated frontmatter"
+    try:
+        data = yaml.load(m.group(1) or "", Loader=UniqueKeyLoader)
+    except yaml.YAMLError as e:
+        return None, None, f"frontmatter is not valid YAML ({type(e).__name__}: {e})"
+    if data is None:
+        data = {}
+    if not isinstance(data, dict):
+        return None, None, "frontmatter must be a mapping"
+    kind, rulings = data.get("kind"), data.get("rulings")
+    if kind is not None and not isinstance(kind, str):
+        return None, None, "kind must be a string"
+    if rulings is not None and not (isinstance(rulings, list) and all(isinstance(r, str) for r in rulings)):
+        return None, None, "rulings must be a list of strings"
+    return kind, rulings, None
+
+
+def changed(base):
+    """(status letter, path) for each page added, modified or renamed (the new path)."""
+    out = git("diff", "-z", "--name-status", "--diff-filter=AMR", f"{base}...HEAD", "--", "Aetheria")
+    fields = out.split("\0")
+    rows, i = [], 0
+    while i < len(fields) and fields[i]:
+        st = fields[i]
+        if st[0] == "R":
+            rows.append(("R", fields[i + 2]))
+            i += 3
+        else:
+            rows.append((st[0], fields[i + 1]))
+            i += 2
+    return rows
+
+
+def main(base):
+    try:
+        rows = changed(base)
+    except subprocess.CalledProcessError as e:
+        print(f"CANON CHECK FAILED: cannot diff against {base!r}: {e.stderr.strip()}")
+        return 1
+    failed = False
+    for st, path in rows:
+        if not path.endswith(".md"):
+            continue
+        try:
+            with open(path, encoding="utf-8-sig") as f:
+                kind, rulings, err = frontmatter(f.read())
+        except (OSError, UnicodeDecodeError) as e:
+            kind, rulings, err = None, None, f"cannot read page ({type(e).__name__})"
+        problem = None
+        if err:
+            problem = err
+        elif kind is None:
+            if st == "A":
+                problem = "new page needs `kind: canon` or `kind: proposal`"
+            else:
+                print(f"notice: {path} has no kind (legacy); tag it when you next touch it")
+        elif kind == "canon":
+            bad = [r for r in (rulings or []) if not ID.fullmatch(r)]
+            if not rulings or bad:
+                problem = ("kind: canon requires `rulings:` with at least one id of the form "
+                           f"<campaign>:ruling:<label>\n  (found: {bad or 'none'})")
+        elif kind == "proposal":
+            if rulings:
+                problem = "kind: proposal may not carry `rulings:`"
+        else:
+            problem = f"kind must be canon or proposal (found: {kind})"
+        if problem:
+            failed = True
+            print(f"CANON CHECK FAILED: {path}\n  {problem}\n"
+                  "  A ruling must exist before its text enters a canon page. "
+                  "Put the text in a `kind: proposal` page until it does.")
+    return 1 if failed else 0
+
+
+if __name__ == "__main__":
+    sys.stdout.reconfigure(encoding="utf-8")
+    sys.exit(main(sys.argv[1]))
diff --git a/scripts/tests/test_check_canon_marker.py b/scripts/tests/test_check_canon_marker.py
new file mode 100644
index 0000000..a97ae6e
--- /dev/null
+++ b/scripts/tests/test_check_canon_marker.py
@@ -0,0 +1,206 @@
+"""Tests for scripts/check_canon_marker.py: each builds a throwaway git repo
+with a main branch, adds pages on a branch, and runs the check against main."""
+import os
+import subprocess
+import tempfile
+import unittest
+
+SCRIPT = os.path.join(os.path.dirname(__file__), "..", "check_canon_marker.py")
+RULING = "aetheria-lore:ruling:megiddo-origin"
+
+
+def git(cwd, *args):
+    subprocess.run(["git", "-c", "user.email=t@t", "-c", "user.name=t", *args],
+                   cwd=cwd, check=True, capture_output=True)
+
+
+class Repo:
+    def __init__(self):
+        self.tmp = tempfile.TemporaryDirectory()
+        self.dir = self.tmp.name
+        git(self.dir, "init", "-b", "main")
+        os.makedirs(os.path.join(self.dir, "Aetheria"))
+        self.write("Aetheria/Legacy.md", "old page, no marker\n" + "line of lore\n" * 20)
+        git(self.dir, "add", "-A")
+        git(self.dir, "commit", "-m", "base")
+        git(self.dir, "checkout", "-b", "pr")
+
+    def write(self, path, text):
+        full = os.path.join(self.dir, path)
+        os.makedirs(os.path.dirname(full), exist_ok=True)
+        with open(full, "w", encoding="utf-8") as f:
+            f.write(text)
+
+    def commit(self):
+        git(self.dir, "add", "-A")
+        git(self.dir, "commit", "-m", "pr")
+
+    def check(self):
+        p = subprocess.run(["python3", SCRIPT, "main"], cwd=self.dir,
+                           capture_output=True, text=True)
+        return p.returncode, p.stdout + p.stderr
+
+
+def canon(rulings, kind="canon"):
+    return f"---\nkind: {kind}\nrulings:\n{rulings}---\nbody\n"
+
+
+class CanonMarker(unittest.TestCase):
+    def run_pages(self, pages, after=None):
+        r = Repo()
+        for path, text in pages.items():
+            r.write(path, text)
+        if after:
+            after(r)
+        r.commit()
+        return r.check()
+
+    def test_a_canon_page_with_a_ruling_id_passes(self):
+        code, out = self.run_pages({"Aetheria/A.md": canon(f"  - {RULING}\n")})
+        self.assertEqual(code, 0, out)
+
+    def test_a_canon_page_without_a_ruling_fails(self):
+        code, out = self.run_pages({"Aetheria/A.md": "---\nkind: canon\n---\nx\n"})
+        self.assertEqual(code, 1, out)
+
+    def test_a_non_ascii_path_is_checked_not_skipped(self):
+        code, out = self.run_pages({"Aetheria/Framgång.md": "---\nkind: canon\n---\nx\n"})
+        self.assertEqual(code, 1, out)
+        self.assertIn("Framgång.md", out)
+
+    def test_a_renamed_page_is_checked(self):
+        def rename(r):
+            git(r.dir, "mv", "Aetheria/Legacy.md", "Aetheria/Moved.md")
+            r.write("Aetheria/Moved.md", "---\nkind: canon\n---\n" + "old page, no marker\n" + "line of lore\n" * 20)
+        code, out = self.run_pages({}, after=rename)
+        self.assertEqual(code, 1, out)
+        self.assertIn("Moved.md", out)
+
+    def test_quoted_values_parse(self):
+        text = f'---\nkind: "canon"\nrulings:\n  - "{RULING}"\n---\nx\n'
+        code, out = self.run_pages({"Aetheria/A.md": text})
+        self.assertEqual(code, 0, out)
+
+    def test_a_trailing_comment_on_a_ruling_is_not_part_of_the_id(self):
+        code, out = self.run_pages({"Aetheria/A.md": canon(f"  - {RULING}  # megiddo\n")})
+        self.assertEqual(code, 0, out)
+
+    def test_a_flow_list_of_rulings_parses(self):
+        text = f"---\nkind: canon\nrulings: [{RULING}, weksa:ruling:other]\n---\nx\n"
+        code, out = self.run_pages({"Aetheria/A.md": text})
+        self.assertEqual(code, 0, out)
+
+    def test_cross_campaign_ruling_ids_count(self):
+        code, out = self.run_pages({"Aetheria/A.md": canon("  - weksa-body:ruling:faction-play-1\n")})
+        self.assertEqual(code, 0, out)
+
+    def test_a_malformed_ruling_id_fails(self):
+        code, out = self.run_pages({"Aetheria/A.md": canon("  - megiddo-origin\n")})
+        self.assertEqual(code, 1, out)
+
+    def test_a_proposal_may_not_carry_rulings(self):
+        code, out = self.run_pages({"Aetheria/A.md": canon(f"  - {RULING}\n", kind="proposal")})
+        self.assertEqual(code, 1, out)
+
+    def test_a_proposal_without_rulings_passes(self):
+        code, out = self.run_pages({"Aetheria/A.md": "---\nkind: proposal\n---\nx\n"})
+        self.assertEqual(code, 0, out)
+
+    def test_an_unknown_kind_fails(self):
+        code, out = self.run_pages({"Aetheria/A.md": "---\nkind: draft\n---\nx\n"})
+        self.assertEqual(code, 1, out)
+
+    def test_a_new_untagged_page_fails(self):
+        code, out = self.run_pages({"Aetheria/A.md": "no frontmatter\n"})
+        self.assertEqual(code, 1, out)
+
+    def test_a_touched_legacy_page_passes_with_a_notice(self):
+        code, out = self.run_pages({"Aetheria/Legacy.md": "old page, edited\n" + "line of lore\n" * 20})
+        self.assertEqual(code, 0, out)
+        self.assertIn("notice", out)
+
+    def test_invalid_yaml_fails_instead_of_crashing(self):
+        code, out = self.run_pages({"Aetheria/A.md": "---\nkind: [canon\n---\nx\n"})
+        self.assertEqual(code, 1, out)
+        self.assertNotIn("Traceback", out)
+
+    def test_a_non_mapping_frontmatter_fails(self):
+        code, out = self.run_pages({"Aetheria/A.md": "---\n- canon\n---\nx\n"})
+        self.assertEqual(code, 1, out)
+        self.assertNotIn("Traceback", out)
+
+    def test_a_python_object_tag_is_refused_not_executed(self):
+        text = "---\nkind: !!python/object/apply:os.getcwd []\n---\nx\n"
+        code, out = self.run_pages({"Aetheria/A.md": text})
+        self.assertEqual(code, 1, out)
+        self.assertNotIn("Traceback", out)
+
+    def test_a_repeated_key_fails(self):
+        text = f"---\nkind: proposal\nkind: canon\nrulings:\n  - {RULING}\n---\nx\n"
+        code, out = self.run_pages({"Aetheria/A.md": text})
+        self.assertEqual(code, 1, out)
+        self.assertIn("duplicate", out)
+
+    def test_a_bom_does_not_hide_the_marker(self):
+        code, out = self.run_pages({"Aetheria/Legacy.md": "\ufeff---\nkind: canon\n---\nx\n"})
+        self.assertEqual(code, 1, out)
+
+    def test_a_ruling_id_with_a_trailing_newline_is_malformed(self):
+        text = f'---\nkind: canon\nrulings:\n  - "{RULING}\\n"\n---\nx\n'
+        code, out = self.run_pages({"Aetheria/A.md": text})
+        self.assertEqual(code, 1, out)
+
+    def test_empty_frontmatter_is_a_kindless_page_not_unterminated(self):
+        code, out = self.run_pages({"Aetheria/A.md": "---\n---\nx\n"})
+        self.assertEqual(code, 1, out)
+        self.assertNotIn("unterminated", out)
+        self.assertIn("needs `kind", out)
+
+    def test_a_lookalike_closing_marker_does_not_end_the_frontmatter(self):
+        text = "---\nkind: proposal\n----\nstill frontmatter?\n---\nx\n"
+        code, out = self.run_pages({"Aetheria/A.md": text})
+        self.assertEqual(code, 1, out)
+
+    def test_a_renamed_legacy_page_passes_with_a_notice(self):
+        def rename(r):
+            git(r.dir, "mv", "Aetheria/Legacy.md", "Aetheria/Moved.md")
+        code, out = self.run_pages({}, after=rename)
+        self.assertEqual(code, 0, out)
+        self.assertIn("notice", out)
+
+    def test_a_page_main_changed_after_the_branch_point_is_not_charged_to_the_pr(self):
+        # Old.md is a bad canon page that main already holds; the PR merges it in
+        # and does not touch it, then main edits it again. Two-dot diffing
+        # would see the PR's copy as modified and fail the PR for main's page.
+        r = Repo()
+        r.write("Aetheria/Mine.md", "---\nkind: proposal\n---\nx\n")
+        r.commit()
+        git(r.dir, "checkout", "main")
+        r.write("Aetheria/Old.md", "---\nkind: canon\n---\nv1\n")
+        git(r.dir, "add", "-A")
+        git(r.dir, "commit", "-m", "main adds Old")
+        git(r.dir, "checkout", "pr")
+        git(r.dir, "merge", "--no-edit", "main")
+        git(r.dir, "checkout", "main")
+        r.write("Aetheria/Old.md", "---\nkind: canon\n---\nv2\n")
+        git(r.dir, "add", "-A")
+        git(r.dir, "commit", "-m", "main edits Old")
+        git(r.dir, "checkout", "pr")
+        code, out = r.check()
+        self.assertEqual(code, 0, out)
+
+    def test_an_unknown_base_ref_fails_loudly_without_a_traceback(self):
+        r = Repo()
+        p = subprocess.run(["python3", SCRIPT, "no-such-ref"], cwd=r.dir,
+                           capture_output=True, text=True)
+        self.assertEqual(p.returncode, 1)
+        self.assertNotIn("Traceback", p.stdout + p.stderr)
+        self.assertIn("no-such-ref", p.stdout + p.stderr)
+
+    def test_pages_outside_aetheria_are_not_checked(self):
+        code, out = self.run_pages({"README2.md": "---\nkind: canon\n---\nx\n"})
+        self.assertEqual(code, 0, out)
+
+
+if __name__ == "__main__":
+    unittest.main()