diff --git a/.github/workflows/promote-baseline.yml b/.github/workflows/promote-baseline.yml
index 476d81f4e..06e229149 100644
--- a/.github/workflows/promote-baseline.yml
+++ b/.github/workflows/promote-baseline.yml
@@ -340,8 +340,8 @@ jobs:
           set -euo pipefail
           # The PR head's objects, to read its tree: fetched, never checked out or run.
           git fetch --quiet origin "$BASE_REF" "+refs/pull/$PR_NUMBER/head:refs/remotes/origin/pr-head"
-          if ! git merge-base --is-ancestor "$MERGE_SHA" "origin/$BASE_REF"; then
-            echo "::notice::merge commit $MERGE_SHA is not on origin/$BASE_REF; skipping"
+          if ! git merge-base --is-ancestor "$MERGE_SHA" "refs/remotes/origin/$BASE_REF"; then
+            echo "::notice::merge commit $MERGE_SHA is not on refs/remotes/origin/$BASE_REF; skipping"
             echo "ok=false" >> "$GITHUB_OUTPUT"; exit 0
           fi
           merge_tree=$(git rev-parse "$MERGE_SHA^{tree}")
diff --git a/scripts/check_doc_reverts.py b/scripts/check_doc_reverts.py
index c8886ef51..0b13a7cd7 100644
--- a/scripts/check_doc_reverts.py
+++ b/scripts/check_doc_reverts.py
@@ -52,16 +52,16 @@
 Without ``--head`` the gate walks the non-merge commits of ``base..HEAD`` one
 by one, each against its parent and with its own message. ``base`` is
 ``--base``, else ``$PINEFORGE_DOC_REVERT_BASE``, else the merge base with
-``origin/main`` or ``main``, else -- in a checkout with no branch refs, as the
-lab's remote hosts have -- the newest first-parent ancestor of ``HEAD`` whose
-subject ends in a pull request number, `` (#N)``: the squash or merge commit
-GitHub wrote on ``main``, which is the merge base CI would find (its branch's
-commits were judged one by one, with their own messages, before it merged; a
-squash commit's message is the pull request's title and cannot name them
-again). Where none resolves, the walk starts at ``HEAD`` and stops at the first
-commit that predates this gate. A commit whose own tree
-has no ``scripts/check_doc_reverts.py`` is never judged: the rule binds from
-the commit that introduced it on. At most ``MAX_COMMITS`` commits are walked.
+``refs/remotes/origin/main`` or ``refs/heads/main``, else -- in a checkout with
+no branch refs, as the lab's remote hosts have -- the newest first-parent
+ancestor of ``HEAD`` whose subject ends in a pull request number, `` (#N)``:
+the squash or merge commit GitHub wrote on ``main``, which is the merge base
+CI would find (its branch's commits were judged one by one, with their own
+messages, before it merged; a squash commit's message is the pull request's
+title and cannot name them again). Where none resolves, the walk starts at
+``HEAD`` and stops at the first commit that predates this gate. A commit whose
+own tree has no ``scripts/check_doc_reverts.py`` is never judged: the rule binds
+from the commit that introduced it on. At most ``MAX_COMMITS`` commits are walked.
 
 Exit 0 when every revert and deletion is named, 1 when one is not, 2 when git
 or a named ref cannot be read.
@@ -352,7 +352,7 @@ def default_base(root: Path) -> str | None:
     env = os.environ.get('PINEFORGE_DOC_REVERT_BASE')
     if env:
         return resolve(env, root)
-    for ref in ('origin/main', 'main'):
+    for ref in ('refs/remotes/origin/main', 'refs/heads/main'):
         tip = git('rev-parse', '--verify', '--quiet', ref + '^{commit}', root=root,
                   check=False).strip()
         if tip:
diff --git a/scripts/ci_preflight.py b/scripts/ci_preflight.py
index 9f3e43c71..26d6e7ff2 100644
--- a/scripts/ci_preflight.py
+++ b/scripts/ci_preflight.py
@@ -1884,7 +1884,7 @@ def ci_workflow_findings(ci: str, native: str, promote: str, cmake: str,
             or 'BASE_REF: main' not in exacttree
             or 'git fetch --quiet origin "$BASE_REF" "+refs/pull/$PR_NUMBER/head:refs/remotes/origin/pr-head"'
             not in exacttree
-            or 'git merge-base --is-ancestor "$MERGE_SHA" "origin/$BASE_REF"' not in exacttree
+            or 'git merge-base --is-ancestor "$MERGE_SHA" "refs/remotes/origin/$BASE_REF"' not in exacttree
             or '--is-ancestor "$HEAD_SHA"' in promote
             or 'merge_tree=$(git rev-parse "$MERGE_SHA^{tree}")' not in exacttree
             or 'head_tree=$(git rev-parse "$HEAD_SHA^{tree}")' not in exacttree
diff --git a/scripts/test_check_doc_reverts.py b/scripts/test_check_doc_reverts.py
index f6536c727..3b2adc1f6 100644
--- a/scripts/test_check_doc_reverts.py
+++ b/scripts/test_check_doc_reverts.py
@@ -18,11 +18,13 @@
 
 import contextlib
 import io
+import os
 from pathlib import Path
 import shutil
 import subprocess
 import tempfile
 import unittest
+from unittest import mock
 
 import check_doc_reverts as guard
 
@@ -91,6 +93,73 @@ def with_keep_handle(page: str = BASE_PAGE) -> str:
     return page + '\n' + KEEP_HANDLE + '\n'
 
 
+class DefaultBase(unittest.TestCase):
+    def setUp(self) -> None:
+        env = mock.patch.dict(os.environ, {'PINEFORGE_DOC_REVERT_BASE': ''})
+        env.start()
+        self.addCleanup(env.stop)
+
+    def test_tags_cannot_shadow_branch_refs(self) -> None:
+        for ref, tag in (('refs/remotes/origin/main', 'origin/main'),
+                         ('refs/heads/main', 'main')):
+            with self.subTest(ref=ref), repo() as r:
+                common = r.commit(BASE_PAGE, 'docs: common ancestor')
+                r.git('checkout', '-q', '-b', 'tag-history')
+                tag_tip = r.commit(with_keep_handle(), 'docs: unrelated tag history')
+                r.git('tag', tag, tag_tip)
+                r.git('checkout', '-q', 'lane')
+                base = r.commit(BASE_PAGE + '\nThe main branch adds its own sentence.\n',
+                                'docs: main branch')
+                r.git('update-ref', ref, base)
+                r.commit(with_keep_handle(), 'docs: lane after main')
+                self.assertEqual(r.git('merge-base', 'HEAD', f'refs/tags/{tag}'), common)
+                self.assertEqual(r.git('merge-base', 'HEAD', ref), base)
+                self.assertNotEqual(common, base)
+                self.assertEqual(guard.default_base(r.root), base)
+
+    def test_remote_tracking_main_precedes_local_main(self) -> None:
+        with repo() as r:
+            remote = r.commit(BASE_PAGE, 'docs: remote main')
+            r.git('update-ref', 'refs/remotes/origin/main', remote)
+            local = r.commit(with_keep_handle(), 'docs: local main')
+            r.git('update-ref', 'refs/heads/main', local)
+            r.commit(BASE_PAGE + '\nThe lane adds a new sentence after local main.\n',
+                     'docs: lane')
+            self.assertEqual(guard.default_base(r.root), remote)
+
+    def test_local_main_is_used_without_remote_tracking_main(self) -> None:
+        with repo() as r:
+            base = r.commit(BASE_PAGE, 'docs: local main')
+            r.git('update-ref', 'refs/heads/main', base)
+            r.commit(with_keep_handle(), 'docs: lane')
+            self.assertEqual(guard.default_base(r.root), base)
+
+    def test_environment_base_precedes_branch_refs(self) -> None:
+        with repo() as r:
+            override = r.commit(BASE_PAGE, 'docs: explicit base')
+            remote = r.commit(with_keep_handle(), 'docs: remote main')
+            r.git('update-ref', 'refs/remotes/origin/main', remote)
+            local = r.commit(BASE_PAGE + '\nLocal main adds another sentence.\n',
+                             'docs: local main')
+            r.git('update-ref', 'refs/heads/main', local)
+            r.commit(with_keep_handle(), 'docs: lane')
+            with mock.patch.dict(os.environ, {'PINEFORGE_DOC_REVERT_BASE': override}):
+                self.assertEqual(guard.default_base(r.root), override)
+
+    def test_merged_pull_request_is_used_without_branch_refs(self) -> None:
+        with repo() as r:
+            r.commit(BASE_PAGE, 'docs: initial guide')
+            base = r.commit(with_keep_handle(), 'docs: merged guide (#12)')
+            r.commit(BASE_PAGE + '\nThe lane adds a new sentence after the merge.\n',
+                     'docs: lane')
+            self.assertEqual(guard.default_base(r.root), base)
+
+    def test_no_branch_ref_or_merged_pull_request_has_no_base(self) -> None:
+        with repo() as r:
+            r.commit(BASE_PAGE, 'docs: initial guide')
+            self.assertIsNone(guard.default_base(r.root))
+
+
 class MustFail(unittest.TestCase):
     def setup_history(self, r: Repo) -> tuple[str, str]:
         base = r.commit(BASE_PAGE, 'docs: the guide')
diff --git a/scripts/test_ci_preflight.py b/scripts/test_ci_preflight.py
index 220b7328a..1729b1523 100644
--- a/scripts/test_ci_preflight.py
+++ b/scripts/test_ci_preflight.py
@@ -868,6 +868,27 @@ def test_ci_contract_pins_parallel_pr_light_and_full_events(self):
                 self.assertNotEqual(changed[index], original[index])
                 self.assertNotEqual(ci_workflow_findings(*changed), [])
 
+    def test_baseline_promotion_accepts_qualified_base_ref(self):
+        sources = self.ci_sources()
+        short = 'git merge-base --is-ancestor "$MERGE_SHA" "origin/$BASE_REF"'
+        qualified = ('git merge-base --is-ancestor "$MERGE_SHA" '
+                     '"refs/remotes/origin/$BASE_REF"')
+        # Supply the same qualified input before and after the workflow fix.
+        sources[2] = sources[2].replace(short, qualified)
+        self.assertEqual(sources[2].count(qualified), 1)
+        self.assertEqual(ci_workflow_findings(*sources, others=self.other_sources()), [])
+
+    def test_baseline_promotion_rejects_short_base_ref(self):
+        sources = self.ci_sources()
+        short = 'git merge-base --is-ancestor "$MERGE_SHA" "origin/$BASE_REF"'
+        qualified = ('git merge-base --is-ancestor "$MERGE_SHA" '
+                     '"refs/remotes/origin/$BASE_REF"')
+        # Supply the old command even when the workflow already uses the fix.
+        sources[2] = sources[2].replace(qualified, short)
+        self.assertEqual(sources[2].count(short), 1)
+        self.assertIn('baseline promotion must admit a squash merge by tree equality only',
+                      ci_workflow_findings(*sources, others=self.other_sources()))
+
     def test_baseline_promotion_rejects_later_pr_head_checkout(self):
         finding = ("baseline promotion must run main's copy and require "
                    "the latest two head statuses")