diff --git a/scripts/action/analyze.sh b/scripts/action/analyze.sh
index 74ad967..f1d1d75 100755
--- a/scripts/action/analyze.sh
+++ b/scripts/action/analyze.sh
@@ -96,14 +96,19 @@ stage() {
   # time rather than carried in the state directory, so a bundle can never
   # inherit the label of the one it was seeded from.
   python3 -c 'import json,os,sys
-json.dump({
-    "kind": sys.argv[2],
+kind = sys.argv[2]
+marker = {
+    "kind": kind,
     "engine_version": os.environ.get("ENGINE_VERSION", ""),
     "cfg_hash": os.environ.get("CFG_HASH", ""),
     "merge_base_sha": os.environ.get("REVIEW_BASE_SHA", ""),
-    "head_sha": os.environ.get("REVIEW_HEAD_SHA", ""),
-    "pr_number": os.environ.get("PR_NUMBER", ""),
-}, open(sys.argv[1], "w"), indent=2)' "$STAGE_DIR/$kind/metadata.json" "$kind"
+}
+# A base describes one commit and is shared by every pull request that forks
+# there, so the run that happened to compute it is not part of its identity.
+if kind == "warmstart":
+    marker["pr_number"] = os.environ.get("PR_NUMBER", "")
+    marker["head_sha"] = os.environ.get("REVIEW_HEAD_SHA", "")
+json.dump(marker, open(sys.argv[1], "w"), indent=2)' "$STAGE_DIR/$kind/metadata.json" "$kind"
 }
 
 analyze_sync() {
diff --git a/tests/test_action_state.py b/tests/test_action_state.py
index 622a445..d2f52bf 100644
--- a/tests/test_action_state.py
+++ b/tests/test_action_state.py
@@ -293,6 +293,20 @@ def test_each_bundle_says_what_it_is(self) -> None:
         warmstart = json.loads((self.stage_dir / "warmstart" / "metadata.json").read_text())
         self.assertEqual(warmstart["kind"], "warmstart")
         self.assertEqual(warmstart["merge_base_sha"], "merge-base-sha")
+        # A warm-start bundle belongs to one pull request; a base does not.
+        self.assertEqual(warmstart["pr_number"], "42")
+
+    def test_a_base_is_not_labelled_with_the_run_that_computed_it(self) -> None:
+        # One base serves every pull request forking from that commit, so the
+        # run that happened to build it is not part of what the bundle is.
+        _state(self.base_dir)
+        self._analyze(RENEW_BASE="true")
+
+        base = json.loads((self.stage_dir / "base" / "metadata.json").read_text())
+        self.assertEqual(base["kind"], "base")
+        self.assertEqual(base["merge_base_sha"], "merge-base-sha")
+        self.assertNotIn("pr_number", base)
+        self.assertNotIn("head_sha", base)
 
     def test_a_bundle_never_inherits_the_label_of_its_seed(self) -> None:
         # A fetched base bundle carries kind=base, and the head is seeded by