diff --git a/tests/test_artifact_identity.py b/tests/test_artifact_identity.py
index 60d086f..fe614bf 100644
--- a/tests/test_artifact_identity.py
+++ b/tests/test_artifact_identity.py
@@ -34,6 +34,15 @@ is genuinely the commit those paths last changed in.
 emit. Two nearly-identical formats are more dangerous than two obviously
 different ones, and the only thing keeping them identical is a test.
 
+**The extension is the one exception, and it is a rendering exception only.**
+AMO's version grammar forbids a leading zero, so the extension emits the same
+numbers unpadded — `2026.8.29.201` where the family says `2026.08.29.0201`
+(#3138, milestone 318 step 8). Rule 148 defines comparison as numeric per
+dot-segment, under which the two are equal, so this is pinned in both
+directions below: the extension must satisfy AMO's grammar, and every artifact
+must derive the same NUMBERS its own commit stamps. An exception left as "the
+extension is different" would drift into being differently different.
+
 The identity-TAG tests this file used to hold are gone with the tag. There is
 no longer a `CHANNELLED` list to drift (the channel is which tag you inspect),
 and no `identity` subcommand to refuse an unqualified call.
@@ -57,6 +66,26 @@ _REVISION = re.compile(r"^[0-9a-f]{12}$")
 # YYYY.MM.DD.HHMM, every segment zero-padded to its full width.
 _VERSION = re.compile(r"^\d{4}\.\d{2}\.\d{2}\.\d{4}$")
 
+# The artifacts that cannot use the padded rendering. Exactly one, and the
+# reason is external: `packaging.sh` derives the extension's version and AMO
+# refuses to sign a padded one.
+AMO_UNPADDED = frozenset({"extension"})
+
+# Mozilla's published grammar for addons.mozilla.org, transcribed from MDN's
+# manifest.json/version page. A segment is the single digit `0` or starts 1-9,
+# and there are at most four. This is the constraint the exception exists for,
+# so it is what the exception is tested against — `2026.08.29.0201` fails it.
+_AMO = re.compile(r"^(0|[1-9][0-9]{0,8})(\.(0|[1-9][0-9]{0,8})){0,3}$")
+
+# YYYY.M.D.HHMM — four segments, none of them zero-padded.
+_UNPADDED = re.compile(r"^\d{4}(\.(0|[1-9]\d*)){3}$")
+
+
+def segments(value: str) -> tuple[int, ...]:
+    """A version as the numbers it denotes, which is how rule 148 says to
+    compare one. `2026.08.29.0201` and `2026.8.29.201` are one value here."""
+    return tuple(int(part) for part in value.split("."))
+
 
 # Everything here goes through artifacts.sh rather than importing a sibling
 # test module. That is the interface build.yml actually calls, so the tests
@@ -126,7 +155,7 @@ def test_revision_is_a_legal_label_value_and_is_stable(artifact):
     assert first == revision(artifact), "revision is not stable across calls"
 
 
-@pytest.mark.parametrize("artifact", ARTIFACTS)
+@pytest.mark.parametrize("artifact", sorted(set(ARTIFACTS) - AMO_UNPADDED))
 def test_version_is_zero_padded_calver(artifact):
     """The family shape, pinned.
 
@@ -148,6 +177,31 @@ def test_version_is_zero_padded_calver(artifact):
     )
 
 
+@pytest.mark.parametrize("artifact", sorted(AMO_UNPADDED))
+def test_the_unpadded_artifacts_derive_something_amo_will_sign(artifact):
+    """The other half of the family shape: the documented exception, tested
+    against the constraint that justifies it rather than against itself.
+
+    A padded value passes `_UNPADDED` on any date with no leading zeros, so
+    that pattern alone would let a regression sit unnoticed until the first
+    single-digit month — at which point the failure is a burned AMO version,
+    not a red lane. AMO's grammar is the assertion that fires immediately.
+    """
+    value = artifacts("version", artifact).strip()
+    assert _AMO.match(value), (
+        f"{artifact} derives {value!r}, which AMO refuses: a segment must be "
+        f"the single digit `0` or start 1-9, and there are at most four. "
+        f"Almost certainly a zero-padded segment — the family pads and this "
+        f"artifact must not (#3138). AMO 409s on re-signing, so a version it "
+        f"rejects is burned."
+    )
+    assert _UNPADDED.match(value), (
+        f"{artifact} derives {value!r}, which is not YYYY.M.D.HHMM. AMO would "
+        f"also accept the pre-318 `1.0.<minutes>`, and that orders below every "
+        f"ext-2026.* release already signed."
+    )
+
+
 @pytest.mark.parametrize("artifact", ARTIFACTS)
 def test_version_and_revision_describe_the_same_commit(artifact):
     """They are derived independently and must not be able to disagree.
@@ -162,5 +216,21 @@ def test_version_and_revision_describe_the_same_commit(artifact):
         capture_output=True, text=True, check=True, cwd=ROOT,
         env={"TZ": "UTC", "PATH": os.environ.get("PATH", "")},
     ).stdout.strip()
-    assert artifacts("version", artifact).strip() == stamped
+    derived = artifacts("version", artifact).strip()
+
+    # Compared as NUMBERS, which is how rule 148 defines comparison and the
+    # only way one assertion can cover both renderings. This is what makes the
+    # extension's exception cosmetic rather than semantic: it must denote
+    # exactly the value its own commit stamps, whatever the padding.
+    assert segments(derived) == segments(stamped), (
+        f"{artifact} derives {derived!r}, but its newest shipped commit "
+        f"{sha[:12]} is {stamped!r}. The instance would name one commit while "
+        f"carrying another's bytes."
+    )
+    if artifact not in AMO_UNPADDED:
+        assert derived == stamped, (
+            f"{artifact} derives {derived!r} where the family shape is "
+            f"{stamped!r} — same numbers, wrong rendering. Only the artifacts "
+            f"in AMO_UNPADDED may differ here."
+        )
     assert sha.startswith(revision(artifact))