diff --git a/src/agentrust_trace/content_marking.py b/src/agentrust_trace/content_marking.py
index a8aa89ee..4ea1eee7 100644
--- a/src/agentrust_trace/content_marking.py
+++ b/src/agentrust_trace/content_marking.py
@@ -145,7 +145,11 @@ def verify_assertion(assertion: dict[str, Any], record_bytes: bytes) -> dict[str
     ref = data.get("record")
     if not isinstance(ref, dict) or not ref.get("url"):
         raise ContentMarkingError("assertion carries no record reference")
-    alg = ref.get("alg", "sha256")
+    alg = ref.get("alg")
+    if not isinstance(alg, str):
+        raise ContentMarkingError(
+            f"unsupported digest algorithm {alg!r}; use sha256 or sha384"
+        )
     expected = ref.get("hash")
     if not _DIGEST_RE.match(str(expected or "")):
         raise ContentMarkingError(f"record.hash {expected!r} is not a sha256:/sha384: digest")
diff --git a/tests/test_content_marking_required_alg.py b/tests/test_content_marking_required_alg.py
new file mode 100644
index 00000000..caa324e2
--- /dev/null
+++ b/tests/test_content_marking_required_alg.py
@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from agentrust_trace.content_marking import ContentMarkingError, build_assertion, verify_assertion
+
+URL = "https://registry.example/records/abc123.json"
+
+
+def _record_bytes() -> bytes:
+    return json.dumps(
+        {
+            "eat_profile": "tag:agentrust-io.com,2026:trace-v0.2",
+            "iat": 1760000000,
+            "subject": "spiffe://example.org/agent/image-bot",
+            "data_class": "public",
+        }
+    ).encode()
+
+
+def test_missing_record_alg_is_refused_instead_of_defaulting_to_sha256() -> None:
+    raw = _record_bytes()
+    assertion = build_assertion(raw, url=URL)
+    del assertion["data"]["record"]["alg"]
+
+    with pytest.raises(ContentMarkingError, match="unsupported digest algorithm None"):
+        verify_assertion(assertion, raw)
+
+
+def test_null_record_alg_is_refused() -> None:
+    raw = _record_bytes()
+    assertion = build_assertion(raw, url=URL)
+    assertion["data"]["record"]["alg"] = None
+
+    # Pins the plausible near-miss where explicit null is treated as "unset"
+    # and silently replaced with the old SHA-256 default.
+    with pytest.raises(ContentMarkingError, match="unsupported digest algorithm None"):
+        verify_assertion(assertion, raw)
+
+
+@pytest.mark.parametrize("bad_alg", [[1], [], {"a": 1}, {}])
+def test_unhashable_record_alg_is_refused_without_type_error(bad_alg) -> None:
+    raw = _record_bytes()
+    assertion = build_assertion(raw, url=URL)
+    assertion["data"]["record"]["alg"] = bad_alg
+    assertion["data"]["record"]["hash"] = "nope"
+
+    with pytest.raises(ContentMarkingError, match="unsupported digest algorithm"):
+        verify_assertion(assertion, raw)
+
+
+def test_explicit_sha256_and_sha384_still_verify() -> None:
+    raw = _record_bytes()
+    verify_assertion(build_assertion(raw, url=URL, alg="sha256"), raw)
+    verify_assertion(build_assertion(raw, url=URL, alg="sha384"), raw)