diff --git a/harness/tests/test_corpus_normalize.py b/harness/tests/test_corpus_normalize.py
index 67bc41e7..96a115e6 100644
--- a/harness/tests/test_corpus_normalize.py
+++ b/harness/tests/test_corpus_normalize.py
@@ -32,7 +32,7 @@ def test_normalize_utf8_bom_newlines_unicode_and_frontmatter():
 
     assert document.content == "Café\nBody\n"
     assert document.title == "Café"
-    assert document.metadata["frontmatter"] == {"tags": ("uno", "due"), "title": "Café"}
+    assert document.metadata["frontmatter"] == {"tags": ["uno", "due"], "title": "Café"}
     assert document.metadata["source"] == {"owner": "docs"}
     assert document.metadata["acquisition"] == {"transport": "http"}
     assert document.source_uri == "https://host/guide.md"
diff --git a/harness/tht/ports/evidence.py b/harness/tht/ports/evidence.py
index 8cd64f89..0f736b0a 100644
--- a/harness/tht/ports/evidence.py
+++ b/harness/tht/ports/evidence.py
@@ -26,6 +26,26 @@ class FrozenDict(dict):
     update = _immutable
 
 
+class FrozenList(list):
+    """A JSON-serializable list whose mutation operations are disabled."""
+
+    def _immutable(self, *args, **kwargs):
+        raise TypeError("frozen JSON metadata cannot be mutated")
+
+    __delitem__ = _immutable
+    __iadd__ = _immutable
+    __imul__ = _immutable
+    __setitem__ = _immutable
+    append = _immutable
+    clear = _immutable
+    extend = _immutable
+    insert = _immutable
+    pop = _immutable
+    remove = _immutable
+    reverse = _immutable
+    sort = _immutable
+
+
 _CAMEL_BOUNDARY = re.compile(r"(?<=[a-z0-9])(?=[A-Z])")
 _SEPARATORS = re.compile(r"[^a-z0-9]+")
 _NAMESPACED_VALUE = re.compile(r"^[a-z][a-z0-9_-]*:[A-Za-z0-9._:-]+$")
@@ -72,7 +92,7 @@ def freeze_json(value):
     if isinstance(value, Mapping):
         return FrozenDict({str(key): freeze_json(child) for key, child in value.items()})
     if isinstance(value, Sequence) and not isinstance(value, (str, bytes, bytearray)):
-        return tuple(freeze_json(child) for child in value)
+        return FrozenList(freeze_json(child) for child in value)
     return value