diff --git a/tests/test_execution_history_events.py b/tests/test_execution_history_events.py
index 0aedd71..76a1474 100644
--- a/tests/test_execution_history_events.py
+++ b/tests/test_execution_history_events.py
@@ -344,3 +344,36 @@ class ExecutionHistoryEventsTests(unittest.TestCase):
             classify_evidence(JOB, command.evidence).reason, "evidence_limit"
         )
         self.assertLessEqual(len(encode_command(command)), 2048)
+
+    def test_oversized_unknown_kind_is_ignored_without_hashing(self):
+        class OversizedKind(str):
+            def __hash__(self):
+                raise AssertionError("Oversized event kind must not be hashed")
+
+        command = finish(
+            [
+                message(OversizedKind("x" * 1_000_000)),
+                message("execution_success", timestamp=300),
+            ]
+        )
+        result = classify_evidence(JOB, command.evidence)
+        self.assertEqual(
+            (result.outcome, result.reason), ("succeeded", "terminal_success")
+        )
+        self.assertLessEqual(len(encode_command(command)), 2048)
+
+    def test_encoder_rejects_oversized_kind_without_hashing(self):
+        class OversizedKind(str):
+            def __hash__(self):
+                raise AssertionError("Oversized event kind must not be hashed")
+
+        command = finish([message("execution_success")])
+        event = LifecycleEvent(
+            OversizedKind("x" * 1_000_000), JOB, None, None, None, None
+        )
+        with self.assertRaises(ValueError):
+            encode_command(
+                replace(
+                    command, evidence=TerminalEvidence((event,), False, False, False)
+                )
+            )
diff --git a/utils/execution_history_events.py b/utils/execution_history_events.py
index 5be4f6b..91eb4c1 100644
--- a/utils/execution_history_events.py
+++ b/utils/execution_history_events.py
@@ -13,6 +13,7 @@ TERMINAL_KINDS = {
     "execution_interrupted": ("interrupted", "terminal_interrupted"),
 }
 EVENT_KINDS = frozenset((*TERMINAL_KINDS, "execution_start"))
+MAX_EVENT_KIND_LENGTH = max(map(len, EVENT_KINDS))
 
 
 @dataclass(frozen=True)
@@ -68,6 +69,14 @@ class Completion:
 RunCommand = RunStart | RunFinish
 
 
+def _valid_event_kind(value: object) -> bool:
+    return (
+        isinstance(value, str)
+        and len(value) <= MAX_EVENT_KIND_LENGTH
+        and value in EVENT_KINDS
+    )
+
+
 def _valid_integer(value: object) -> bool:
     return type(value) is int and 0 <= value <= MAX_INTEGER
 
@@ -159,7 +168,7 @@ def make_finish(
             if not malformed:
                 kind, data = message
                 malformed = not isinstance(kind, str)
-                if not malformed and kind not in EVENT_KINDS:
+                if not malformed and not _valid_event_kind(kind):
                     continue
                 malformed |= usable_job_id is None
             if not malformed:
@@ -302,8 +311,7 @@ def _valid_evidence(evidence: object) -> bool:
         if type(event) is not LifecycleEvent:
             return False
         if (
-            not isinstance(event.kind, str)
-            or event.kind not in EVENT_KINDS
+            not _valid_event_kind(event.kind)
             or not _valid_uuid(event.job_id)
             or (event.timestamp is not None and not _valid_integer(event.timestamp))
         ):