diff --git a/agent_assembly/core/runtime_interceptor.py b/agent_assembly/core/runtime_interceptor.py
index 4eb7ded..819d3de 100644
--- a/agent_assembly/core/runtime_interceptor.py
+++ b/agent_assembly/core/runtime_interceptor.py
@@ -55,10 +55,11 @@
 
 # Native decisions that authoritatively permit the tool to proceed. ``deny`` and
 # ``pending`` are handled explicitly; only these are treated as an allow. Anything
-# else — an unknown string, an empty value, or a missing ``decision`` key — is not
-# an authoritative allow and must fail closed under ``enforce`` (AAASM-4014); the
-# runtime remains the authority on redaction, so ``redact`` proceeds here.
-_ALLOW_DECISIONS = frozenset({"allow", "redact", "unspecified"})
+# else — the proto3 zero value ``unspecified`` ("no decision rendered"), an unknown
+# string, an empty value, or a missing ``decision`` key — is not an authoritative
+# allow and must fail closed under ``enforce`` (AAASM-4014, AAASM-4166), matching the
+# Node SDK; the runtime remains the authority on redaction, so ``redact`` proceeds.
+_ALLOW_DECISIONS = frozenset({"allow", "redact"})
 
 
 def _local_posture_is_enforce(enforcement_mode: str | None) -> bool:
@@ -247,11 +248,13 @@ def check_tool_start(
         * ``"deny"`` → ``{"status": "deny", "reason": ...}``.
         * ``"pending"`` → ``{"status": "pending", "reason": ...}`` so the
           adapter's existing approval path runs.
-        * ``"allow"`` / ``"redact"`` / ``"unspecified"`` → ``{"status": "allow"}``.
-          The runtime redacts authoritatively; this layer never redacts.
-        * A raising ``query_policy`` or an error-sentinel ``decision``
-          (``query_failed`` / ``channel_closed`` / ``shutdown``) → ``deny`` under
-          ``enforce`` (fail closed, AAASM-3106), else ``allow`` (fail open).
+        * ``"allow"`` / ``"redact"`` → ``{"status": "allow"}``. The runtime
+          redacts authoritatively; this layer never redacts.
+        * A raising ``query_policy``, the proto3 zero value ``"unspecified"``
+          ("no decision rendered"), an error-sentinel ``decision``
+          (``query_failed`` / ``channel_closed`` / ``shutdown``), or any
+          unrecognized decision → ``deny`` under ``enforce`` (fail closed,
+          AAASM-3106 / AAASM-4166, matching Node), else ``allow`` (fail open).
 
         Before any of the above, the live op-control kill switch (AAASM-3491) is
         consulted when an ``op_id`` is supplied and a subscriber is wired: a
diff --git a/test/unit/core/test_runtime_interceptor.py b/test/unit/core/test_runtime_interceptor.py
index d2c7603..77eff0e 100644
--- a/test/unit/core/test_runtime_interceptor.py
+++ b/test/unit/core/test_runtime_interceptor.py
@@ -615,10 +615,11 @@ def query_policy(self, *_args: Any, **_kwargs: Any) -> dict[str, str]:
     assert result["status"] == "deny"
 
 
-@pytest.mark.parametrize("decision", ["allow", "redact", "unspecified"])
+@pytest.mark.parametrize("decision", ["allow", "redact"])
 def test_known_good_decisions_allow_under_enforce(decision: str) -> None:
     """Authoritative allow verdicts still proceed under enforce; the runtime
-    remains the authority on redaction, so ``redact`` proceeds here."""
+    remains the authority on redaction, so ``redact`` proceeds here. ``unspecified``
+    is no longer among them — it fails closed (AAASM-4166)."""
     interceptor = RuntimeQueryInterceptor(_FakeGatewayClient(), _FakeRuntimeClient(decision), "agent-001", enforce=True)
 
     result = interceptor.check_tool_start(serialized={"name": "t"}, input_str="i")
@@ -626,6 +627,31 @@ def test_known_good_decisions_allow_under_enforce(decision: str) -> None:
     assert result == {"status": "allow"}
 
 
+def test_unspecified_decision_denies_under_enforce() -> None:
+    """AAASM-4166: the proto3 zero value ``unspecified`` ("no decision rendered")
+    is not an authoritative allow — it must fail closed under enforce (deny),
+    matching the Node SDK, rather than being folded onto allow as before."""
+    interceptor = RuntimeQueryInterceptor(
+        _FakeGatewayClient(), _FakeRuntimeClient("unspecified"), "agent-001", enforce=True
+    )
+
+    result = interceptor.check_tool_start(serialized={"name": "t"}, input_str="i")
+
+    assert result["status"] == "deny"
+
+
+def test_unspecified_decision_allows_under_observe() -> None:
+    """Under observe the ``unspecified`` verdict still proceeds (fail open), like
+    any other non-authoritative decision."""
+    interceptor = RuntimeQueryInterceptor(
+        _FakeGatewayClient(), _FakeRuntimeClient("unspecified"), "agent-001", enforce=False
+    )
+
+    result = interceptor.check_tool_start(serialized={"name": "t"}, input_str="i")
+
+    assert result == {"status": "allow"}
+
+
 def test_unknown_decision_allows_under_observe() -> None:
     """Under observe the unknown-decision path still proceeds (fail open)."""
     interceptor = RuntimeQueryInterceptor(