diff --git a/src/ca2a_runtime/response.py b/src/ca2a_runtime/response.py
index 0d36ee8d..660e916e 100644
--- a/src/ca2a_runtime/response.py
+++ b/src/ca2a_runtime/response.py
@@ -32,8 +32,14 @@ class AuthenticatedPeerError(CA2AError):
     """An authenticated peer denial, not proof that no side effect occurred."""
 
     def __init__(self, response: VerifiedResponse) -> None:
-        error = response.body["error"]
-        super().__init__(str(error.get("message", "peer denial")))
+        error = response.body.get("error")
+        if not isinstance(error, dict):
+            raise _fail()
+        detail = error.get("detail")
+        super().__init__(
+            str(error.get("message", "peer denial")),
+            detail=detail if isinstance(detail, str) else None,
+        )
         self.code = str(error.get("code", "PEER_DENIAL"))
         self.http_status = response.status
         self.response_authentication = response.authentication()
@@ -81,10 +87,14 @@ def check(item: Any, depth: int = 0) -> None:
 
 def decode(raw: bytes) -> dict[str, Any]:
     """Reject ambiguous/oversized JSON before it can be authenticated."""
-    if len(raw) > MAX_BYTES:
+    if type(raw) is not bytes or len(raw) > MAX_BYTES:
         raise _fail()
     try:
-        value = json.loads(raw.decode("utf-8"), object_pairs_hook=_object)
+        value = json.loads(
+            raw.decode("utf-8"),
+            object_pairs_hook=_object,
+            parse_constant=lambda _token: (_ for _ in ()).throw(_fail()),
+        )
         if not isinstance(value, dict):
             raise _fail()
         encode(value)
@@ -158,6 +168,14 @@ def __init__(self, private: X25519PrivateKey, envelope: dict[str, Any]) -> None:
         self._key = _key(private, self.envelope["recipient"], self.request_sha256)
 
     def finish(self, status: int, body: dict[str, Any]) -> dict[str, Any]:
+        if (
+            type(status) is not int
+            or not (status == 200 or 400 <= status <= 599)
+            or not isinstance(body, dict)
+            or (status == 200 and body.get("accepted") is not True)
+            or (status != 200 and not isinstance(body.get("error"), dict))
+        ):
+            raise _fail()
         statement = {
             "profile": PROFILE,
             "request_sha256": self.request_sha256,
@@ -186,7 +204,12 @@ def __init__(
         timeout: float = 30.0,
         clock: Callable[[], float] = time.monotonic,
     ) -> None:
-        if not math.isfinite(timeout) or not 0 < timeout <= 300:
+        if (
+            isinstance(timeout, bool)
+            or not isinstance(timeout, (int, float))
+            or not math.isfinite(timeout)
+            or not 0 < timeout <= 300
+        ):
             raise ValueError("response timeout must be finite and in (0, 300]")
         private = X25519PrivateKey.generate()
         self._request = encode(
diff --git a/tests/unit/test_response_authentication.py b/tests/unit/test_response_authentication.py
index 5170a827..91685c9e 100644
--- a/tests/unit/test_response_authentication.py
+++ b/tests/unit/test_response_authentication.py
@@ -121,7 +121,7 @@ def test_deadline_clock_rollback_and_invalid_clock_fail_closed(now):
         pending.verify(200, encode(success(producer)))
 
 
-@pytest.mark.parametrize("timeout", [0, -1, 301, float("nan"), float("inf")])
+@pytest.mark.parametrize("timeout", [0, -1, 301, float("nan"), float("inf"), True, False, "30"])
 def test_invalid_timeout(timeout):
     private = X25519PrivateKey.generate()
     with pytest.raises(ValueError):
@@ -132,6 +132,34 @@ def test_invalid_timeout(timeout):
         )
 
 
+@pytest.mark.parametrize(
+    ("status", "body"),
+    [
+        (True, {"accepted": True}),
+        (201, {"accepted": True}),
+        (302, {"error": {"code": "X"}}),
+        (200, {"accepted": False}),
+        (200, {}),
+        (403, {"error": "not-a-dict"}),
+        (403, {}),
+    ],
+)
+def test_response_producer_finish_rejects_invalid_status_or_body(status, body):
+    _, _, producer = exchange()
+    with pytest.raises(ResponseAuthenticationFailed):
+        producer.finish(status, body)
+
+
+def test_decode_rejects_non_bytes_and_authenticated_peer_error_preserves_detail():
+    with pytest.raises(ResponseAuthenticationFailed):
+        decode("{}")  # type: ignore[arg-type]
+    _, pending, producer = exchange()
+    body = {"error": {"code": "DENIED", "message": "denied", "detail": "missing cap:write"}}
+    response = pending.verify(403, encode(producer.finish(403, body)))
+    err = AuthenticatedPeerError(response)
+    assert err.detail == "missing cap:write"
+
+
 def test_concurrent_consumption_has_exactly_one_success():
     _, pending, producer = exchange()
     raw = encode(success(producer))