diff --git a/cirq-core/cirq/contrib/bayesian_network/bayesian_network_gate.py b/cirq-core/cirq/contrib/bayesian_network/bayesian_network_gate.py
index 66a1f265fad..fe54988de4e 100644
--- a/cirq-core/cirq/contrib/bayesian_network/bayesian_network_gate.py
+++ b/cirq-core/cirq/contrib/bayesian_network/bayesian_network_gate.py
@@ -180,11 +180,7 @@ def _value_equality_values_(self):
         return self._init_probs, self._arc_probs
 
     def _json_dict_(self) -> Dict[str, Any]:
-        return {
-            'cirq_type': self.__class__.__name__,
-            'init_probs': self._init_probs,
-            'arc_probs': self._arc_probs,
-        }
+        return {'init_probs': self._init_probs, 'arc_probs': self._arc_probs}
 
     @classmethod
     def _from_json_dict_(
diff --git a/cirq-core/cirq/protocols/json_serialization.py b/cirq-core/cirq/protocols/json_serialization.py
index bf411b17785..1d093b28741 100644
--- a/cirq-core/cirq/protocols/json_serialization.py
+++ b/cirq-core/cirq/protocols/json_serialization.py
@@ -17,7 +17,6 @@
 import json
 import numbers
 import pathlib
-import warnings
 from typing import (
     Any,
     Callable,
@@ -188,8 +187,7 @@ def dataclass_json_dict(obj: Any) -> Dict[str, Any]:
 def _json_dict_with_cirq_type(obj: Any):
     base_dict = obj._json_dict_()
     if 'cirq_type' in base_dict:
-        # TODO: upgrade to ValueError in v0.15
-        warnings.warn(
+        raise ValueError(
             f"Found 'cirq_type': '{base_dict['cirq_type']}' in user-specified _json_dict_. "
             "'cirq_type' is now automatically generated from the class's name and its "
             "_json_namespace_ method as `cirq_type: '[<namespace>.]<class_name>'`."
@@ -201,10 +199,8 @@ def _json_dict_with_cirq_type(obj: Any):
             "For backwards compatibility, third-party classes whose old 'cirq_type' value "
             "does not match the new value must appear under BOTH values in the resolver "
             "for that package. For details on defining custom resolvers, see the "
-            "DEFAULT_RESOLVER docstring in cirq-core/cirq/protocols/json_serialization.py.",
-            DeprecationWarning,
+            "DEFAULT_RESOLVER docstring in cirq-core/cirq/protocols/json_serialization.py."
         )
-        return base_dict
     return {'cirq_type': json_cirq_type(type(obj)), **base_dict}
 
 
diff --git a/cirq-core/cirq/protocols/json_serialization_test.py b/cirq-core/cirq/protocols/json_serialization_test.py
index fab73078f48..47b07fce7ef 100644
--- a/cirq-core/cirq/protocols/json_serialization_test.py
+++ b/cirq-core/cirq/protocols/json_serialization_test.py
@@ -99,7 +99,7 @@ def custom_resolver(name):
             return HasOldJsonDict
 
     test_resolvers = [custom_resolver] + cirq.DEFAULT_RESOLVERS
-    with cirq.testing.assert_deprecated("Found 'cirq_type'", deadline='v0.15'):
+    with pytest.raises(ValueError, match="Found 'cirq_type'"):
         assert_json_roundtrip_works(HasOldJsonDict(), resolvers=test_resolvers)