diff --git a/agent/relay_llm.py b/agent/relay_llm.py
index c6c6affa78c..85286c47dae 100644
--- a/agent/relay_llm.py
+++ b/agent/relay_llm.py
@@ -706,23 +706,7 @@ class AnthropicStreamAccumulator:
     def response(self, base: Any = None) -> Any:
         """Return the attribute-shaped response consumed by Hermes."""
         assembled = self.finalize()
-        if base is not None and base.__class__.__module__ == "unittest.mock":
-            base_payload = {}
-            for key in (
-                "id",
-                "type",
-                "role",
-                "model",
-                "content",
-                "stop_reason",
-                "stop_sequence",
-                "usage",
-            ):
-                value = getattr(base, key, None)
-                if value is not None and value.__class__.__module__ != "unittest.mock":
-                    base_payload[key] = _jsonable(value)
-        else:
-            base_payload = _jsonable(base)
+        base_payload = _jsonable(base)
         if not isinstance(base_payload, dict):
             base_payload = {}
         content = assembled.pop("content", [])
@@ -1072,25 +1056,25 @@ def _codec(relay: Any, metadata: dict[str, Any] | None) -> Any:
 def _jsonable(value: Any) -> Any:
     if value is None or isinstance(value, (str, int, float, bool)):
         return value
-    # Test doubles synthesize arbitrary callable attributes such as
-    # ``model_dump``. Treat them as opaque instead of recursively invoking an
-    # endless chain of child mocks.
-    if value.__class__.__module__ == "unittest.mock":
-        return str(value)
     if isinstance(value, dict):
         return {str(key): _jsonable(item) for key, item in value.items()}
     if isinstance(value, (list, tuple, set)):
         return [_jsonable(item) for item in value]
-    model_dump = getattr(value, "model_dump", None)
+    model_dump = getattr(type(value), "model_dump", None)
     if callable(model_dump):
         try:
-            return _jsonable(model_dump(mode="json"))
+            return _jsonable(value.model_dump(mode="json"))
         except Exception:
             pass
     try:
-        return _jsonable(vars(value))
+        attributes = {
+            str(key): item
+            for key, item in vars(value).items()
+            if not str(key).startswith("_")
+        }
     except (TypeError, AttributeError):
         return str(value)
+    return _jsonable(attributes) if attributes else str(value)
 
 
 def _namespace(value: Any) -> Any:
diff --git a/tests/agent/test_relay_llm.py b/tests/agent/test_relay_llm.py
index 32e4788a974..0266d21bd2f 100644
--- a/tests/agent/test_relay_llm.py
+++ b/tests/agent/test_relay_llm.py
@@ -281,6 +281,52 @@ def test_anthropic_stream_accumulator_merges_terminal_usage():
     }
 
 
+def test_anthropic_stream_accumulator_merges_plain_provider_object():
+    accumulator = relay_llm.AnthropicStreamAccumulator()
+    accumulator.observe({
+        "type": "message_start",
+        "message": {
+            "id": "message-1",
+            "type": "message",
+            "role": "assistant",
+            "model": "claude-test",
+            "usage": {"input_tokens": 10},
+        },
+    })
+    accumulator.observe({
+        "type": "content_block_start",
+        "index": 0,
+        "content_block": {"type": "text", "text": "hello"},
+    })
+
+    response = accumulator.response(
+        SimpleNamespace(
+            id="message-1",
+            type="message",
+            role="assistant",
+            model="claude-test",
+            content=[],
+            stop_reason=None,
+            usage={"input_tokens": 10},
+        )
+    )
+
+    assert response.id == "message-1"
+    assert response.content[0].text == "hello"
+    assert response.usage.input_tokens == 10
+
+
+def test_jsonable_does_not_probe_dynamic_attributes():
+    class DynamicProviderObject:
+        def __getattr__(self, name):
+            raise AssertionError(f"unexpected dynamic attribute lookup: {name}")
+
+        def __str__(self):
+            return "opaque-provider-object"
+
+    assert relay_llm._jsonable(DynamicProviderObject()) == "opaque-provider-object"
+
+
 def test_non_stream_preserves_raw_provider_response_identity(relay_turn):
     _relay, _turn = relay_turn
     raw_response = SimpleNamespace(model="test-model", content="raw")