fix(a2a): don't claim sampling params the protocol has no field for

get_supported_openai_params listed temperature, max_tokens and top_p.
map_openai_params only ever reads stream, and transform_request builds a
JSON-RPC body of just {message}, so the three values went nowhere.

A2A has no sampling controls by design: SendMessageConfiguration is
acceptedOutputModes, taskPushNotificationConfig, historyLength and
returnImmediately, because the remote agent owns its own model config.
Advertising them suppressed the UnsupportedParamsError and dropped them
in silence.

stream stays, it selects message/stream over message/send.

litellm/llms/a2a/chat/transformation.py

@@ -91,12 +91,16 @@ class A2AConfig(BaseConfig):
         return api_base, api_key, headers
 
     def get_supported_openai_params(self, model: str) -> list[str]:
-        """Return list of supported OpenAI parameters"""
+        """Return list of supported OpenAI parameters.
+
+        A2A has no sampling controls. SendMessageConfiguration carries
+        acceptedOutputModes, taskPushNotificationConfig, historyLength and
+        returnImmediately, nothing else, because the remote agent owns its own
+        model config. Only stream is real here: it picks message/stream over
+        message/send.
+        """
         return [
             "stream",
-            "temperature",
-            "max_tokens",
-            "top_p",
         ]
 
     def map_openai_params(

tests/llm_translation/test_a2a.py

@@ -130,3 +130,38 @@ def test_a2a_completion_sync_streaming():
         pytest.skip(f"A2A agent not reachable at {api_base}: {e}")
     except Exception as e:
         pytest.fail(f"Error occurred: {e}")
+
+
+def test_a2a_only_claims_stream():
+    """A2A has no sampling params, so it must not advertise any.
+
+    Claiming one suppresses the UnsupportedParamsError and the value is
+    dropped without telling the caller.
+    """
+    from litellm.llms.a2a.chat.transformation import A2AConfig
+
+    supported = A2AConfig().get_supported_openai_params("test-agent")
+    assert "stream" in supported
+    for param in ("temperature", "max_tokens", "top_p"):
+        assert param not in supported
+
+
+@pytest.mark.parametrize("param,value", [("temperature", 0.5), ("max_tokens", 100), ("top_p", 0.9)])
+def test_a2a_sampling_params_raise(param, value):
+    """Sampling params should surface an error rather than vanish."""
+    from litellm.utils import get_optional_params
+
+    with pytest.raises(litellm.UnsupportedParamsError):
+        get_optional_params(
+            model="test-agent", custom_llm_provider="a2a", **{param: value}
+        )
+
+
+def test_a2a_stream_still_maps():
+    """stream is the one real param, it must survive."""
+    from litellm.utils import get_optional_params
+
+    params = get_optional_params(
+        model="test-agent", custom_llm_provider="a2a", stream=True
+    )
+    assert params["stream"] is True