fix(a2a): keep the missing a2a-sdk error reachable

a2a-sdk is optional and litellm[proxy] does not install it. The card
resolver fell back to None and then subclassed that placeholder at
module scope, so importing the package raised TypeError before anything
could read A2A_SDK_AVAILABLE

Agent calls therefore reported "NoneType takes no arguments" instead of
the message naming the package to install, which has been unreachable
since the resolver was introduced

Fall back to a subclassable placeholder so the availability check runs

Signed-off-by: Aryan Gorde <aryangorde8@gmail.com>

litellm/a2a_protocol/card_resolver.py

@@ -13,7 +13,7 @@ if TYPE_CHECKING:
     from a2a.types import AgentCard
 
 # Runtime imports with availability check
-_A2ACardResolver: Any = None
+_A2ACardResolver: Any = object
 AGENT_CARD_WELL_KNOWN_PATH: str = "/.well-known/agent-card.json"
 PREV_AGENT_CARD_WELL_KNOWN_PATH: str = "/.well-known/agent.json"
 

tests/test_litellm/a2a_protocol/test_card_resolver.py

@@ -4,6 +4,10 @@ Mock tests for LiteLLMA2ACardResolver.
 Tests that the card resolver tries both old and new well-known paths.
 """
 
+import os
+import subprocess
+import sys
+import textwrap
 from types import SimpleNamespace
 from unittest.mock import MagicMock, patch
 
@@ -17,6 +21,45 @@ from litellm.a2a_protocol.card_resolver import (
 )
 
 
+def test_a2a_protocol_imports_when_a2a_sdk_is_missing():
+    """
+    a2a-sdk is an optional dependency, so the proxy degrades by reading A2A_SDK_AVAILABLE and
+    returning a JSON-RPC "'a2a' package not installed" error. Reading that flag imports this
+    module, so importing it with the SDK absent must not raise.
+
+    Runs in a subprocess because the check is about import time, and a2a-sdk is installed in CI.
+    """
+    script = textwrap.dedent(
+        """
+        import sys
+
+        class _BlockA2A:
+            def find_spec(self, name, path=None, target=None):
+                if name == "a2a" or name.startswith("a2a."):
+                    raise ModuleNotFoundError(f"No module named '{name}'")
+                return None
+
+        sys.meta_path.insert(0, _BlockA2A())
+
+        from litellm.a2a_protocol import asend_message_streaming
+        from litellm.a2a_protocol.main import A2A_SDK_AVAILABLE
+
+        print(f"A2A_SDK_AVAILABLE={A2A_SDK_AVAILABLE}")
+        """
+    )
+
+    result = subprocess.run(
+        [sys.executable, "-c", script],
+        capture_output=True,
+        text=True,
+        timeout=300,
+        env={**os.environ, "LITELLM_LOCAL_MODEL_COST_MAP": "True"},
+    )
+
+    assert result.returncode == 0, f"importing litellm.a2a_protocol without a2a-sdk failed:\n{result.stderr}"
+    assert "A2A_SDK_AVAILABLE=False" in result.stdout
+
+
 @pytest.mark.asyncio
 async def test_card_resolver_fallback_from_new_to_old_path():
     """