fix(a2a): forward signature_verifier and type the resolver fallback

Annotating the absent-SDK fallback as Any meant type checkers could not
validate the subclass at all, which is what the previous commit's fallback
relied on. Declare the base under TYPE_CHECKING and fall back to object only
at runtime, matching the idiom already used in types/utils.py and
llms/bedrock.py

With the real base class visible, basedpyright immediately caught that
get_agent_card drops the SDK's signature_verifier parameter, so any caller
passing it hit a TypeError. Forward it to both super() calls

basedpyright on this file goes from 25 errors to 14

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

litellm/a2a_protocol/card_resolver.py

@@ -4,21 +4,26 @@ Custom A2A Card Resolver for LiteLLM.
 Extends the A2A SDK's card resolver to support multiple well-known paths.
 """
 
+from collections.abc import Callable
 from typing import TYPE_CHECKING, Any, Final
 
 from litellm._logging import verbose_logger
 from litellm.constants import LOCALHOST_URL_PATTERNS
 
 if TYPE_CHECKING:
+    from a2a.client import A2ACardResolver as _A2ACardResolver
     from a2a.types import AgentCard
+else:
+    try:
+        from a2a.client import A2ACardResolver as _A2ACardResolver
+    except ImportError:
+        _A2ACardResolver = object
 
 # Runtime imports with availability check
-_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"
 
 try:
-    from a2a.client import A2ACardResolver as _A2ACardResolver
     from a2a.utils.constants import (
         AGENT_CARD_WELL_KNOWN_PATH,
         PREV_AGENT_CARD_WELL_KNOWN_PATH,
@@ -115,6 +120,7 @@ class LiteLLMA2ACardResolver(_A2ACardResolver):
         self,
         relative_card_path: str | None = None,
         http_kwargs: dict[str, Any] | None = None,
+        signature_verifier: Callable[["AgentCard"], None] | None = None,
     ) -> "AgentCard":
         """
         Fetch the agent card, trying multiple well-known paths.
@@ -125,6 +131,8 @@ class LiteLLMA2ACardResolver(_A2ACardResolver):
             relative_card_path: Optional path to the agent card endpoint.
                 If None, tries both well-known paths.
             http_kwargs: Optional dictionary of keyword arguments to pass to httpx.get
+            signature_verifier: Optional callable forwarded to the SDK to verify
+                the resolved card's signature.
 
         Returns:
             AgentCard from the A2A agent
@@ -137,6 +145,7 @@ class LiteLLMA2ACardResolver(_A2ACardResolver):
             return await super().get_agent_card(
                 relative_card_path=relative_card_path,
                 http_kwargs=http_kwargs,
+                signature_verifier=signature_verifier,
             )
 
         # Try both well-known paths
@@ -152,6 +161,7 @@ class LiteLLMA2ACardResolver(_A2ACardResolver):
                 return await super().get_agent_card(
                     relative_card_path=path,
                     http_kwargs=http_kwargs,
+                    signature_verifier=signature_verifier,
                 )
             except Exception as e:
                 verbose_logger.debug("Failed to fetch agent card from %s%s: %s", self.base_url, path, e)

tests/test_litellm/a2a_protocol/test_card_resolver.py

@@ -76,7 +76,7 @@ async def test_card_resolver_fallback_from_new_to_old_path():
 
     # Create a mock for the parent's get_agent_card method
     async def mock_parent_get_agent_card(
-        self, relative_card_path=None, http_kwargs=None
+        self, relative_card_path=None, http_kwargs=None, signature_verifier=None
     ):
         paths_called.append(relative_card_path)
         if relative_card_path == "/.well-known/agent-card.json":
@@ -111,6 +111,39 @@ async def test_card_resolver_fallback_from_new_to_old_path():
         assert result.name == "Test Agent"
 
 
+@pytest.mark.asyncio
+async def test_get_agent_card_forwards_signature_verifier():
+    """
+    The SDK's get_agent_card accepts a signature_verifier. Dropping it from this override
+    made the call raise TypeError for any caller passing it, so it must be forwarded.
+    """
+    received = {}
+
+    async def mock_parent_get_agent_card(
+        self, relative_card_path=None, http_kwargs=None, signature_verifier=None
+    ):
+        received["signature_verifier"] = signature_verifier
+        return MagicMock()
+
+    def verifier(card):
+        return None
+
+    with patch.object(
+        LiteLLMA2ACardResolver.__bases__[0],
+        "get_agent_card",
+        mock_parent_get_agent_card,
+    ):
+        resolver = LiteLLMA2ACardResolver(
+            httpx_client=MagicMock(), base_url="http://test-agent:8000"
+        )
+        await resolver.get_agent_card(
+            relative_card_path="/.well-known/agent-card.json",
+            signature_verifier=verifier,
+        )
+
+    assert received["signature_verifier"] is verifier
+
+
 def test_is_localhost_or_internal_url():
     """Test that localhost/internal URLs are correctly detected."""
     # Should return True for localhost variants