refactor(client): move agent card signature verification to BaseClient (#793)

This PR refactors the signature verification logic for Extended Agent
Cards to be handled at the client level (BaseClient) rather than being
duplicated across each individual transport implementation
(GrpcTransport, JsonRpcTransport, RestTransport).

Changes:

- BaseClient: Added signature verification step and state mutation logic
(self._card = card) inside get_extended_agent_card().
- Transports (gRPC, JSON-RPC, REST, TenantDecorator): Removed the
signature_verifier parameter from get_extended_agent_card().
- Removed internal caching mechanism and state (self.agent_card,
self._needs_extended_card) from the get_extended_agent_card() flow.

Author: guglielmo-san

src/a2a/client/base_client.py

@@ -303,8 +303,10 @@ async def get_extended_agent_card(
         card = await self._transport.get_extended_agent_card(
             request,
             context=context,
-            signature_verifier=signature_verifier,
         )
+        if signature_verifier:
+            signature_verifier(card)
+
         self._card = card
         return card
 

src/a2a/client/transports/base.py

@@ -1,5 +1,5 @@
 from abc import ABC, abstractmethod
-from collections.abc import AsyncGenerator, Callable
+from collections.abc import AsyncGenerator
 from types import TracebackType
 
 from typing_extensions import Self
@@ -141,7 +141,6 @@ async def get_extended_agent_card(
         request: GetExtendedAgentCardRequest,
         *,
         context: ClientCallContext | None = None,
-        signature_verifier: Callable[[AgentCard], None] | None = None,
     ) -> AgentCard:
         """Retrieves the Extended AgentCard."""
 

src/a2a/client/transports/grpc.py

@@ -105,9 +105,6 @@ def __init__(
         self.agent_card = agent_card
         self.channel = channel
         self.stub = a2a_pb2_grpc.A2AServiceStub(channel)
-        self._needs_extended_card = (
-            agent_card.capabilities.extended_agent_card if agent_card else True
-        )
 
     @classmethod
     def create(
@@ -270,22 +267,18 @@ async def get_extended_agent_card(
         request: GetExtendedAgentCardRequest,
         *,
         context: ClientCallContext | None = None,
-        signature_verifier: Callable[[AgentCard], None] | None = None,
     ) -> AgentCard:
         """Retrieves the agent's card."""
-        card = await self._call_grpc(
+        card = self.agent_card
+        if card and not card.capabilities.extended_agent_card:
+            return card
+
+        return await self._call_grpc(
             self.stub.GetExtendedAgentCard,
             request,
             context,
         )
 
-        if signature_verifier:
-            signature_verifier(card)
-
-        self.agent_card = card
-        self._needs_extended_card = False
-        return card
-
     async def close(self) -> None:
         """Closes the gRPC channel."""
         await self.channel.close()

src/a2a/client/transports/jsonrpc.py

@@ -1,6 +1,6 @@
 import logging
 
-from collections.abc import AsyncGenerator, Callable
+from collections.abc import AsyncGenerator
 from typing import Any
 from uuid import uuid4
 
@@ -62,7 +62,6 @@ def __init__(
         self.httpx_client = httpx_client
         self.agent_card = agent_card
         self.interceptors = interceptors or []
-        self._needs_extended_card = agent_card.capabilities.extended_agent_card
 
     async def send_message(
         self,
@@ -284,11 +283,9 @@ async def get_extended_agent_card(
         request: GetExtendedAgentCardRequest,
         *,
         context: ClientCallContext | None = None,
-        signature_verifier: Callable[[AgentCard], None] | None = None,
     ) -> AgentCard:
         """Retrieves the agent's card."""
         card = self.agent_card
-
         if not card.capabilities.extended_agent_card:
             return card
 
@@ -312,11 +309,7 @@ async def get_extended_agent_card(
         response: AgentCard = json_format.ParseDict(
             json_rpc_response.result, AgentCard()
         )
-        if signature_verifier:
-            signature_verifier(response)
 
-        self.agent_card = response
-        self._needs_extended_card = False
         return response
 
     async def close(self) -> None:

src/a2a/client/transports/rest.py

@@ -1,7 +1,7 @@
 import json
 import logging
 
-from collections.abc import AsyncGenerator, Callable
+from collections.abc import AsyncGenerator
 from typing import Any, NoReturn
 
 import httpx
@@ -61,7 +61,6 @@ def __init__(
         self.httpx_client = httpx_client
         self.agent_card = agent_card
         self.interceptors = interceptors or []
-        self._needs_extended_card = agent_card.capabilities.extended_agent_card
 
     async def send_message(
         self,
@@ -265,26 +264,17 @@ async def get_extended_agent_card(
         request: GetExtendedAgentCardRequest,
         *,
         context: ClientCallContext | None = None,
-        signature_verifier: Callable[[AgentCard], None] | None = None,
     ) -> AgentCard:
         """Retrieves the Extended AgentCard."""
         card = self.agent_card
-
         if not card.capabilities.extended_agent_card:
             return card
 
         response_data = await self._execute_request(
             'GET', '/extendedAgentCard', request.tenant, context=context
         )
-        response: AgentCard = ParseDict(response_data, AgentCard())
-
-        if signature_verifier:
-            signature_verifier(response)
 
-        # Update the transport's agent_card
-        self.agent_card = response
-        self._needs_extended_card = False
-        return response
+        return ParseDict(response_data, AgentCard())
 
     async def close(self) -> None:
         """Closes the httpx client."""

src/a2a/client/transports/tenant_decorator.py

@@ -1,4 +1,4 @@
-from collections.abc import AsyncGenerator, Callable
+from collections.abc import AsyncGenerator
 
 from a2a.client.middleware import ClientCallContext
 from a2a.client.transports.base import ClientTransport
@@ -154,14 +154,12 @@ async def get_extended_agent_card(
         request: GetExtendedAgentCardRequest,
         *,
         context: ClientCallContext | None = None,
-        signature_verifier: Callable[[AgentCard], None] | None = None,
     ) -> AgentCard:
         """Retrieves the Extended AgentCard."""
         request.tenant = self._resolve_tenant(request.tenant)
         return await self._base.get_extended_agent_card(
             request,
             context=context,
-            signature_verifier=signature_verifier,
         )
 
     async def close(self) -> None:

src/a2a/compat/v0_3/grpc_transport.py

@@ -338,7 +338,6 @@ async def get_extended_agent_card(
         request: a2a_pb2.GetExtendedAgentCardRequest,
         *,
         context: ClientCallContext | None = None,
-        signature_verifier: Callable[[a2a_pb2.AgentCard], None] | None = None,
     ) -> a2a_pb2.AgentCard:
         """Retrieves the agent's card (v0.3)."""
         req_proto = a2a_v0_3_pb2.GetAgentCardRequest()
@@ -350,9 +349,6 @@ async def get_extended_agent_card(
             proto_utils.FromProto.agent_card(resp_proto)
         )
 
-        if signature_verifier:
-            signature_verifier(card)
-
         self.agent_card = card
         return card