feat: Allow agent cards (default and extended) to be dynamic (#365)

Adds support for dynamic agent card serving for default and
extended-authenticated cards. Also fix some build errors. Add tests.

---------

Co-authored-by: Holt Skinner <13262395+holtskinner@users.noreply.github.com>
Co-authored-by: Holt Skinner <holtskinner@google.com>
Co-authored-by: Krishna Thota <kcthota@google.com>

Author: 3 people

src/a2a/server/apps/jsonrpc/fastapi_app.py

@@ -7,11 +7,9 @@
 from fastapi import FastAPI
 
 from a2a.server.apps.jsonrpc.jsonrpc_app import (
-    CallContextBuilder,
     JSONRPCApplication,
 )
-from a2a.server.request_handlers.jsonrpc_handler import RequestHandler
-from a2a.types import A2ARequest, AgentCard
+from a2a.types import A2ARequest
 from a2a.utils.constants import (
     AGENT_CARD_WELL_KNOWN_PATH,
     DEFAULT_RPC_URL,
@@ -31,32 +29,6 @@ class A2AFastAPIApplication(JSONRPCApplication):
     (SSE).
     """
 
-    def __init__(
-        self,
-        agent_card: AgentCard,
-        http_handler: RequestHandler,
-        extended_agent_card: AgentCard | None = None,
-        context_builder: CallContextBuilder | None = None,
-    ) -> None:
-        """Initializes the A2AStarletteApplication.
-
-        Args:
-            agent_card: The AgentCard describing the agent's capabilities.
-            http_handler: The handler instance responsible for processing A2A
-              requests via http.
-            extended_agent_card: An optional, distinct AgentCard to be served
-              at the authenticated extended card endpoint.
-            context_builder: The CallContextBuilder used to construct the
-              ServerCallContext passed to the http_handler. If None, no
-              ServerCallContext is passed.
-        """
-        super().__init__(
-            agent_card=agent_card,
-            http_handler=http_handler,
-            extended_agent_card=extended_agent_card,
-            context_builder=context_builder,
-        )
-
     def add_routes_to_app(
         self,
         app: FastAPI,
@@ -90,13 +62,13 @@ def add_routes_to_app(
         )(self._handle_requests)
         app.get(agent_card_url)(self._handle_get_agent_card)
 
-        # add deprecated path only if the agent_card_url uses default well-known path
         if agent_card_url == AGENT_CARD_WELL_KNOWN_PATH:
-            app.get(PREV_AGENT_CARD_WELL_KNOWN_PATH, include_in_schema=False)(
-                self.handle_deprecated_agent_card_path
+            # For backward compatibility, serve the agent card at the deprecated path as well.
+            # TODO: remove in a future release
+            app.get(PREV_AGENT_CARD_WELL_KNOWN_PATH)(
+                self._handle_get_agent_card
             )
 
-        # TODO: deprecated endpoint to be removed in a future release
         if self.agent_card.supports_authenticated_extended_card:
             app.get(extended_agent_card_url)(
                 self._handle_get_authenticated_extended_agent_card

src/a2a/server/apps/jsonrpc/jsonrpc_app.py

@@ -4,7 +4,7 @@
 import traceback
 
 from abc import ABC, abstractmethod
-from collections.abc import AsyncGenerator
+from collections.abc import AsyncGenerator, Callable
 from typing import Any
 
 from fastapi import FastAPI
@@ -123,12 +123,17 @@ class JSONRPCApplication(ABC):
     (SSE).
     """
 
-    def __init__(
+    def __init__(  # noqa: PLR0913
         self,
         agent_card: AgentCard,
         http_handler: RequestHandler,
         extended_agent_card: AgentCard | None = None,
         context_builder: CallContextBuilder | None = None,
+        card_modifier: Callable[[AgentCard], AgentCard] | None = None,
+        extended_card_modifier: Callable[
+            [AgentCard, ServerCallContext], AgentCard
+        ]
+        | None = None,
     ) -> None:
         """Initializes the A2AStarletteApplication.
 
@@ -141,17 +146,26 @@ def __init__(
             context_builder: The CallContextBuilder used to construct the
               ServerCallContext passed to the http_handler. If None, no
               ServerCallContext is passed.
+            card_modifier: An optional callback to dynamically modify the public
+              agent card before it is served.
+            extended_card_modifier: An optional callback to dynamically modify
+              the extended agent card before it is served. It receives the
+              call context.
         """
         self.agent_card = agent_card
         self.extended_agent_card = extended_agent_card
+        self.card_modifier = card_modifier
+        self.extended_card_modifier = extended_card_modifier
         self.handler = JSONRPCHandler(
             agent_card=agent_card,
             request_handler=http_handler,
             extended_agent_card=extended_agent_card,
+            extended_card_modifier=extended_card_modifier,
         )
         if (
             self.agent_card.supports_authenticated_extended_card
             and self.extended_agent_card is None
+            and self.extended_card_modifier is None
         ):
             logger.error(
                 'AgentCard.supports_authenticated_extended_card is True, but no extended_agent_card was provided. The /agent/authenticatedExtendedCard endpoint will return 404.'
@@ -448,24 +462,23 @@ async def _handle_get_agent_card(self, request: Request) -> JSONResponse:
         Returns:
             A JSONResponse containing the agent card data.
         """
-        # The public agent card is a direct serialization of the agent_card
-        # provided at initialization.
+        if request.url.path == PREV_AGENT_CARD_WELL_KNOWN_PATH:
+            logger.warning(
+                f"Deprecated agent card endpoint '{PREV_AGENT_CARD_WELL_KNOWN_PATH}' accessed. "
+                f"Please use '{AGENT_CARD_WELL_KNOWN_PATH}' instead. This endpoint will be removed in a future version."
+            )
+
+        card_to_serve = self.agent_card
+        if self.card_modifier:
+            card_to_serve = self.card_modifier(card_to_serve)
+
         return JSONResponse(
-            self.agent_card.model_dump(
+            card_to_serve.model_dump(
                 exclude_none=True,
                 by_alias=True,
             )
         )
 
-    async def handle_deprecated_agent_card_path(
-        self, request: Request
-    ) -> JSONResponse:
-        """Handles GET requests for the deprecated agent card endpoint."""
-        logger.warning(
-            f"Deprecated agent card endpoint '{PREV_AGENT_CARD_WELL_KNOWN_PATH}' accessed. Please use '{AGENT_CARD_WELL_KNOWN_PATH}' instead. This endpoint will be removed in a future version."
-        )
-        return await self._handle_get_agent_card(request)
-
     async def _handle_get_authenticated_extended_agent_card(
         self, request: Request
     ) -> JSONResponse:
@@ -480,17 +493,24 @@ async def _handle_get_authenticated_extended_agent_card(
                 status_code=404,
             )
 
-        # If an explicit extended_agent_card is provided, serve that.
-        if self.extended_agent_card:
+        card_to_serve = self.extended_agent_card
+
+        if self.extended_card_modifier:
+            context = self._context_builder.build(request)
+            # If no base extended card is provided, pass the public card to the modifier
+            base_card = card_to_serve if card_to_serve else self.agent_card
+            card_to_serve = self.extended_card_modifier(base_card, context)
+
+        if card_to_serve:
             return JSONResponse(
-                self.extended_agent_card.model_dump(
+                card_to_serve.model_dump(
                     exclude_none=True,
                     by_alias=True,
                 )
             )
-        # If supports_authenticated_extended_card is true, but no specific
-        # extended_agent_card was provided during server initialization,
-        # return a 404
+        # If supports_authenticated_extended_card is true, but no
+        # extended_agent_card was provided, and no modifier produced a card,
+        # return a 404.
         return JSONResponse(
             {
                 'error': 'Authenticated extended agent card is supported but not configured on the server.'

src/a2a/server/apps/jsonrpc/starlette_app.py

@@ -6,11 +6,8 @@
 from starlette.routing import Route
 
 from a2a.server.apps.jsonrpc.jsonrpc_app import (
-    CallContextBuilder,
     JSONRPCApplication,
 )
-from a2a.server.request_handlers.jsonrpc_handler import RequestHandler
-from a2a.types import AgentCard
 from a2a.utils.constants import (
     AGENT_CARD_WELL_KNOWN_PATH,
     DEFAULT_RPC_URL,
@@ -30,32 +27,6 @@ class A2AStarletteApplication(JSONRPCApplication):
     (SSE).
     """
 
-    def __init__(
-        self,
-        agent_card: AgentCard,
-        http_handler: RequestHandler,
-        extended_agent_card: AgentCard | None = None,
-        context_builder: CallContextBuilder | None = None,
-    ) -> None:
-        """Initializes the A2AStarletteApplication.
-
-        Args:
-            agent_card: The AgentCard describing the agent's capabilities.
-            http_handler: The handler instance responsible for processing A2A
-              requests via http.
-            extended_agent_card: An optional, distinct AgentCard to be served
-              at the authenticated extended card endpoint.
-            context_builder: The CallContextBuilder used to construct the
-              ServerCallContext passed to the http_handler. If None, no
-              ServerCallContext is passed.
-        """
-        super().__init__(
-            agent_card=agent_card,
-            http_handler=http_handler,
-            extended_agent_card=extended_agent_card,
-            context_builder=context_builder,
-        )
-
     def routes(
         self,
         agent_card_url: str = AGENT_CARD_WELL_KNOWN_PATH,
@@ -87,14 +58,15 @@ def routes(
             ),
         ]
 
-        # add deprecated path only if the agent_card_url uses default well-known path
         if agent_card_url == AGENT_CARD_WELL_KNOWN_PATH:
+            # For backward compatibility, serve the agent card at the deprecated path as well.
+            # TODO: remove in a future release
             app_routes.append(
                 Route(
                     PREV_AGENT_CARD_WELL_KNOWN_PATH,
-                    self.handle_deprecated_agent_card_path,
+                    self._handle_get_agent_card,
                     methods=['GET'],
-                    name='agent_card_path_deprecated',
+                    name='deprecated_agent_card',
                 )
             )
 

src/a2a/server/request_handlers/grpc_handler.py

@@ -18,6 +18,8 @@
         "'pip install a2a-sdk[grpc]'"
     ) from e
 
+from collections.abc import Callable
+
 import a2a.grpc.a2a_pb2_grpc as a2a_grpc
 
 from a2a import types
@@ -87,6 +89,7 @@ def __init__(
         agent_card: AgentCard,
         request_handler: RequestHandler,
         context_builder: CallContextBuilder | None = None,
+        card_modifier: Callable[[AgentCard], AgentCard] | None = None,
     ):
         """Initializes the GrpcHandler.
 
@@ -96,10 +99,13 @@ def __init__(
                              delegate requests to.
             context_builder: The CallContextBuilder object. If none the
                              DefaultCallContextBuilder is used.
+            card_modifier: An optional callback to dynamically modify the public
+              agent card before it is served.
         """
         self.agent_card = agent_card
         self.request_handler = request_handler
         self.context_builder = context_builder or DefaultCallContextBuilder()
+        self.card_modifier = card_modifier
 
     async def SendMessage(
         self,
@@ -331,7 +337,10 @@ async def GetAgentCard(
         context: grpc.aio.ServicerContext,
     ) -> a2a_pb2.AgentCard:
         """Get the agent card for the agent served."""
-        return proto_utils.ToProto.agent_card(self.agent_card)
+        card_to_serve = self.agent_card
+        if self.card_modifier:
+            card_to_serve = self.card_modifier(card_to_serve)
+        return proto_utils.ToProto.agent_card(card_to_serve)
 
     async def abort_context(
         self, error: ServerError, context: grpc.aio.ServicerContext

src/a2a/server/request_handlers/jsonrpc_handler.py

@@ -1,6 +1,6 @@
 import logging
 
-from collections.abc import AsyncIterable
+from collections.abc import AsyncIterable, Callable
 
 from a2a.server.context import ServerCallContext
 from a2a.server.request_handlers.request_handler import RequestHandler
@@ -62,17 +62,25 @@ def __init__(
         agent_card: AgentCard,
         request_handler: RequestHandler,
         extended_agent_card: AgentCard | None = None,
+        extended_card_modifier: Callable[
+            [AgentCard, ServerCallContext], AgentCard
+        ]
+        | None = None,
     ):
         """Initializes the JSONRPCHandler.
 
         Args:
             agent_card: The AgentCard describing the agent's capabilities.
             request_handler: The underlying `RequestHandler` instance to delegate requests to.
             extended_agent_card: An optional, distinct Extended AgentCard to be served
+            extended_card_modifier: An optional callback to dynamically modify
+              the extended agent card before it is served. It receives the
+              call context.
         """
         self.agent_card = agent_card
         self.request_handler = request_handler
         self.extended_agent_card = extended_agent_card
+        self.extended_card_modifier = extended_card_modifier
 
     async def on_message_send(
         self,
@@ -417,16 +425,27 @@ async def get_authenticated_extended_card(
         Returns:
             A `GetAuthenticatedExtendedCardResponse` object containing the config or a JSON-RPC error.
         """
-        if self.extended_agent_card is None:
+        if (
+            self.extended_agent_card is None
+            and self.extended_card_modifier is None
+        ):
             return GetAuthenticatedExtendedCardResponse(
                 root=JSONRPCErrorResponse(
                     id=request.id,
                     error=AuthenticatedExtendedCardNotConfiguredError(),
                 )
             )
 
+        base_card = self.extended_agent_card
+        if base_card is None:
+            base_card = self.agent_card
+
+        card_to_serve = base_card
+        if self.extended_card_modifier and context:
+            card_to_serve = self.extended_card_modifier(base_card, context)
+
         return GetAuthenticatedExtendedCardResponse(
             root=GetAuthenticatedExtendedCardSuccessResponse(
-                id=request.id, result=self.extended_agent_card
+                id=request.id, result=card_to_serve
             )
         )