refactor!: Make the FastAPI dependency optional

With this update, using A2AFastAPIApplication requires either adding
the FastAPI package directly to the project or indicating the "fastapi"
group when installing a2a-sdk, e.g., with uv "uv add a2a-sdk[fastapi]"
or with pip "pip install a2a-sdk[fastapi]"

Author: darkhaniop

pyproject.toml

@@ -8,7 +8,6 @@ authors = [{ name = "Google LLC", email = "[email protected]" }]
 requires-python = ">=3.10"
 keywords = ["A2A", "A2A SDK", "A2A Protocol", "Agent2Agent"]
 dependencies = [
-    "fastapi>=0.115.2",
     "httpx>=0.28.1",
     "httpx-sse>=0.4.0",
     "google-api-core>=1.26.0",
@@ -80,6 +79,10 @@ dev = [
     "uv-dynamic-versioning>=0.8.2",
     "types-protobuf",
     "types-requests",
+    "fastapi>=0.115.2",
+]
+fastapi = [
+    "fastapi>=0.115.2",
 ]
 
 [[tool.uv.index]]

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

@@ -2,8 +2,11 @@
 
 from typing import Any
 
-from fastapi import FastAPI, Request, Response
-
+from a2a.server.apps.jsonrpc.fastapi_import_helpers import (
+    FastAPI,
+    Request,
+    Response,
+)
 from a2a.server.apps.jsonrpc.jsonrpc_app import (
     CallContextBuilder,
     JSONRPCApplication,

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

@@ -0,0 +1,32 @@
+"""Helper functions for handling optional FastAPI package imports."""
+
+try:
+    from fastapi import FastAPI, Request, Response
+except ImportError:
+    _FASTAPI_DEPENDENCY_ERROR_MSG = """
+    The A2A Python SDK FastAPI app requires the FastAPI package, which
+    is an optional dependency. To fix this issue, please add the fastapi
+    package to your project, e.g. by executing
+
+        uv add fastapi
+
+    or install the A2A SDK with the optional FastAPI dependency:
+
+        uv add a2a-sdk[fastapi]
+    """
+
+    class _DummyFastAPIClasses:
+        """Parent class for dummy fastapi.* class declarations."""
+
+        def __init__(self) -> None:
+            """Raises ImportError when initiating a dummy fastapi.* instance."""
+            raise ImportError(_FASTAPI_DEPENDENCY_ERROR_MSG)
+
+    class FastAPI(_DummyFastAPIClasses):
+        """A dummy fastapi.FastAPI declaration."""
+
+    class Request(_DummyFastAPIClasses):
+        """A dummy fastapi.Request declaration."""
+
+    class Response(_DummyFastAPIClasses):
+        """A dummy fastapi.Response declaration."""

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

@@ -7,7 +7,6 @@
 from collections.abc import AsyncGenerator
 from typing import Any
 
-from fastapi import FastAPI
 from pydantic import ValidationError
 from sse_starlette.sse import EventSourceResponse
 from starlette.applications import Starlette
@@ -17,6 +16,7 @@
 
 from a2a.auth.user import UnauthenticatedUser
 from a2a.auth.user import User as A2AUser
+from a2a.server.apps.jsonrpc.fastapi_import_helpers import FastAPI
 from a2a.server.context import ServerCallContext
 from a2a.server.request_handlers.jsonrpc_handler import JSONRPCHandler
 from a2a.server.request_handlers.request_handler import RequestHandler