refactor!: Make the FastAPI and Starlette dependencies optional

.github/actions/spelling/allow.txt

@@ -47,6 +47,7 @@ opensource
 protoc
 pyi
 pyversions
+redef
 respx
 resub
 socio

README.md

@@ -3,7 +3,7 @@
 [![License](https://img.shields.io/badge/License-Apache_2.0-blue.svg)](LICENSE)
 [![PyPI version](https://img.shields.io/pypi/v/a2a-sdk)](https://pypi.org/project/a2a-sdk/)
 ![PyPI - Python Version](https://img.shields.io/pypi/pyversions/a2a-sdk)
 [![PyPI - Downloads](https://img.shields.io/pypi/dw/a2a-sdk)](https://pypistats.org/packages/a2a-sdk)
 
 <!-- markdownlint-disable no-inline-html -->
 
@@ -27,20 +27,30 @@
 
 ### Using `uv`
 
-When you're working within a uv project or a virtual environment managed by uv, the preferred way to add packages is using uv add.
-
 ```bash
 uv add a2a-sdk
 ```
 
+To include the optional HTTP server components (FastAPI, Starlette), install the `http-server` extra:
+
+```bash
+uv add a2a-sdk[http-server]
+```
+
 ### Using `pip`
 
-If you prefer to use pip, the standard Python package installer, you can install `a2a-sdk` as follows
+You can install `a2a-sdk` using pip, the standard Python package installer.
 
 ```bash
 pip install a2a-sdk
 ```
 
+To include the optional HTTP server components (FastAPI, Starlette), install the `http-server` extra:
+
+```bash
+pip install a2a-sdk[http-server]
+```
+
 ## Examples
 
 ### [Helloworld Example](https://github.com/a2aproject/a2a-samples/tree/main/samples/python/agents/helloworld)

pyproject.toml

@@ -8,15 +8,12 @@ authors = [{ name = "Google LLC", email = "googleapis-packages@google.com" }]
 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",
   "opentelemetry-api>=1.33.0",
   "opentelemetry-sdk>=1.33.0",
   "pydantic>=2.11.3",
-  "sse-starlette",
-  "starlette",
   "grpcio>=1.60",
   "grpcio-tools>=1.60",
   "grpcio_reflection>=1.7.0",
@@ -36,6 +33,9 @@ classifiers = [
   "License :: OSI Approved :: Apache Software License",
 ]
 
+[project.optional-dependencies]
+http-server = ["fastapi>=0.115.2", "sse-starlette", "starlette"]
+
 [project.urls]
 homepage = "https://a2aproject.github.io/A2A/"
 repository = "https://github.com/a2aproject/a2a-python"
@@ -85,6 +85,9 @@ dev = [
   "types-protobuf",
   "types-requests",
   "pre-commit",
+  "fastapi>=0.115.2",
+  "sse-starlette",
+  "starlette",
 ]
 
 [[tool.uv.index]]

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

@@ -1,8 +1,17 @@
 import logging
 
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
-from fastapi import FastAPI, Request, Response
+
+if TYPE_CHECKING:
+    from fastapi import FastAPI, Request, Response
+else:
+    try:
+        from fastapi import FastAPI, Request, Response
+    except ImportError:
+        FastAPI = Any
+        Request = Any
+        Response = Any
 
 from a2a.server.apps.jsonrpc.jsonrpc_app import (
     CallContextBuilder,
@@ -30,7 +39,7 @@ def __init__(
         extended_agent_card: AgentCard | None = None,
         context_builder: CallContextBuilder | None = None,
     ):
-        """Initializes the A2AStarletteApplication.
+        """Initializes the A2AFastAPIApplication.
 
         Args:
             agent_card: The AgentCard describing the agent's capabilities.
@@ -49,6 +58,17 @@ def __init__(
             context_builder=context_builder,
         )
 
+        self._check_fastapi_dependency()
+
+    def _check_fastapi_dependency(self) -> None:
+        """Checks if the FastAPI package is installed.
+
+        If instead of the actual FastAPI class, a dummy implementation from
+        ./fastapi_import_helpers.py is imported, initializing FastAPI() would
+        raise ImportError.
+        """
+        _app = FastAPI()
+
     def add_routes_to_app(
         self,
         app: FastAPI,

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

@@ -5,9 +5,8 @@
 
 from abc import ABC, abstractmethod
 from collections.abc import AsyncGenerator
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
-from fastapi import FastAPI
 from pydantic import ValidationError
 from sse_starlette.sse import EventSourceResponse
 from starlette.applications import Starlette
@@ -45,6 +44,37 @@
 
 logger = logging.getLogger(__name__)
 
+if TYPE_CHECKING:
+    from fastapi import FastAPI
+    from sse_starlette.sse import EventSourceResponse
+    from starlette.applications import Starlette
+    from starlette.authentication import BaseUser
+    from starlette.requests import Request
+    from starlette.responses import JSONResponse, Response
+
+    _http_server_installed = True
+else:
+    try:
+        from fastapi import FastAPI
+        from sse_starlette.sse import EventSourceResponse
+        from starlette.applications import Starlette
+        from starlette.authentication import BaseUser
+        from starlette.requests import Request
+        from starlette.responses import JSONResponse, Response
+
+        _http_server_installed = True
+    except ImportError:
+        _http_server_installed = False
+        # Provide placeholder types for runtime type hinting when dependencies are not installed.
+        # These will not be used if the code path that needs them is guarded by _http_server_installed.
+        FastAPI = Any
+        EventSourceResponse = Any
+        Starlette = Any
+        BaseUser = Any
+        Request = Any
+        JSONResponse = Any
+        Response = Any
+
 
 class StarletteUserProxy(A2AUser):
     """Adapts the Starlette User class to the A2A user representation."""
@@ -108,7 +138,7 @@ def __init__(
         extended_agent_card: AgentCard | None = None,
         context_builder: CallContextBuilder | None = None,
     ):
-        """Initializes the A2AStarletteApplication.
+        """Initializes the JSONRPCApplication.
 
         Args:
             agent_card: The AgentCard describing the agent's capabilities.
@@ -120,6 +150,10 @@ def __init__(
               ServerCallContext passed to the http_handler. If None, no
               ServerCallContext is passed.
         """
+        if not _http_server_installed:
+            raise ImportError(
+                'The `a2a-sdk[http-server]` package is required to use the `JSONRPCApplication`.'
+            )
         self.agent_card = agent_card
         self.extended_agent_card = extended_agent_card
         self.handler = JSONRPCHandler(

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

@@ -1,9 +1,18 @@
 import logging
 
-from typing import Any
-
-from starlette.applications import Starlette
-from starlette.routing import Route
+from typing import TYPE_CHECKING, Any
+
+
+if TYPE_CHECKING:
+    from starlette.applications import Starlette
+    from starlette.routing import Route
+else:
+    try:
+        from starlette.applications import Starlette
+        from starlette.routing import Route
+    except ImportError:
+        Starlette = Any
+        Route = Any
 
 from a2a.server.apps.jsonrpc.jsonrpc_app import (
     CallContextBuilder,