feat: Add FastAPI Application

[martimfasantos]

Description

Summary

This PR introduces a A2AFastAPIApplication class that enables serving A2A endpoints using a FastAPI application, while preserving compatibility with the existing Starlette-based architecture.

Motivation

While the SDK currently provides a Starlette-based server app for A2A agent communication, many production Python APIs are built with FastAPI due to its support for extensibility, automatic OpenAPI schema generation, and strong async capabilities. Providing native FastAPI support enables seamless integration into such environments without requiring users to manually wrap the existing Starlette app.

This addition does not introduce any breaking changes. FastAPI is treated as an optional integration, and the core A2A logic continues to rely on shared base classes and request handlers.

Implementation Details

• Introduced DefaultA2AApplication, an abstract base class that encapsulates the shared request-handling logic for both Starlette and FastAPI applications. This isolates the common behavior, with the only required customization being the build(...) method used to register routes and return the appropriate application instance.

• Added A2AFastAPIApplication, a concrete implementation of DefaultA2AApplication that constructs a FastAPI app with the appropriate routes:

  • POST / (or custom RPC endpoint) for handling A2A JSON-RPC messages.
  • GET /.well-known/agent.json (or custom) for serving the agent card.

• All request processing is shared via _handle_requests and _handle_get_agent_card.

• SSE streaming support is preserved for SendStreamingMessageRequest and TaskResubscriptionRequest.

• Added fastapi to the dependencies in pyproject.toml.

Usage Example

from a2a.server.apps.fastapi_app import A2AFastAPIApplication

app = A2AFastAPIApplication(agent_card=my_agent_card, http_handler=my_handler).build()

Hello World Example (examples/helloword/)

1. Change A2AStarletteApplication to A2AFastAPIApplication

2. Start the server

   uv run .

3. Run the test client

 uv run test_client.py

---

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• Ensure the tests and linter pass (Run nox -s format from the repository root to format)
• Appropriate docs were updated (if necessary)

Fixes #21 🦕

[pstephengoogle]

overall this is good but can we name this more like DefaultJSONRPCApplication? We are introducing different transport solutions soon and don't want to bias that one transport is dominant over another (also remove A2A from class names to avoid duplication - they will already be importing as from a2a... import DefaultJSONRPCApplication so the scope is clear

[pstephengoogle]

some comment on the file. scope it to the transport and not a general 'default'

[martimfasantos]

@pstephengoogle Accidentally closed this PR — I'll be opening another one soon with the comments addressed :)