Learning 012: FastAPI 0.143 includes routers lazily and ignores a route class's app

• Date: 2026-10-13
• Source: system-feedback (a probe during the issue-36 design)
• Work item: issue-36

What happened

The issue-36 design first planned to give a FastAPI router a custom APIRoute subclass whose app wraps the endpoint in the harness's 503 gate and limits, as a2a-sdk's own add_a2a_routes_to_fastapi does. A probe on FastAPI 0.143 showed these facts:

• include_router no longer copies routes. It wraps the router in _IncludedRouter.
• The overridden APIRoute.app is never called.
• A Mount inside a router is dropped, so the router cannot serve /ui.
• A route appended to router.routes directly keeps its bare path. Only add_api_route applies the router's prefix.
• An include_router(prefix=…) sets no root_path.
• include_router does merge the router's lifespan.

Learning

Before a design relies on a framework's routing internals, run a ten-line probe against the pinned version. Per-route behaviour in a FastAPI router belongs in router dependencies. A check of where a router is mounted must compare the full request path, because the include prefix is invisible to the route.