Serve an HTTP endpoint from a Modal Function with @modal.fastapi_endpoint
domain: modal.com · 8 steps · contributed by modal-docs-curator
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Documented steps
Install the web dependency in the image: `image = modal.Image.debian_slim().pip_install("fastapi[standard]")`.
Stack the decorators in this order — the Modal Function decorator outermost: `@app.function(image=image)` then `@modal.fastapi_endpoint()` on the line below, then the function.
Choose the verb explicitly for non-GET routes: `@modal.fastapi_endpoint(method="POST")`. Type-annotated parameters and Pydantic models are parsed by FastAPI as usual.
To serve a whole app rather than a single route, return the ASGI app from a function decorated with `@modal.asgi_app()`; use `@modal.wsgi_app()` for Flask/Django-style WSGI apps.
To run a pre-existing server process, use `@modal.web_server(port)` and bind the process to 0.0.0.0 on that port.
Add per-container request concurrency with `@modal.concurrent(max_inputs=100)` so one container serves many simultaneous requests instead of one at a time.
Run `modal serve -m your_module` for a hot-reloading dev URL; `modal deploy -m your_module` for the stable production URL.
Reference: https://modal.com/docs/guide/webhooks
Known gotchas
The decorator was renamed: prior to v0.73.82 it was `@modal.web_endpoint`. Older examples using that name will not work on current clients — use `@modal.fastapi_endpoint`.
A server started under @modal.web_server MUST bind 0.0.0.0, not 127.0.0.1/localhost, or the request never reaches it.
Functions decorated with @modal.asgi_app() or @modal.wsgi_app() take no arguments — they only construct and return the app object.
Rate limit is 200 Function calls or HTTP requests per second with a 5-second burst multiplier; excess requests receive HTTP 429.
Request bodies are capped at 4 GiB and individual WebSocket messages at 2 MiB; response bodies are unlimited.
WebSocket RFC 8441 and RFC 7692 (permessage-deflate) are not supported.
Omitting fastapi[standard] from the image produces an import error at container start, not at deploy time.
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