The 256 MiB ceiling was only enforced inside _dump_payload, which runs after
FastAPI has parsed the request and after json.dumps has re-serialised it. By
that point the payload has been materialised several times over, so the limit
rejected an allocation it had already paid for.
Check Content-Length first, on both write routes. A request that omits or lies
about the header still reaches the exact byte count, which remains
authoritative.