There are two versions that have been developed, reasonably feature complete, pending testing. One presents an OpenAPI schema of application/x-www-form-urlencoded (multipart forms); the other, application/json. The latter uses middleware (strictly speaking, a custom request and route class, instead of the Starlette middleware capability, although that may change) to intercept multipart requests, so as to remain backwards compatible.

https://github.com/jestabro/vyos-1x/tree/fastapi-multipart
https://github.com/jestabro/vyos-1x/tree/fastapi-json

Pending completion/confirmation of the compatibility layer, the latter version will be considered.

Draft PR linked below; it is a draft pending the addition of a supporting package (vyos-http-api-tools). All data validation for application/json requests is done implicitly using Pydantic models/validators; for multipart forms requests, all explicit validation has been moved out of the endpoint function bodies and into the middleware --- this is a pedantic block of code, but allows consolidation and review of all manual checks. An OpenAPI 3.0.2 schema is automatically generated and presented at server-name/docs (Swagger) and server-name/redoc (ReDoc version; slightly more pleasing to the eye).

https://github.com/vyos/vyos-1x/pull/785