I'm a little late to the party but here's how you can do it:

from fastapi import FastAPI

app = FastAPI()

@app.get('/', openapi_extra = { "summary": "ABC My Wonderful Function" })
def abc_my_wonderful_function():
  return {"Wonderful": "Function"}

References:

• https://fastapi.tiangolo.com/advanced/path-operation-advanced-configuration/#custom-openapi-path-operation-schema
• https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#operation-object