0.137.0: empty-path route nested under prefix-less outer include raises 'Prefix and path cannot be both empty'

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Example Code

from fastapi import FastAPI, APIRouter

leaf = APIRouter()

@leaf.get("")                                # empty path
def list_items():
    return []

inner = APIRouter()
inner.include_router(leaf, prefix="/items")  # inner include HAS a prefix

app = FastAPI()
app.include_router(inner)                     # outer include has NO prefix

Description

On 0.137.0, the last line raises at app-construction time:
fastapi.exceptions.FastAPIError: Prefix and path cannot be both empty (path operation: list_items)

On 0.136.3 and earlier the same code works and registers the route at /items.

This is a regression introduced by the route-tree refactor in #15745 (preserving APIRouter/APIRoute instances instead of flattening on include).

It only happens when an empty-path route (path="") is nested and at least one include hop in its ancestry has no prefix. If every hop carries a prefix it still works on 0.137.0. Verified on both versions:

setup	0.136.3	0.137.0
app.include_router(leaf, prefix="/items") (single include)	/items	works
inner and app both include with a prefix	/v1/items	works
inner has prefix, app.include_router(inner) has no prefix	/items	raises

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.137.0

Pydantic Version

2.13.4

Python Version

3.12.13

Additional Context

Root-cause pointer (from a quick read of 0.137.0 fastapi/routing.py): in include_router, when prefix is empty, it recursively iterates the included route candidates and raises if any route's raw path is empty — without composing the prefixes contributed by the intermediate included routers.
So a leaf that was included with a prefix at an inner hop still trips the check at a prefix-less outer include.
Whether the right fix is to validate the effective (prefix-composed) path, or to defer this validation to route-resolution time, is a design call for the refactor author.

[svlandeg]

Thanks for the report, we'll look into this!

[tiangolo]

I'll create an issue to track it here: #15762

Fix incoming in a few minutes. 🤓

[tiangolo]

This should be fixed in #15763 , available in FastAPI 0.137.1, just released. 🚀