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docs/en/docs/release-notes.md

@@ -7,6 +7,13 @@ hide:
 
 ## Latest Changes
 
+## 0.137.1 (2026-06-15)
+
+### Fixes
+
+* 🚨 Fix typing checks for APIRoute. PR [#15765](https://github.com/fastapi/fastapi/pull/15765) by [@tiangolo](https://github.com/tiangolo).
+* 🐛 Fix bug, allow empty path in path operation in prefixless router. PR [#15763](https://github.com/fastapi/fastapi/pull/15763) by [@tiangolo](https://github.com/tiangolo).
+
 ## 0.137.0 (2026-06-14)
 
 ### Breaking Changes

fastapi/__init__.py

@@ -1,6 +1,6 @@
 """FastAPI framework, high performance, easy to learn, fast to code, ready for production"""
 
-__version__ = "0.137.0"
+__version__ = "0.137.1"
 
 from starlette import status as status
 

fastapi/routing.py

@@ -930,28 +930,6 @@ def _populate_api_route_state(
     route.path = path
     route.endpoint = endpoint
     route.stream_item_type = None
-    if isinstance(response_model, DefaultPlaceholder):
-        return_annotation = get_typed_return_annotation(endpoint)
-        if lenient_issubclass(return_annotation, Response):
-            response_model = None
-        else:
-            stream_item = get_stream_item_type(return_annotation)
-            if stream_item is not None:
-                # Extract item type for JSONL or SSE streaming when
-                # response_class is DefaultPlaceholder (JSONL) or
-                # EventSourceResponse (SSE).
-                # ServerSentEvent is excluded: it's a transport
-                # wrapper, not a data model, so it shouldn't feed
-                # into validation or OpenAPI schema generation.
-                if (
-                    isinstance(response_class, DefaultPlaceholder)
-                    or lenient_issubclass(response_class, EventSourceResponse)
-                ) and not lenient_issubclass(stream_item, ServerSentEvent):
-                    route.stream_item_type = stream_item
-                response_model = None
-            else:
-                response_model = return_annotation
-    route.response_model = response_model
     route.summary = summary
     route.response_description = response_description
     route.deprecated = deprecated
@@ -987,27 +965,6 @@ def _populate_api_route_state(
     if isinstance(status_code, IntEnum):
         status_code = int(status_code)
     route.status_code = status_code
-    if route.response_model:
-        assert is_body_allowed_for_status_code(status_code), (
-            f"Status code {status_code} must not have a response body"
-        )
-        response_name = "Response_" + route.unique_id
-        route.response_field = create_model_field(
-            name=response_name,
-            type_=route.response_model,
-            mode="serialization",
-        )
-    else:
-        route.response_field = None
-    if route.stream_item_type:
-        stream_item_name = "StreamItem_" + route.unique_id
-        route.stream_item_field = create_model_field(
-            name=stream_item_name,
-            type_=route.stream_item_type,
-            mode="serialization",
-        )
-    else:
-        route.stream_item_field = None
     route.dependencies = list(dependencies or [])
     route.description = description or inspect.cleandoc(route.endpoint.__doc__ or "")
     # if a "form feed" character (page break) is found in the description text,
@@ -1059,9 +1016,88 @@ def _populate_api_route_state(
     route.is_json_stream = is_generator and isinstance(
         response_class, DefaultPlaceholder
     )
+    if isinstance(response_model, DefaultPlaceholder):
+        return_annotation = get_typed_return_annotation(endpoint)
+        if lenient_issubclass(return_annotation, Response):
+            response_model = None
+        else:
+            stream_item = get_stream_item_type(return_annotation)
+            if stream_item is not None and is_generator:
+                # Extract item type for JSONL or SSE streaming for
+                # generator endpoints when response_class is
+                # DefaultPlaceholder (JSONL) or EventSourceResponse
+                # (SSE).
+                # ServerSentEvent is excluded: it's a transport
+                # wrapper, not a data model, so it shouldn't feed
+                # into validation or OpenAPI schema generation.
+                if (
+                    isinstance(response_class, DefaultPlaceholder)
+                    or lenient_issubclass(response_class, EventSourceResponse)
+                ) and not lenient_issubclass(stream_item, ServerSentEvent):
+                    route.stream_item_type = stream_item
+                response_model = None
+            else:
+                response_model = return_annotation
+    route.response_model = response_model
+    if route.response_model:
+        assert is_body_allowed_for_status_code(status_code), (
+            f"Status code {status_code} must not have a response body"
+        )
+        response_name = "Response_" + route.unique_id
+        route.response_field = create_model_field(
+            name=response_name,
+            type_=route.response_model,
+            mode="serialization",
+        )
+    else:
+        route.response_field = None
+    if route.stream_item_type:
+        stream_item_name = "StreamItem_" + route.unique_id
+        route.stream_item_field = create_model_field(
+            name=stream_item_name,
+            type_=route.stream_item_type,
+            mode="serialization",
+        )
+    else:
+        route.stream_item_field = None
 
 
 class APIRoute(routing.Route):
+    stream_item_type: Any | None
+    response_model: Any
+    summary: str | None
+    response_description: str
+    deprecated: bool | None
+    operation_id: str | None
+    response_model_include: IncEx | None
+    response_model_exclude: IncEx | None
+    response_model_by_alias: bool
+    response_model_exclude_unset: bool
+    response_model_exclude_defaults: bool
+    response_model_exclude_none: bool
+    include_in_schema: bool
+    response_class: type[Response] | DefaultPlaceholder
+    dependency_overrides_provider: Any | None
+    callbacks: list[BaseRoute] | None
+    openapi_extra: dict[str, Any] | None
+    generate_unique_id_function: Callable[[Any], str] | DefaultPlaceholder
+    strict_content_type: bool | DefaultPlaceholder
+    tags: list[str | Enum]
+    responses: dict[int | str, dict[str, Any]]
+    unique_id: str
+    status_code: int | None
+    response_field: ModelField | None
+    stream_item_field: ModelField | None
+    dependencies: list[params.Depends]
+    description: str
+    response_fields: dict[int | str, ModelField]
+    dependant: Dependant
+    _flat_dependant: Dependant
+    _embed_body_fields: bool
+    body_field: ModelField | None
+    is_sse_stream: bool
+    is_json_stream: bool
+
     def __init__(
         self,
         path: str,
@@ -2435,9 +2471,16 @@ class APIRouter(routing.Router):
                 "A path prefix must not end with '/', as the routes will start with '/'"
             )
         else:
-            for r in _iter_included_route_candidates(router.routes):
-                path = getattr(r, "path", None)
-                name = getattr(r, "name", "unknown")
+            for route, route_context in _iter_routes_with_context(router.routes):
+                if route_context is None:
+                    path = getattr(route, "path", None)
+                    name = getattr(route, "name", "unknown")
+                elif route_context.starlette_route is not None:
+                    path = getattr(route_context.starlette_route, "path", None)
+                    name = getattr(route_context.starlette_route, "name", "unknown")
+                else:
+                    path = route_context.path
+                    name = route_context.name
                 if path is not None and not path:
                     raise FastAPIError(
                         f"Prefix and path cannot be both empty (path operation: {name})"

tests/test_router_include_context.py

@@ -2,6 +2,7 @@ from typing import Annotated, cast
 
 import pytest
 from fastapi import APIRouter, Body, Depends, FastAPI, Request
+from fastapi.exceptions import FastAPIError
 from fastapi.responses import HTMLResponse, JSONResponse, PlainTextResponse
 from fastapi.routing import (
     APIRoute,
@@ -807,6 +808,60 @@ def test_no_prefix_include_validation_sees_effective_starlette_route_candidates(
     assert cast(Route, candidates[0]).path == "/child/items"
 
 
+def test_no_prefix_include_validation_sees_effective_api_route_path():
+    leaf_router = APIRouter()
+
+    @leaf_router.get("")
+    def read_items():
+        return []
+
+    parent_router = APIRouter()
+    parent_router.include_router(leaf_router, prefix="/items")
+
+    # for coverage
+    candidates = list(_iter_included_route_candidates(parent_router.routes))
+    assert cast(APIRoute, candidates[0]).path == ""
+
+    app = FastAPI()
+    app.include_router(parent_router)
+    client = TestClient(app)
+
+    response = client.get("/items")
+
+    assert response.status_code == 200, response.text
+    assert response.json() == []
+
+
+def test_no_prefix_include_validation_sees_effective_starlette_route_path():
+    def endpoint(request):
+        return PlainTextResponse("ok")
+
+    child_router = APIRouter(routes=[Route("/items", endpoint, name="read_items")])
+    parent_router = APIRouter()
+    parent_router.include_router(child_router, prefix="/child")
+
+    app = FastAPI()
+    app.include_router(parent_router)
+    client = TestClient(app)
+
+    response = client.get("/child/items")
+
+    assert response.status_code == 200, response.text
+    assert response.text == "ok"
+
+
+def test_no_prefix_include_validation_rejects_empty_effective_api_route_path():
+    router = APIRouter()
+
+    @router.get("")
+    def read_items():  # pragma: no cover
+        return []
+
+    app = FastAPI()
+    with pytest.raises(FastAPIError):
+        app.include_router(router)
+
+
 def test_apirouter_matches_fallback_without_include_context():
     router = APIRouter()
 

tests/test_skip_defaults.py

@@ -1,3 +1,5 @@
+from collections.abc import Iterable
+
 from fastapi import FastAPI
 from fastapi.testclient import TestClient
 from pydantic import BaseModel
@@ -65,6 +67,21 @@ def get_exclude_unset_none() -> ModelDefaults:
     return ModelDefaults(x=None, y="y")
 
 
+@app.get("/iterable_exclude_unset", response_model_exclude_unset=True)
+def get_iterable_exclude_unset() -> Iterable[ModelDefaults]:
+    return [ModelDefaults(x=None, y="y")]
+
+
+@app.get("/iterable_exclude_defaults", response_model_exclude_defaults=True)
+def get_iterable_exclude_defaults() -> Iterable[ModelDefaults]:
+    return [ModelDefaults(x=None, y="y")]
+
+
+@app.get("/iterable_exclude_none", response_model_exclude_none=True)
+def get_iterable_exclude_none() -> Iterable[ModelDefaults]:
+    return [ModelDefaults(x=None, y="y")]
+
+
 client = TestClient(app)
 
 
@@ -91,3 +108,18 @@ def test_return_exclude_none():
 def test_return_exclude_unset_none():
     response = client.get("/exclude_unset_none")
     assert response.json() == {"y": "y"}
+
+
+def test_return_iterable_exclude_unset():
+    response = client.get("/iterable_exclude_unset")
+    assert response.json() == [{"x": None, "y": "y"}]
+
+
+def test_return_iterable_exclude_defaults():
+    response = client.get("/iterable_exclude_defaults")
+    assert response.json() == [{}]
+
+
+def test_return_iterable_exclude_none():
+    response = client.get("/iterable_exclude_none")
+    assert response.json() == [{"y": "y", "z": "z"}]