fix: preserve FastStream schema metadata in FastAPI dependants

src/faststream_fastapi/_internal/get_dependant.py

@@ -1,7 +1,9 @@
 import inspect
 from collections.abc import Callable, Iterable
+from dataclasses import fields
 from typing import Annotated, Any, Final, cast, get_args, get_origin
 
+from fast_depends.library.serializer import OptionItem
 from fast_depends.utils import get_typed_annotation
 from fastapi.dependencies.models import Dependant
 from fastapi.dependencies.utils import (
@@ -10,15 +12,30 @@
     get_typed_signature,
 )
 from fastapi.params import Depends
-from pydantic import Field
+from pydantic import Field, create_model
 
 from faststream_fastapi._internal.fs_re_exports._compat import PYDANTIC_V2, PydanticUndefined
 
 
+class _FastStreamDependant(Dependant):
+    """FastAPI dependant extended with fields required by FastStream."""
+
+    model: type[Any]
+    custom_fields: dict[str, Any]
+    flat_params: list[OptionItem]
+
+
+def _extend_fastapi_dependant(dependant: Dependant) -> _FastStreamDependant:
+    field_values = {
+        field.name: getattr(dependant, field.name) for field in fields(dependant) if field.init
+    }
+    return _FastStreamDependant(**field_values)
+
+
 def get_fastapi_dependant(
     orig_call: Callable[..., Any],
     dependencies: Iterable[Depends],
-) -> Dependant:
+) -> _FastStreamDependant:
     dependent = get_fastapi_native_dependant(orig_call=orig_call, dependencies=dependencies)
     return _patch_fastapi_dependent(dependent)
 
@@ -41,7 +58,8 @@ def get_fastapi_native_dependant(
     return dependent
 
 
-def _patch_fastapi_dependent(dependant: Dependant) -> Dependant:
+def _patch_fastapi_dependent(dependant: Dependant) -> _FastStreamDependant:
+    dependant = _extend_fastapi_dependant(dependant)
     params = dependant.query_params + dependant.body_params
 
     for d in dependant.dependencies:
@@ -117,6 +135,15 @@ def _patch_fastapi_dependent(dependant: Dependant) -> Dependant:
                 f,
             )
 
+    dependant.model = create_model(
+        getattr(call, "__name__", type(call).__name__),
+    )
+    dependant.custom_fields = {}
+    dependant.flat_params = [
+        OptionItem(field_name=name, field_type=type_, default_value=default)
+        for name, (type_, default) in params_unique.items()
+    ]
+
     return dependant
 
 

tests/test_get_fastapi_dependant.py

@@ -0,0 +1,25 @@
+from typing import Annotated
+
+from fastapi import Depends
+
+from faststream_fastapi._internal.get_dependant import get_fastapi_dependant
+
+
+def test_get_fastapi_dependant_has_faststream_fields() -> None:
+    async def dependency() -> str:
+        return "dependency"
+
+    async def handler(
+        message: str,
+        dependency_value: Annotated[str, Depends(dependency)],
+    ) -> None:
+        pass
+
+    dependant = get_fastapi_dependant(handler, ())
+
+    assert dependant.call is handler
+    assert len(dependant.dependencies) == 1
+    assert dependant.dependencies[0].call is dependency
+    assert dependant.model.__name__ == "handler"
+    assert dependant.custom_fields == {}
+    assert [field.field_name for field in dependant.flat_params] == ["message"]