From 2b549d99b69a79caf91502081306d2ec14928059 Mon Sep 17 00:00:00 2001
From: Lukas Erlbacher <lukas.erlbacher97@gmail.com>
Date: Mon, 2 Feb 2026 16:00:43 +0100
Subject: [PATCH] fixed bug introduced in 0.124.1

---
 fastapi/_compat/v2.py                         |  4 +-
 tests/test_union_single_root_model_openapi.py | 88 +++++++++++++++++++
 2 files changed, 90 insertions(+), 2 deletions(-)
 create mode 100644 tests/test_union_single_root_model_openapi.py

diff --git a/fastapi/_compat/v2.py b/fastapi/_compat/v2.py
index 25b6814536..a2a92d5e09 100644
--- a/fastapi/_compat/v2.py
+++ b/fastapi/_compat/v2.py
@@ -22,7 +22,7 @@ from pydantic import ValidationError as ValidationError
 from pydantic._internal._schema_generation_shared import (  # type: ignore[attr-defined]
     GetJsonSchemaHandler as GetJsonSchemaHandler,
 )
-from pydantic._internal._typing_extra import eval_type_lenient
+from pydantic._internal._typing_extra import annotated_type, eval_type_lenient
 from pydantic._internal._utils import lenient_issubclass as lenient_issubclass
 from pydantic.fields import FieldInfo as FieldInfo
 from pydantic.json_schema import GenerateJsonSchema as GenerateJsonSchema
@@ -460,7 +460,7 @@ def create_body_model(
 def get_model_fields(model: type[BaseModel]) -> list[ModelField]:
     model_fields: list[ModelField] = []
     for name, field_info in model.model_fields.items():
-        type_ = field_info.annotation
+        type_ = annotated_type(field_info.annotation) or field_info.annotation
         if lenient_issubclass(type_, (BaseModel, dict)) or is_dataclass(type_):
             model_config = None
         else:
diff --git a/tests/test_union_single_root_model_openapi.py b/tests/test_union_single_root_model_openapi.py
new file mode 100644
index 0000000000..a2747074aa
--- /dev/null
+++ b/tests/test_union_single_root_model_openapi.py
@@ -0,0 +1,88 @@
+from typing import Annotated, Literal, Union
+
+import pytest
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+from inline_snapshot import snapshot
+
+
+@pytest.fixture(name="client")
+def get_client():
+    from pydantic import BaseModel, Field, RootModel
+
+    class Base(BaseModel):
+        type: Literal["BASE"] = "BASE"
+        value: str
+
+    class MyModel(RootModel[Annotated[Union[Base], Field(discriminator="type")]]):
+        pass
+
+    app = FastAPI()
+
+    @app.get("/")
+    def test() -> MyModel:
+        return MyModel.model_validate(Base(value="test"))
+
+    client = TestClient(app)
+    return client
+
+
+def test_get(client: TestClient):
+    response = client.get("/")
+    assert response.json() == {"value": "test", "type": "BASE"}
+
+
+def test_openapi_schema(client: TestClient):
+    response = client.get("openapi.json")
+    assert response.json() == snapshot(
+        {
+            "openapi": "3.1.0",
+            "info": {"title": "FastAPI", "version": "0.1.0"},
+            "paths": {
+                "/": {
+                    "get": {
+                        "summary": "Test",
+                        "operationId": "test__get",
+                        "responses": {
+                            "200": {
+                                "description": "Successful Response",
+                                "content": {
+                                    "application/json": {
+                                        "schema": {
+                                            "$ref": "#/components/schemas/MyModel"
+                                        }
+                                    }
+                                },
+                            }
+                        },
+                    }
+                }
+            },
+            "components": {
+                "schemas": {
+                    "Base": {
+                        "properties": {
+                            "type": {
+                                "type": "string",
+                                "const": "BASE",
+                                "title": "Type",
+                                "default": "BASE",
+                            },
+                            "value": {"type": "string", "title": "Value"},
+                        },
+                        "type": "object",
+                        "required": ["value"],
+                        "title": "Base",
+                    },
+                    "MyModel": {
+                        "oneOf": [{"$ref": "#/components/schemas/Base"}],
+                        "title": "MyModel",
+                        "discriminator": {
+                            "propertyName": "type",
+                            "mapping": {"BASE": "#/components/schemas/Base"},
+                        },
+                    },
+                }
+            },
+        }
+    )