From cda2003ff55b6aa38d8dbb253bbdf281d5c16d33 Mon Sep 17 00:00:00 2001
From: Yurii Motov <yurii.motov.monte@gmail.com>
Date: Mon, 9 Feb 2026 12:04:36 +0100
Subject: [PATCH] Fix

---
 fastapi/_compat/shared.py                  |   8 ++
 tests/test_nested_annotated_in_sequence.py | 136 +++++++++++++++++++++
 2 files changed, 144 insertions(+)
 create mode 100644 tests/test_nested_annotated_in_sequence.py

diff --git a/fastapi/_compat/shared.py b/fastapi/_compat/shared.py
index c009da8fd..db4cb541b 100644
--- a/fastapi/_compat/shared.py
+++ b/fastapi/_compat/shared.py
@@ -65,6 +65,10 @@ def _annotation_is_sequence(annotation: Union[type[Any], None]) -> bool:
 
 def field_annotation_is_sequence(annotation: Union[type[Any], None]) -> bool:
     origin = get_origin(annotation)
+
+    if origin is Annotated:
+        return field_annotation_is_sequence(get_args(annotation)[0])
+
     if origin is Union or origin is UnionType:
         for arg in get_args(annotation):
             if field_annotation_is_sequence(arg):
@@ -110,6 +114,10 @@ def field_annotation_is_scalar(annotation: Any) -> bool:
 
 def field_annotation_is_scalar_sequence(annotation: Union[type[Any], None]) -> bool:
     origin = get_origin(annotation)
+
+    if origin is Annotated:
+        return field_annotation_is_scalar_sequence(get_args(annotation)[0])
+
     if origin is Union or origin is UnionType:
         at_least_one_scalar_sequence = False
         for arg in get_args(annotation):
diff --git a/tests/test_nested_annotated_in_sequence.py b/tests/test_nested_annotated_in_sequence.py
new file mode 100644
index 000000000..09e4363e2
--- /dev/null
+++ b/tests/test_nested_annotated_in_sequence.py
@@ -0,0 +1,136 @@
+from typing import Annotated, Union
+
+from dirty_equals import IsList
+from fastapi import FastAPI, Query
+from fastapi.testclient import TestClient
+from pydantic import Field
+
+MaxSizedSet = Annotated[set[str], Field(max_length=3)]
+
+app = FastAPI()
+
+
+@app.get("/")
+def read_root(foo: Annotated[Union[MaxSizedSet, None], Query()] = None):
+    return {"foo": foo}
+
+
+client = TestClient(app)
+
+
+def test_endpoint_none():
+    response = client.get("/")
+    assert response.status_code == 200
+    assert response.json() == {"foo": None}
+
+
+def test_endpoint_valid():
+    response = client.get("/", params={"foo": ["a", "b"]})
+    assert response.status_code == 200
+    assert response.json() == {"foo": IsList("a", "b", check_order=False)}
+
+
+def test_endpoint_too_long():
+    response = client.get("/", params={"foo": ["a", "b", "c", "d"]})
+    assert response.status_code == 422
+    assert response.json() == {
+        "detail": [
+            {
+                "type": "too_long",
+                "loc": ["query", "foo"],
+                "msg": "Set should have at most 3 items after validation, not more",
+                "input": IsList("a", "b", "c", "d", check_order=False),
+                "ctx": {
+                    "actual_length": None,
+                    "field_type": "Set",
+                    "max_length": 3,
+                },
+            }
+        ]
+    }
+
+
+def test_openapi():
+    assert app.openapi() == {
+        "components": {
+            "schemas": {
+                "HTTPValidationError": {
+                    "properties": {
+                        "detail": {
+                            "items": {"$ref": "#/components/schemas/ValidationError"},
+                            "title": "Detail",
+                            "type": "array",
+                        },
+                    },
+                    "title": "HTTPValidationError",
+                    "type": "object",
+                },
+                "ValidationError": {
+                    "properties": {
+                        "ctx": {"title": "Context", "type": "object"},
+                        "input": {"title": "Input"},
+                        "loc": {
+                            "items": {
+                                "anyOf": [{"type": "string"}, {"type": "integer"}],
+                            },
+                            "title": "Location",
+                            "type": "array",
+                        },
+                        "msg": {"title": "Message", "type": "string"},
+                        "type": {"title": "Error Type", "type": "string"},
+                    },
+                    "required": ["loc", "msg", "type"],
+                    "title": "ValidationError",
+                    "type": "object",
+                },
+            },
+        },
+        "info": {
+            "title": "FastAPI",
+            "version": "0.1.0",
+        },
+        "openapi": "3.1.0",
+        "paths": {
+            "/": {
+                "get": {
+                    "operationId": "read_root__get",
+                    "parameters": [
+                        {
+                            "in": "query",
+                            "name": "foo",
+                            "required": False,
+                            "schema": {
+                                "anyOf": [
+                                    {
+                                        "items": {"type": "string"},
+                                        "maxItems": 3,
+                                        "type": "array",
+                                        "uniqueItems": True,
+                                    },
+                                    {"type": "null"},
+                                ],
+                                "title": "Foo",
+                            },
+                        },
+                    ],
+                    "responses": {
+                        "200": {
+                            "content": {"application/json": {"schema": {}}},
+                            "description": "Successful Response",
+                        },
+                        "422": {
+                            "content": {
+                                "application/json": {
+                                    "schema": {
+                                        "$ref": "#/components/schemas/HTTPValidationError",
+                                    },
+                                },
+                            },
+                            "description": "Validation Error",
+                        },
+                    },
+                    "summary": "Read Root",
+                },
+            },
+        },
+    }