Fix duplicate operationId when route has multiple methods

When a route is registered with multiple HTTP methods via add_api_route(),
generate_unique_id() uses list(route.methods)[0] which is non-deterministic
(methods is a set) and the same for every method in the route. This causes
get_openapi_operation_metadata to assign identical operationIds to POST and
DELETE (or any other method pair), triggering "Duplicate Operation ID" warnings.

Two fixes:
- generate_unique_id: use sorted(route.methods)[0] so the fallback unique_id
  is deterministic for single-method routes
- get_openapi_operation_metadata: for multi-method routes without an explicit
  operation_id, derive the operationId from the route name/path plus the
  per-iteration method instead of reusing route.unique_id

Fixes #13175

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

fastapi/openapi/utils.py

@@ -1,6 +1,7 @@
 import copy
 import http.client
 import inspect
+import re
 import warnings
 from collections.abc import Sequence
 from typing import Any, Literal, cast
@@ -243,7 +244,13 @@ def get_openapi_operation_metadata(
     operation["summary"] = generate_operation_summary(route=route, method=method)
     if route.description:
         operation["description"] = route.description
-    operation_id = route.operation_id or route.unique_id
+    if route.operation_id:
+        operation_id = route.operation_id
+    elif route.methods and len(route.methods) > 1:
+        base = re.sub(r"\W", "_", f"{route.name}{route.path_format}")
+        operation_id = f"{base}_{method.lower()}"
+    else:
+        operation_id = route.unique_id
     if operation_id in operation_ids:
         message = (
             f"Duplicate Operation ID {operation_id} for function "

fastapi/utils.py

@@ -96,7 +96,7 @@ def generate_unique_id(route: "APIRoute") -> str:
     operation_id = f"{route.name}{route.path_format}"
     operation_id = re.sub(r"\W", "_", operation_id)
     assert route.methods
-    operation_id = f"{operation_id}_{list(route.methods)[0].lower()}"
+    operation_id = f"{operation_id}_{sorted(route.methods)[0].lower()}"
     return operation_id
 
 

tests/test_generate_unique_id_function.py

@@ -1697,3 +1697,32 @@ def test_warn_duplicate_operation_id():
         ]
         assert len(duplicate_warnings) > 0
         assert "Duplicate Operation ID" in str(duplicate_warnings[0].message)
+
+
+def test_no_duplicate_operation_id_for_multi_method_route():
+    app = FastAPI()
+
+    def clear():
+        return "cleared"  # pragma: nocover
+
+    app.router.add_api_route("/clear", clear, methods=["POST", "DELETE"])
+
+    client = TestClient(app)
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        response = client.get("/openapi.json")
+        duplicate_warnings = [
+            warning
+            for warning in w
+            if issubclass(warning.category, UserWarning)
+            and "Duplicate Operation ID" in str(warning.message)
+        ]
+        assert len(duplicate_warnings) == 0
+
+    schema = response.json()
+    clear_path = schema["paths"]["/clear"]
+    assert "post" in clear_path
+    assert "delete" in clear_path
+    assert clear_path["post"]["operationId"] != clear_path["delete"]["operationId"]
+    assert clear_path["post"]["operationId"] == "clear_clear_post"
+    assert clear_path["delete"]["operationId"] == "clear_clear_delete"