Fix duplicate OperationID when adding route with multiple methods

When a route handles multiple HTTP methods (e.g. methods=["POST", "DELETE"]),
the operation ID was generated once from the first method in the set, causing
duplicate operation IDs in the OpenAPI schema and triggering warnings.

Now, for routes with multiple methods, the current method is appended to the
unique_id to ensure each method gets a distinct operation ID.

Closes #13175

fastapi/openapi/utils.py

@@ -242,7 +242,14 @@ 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 is not None and len(route.methods) > 1:
+        # When a route handles multiple HTTP methods, append the current
+        # method to create unique operation IDs per method
+        operation_id = f"{route.unique_id}_{method.lower()}"
+    else:
+        operation_id = route.unique_id
     if operation_id in operation_ids:
         message = (
             f"Duplicate Operation ID {operation_id} for function "

tests/test_duplicate_operation_id_multiple_methods.py

@@ -0,0 +1,71 @@
+import warnings
+
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+
+def test_no_duplicate_operation_id_with_multiple_methods():
+    """Ensure routes with multiple methods get unique operation IDs."""
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+
+        app = FastAPI()
+
+        def handler():
+            return {"status": "ok"}  # pragma: nocover
+
+        app.add_api_route("/items", handler, methods=["POST", "DELETE"])
+
+        client = TestClient(app)
+        response = client.get("/openapi.json")
+        assert response.status_code == 200
+        schema = response.json()
+
+        post_op = schema["paths"]["/items"]["post"]
+        delete_op = schema["paths"]["/items"]["delete"]
+
+        assert post_op["operationId"] != delete_op["operationId"]
+        assert "post" in post_op["operationId"]
+        assert "delete" in delete_op["operationId"]
+
+        dup_warnings = [x for x in w if "Duplicate" in str(x.message)]
+        assert len(dup_warnings) == 0
+
+
+def test_single_method_route_operation_id_unchanged():
+    """Ensure single-method routes still generate the same operation IDs."""
+    app = FastAPI()
+
+    @app.get("/items")
+    def get_items():
+        return []  # pragma: nocover
+
+    client = TestClient(app)
+    response = client.get("/openapi.json")
+    schema = response.json()
+
+    get_op = schema["paths"]["/items"]["get"]
+    assert get_op["operationId"] == "get_items_items_get"
+
+
+def test_three_methods_unique_operation_ids():
+    """Ensure routes with three methods all get unique operation IDs."""
+    app = FastAPI()
+
+    def handler():
+        return {"status": "ok"}  # pragma: nocover
+
+    app.add_api_route("/items", handler, methods=["GET", "POST", "DELETE"])
+
+    client = TestClient(app)
+    response = client.get("/openapi.json")
+    schema = response.json()
+
+    get_op = schema["paths"]["/items"]["get"]
+    post_op = schema["paths"]["/items"]["post"]
+    delete_op = schema["paths"]["/items"]["delete"]
+
+    assert "get" in get_op["operationId"]
+    assert "post" in post_op["operationId"]
+    assert "delete" in delete_op["operationId"]
+    assert len({get_op["operationId"], post_op["operationId"], delete_op["operationId"]}) == 3