Fix duplicate operation IDs for routes with multiple methods

When a route is registered with multiple HTTP methods (e.g., methods=['POST', 'DELETE']),
the default generate_unique_id function only uses the first method, causing duplicate
operation IDs in the OpenAPI schema.

This fix ensures that when a route has multiple methods and no explicit operation_id,
each method gets a unique operation ID by appending the current method name.

Fixes #13175

fastapi/openapi/utils.py

@@ -242,7 +242,20 @@ 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 len(route.methods) > 1:
+        # When a route handles multiple methods, the default unique_id is based on
+        # the first method only (via generate_unique_id). Append the actual method
+        # to avoid duplicate operation IDs across methods. See #13175.
+        base = route.unique_id
+        # Remove the trailing _<method> suffix that generate_unique_id appends
+        first_method = f"_{list(route.methods)[0].lower()}"
+        if base.endswith(first_method):
+            base = base[: -len(first_method)]
+        operation_id = f"{base}_{method.lower()}"
+    else:
+        operation_id = route.unique_id
     if operation_id in operation_ids:
         endpoint_name = getattr(route.endpoint, "__name__", "<unnamed_endpoint>")
         message = f"Duplicate Operation ID {operation_id} for function {endpoint_name}"

tests/test_generate_unique_id_function.py

@@ -1697,3 +1697,30 @@ def test_warn_duplicate_operation_id():
         ]
         assert len(duplicate_warnings) > 0
         assert "Duplicate Operation ID" in str(duplicate_warnings[0].message)
+
+
+def test_unique_operation_id_for_multi_method_route():
+    """A route with multiple methods should get unique operation IDs per method.
+
+    Regression test for https://github.com/fastapi/fastapi/issues/13175
+    """
+    app = FastAPI()
+
+    def handler():
+        return {"msg": "ok"}
+
+    app.add_api_route("/clear", handler, methods=["POST", "DELETE"])
+
+    client = TestClient(app)
+    response = client.get("/openapi.json")
+    data = response.json()
+    paths = data["paths"]
+
+    op_ids = []
+    for methods in paths.values():
+        for details in methods.values():
+            op_ids.append(details["operationId"])
+
+    assert len(op_ids) == 2, f"Expected 2 operations, got {len(op_ids)}"
+    assert op_ids[0] != op_ids[1], f"Duplicate operation IDs: {op_ids}"
+    assert set(op_ids) == {"handler_clear_post", "handler_clear_delete"}