app/api/v1/rest_router.py

@@ -8,8 +8,10 @@ from functools import wraps
 from typing import Any, Generic, TypeVar, get_args, get_origin
 
 from fastapi import APIRouter, Depends, Query
+from fastapi.encoders import jsonable_encoder
 from fastapi.routing import APIRoute
 from pydantic import BaseModel, JsonValue, create_model
+from starlette.responses import Response
 
 from app.api.problem_details import ProblemDetails
 from app.api.v1.router import api_router as handler_api_router
@@ -263,6 +265,21 @@ def _with_pagination(endpoint):
     return wrapper
 
 
+def _with_jsonable_response(endpoint):
+    """Normalize untyped handler results before JsonValue validation."""
+
+    @wraps(endpoint)
+    async def wrapper(*args, **kwargs):
+        result = endpoint(*args, **kwargs)
+        if inspect.isawaitable(result):
+            result = await result
+        if isinstance(result, Response):
+            return result
+        return jsonable_encoder(result)
+
+    return wrapper
+
+
 def _adapt_route(route: APIRoute) -> APIRoute:
     methods = route.methods or set()
     if len(methods) != 1:
@@ -286,6 +303,9 @@ def _adapt_route(route: APIRoute) -> APIRoute:
 
     endpoint = _with_header_project_context(route.endpoint, route.name)
     response_model = route.response_model
+    has_untyped_response = response_model is None
+    if has_untyped_response:
+        endpoint = _with_jsonable_response(endpoint)
     if get_origin(response_model) is list:
         item_type = get_args(response_model)[0] if get_args(response_model) else JsonValue
         response_model = Page[item_type]

tests/api/test_openapi_contract.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from datetime import datetime, timezone
 from pathlib import Path
 from uuid import uuid4
 
@@ -282,3 +283,35 @@ def test_rest_runtime_wraps_handler_paginated_list() -> None:
         "limit": 2,
         "offset": 1,
     }
+
+
+def test_rest_runtime_json_encodes_untyped_datetime_response() -> None:
+    source_router = APIRouter()
+
+    @source_router.get("/simulation-result")
+    async def simulation_result():
+        return {
+            "result": [
+                {
+                    "time": datetime(2026, 7, 30, 4, tzinfo=timezone.utc),
+                    "id": "4277",
+                }
+            ]
+        }
+
+    app = FastAPI(redirect_slashes=False)
+    app.include_router(build_rest_router(source_router.routes), prefix="/api/v1")
+
+    response = TestClient(app, raise_server_exceptions=False).get(
+        "/api/v1/simulation-result"
+    )
+
+    assert response.status_code == 200
+    assert response.json() == {
+        "result": [
+            {
+                "time": "2026-07-30T04:00:00+00:00",
+                "id": "4277",
+            }
+        ]
+    }