The Fix
Fixes the issue where routing to a sub-application added via .add_domain() does not work if the same path exists on the parent app.
Based on closed aio-libs/aiohttp issue #11665 · PR/commit linked
Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.
@@ -0,0 +1,2 @@
@@ -0,0 +1,2 @@
+Fixed routing to a sub-application added via ``.add_domain()`` not working
+if the same path exists on the parent app. -- by :user:`Dreamsorcerer`.
diff --git a/aiohttp/web_urldispatcher.py b/aiohttp/web_urldispatcher.py
repro.py
async def test_subapp_domain_routing(aiohttp_client: AiohttpClient) -> None:
app = web.Application()
subApp = web.Application()
class MyViewWrong(web.View):
async def get(self) -> web.Response:
return web.Response(status=404)
class MyViewCorrect(web.View):
async def get(self) -> web.Response:
return web.Response()
app.router.add_routes([web.view("/", MyViewWrong)])
subApp.router.add_routes([web.view("/", MyViewCorrect)])
app.add_domain('different.example.com', subApp)
client = await aiohttp_client(app)
headers = {'Host': 'different.example.com'}
async with client.get("/", headers=headers) as r:
assert r.status == 200
verify
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
fix.md
Option A — Apply the official fix\nFixes the issue where routing to a sub-application added via .add_domain() does not work if the same path exists on the parent app.\nWhen NOT to use: This fix is not applicable if the main app should handle the same path as the sub-app.\n\n
Why This Fix Works in Production
- Trigger: E AssertionError: assert 404 == 200
- Mechanism: Fixes the issue where routing to a sub-application added via .add_domain() does not work if the same path exists on the parent app.
Production impact:
- If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.
Why This Breaks in Prod
- Shows up under Python 3.13.7 in real deployments (not just unit tests).
- Surfaces as: /home/me/aiohttp-1/tests/test_web_urldispatcher.py::test_subapp_domain_routing[pyloop] failed: aiohttp_client = <function aiohttp_client.<locals>.go at 0x775d3d3b56c0>
Proof / Evidence
- GitHub issue: #11665
- Fix PR: https://github.com/aio-libs/aiohttp/pull/11673
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.80
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.33
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“When tweaking the tests, I noticed that it works correctly if you don't have a handler on the main app. So, the routing is still…”
Failure Signature (Search String)
- E AssertionError: assert 404 == 200
Error Message
Stack trace
error.txt
Error Message
-------------
/home/me/aiohttp-1/tests/test_web_urldispatcher.py::test_subapp_domain_routing[pyloop] failed: aiohttp_client = <function aiohttp_client.<locals>.go at 0x775d3d3b56c0>
async def test_subapp_domain_routing(aiohttp_client: AiohttpClient) -> None:
app = web.Application()
subApp = web.Application()
class MyViewWrong(web.View):
async def get(self) -> web.Response:
return web.Response(status=404)
class MyViewCorrect(web.View):
async def get(self) -> web.Response:
return web.Response()
app.router.add_routes([web.view("/", MyViewWrong)])
subApp.router.add_routes([web.view("/", MyViewCorrect)])
app.add_domain('different.example.com', subApp)
client = await aiohttp_client(app)
headers = {'Host': 'different.example.com'}
async with client.get("/", headers=headers) as r:
assert r.status == 200
E AssertionError: assert 404 == 200
E + where 404 = <ClientResponse(http://127.0.0.1:36377/) [404 Not Found]>\n<CIMultiDictProxy('Content-Length': '0', 'Date': 'Thu, 16 Oct 2025 05:35:55 GMT', 'Server': 'Python/3.13 aiohttp/4.0.0a2.dev0')>\n.status
MyViewCorrect = <class 'test_web_urldispatcher.test_subapp_domain_routing.<locals>.MyViewCorrect'>
MyViewWrong = <class 'test_web_urldispatcher.test_subapp_dom
... (truncated) ...
Minimal Reproduction
repro.py
async def test_subapp_domain_routing(aiohttp_client: AiohttpClient) -> None:
app = web.Application()
subApp = web.Application()
class MyViewWrong(web.View):
async def get(self) -> web.Response:
return web.Response(status=404)
class MyViewCorrect(web.View):
async def get(self) -> web.Response:
return web.Response()
app.router.add_routes([web.view("/", MyViewWrong)])
subApp.router.add_routes([web.view("/", MyViewCorrect)])
app.add_domain('different.example.com', subApp)
client = await aiohttp_client(app)
headers = {'Host': 'different.example.com'}
async with client.get("/", headers=headers) as r:
assert r.status == 200
Environment
- Python: 3.13.7
What Broke
Requests to sub-app domains return 404 instead of the expected response.
Fix Options (Details)
Option A — Apply the official fix
Fixes the issue where routing to a sub-application added via .add_domain() does not work if the same path exists on the parent app.
When NOT to use: This fix is not applicable if the main app should handle the same path as the sub-app.
Fix reference: https://github.com/aio-libs/aiohttp/pull/11673
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not applicable if the main app should handle the same path as the sub-app.
Verify Fix
verify
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
Did This Fix Work in Your Case?
Quick signal helps us prioritize which fixes to verify and improve.
Prevention
- Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
- Pin production dependencies and upgrade only with a reproducible test that hits the failing path.
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.