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The shared proxy client keeps httpx's default limit of 100 connections, and streamed responses hold theirs until they finish. Once all of them are busy, a new proxied request waits for the client timeout and then raises httpx.PoolTimeout, which none of the backend failure handlers catches, so the client got a 500 that looks like a server bug. Map it to 503 PROXY_POOL_EXHAUSTED: the server is out of capacity, and the sandbox itself was never reached.
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October 31, 2026 08:41
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#2084 mapped proxied backend failures to gateway statuses (502
BACKEND_CONNECTION_FAILED, 504BACKEND_TIMEOUT, 502BACKEND_RESPONSE_FAILED), so that a sandbox problem no longer looks like a server bug. One timeout was left out:httpx.PoolTimeout.The proxy shares one
httpx.AsyncClient(timeout=180.0), which keeps httpx's default limit of 100 connections. A streamed response (an SSE stream, a long poll, a large download) holds its connection until it finishes. Once all of them are busy, the next proxied request waits 180 seconds for a free connection and then raisesPoolTimeout. That isn't caught by any of the backend handlers, so it ends in the generic handler and the client gets500 GENERAL::UNKNOWN_ERROR, "An internal error occurred in the proxy".Summary
httpx.PoolTimeoutnow returns 503 with codePROXY_POOL_EXHAUSTED. The server is out of capacity and the sandbox was never contacted, so 503 fits better than 504, and clients can retry.Testing
Not run (explain why)
Unit tests
Integration tests
e2e / manual verification
Added a
PoolTimeoutcase totest_proxy_maps_backend_failure_after_connect_to_gateway_error. It fails on current main with 500.The full server suite passes (2698 passed).
ruffandpyrightare clean.Breaking Changes
Checklist