The custom ``httpx.HTTPTransport(socket_options=[SO_KEEPALIVE, ...])``
in ``_build_keepalive_http_client()`` was introduced to fix CLOSE-WAIT
socket accumulation on long-lived connections (#10324).
That approach broke streaming for providers behind reverse proxies
(OpenResty, Cloudflare, etc.) because the custom socket options
conflict with the proxy's chunked-transfer handling (#54049, #12952).
It also stripped TCP_NODELAY, stalling TLS handshakes and SSE encoding.
Narrow per-provider bypasses were added for Copilot (#50298), Codex
(#36623, #12953), but the root cause remained.
The fix moves connection lifecycle management from the socket layer to
the HTTP pool layer:
- ``httpx.Limits(keepalive_expiry=20.0)`` tells httpx to close idle
pooled connections at 20 s, before a reverse proxy's typical 30-60 s
timeout drops them and causes CLOSE-WAIT accumulation.
- The default httpx transport preserves OS TCP defaults (including
TCP_NODELAY), so TLS handshakes and SSE chunked encoding work
correctly.
- ``trust_env=False`` prevents httpx from double-dipping on env vars
(we handle proxy detection ourselves via ``_get_proxy_for_base_url``
which respects NO_PROXY).
- The Copilot host bypass (line 3632) is no longer needed since all
providers now use the same standard httpx.Client.
Closes#54049. Supersedes #12010, #36623, #12953, #50298.
