DeepSeek native Responses: upstream SSE socket closed unexpectedly (upstream_reset), deterministic #6187

Client or integration

Codex App

Provider or upstream service

deepseek

OpenCodex version

2.69.0

Endpoint or capability

streaming

Current behaviour

A request with stream:true deterministically ends in response.failed:
{"type":"upstream_error","code":"upstream_reset","message":"Upstream stream terminated unexpectedly: The socket connection was closed unexpectedly."}
Identical on every run. Via the ChatGPT client it surfaces as 502 upstream_server_error, failureStage=protocol-prelude, disconnecting after ~336ms.

Expected behaviour

The upstream SSE stream should stay alive — at least no worse than a direct curl, which streams DeepSeek /responses end-to-end without errors.

Minimal redacted request or reproduction

$b = @{ model = "deepseek/deepseek-flash"; input = "hi"; max_output_tokens = 10; stream = $true } | ConvertTo-Json -Depth 5
Invoke-RestMethod -Uri "http://localhost:10100/v1/responses" -Method Post -Body $b -ContentType "application/json" -TimeoutSec 60

Actual response or error

event: response.failed
data: {"type":"response.failed","response":{"status":"failed","error":{"type":"upstream_error","code":"upstream_reset","message":"Upstream stream terminated unexpectedly: The socket connection was closed unexpectedly."}}}
data: [DONE]
 HTTP 502 upstream_server_error.

Upstream documentation

https://api-docs.deepseek.com/guides/responses_api
Consider re-applying the bounded-JSON upstream policy (#1026) for DeepSeek as a fallback when upstream SSE fails in proxied environments.

Suggested mapping or implementation notes

No response

Additional context and attachments

• OpenCodex 2.69.0, Windows, PowerShell
• Transparent TUN proxy in use; api.deepseek.com resolves to fake-IP 198.18.0.27
• Failure is deterministic: identical on every run
• Control cases: non-streaming via OpenCodex OK; direct curl -N streaming to DeepSeek OK
• Related: fix(deepseek): transport-neutral bounded-JSON policy with terminal SSE synthesis (#875) #1026 (bounded-JSON policy), fix(responses): restore DeepSeek streaming and routed code-mode tools #1361 (restored upstream streaming), fix(provider): treat deepseek-flash as native multimodal #4500 (multimodal)

Checks

• I searched existing provider and compatibility issues.
• The request and response were redacted.
• The expected behaviour is based on an upstream specification or a concrete client requirement.