[Bug] OpenCode Go: deepseek-v4.1-flash 403 "Upstream response was not valid JSON" with large payload #50897

Description

On OpenCode Go (subscription), deepseek-v4.1-flash returns HTTP 403 Upstream request failed: [server_error] Upstream response was not valid JSON ONLY when the request combines reasoning_effort: "high" with a large payload (~600 KB / ~150k tokens).
Same request with reasoning_effort: "low" or without the field succeeds. Small payloads with "high" succeed. Failing intermittently since 2026-09-23 09:15 UTC; the model worked fine before (800+ successful calls on the same account).

Server-side console log of a failing request:

{ "requestedModel": "deepseek-v4.1-flash", "model": "deepseek-v4p1-flash",
  "provider": "opencode", "statusCode": 403,
  "errorCode": "upstream_response_status_not_200",
  "reasoningMode": "effort", "reasoningEffort": "high", "stream": true,
  "path": "/inference/go/openai/v1/chat/completions",
  "attempts": [{ "provider": "opencode", "model": "deepseek-v4p1-flash",
                 "statusCode": 403, "durationMs": 44,
                 "errorCode": "upstream_response_status_not_200" }] 
}

Note: deepseek-v4p1-flash (the id actually served) is NOT listed in GET /zen/go/v1/models.

Plugins

None (client is OpenAI-compatible, not the opencode CLI)

OpenCode version

N/A as CLI — failing client is Hermes Agent 0.21.3 (User-Agent: HermesAgent/0.21.3) hitting the Go server API

Steps to reproduce

1. POST https://opencode.ai/zen/go/v1/chat/completions (same as /inference/go/openai/v1/chat/completions) with model=deepseek-v4.1-flash, stream: true, tools, messages ≈ 600 KB (~150k tokens), and reasoning_effort: "high". Header x-opencode-session: .
2. Response: HTTP 403 {"error":{"type":"server_error","code":"server_error", "message":"Upstream request failed: [server_error] Upstream response was not valid JSON"}}.
3. Repeat the identical request with reasoning_effort: "low" → HTTP 200, full streamed reply.
4. Repeat without reasoning_effort → HTTP 200.
5. Small payload + reasoning_effort: "high" → HTTP 200.
   This A/B was verified on 2026-09-23 ~11:35 UTC.

Screenshot and/or share link

No response

Operating System

Linux (headless container, Debian-based)

Terminal

N/A (server-side API call, not a terminal client)