bug: Hermes Agent — gpt-5.6-luna via opencode-go provider returns finish_reason:null (no [DONE])

Description

When using opencode-go as a provider in Hermes Agent, every gpt-5.6-luna response fails. The OpenCode Go gateway (https://opencode.ai/zen/go/v1/chat/completions) never sends a terminal finish_reason for this model — streamed or non-streamed: non-streaming returns "finish_reason":null on a complete response, and streaming ends with data: {"choices":[],"cost":"0"} and no [DONE] marker. Hermes requires a terminal event and retries, then errors with "Response remained truncated after 4 continuation attempts" — gpt-5.6-luna is unusable via this provider. All other Go models tested (deepseek-v4-flash, deepseek-v4-pro, kimi-k2.7-code, glm-5.2) return finish_reason: "stop" correctly. Related to but distinct from #36766 (truncated tool args on native OpenAI Responses) — this is the Go gateway omitting the terminal event.

Plugins

N/A — no OpenCode plugins. Consumer is Hermes Agent v0.19.1 (docker, Linux), which fails on every gpt-5.6-luna response.

OpenCode version

N/A — issue is against the hosted Go gateway endpoint, not a local OpenCode install.

Steps to reproduce

1. POST /chat/completions, model: gpt-5.6-luna, stream: false → HTTP 200, content correct, choices[0].finish_reason: null
2. Same with stream: true → content chunk arrives with finish_reason: null, then final chunk {"choices":[],"cost":"0"} — no [DONE]
3. Control (same calls, deepseek-v4-flash / kimi-k2.7-code / glm-5.2 / deepseek-v4-pro) → finish_reason: "stop" and a proper [DONE] chunk

Screenshot and/or share link

No UI screenshot applicable (API-level bug). Verbatim captures from a fresh repro run:

Non-streaming:

{"id":"gen-1785843293-273noTConb4mUERLVDI2","object":"chat.completion","created":1785843294,"model":"gpt-5.6-luna","choices":[{"index":0,"message":{"role":"assistant","content":"OK"},"finish_reason":null}],"usage":{"prompt_tokens":11,"completion_tokens":5,"total_tokens":16}}

Streaming (all data: lines):

data: {"id":"gen-...","object":"chat.completion.chunk","choices":[]}
data: {"id":"","object":"chat.completion.chunk","choices":[{"index":0,"delta":{"content":"OK"},"finish_reason":null}]}
data: {"id":"gen-...","choices":[],"usage":{"prompt_tokens":11,"completion_tokens":5,"total_tokens":16}}
data: {"choices":[],"cost":"0"}

Operating System

Linux (Docker container)

Terminal

N/A — no terminal UI involved. Reproduced with plain curl from a non-interactive container shell (TERM=dumb, no TTY).

Start with the OpenCode Go gateway's /chat/completions behavior for gpt-5.6-luna, using the curl reproduction and comparing it with the listed control models. Check both streaming and non-streaming responses; done means a terminal finish_reason is returned and streaming ends with [DONE], so Hermes no longer retries and reports truncation.