[Zen/GO Gateway] Short reply via Anthropic-compat endpoint missing stop_reason -> client triggers false retry (duplicate output)

Summary

When using the Zen/GO gateway (https://opencode.ai/zen/go/v1) via its Anthropic-compatible endpoint, very short completions cause the client to think the stream was truncated and auto-retry with Continue exactly where you left off, producing garbled duplicate output.

Root cause appears to be OpenAI -> Anthropic streaming translation: the final stop_reason / message_stop event is missing or not forwarded for short replies, so Anthropic clients never receive a clean termination.

Repro

• Gateway: https://opencode.ai/zen/go/v1 (Zen/GO)
• Client: Hermes Agent Desktop (Anthropic-compatible client)
• Model: muse-spark-1.2-contributor via opencode-go (DeepSeek channel also affected, but repro here is Muse Spark)
• Prompt: trivial short prompt, e.g. "测试新模型是否能正常对话，不用过多的回复" / "test if model can talk normally, short reply only"
• Expected: content_block_stop + message_stop with stop_reason: "end_turn" and stream ends cleanly.
• Actual: content arrives correctly ("正常，能对话" / single short sentence), but stream ends without proper stop_reason. Client interprets as network truncation.

Observed behavior

Hermes injects system message:

[System: The previous response was cut off by a network error mid-stream. Continue exactly where you left off. Do not restart or repeat prior text. Finish the answer directly.]

Model then emits a duplicate/continuation (e.g. "✓", "正常", then repeated system injections), visible as "乱七八糟东西" to user. Turning off Hermes retry still leaves the stream technically incomplete per Anthropic spec.

Longer replies are less likely to trigger this; short 1-2 token replies reproduce reliably (3/3 in session).

Hypothesis

Zen gateway translates OpenAI streaming (choices[0].finish_reason + data: [DONE]) to Anthropic SSE (message_delta with stop_reason, message_stop). For very short completions the final delta is dropped or stop_reason is null, so message_stop never fires.

Expected fix

Ensure for every OpenAI stream termination (including finish_reason: "stop" on short content), the gateway always emits:

event: message_delta
data: {"delta":{"stop_reason":"end_turn","stop_sequence":null}, ...}
event: message_stop

even when content is 1-2 tokens.

Evidence

• Gateway base_url: https://opencode.ai/zen/go/v1 with User-Agent: OpenCode/1.0
• Token: opencode-go subscription ($10/mo, Zen)
• Client logs show Hermes adding the continuation prompt only after GO stream ends without stop signal (not a client network drop).
• Same model via native OpenAI endpoint does not exhibit this (only via Anthropic-compat translation).

Impact

Any Anthropic-compat client (Hermes, Claude Code, OpenCode) using Zen/GO will see occasional duplicate/garbled output on short replies. Looks like "GO bug + Hermes over-eager retry" but primary fix is in gateway translation.

Happy to provide a minimal SSE dump if needed.