[muse] chat/completions returns empty content for muse-spark models, responses works #5426

Summary

Models served through the 9Router muse provider (upstream: Moonshot/Kimi muse-spark family) return empty choices[0].message.content on POST /v1/chat/completions, both streaming and non-streaming. The same models, same key, same gateway return correct answers on POST /v1/responses.

Environment

• 9Router version: 0.5.112 (-p 20128 -H 0.0.0.0 -n --skip-update)
• OS: Linux x86_64, kernel 6.18.54-instabox
• Provider in 9Router: muse (upstream Moonshot/Kimi)
• Auth: API key (redacted, placeholder below)

Affected models (all fail identically on chat/completions)

• muse/muse-spark-1.1
• muse/muse-spark-1.2
• muse/muse-spark-1.2-contributor
• muse/muse-spark-1.3
• muse/muse-spark-1.3-contributor

Actual behavior — chat/completions (non-stream)

{"id":"resp_xxx","object":"chat.completion","created":1793694104,"model":"muse-spark-1.3","choices":[{"index":0,"message":{"role":"assistant","content":""},"finish_reason":"in_progress"}],"usage":{"prompt_tokens":0,"completion_tokens":0,"all_tokens":0}}

Empty content, zero usage, finish_reason: in_progress.

Actual behavior — chat/completions (stream)

Single chunk {"delta":{},"finish_reason":"stop"}, no text deltas.

Variations tried (all still empty)

System prompt, tools: [], reasoning_effort: medium, temperature: 0.7 — identical empty result.

Control test — /v1/responses WORKS (same key, same gateway)

{"model":"muse/muse-spark-1.3","input":"Tulis fungsi python faktorial(n).","max_output_tokens":500,"reasoning":{"effort":"low"}}

Result: status: completed, correct factorial code, output_tokens: 364 (reasoning 302).

Free-tier variants (separate failure mode)

• oc/muse-spark-1.2-contributor-free
• oc/muse-spark-1.3-contributor-free
• oc/union-alpha
• All return 400 Upstream request failed: Model is unavailable.

Minimal reproduction (replace placeholder with a real key)

curl -s http://localhost:20128/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_9ROUTER_KEY" \
  -d '{"model":"muse/muse-spark-1.3","messages":[{"role":"user","content":"Katakan OK"}],"max_tokens":100}'

Expected: choices[0].message.content contains "OK". Actual: empty string.

Hypothesis

The muse provider's Responses-to-chat/completions translation does not map the upstream output message into choices[0].message.content.