Zen gateway does not forward reasoning/thinking stream for Muse Spark 1.2

Summary

Muse Spark 1.2 (muse-spark-1.2, muse-spark-1.2-contributor) supports reasoning/thinking, but the Zen gateway (https://opencode.ai/zen/go/v1) does not expose the reasoning stream to OpenAI-compatible clients. When streaming via either /v1/chat/completions or /v1/responses, clients receive only the final assistant text; no reasoning_content / reasoning_details / thinking delta is ever emitted, so UIs cannot render a thinking bubble even though the model is reasoning on the server side.

This is distinct from #43379 (missing finish_reason on streaming completions for muse-*). That issue blocks chat/completions entirely. After pinning muse-spark to responses (as done in oh-my-pi #8980), calls succeed but the thinking remains invisible.

Steps to Reproduce

1. Subscribe to OpenCode Go and obtain a Zen API key.
2. Stream a request for muse-spark-1.2-contributor via the Zen gateway:
   • POST https://opencode.ai/zen/go/v1/responses with "stream": true and reasoning: { effort: "high" } (Responses API), or
   • POST https://opencode.ai/zen/go/v1/chat/completions with "stream": true (OpenAI chat completions)
3. Observe the SSE stream.

Expected Behavior

Per OpenAI Responses / chat completions streaming conventions, the gateway should forward reasoning deltas when the upstream model produces them, e.g.:

• Responses API: response.output_text.delta for reasoning or a dedicated reasoning event
• Chat completions: choices[0].delta.reasoning_content (or equivalent reasoning field)

Clients that render thinking (Hermes, oh-my-pi, other agent UIs) rely on these deltas to show progress.

Actual Behavior

• Only output_text / choices[0].delta.content is streamed.
• No reasoning delta is emitted at any point, even with reasoning.effort = high.
• The model does reason server-side (higher latency and token usage vs effort: none), but the thinking is not observable client-side.

Environment

• Endpoint: https://opencode.ai/zen/go/v1/{responses,chat/completions}
• Models: muse-spark-1.2, muse-spark-1.2-contributor (alias)
• Client: any OpenAI-compatible streaming client (Hermes Agent, oh-my-pi)
• Auth: Zen Go API key

Impact

• Users cannot see or debug the model's reasoning, making long turns appear as hangs.
• Agent UIs that gate on reasoning (e.g., showing a spinner/thinking block) show no progress until the final answer arrives.

Suggested Fix

• For /v1/responses: forward upstream reasoning events as Responses API reasoning deltas.
• For /v1/chat/completions: map reasoning to delta.reasoning_content (or the gateway's chosen field) when streaming.
• If upstream does not provide a structured reasoning stream, consider exposing it once available rather than swallowing it.

Related

• #43379 — missing finish_reason for muse-* on chat/completions (blocking, open)
• can1357/oh-my-pi#8957 / #8980 — pinned muse-spark to responses to work around #43379 (merged)
• This issue remains after that workaround: calls succeed, but reasoning is still not forwarded.

Happy to provide additional logs or test against a staging gateway if helpful.