[bug] opencode provider: Muse Spark contributor-free fails with "Invalid upload request", other free models work

Description

Summary

Streaming with muse-spark-1.3-contributor-free (and 1.2) via the
opencode provider fails with
[invalid_request_error] Invalid upload request.
It happens on old AND brand-new sessions. mimo-v2.5-free on the same
setup works fine, so this looks like an upstream (Console → Muse Spark
free tier) rejection, not local config.

Error

AI_APICallError: Error from provider (Console): Upstream request failed:
[invalid_request_error] Invalid upload request.

Repro

1. OpenCode Desktop, provider opencode, model
   muse-spark-1.3-contributor-free
2. New empty session, send any message
3. Stream fails within seconds with the error above
   (intermittent: occasionally a stream succeeds, e.g. session
   ses_f9a6... worked while ses_f94e... failed minutes apart)

Evidence (opencode.log)

2026-09-04T06:07:48Z stream error modelID=muse-spark-1.3-contributor-free
session.id=ses_fa2a86d1bffefzmljvvD0sB5l4
2026-09-04T06:08:39Z stream error modelID=muse-spark-1.2-contributor-free
(same session)
2026-09-04T06:09:16-49Z modelID=mimo-v2.5-free streams OK
(compaction + build, same session)
2026-09-04T06:32:21Z stream error modelID=muse-spark-1.3-contributor-free
session.id=ses_f94e20a8bffeakd3FQqb8L5HT1 (fresh session)

Tried, did not help

• New sessions, app restart
• Cleared %USERPROFILE%\.cache\opencode (renamed, rebuilt on launch)
• Desktop is on latest channel

Environment

• OpenCode Desktop 1.18.27, Windows (win32)
• providerID=opencode, failing: muse-spark-1.2/1.3-contributor-free,
  working: mimo-v2.5-free

Plugins

No response

OpenCode version

No response

Steps to reproduce

No response

Screenshot and/or share link

No response

Operating System

No response

Terminal

No response