subagent: child sessions fail on providers that only support tool_choice auto #54772

Summary

Subagent child sessions fail immediately with only "auto" is supported for tool_choice on providers/models that don't accept tool_choice values "none", "required", or named function choices. This makes file-based subagents unusable on those providers and blocks autonomous multi-agent workflows.

Environment

• opencode version: 2.0.26
• OS: Windows_NT 10.0.19045 (win32 x64)
• Terminal: TERM=xterm-256color
• Shell: C:\WINDOWS\system32\cmd.exe
• Install/channel: latest
• Active plugins: None found in config

Reproduction

1. Define a file-based subagent (.opencode/agents/*.md, mode: subagent) with model: pointing at a provider that only supports tool_choice: "auto" (observed with opencode/muse-spark-1.3-contributor-free, all variants and no-variant).
2. Invoke it via the subagent tool from a parent session.
3. Observe immediate failure; repeated across 6+ invocations and 3 different agent definitions.

Expected Behavior

The child session runs with tool_choice: "auto" (or the runner avoids setting an unsupported value), so subagents work on any provider.

Actual Behavior

Subagent failed (sessionID: ses_...): only `"auto"` is supported for
`tool_choice`. `"none"`, `"required"`, and named function choices are
not currently supported

Failure is intermittent per call (2 of ~8 calls partially/completely succeeded and produced correct file output before the error surfaced), suggesting the unsupported value is set on some code paths (possibly final-step tool removal or a specific tool call) rather than unconditionally. No user-side workaround: model variants, prompt changes, and service restarts don't help.

Additional Context

• Frequency: consistent-but-intermittent — same agent+prompt fails instantly on some attempts and executes (partially or fully) on others.
• Related misleading error: when the same runner hits adacode API 500s on completions, opencode surfaces Unauthorized: API key tidak dikenali although the key is valid (verified via direct /v1/models call, HTTP 200). Possibly a separate error-mapping issue.
• Impact: blocks the documented primary→subagent orchestration pattern on affected providers; only provider with reliable subagent runs in our testing was adacode post-purchase (when its API wasn't 500ing).
• Workarounds tried (no help): removing model #variant, opencode reload, opencode service restart (pids cycled, port stable).