agents: mode:all configured model ignored at subagent spawn — parent model inherited #50925

Summary

A markdown agent with mode: all and a frontmatter model: (no #variant) has its configured model silently ignored when invoked via the subagent tool — the child session inherits the parent session's model. The same agent works correctly when its frontmatter says mode: subagent. Same symptom family as #50849, but a distinct trigger (see Additional Context).

Environment

• opencode version: 2.0.10
• OS: Linux 7.0.14-17-pve (linux x64)
• Terminal: xterm-256color (headless opencode serve + TUI session; TERM_PROGRAM not set)
• Shell: /bin/bash
• Install/channel: latest
• Active plugins: none in config (custom plugin files exist in plugins-v2/ but are not referenced from config)

Reproduction

Two custom openai-compatible providers (npm: "@ai-sdk/openai-compatible"), each with a declared model in its models map; both models resolvable (present in GET /api/model). Two agents under ~/.config/opencode/agents/, both using legacy (v1) frontmatter keys (permission: map, temperature):

• A.md: mode: subagent, model: <providerA/modelA>
• B.md: mode: all, model: <providerB/modelB>

Parent session runs model M (different from both). For each configuration, invoke the subagent tool with only agent/description/prompt (no model parameter), then inspect the child via GET /api/session/{childID} and the assistant message metadata in the session store.

Each row was a fresh spawn; only the single named variable changed between rows.

Expected Behavior

Case 1 should run on providerB/modelB — the configured model: applies at spawn regardless of the agent's mode.

Actual Behavior

Case 1 creates the child session with the parent's model M. No error, no warning, nothing in the server log. GET /api/agent/B correctly reports the configured model, so parsing/registry is fine — only spawn-time resolution is wrong.

Additional Context

• Consistently reproducible (2/2 spawns in case 1, v2.0.10, Linux, opencode serve).
• Distinct from core: legacy Markdown agent silently drops "model: provider/model#variant" — subagent inherits parent model #50849 (opened 2026-09-23): that issue is the v1-migration modelSelection() regex rejecting embedded model: provider/model#variant. Here the model string contains no # (so it passes that regex), and the failure tracks the agent's mode — a mode: subagent agent with the identical model string and identical legacy frontmatter resolves correctly (rows 2–4). If the v1 migration path also mishandles mode: all, the two would share an upstream location.
• Related family: Legacy agent Markdown loses a separate model variant #47413 (separate variant: key dropped, fixed by f1eed8b), Subagent spawned via Task tool without its own variant inherits parent's variant concatenated onto model ID (model#variant), causing "Model not found" errors #50698 (parent variant concatenated onto subagent model ID).
• Not tested: mode: all with native (non-legacy) v2 frontmatter — unclear whether the legacy keys interact.
• Workaround: set mode: subagent on the agent (verified, row 4).
• Impact: mode: all is intended for agents usable both as primary and as subagent; the silent fallback to the parent's model makes misconfiguration hard to notice (subagents quietly run on a different/larger model than configured, with no diagnostic anywhere).