v2: subagent spawn ignores agents.*.model and uses parent session model

Description

On OpenCode 2.0.8, agents.<name>.model is loaded into the agent catalog but is not applied when a subagent is spawned.

GET /api/agent shows the configured model on general / explore. The child session still runs with the parent session model. The parent variant is also dropped (#xhigh → #default).

This matches the V2 agents docs ("a subagent uses its configured model, or inherits the parent session model when none is configured") but not the current runner, which resolves only session.model.

The parent subagent tool schema also says not to pass model unless the user names one, so the configured agent model is never applied on the default path.

Related: #21952 #36250 #36289 #37797. Those were mostly V1; this is a V2 2.0.8 regression/gap.

Plugins

None

OpenCode version

2.0.8 (OpenCode Desktop, latest)

Steps to reproduce

1. Set the primary session to model A, with a non-default variant (example: provider-a/model-a#xhigh).
2. Pin a different model B on the built-in subagents in opencode.jsonc:

{
  "agents": {
    "general": {
      "mode": "subagent",
      "model": "provider-b/model-b#high"
    },
    "explore": {
      "mode": "subagent",
      "model": "provider-b/model-b#high"
    }
  }
}

3. Start a new session. Confirm the catalog:

GET /api/agent

general / explore resolve to provider-b/model-b with variant high.

4. Ask the primary agent to spawn general or explore. Do not name a model in the prompt (so the subagent tool does not receive model).
5. Inspect the child session:

GET /api/session/{childID}

Expected Behavior

Child session uses the agent catalog model: provider-b/model-b#high.

Actual Behavior

Child session uses the parent model with variant reset:

• parent: provider-a/model-a#xhigh
• child general sessions: provider-a/model-a#default

Observed on three child sessions from one parent. Reproducible, not intermittent. Custom providers were used in the original run; the mismatch is catalog model B vs spawned session model A.

packages/core/src/session/runner/model.ts (SessionRunnerModel.resolve) selects from session.model only. agent.model is never consulted. packages/core/src/tool/builtins.ts still lists task as not ported.

Screenshot and/or share link

N/A. API evidence (IDs generalized):

GET /api/agent
  → general.model = { id: model-b, providerID: provider-b, variant: high }

GET /api/session/{child}
  → agent=general, model={ id: model-a, providerID: provider-a, variant: default }

Operating System

Windows 11 (10.0.26200.9457)

Terminal

OpenCode Desktop (@opencode-aidesktop)

Start with packages/core/src/session/runner/model.ts and inspect SessionRunnerModel.resolve, then trace the subagent spawn entry point in packages/core/src/tool/builtins.ts. Reproduce the issue with GET /api/agent and GET /api/session/{childID}. Done means a configured agent model and variant are used for the child session, while an unconfigured agent still inherits the parent model.