[Bug] Model parameter ignored when launching subagent with agent name

[iceblove]

Summary

When launching a subagent session via the SDK with both agent and model parameters, opencode ignores the model parameter and uses its built-in fallback chain for that agent instead.

Environment

• opencode version: 1.2.27
• Platform: macOS (darwin)

Steps to Reproduce

1. Configure a custom provider with models:

// ~/.config/opencode/opencode.json
{
  "provider": {
    "opencode-go": {
      "npm": "@ai-sdk/anthropic",
      "options": {
        "baseURL": "https://cloud.infini-ai.com/maas/coding/v1",
        "apiKey": "{env:GENSTUDIO_API_KEY}"
      },
      "models": {
        "minimax-m2.7": { "name": "MiniMax M2.7" },
        "glm-5": { "name": "GLM-5" }
      }
    }
  }
}

2. Verify the provider and models work correctly:

$ opencode models opencode-go
opencode-go/glm-5
opencode-go/minimax-m2.7

$ opencode run -m opencode-go/minimax-m2.7 "Say: test"
# Works correctly! Uses opencode-go/minimax-m2.7

3. Launch a subagent via SDK with explicit model parameter:

await client.session.promptAsync({
  path: { id: sessionID },
  body: {
    agent: "explore",
    model: { providerID: "opencode-go", modelID: "minimax-m2.7" },
    parts: [{ type: "text", text: "Say what model you are using" }]
  }
});

4. Check the database record:

SELECT data FROM message WHERE agent = 'explore' AND role = 'user' ORDER BY time_created DESC LIMIT 1;
-- Result: {"agent":"explore","model":{"providerID":"opencode","modelID":"minimax-m2.5-free"},...}
-- The model is WRONG! It's using opencode/minimax-m2.5-free instead of opencode-go/minimax-m2.7

Expected Behavior

When model parameter is explicitly provided, it should be used as-is. The agent's built-in fallback chain should only be used when NO model parameter is provided.

Actual Behavior

The model parameter is ignored when an agent is specified. OpenCode uses the agent's built-in fallback chain, which resolves to opencode/minimax-m2.5-free (a free built-in model).

Evidence

Database records showing the issue:

1. Direct model usage (works correctly):

SELECT modelID, providerID FROM message WHERE role = 'assistant' ORDER BY time_created DESC LIMIT 1;
-- Result: modelID = "minimax-m2.7", providerID = "opencode-go" ✓

2. Subagent with agent name (broken):

SELECT json_extract(data, '$.model') FROM message WHERE agent = 'explore' AND role = 'user' ORDER BY time_created DESC LIMIT 1;
-- Result: {"providerID":"opencode","modelID":"minimax-m2.5-free"} ✗

Root Cause

OpenCode has built-in fallback chains for agents like explore, librarian, etc. When processing a session prompt:

• If agent is specified, OpenCode appears to use the agent's fallback chain
• The explicitly provided model parameter is overridden

Why This Matters

1. Plugin compatibility: Plugins like oh-my-opencode configure custom models for agents, but these are ignored
2. User choice: Users cannot override agent models even when explicitly specifying them
3. Cost control: Users may want to use their own API keys with specific providers, but fall back to built-in free models instead

Suggested Fix

The priority should be:

1. User-provided model (highest priority) - use as-is
2. Agent's built-in fallback chain (fallback) - only when no model is provided
3. System default (last resort)

Related Issue

Also reported to oh-my-opencode: code-yeongyu/oh-my-openagent#2741

Initially thought it was an oh-my-opencode issue, but database evidence shows the model is already wrong in the user message record, indicating the issue is in opencode's session handling.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Server API ignores agent's configured model when only agent parameter is passed #7099: Server API ignores agent's configured model when only agent parameter is passed — closely related; covers the broader pattern of model selection being ignored when an agent is specified via the API
• [Bug] Subagent spawned via Task tool uses global config model instead of inheriting parent session's active model #17870: Subagent spawned via Task tool uses global config model instead of inheriting parent session's active model — related model-override-being-ignored behavior

Your issue has a distinct angle (explicitly providing model in the SDK call alongside agent, and still having it overridden), so it may warrant staying open as a separate report if the root cause differs.

[iceblove]

Thank you for pointing out the related issues. I've reviewed them and believe this issue has a distinct angle that warrants staying open.

Comparison with Related Issues

#7099 (Server API ignores agent's configured model when only agent parameter is passed)

• Issue: When passing agent WITHOUT model, the agent's configured default model is ignored
• Our case: We're passing BOTH agent AND model explicitly, and the model is still being overridden

#17870 (Subagent spawned via Task tool uses global config model instead of inheriting parent session's active model)

• Issue: Subagent doesn't inherit parent session's model when no explicit model is passed
• Our case: We're explicitly passing model in the SDK call, but it's being ignored

Distinct Angle

This issue is specifically about: Explicit model parameter being ignored when agent parameter is also present.

// Both parameters are explicitly provided
await client.session.promptAsync({
  body: {
    agent: "explore",
    model: { providerID: "opencode-go", modelID: "minimax-m2.7" },  // ← Explicitly provided
    parts: [{ type: "text", text: "..." }]
  }
});

// Expected: Use opencode-go/minimax-m2.7
// Actual: Uses opencode/minimax-m2.5-free (from agent's built-in fallback chain)

Root Cause Hypothesis

It appears that opencode's model resolution logic has this priority:

1. Agent's built-in fallback chain (when agent is present)
2. User-provided model parameter ← This should have higher priority
3. System default

The fix should ensure that an explicitly provided model parameter always takes precedence over any agent-based model selection.

Evidence

Database record shows the model is already wrong in the user message:

{
  "agent": "explore",
  "model": {"providerID": "opencode", "modelID": "minimax-m2.5-free"}  // Wrong!
}

This indicates the model is overridden at the session handling level, before the message is even stored.

---

Happy to close this as a duplicate if you determine the root cause is the same. However, I believe the fix for this case (respect explicit model parameter) may differ from the other issues (respect agent config or inherit parent session model).