Support OPENCODE_API_KEY passthrough and model selection for OpenCode agent

[NathanFlurry]

Context

When running OpenCode CLI locally (without Sandbox Agent), it can use the OPENCODE_API_KEY env var set to a Zen API key. Running opencode auth list shows provider "zen" through the env var. Selecting a model like gemini-3-pro works (though may require billing).

Requests

1. Pass OPENCODE_API_KEY to the OpenCode agent

Sandbox Agent already passes CODEX_API_KEY for the codex agent. It should similarly pass OPENCODE_API_KEY to the OpenCode agent process.

An alternative approach is writing ~/.local/share/opencode/auth.json:

{
  "opencode": {
    "type": "api",
    "key": "sk-..."
  }
}

2. Support model selection per agent in sessions

There's currently no way to specify which model an agent should use when creating a session via the SDK. This blocks testing providers like Zen that require a specific model to be selected.

Related

• Gemini cli api key support for OpenCode #185 — Gemini CLI API key support for OpenCode
• E2B + codex auth using env vars #154 — E2B + codex auth using env vars

[linear]

RVT-6014