Web UI ignores default_agent — new sessions always start on built-in build agent #45873

Summary

The default_agent config option does not apply to the Web UI (opencode web): new sessions started in the browser always default to the built-in build agent, even when default_agent is set to a valid custom primary agent.

Docs vs behavior

The default_agent docs state:

This setting applies across all interfaces: TUI, CLI (opencode run), desktop app, and GitHub Action.

The Web UI is not listed, and in practice it does not honor default_agent — it starts new sessions on build regardless of the configured default.

Reproduction

1. Set in ~/.config/opencode/opencode.json:

   { "default_agent": "my-agent" }

   where my-agent is a defined custom agent with "mode": "primary".
2. TUI and opencode run start on my-agent. ✅
3. Start opencode web, create a new session → it starts on build. ❌

Expected behavior

Either:

• the Web UI honors default_agent for new sessions like the TUI/CLI do, or
• the docs explicitly state that the Web UI has a different (non-configurable) default, so users know to expect it.

Environment

• opencode version: 1.18.23
• global config, no project-level config overriding
• agent is a valid primary agent (not a subagent, exists) — so the documented "fall back to build with a warning" path is not triggered