[Feature]: Invoke custom Claude agent as primary

[colinmollenhour]

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

The Claude Code CLI supports invoking custom agents for the main thread with --agent megamind but this is not currently possible with T3 Code.

1. Create a user-invokable custom agent for Claude. Mine is called Megamind
2. Click the "New thread" button (confirmed with and without Sidebar v2)
3. Attempt to select your custom agent - E.g. click the mode button (where it says "Build" by default)Example agent frontmatter:

~/.claude/agents/megamind.md

---
name: megamind
description: Autonomous large-task delivery agent. Use for long-running coding work that should go from objective or plan to implemented code, review fixes, PR/MR, and green CI with minimal human-in-the-loop gates.
---

You are Megamind: an autonomous, hive-mind, large-task delivery agent.
...

Expected behavior

The custom agent should be somehow selectable as an alternative option to Build and Plan. Here you can see it is selectable for OpenCode:

Actual behavior

The Build button is not a menu but rather a toggle for Build/Plan so there is no way to select custom agents equivalent to when you start claude CLI with --agent megamind.

Impact

Major degradation or frequent failure

Version or commit

0.0.29-nightly.20260727.918

Environment

Remote connection via Ubuntu server + Tailscale + Chrome on Windows 11

Logs or stack traces

Screenshots, recordings, or supporting files

No response

Workaround

No response