[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

[lukidoescode]

Not every agent needs every skill and every permission. Steering the conversation by forcing an entry point agent to deploy specialist agents and being able to control what context they each get is a powerful tool for significantly raising the quality of the outputs. This would be a killer feature for me, as --agent keeps me consistently coming back to the terminal.