Mention a provider, model, and effort in the composer with @ to start a subagent or new thread on it

[itsfaqih]

When I want the agent to hand work to a specific setup, for example "have Codex review this on a high-effort model" or "start a new thread on Claude Opus for the migration", I have to spell out the provider, model, and effort in prose. The agent then has to map that to the right delegate_task options. Nothing in the composer helps me say it precisely, and with several provider instances (multiple accounts, custom instances) a name in prose can be ambiguous.

Proposal

Typing @ in the composer would also offer the user's enabled providers and provider instances, next to the existing file results. Picking one inserts a mention chip such as @Codex or @Claude (work). The chip can be narrowed in the same flow:

• Provider instance: @Claude (work)
• Model: @Claude (work) · Opus
• Effort: @Claude (work) · Opus · high

Each step after the provider is optional. If I stop at the provider, the model and effort fall back to that instance's defaults, as they do today. When the message is sent, the mention resolves to an exact provider instance, model, and effort. The agent receives it as an unambiguous target for delegate_task or a new thread. It does not have to guess from free text.

Example: @Codex · gpt-5 · high review the diff on this branch leads the agent to start a subagent on that Codex instance with that model and effort. The current thread stays on its own provider.