Subagent "Couldn't start" error when switching model mid-conversation

Support Bug Reports
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1

Where does the bug appear (feature/product)?

Cursor IDE

Describe the Bug

When switching the AI model mid-conversation in Composer, any subsequently dispatched subagents fail to start. Instead of running their assigned task, they immediately show a “Couldn’t start” error with no additional details or fallback. This happens consistently after a model switch — the subagents are queued and dispatched but never actually initialize. The issue makes it impossible to use multi-agent workflows once the model has been changed during an ongoing session.

Steps to Reproduce

Open Cursor and start a conversation in Composer

Mid-conversation, switch to a different AI model (e.g., from one model to Gemini 3.5 Flash)

Trigger or wait for a subagent to be dispatched

Observe that the subagent shows “Couldn’t start” and fails to run

Expected Behavior

The subagent should start successfully and complete its assigned task, regardless of whether the model was switched mid-conversation.

Screenshots / Screen Recordings

Screenshot_20260622_141503.png
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Operating System

Linux

Version Information

Version: 3.8.11
VS Code Extension API: 1.105.1
Commit: e56ad3440df06d22ca7501e65fd518e905486ef0
Date: 2026-06-18T01:40:18.333Z
Layout: editor
Build Type: Stable
Release Track: Default
Electron: 40.10.3
Chromium: 144.0.7559.236
Node.js: 24.15.0
V8: 14.4.258.32-electron.0
xterm.js: 6.1.0-beta.256
OS: Linux x64 6.12.90+deb12.1-amd64

For AI issues: which model did you use?

Composer 2.5 and Gemini 3.5 Flash

For AI issues: add Request ID with privacy disabled

faebd769-36b0-49be-b29b-89f83ecfb9ca

Does this stop you from using Cursor

Sometimes - I can sometimes use Cursor

5

Hey, thanks for the report. This isn’t a server issue. It’s how subagents pick a model.

A subagent can only start on a model that’s Enabled in your Settings → Models and supports agent mode. If you switch the active model mid-conversation to one that isn’t in that set, in your case Gemini Flash, the subagent won’t have any valid model to start on. That’s why you get the generic “Couldn’t start”.

What you can do:

  • Before dispatching a subagent, switch back to a subagent-capable model, for example the Composer model.
  • In Settings → Models, enable the model you want.
  • If the option is there, set a default subagent model in Settings → Agents → Subagents.

On the Free plan, the list of subagent-capable models is pretty limited, so you’ll have fewer options.

This is a known limitation of the current model binding for subagents and it’s on our radar. Let me know if switching back to the Composer model fixed it.

6

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