Cursor Projects: the main agent of every existing project shows "Agent encountered an error" after an update; reconnecting GitHub and picking a default repository fixed it

Context

Source(s): https://forum.cursor.com/t/cursor-projects-agent-encountered-error-after-latest-update/174068 Signal: 59 views; the original poster's own fix reply is the accepted answer, posted 12 minutes after the report (October 2026). Status: reported working by source, not reproduced by us.

Problem

After the 2026-10-08 Cursor update, sending a message to the main agent of any project created before the update shows an "Agent encountered an error" popup, and retrying or sending a new request shows it again. Creating a new agent inside the project avoids the popup, but its chat disappears after sending a message. Reproduced on macOS and on the web.

Error (redacted / normalized)

Agent encountered an error

Applies when

Cursor 3.23.23 (Universal) on macOS, Agent Window layout, and Cursor on the web; projects started in the macOS IDE before the update.

Confirm it is this problem

1. Open a project created before the update and send a message to its main agent. → expect: the "Agent encountered an error" popup every time.
2. Create a new agent in the same project and send a message. → expect: no popup, but the chat disappears.

Not this when / boundary

The popup also appears in projects that never used a GitHub-backed environment, or GitHub is connected and a default repository is already set (our inference).

Root cause

Not explained by Cursor. The fix suggests the project's environment lost its GitHub connection or default repository during the update (our inference).

Fix

1. Reconnect your GitHub account in Cursor's settings (the original poster; they did not give the exact menu path).
2. Select a default repository for the environment in Cursor's settings (the original poster). → expect: the main agents of the existing projects work again.

How to verify

Sending a message to an existing project's main agent gets a normal reply with no popup.

What did not work

Retrying the request and sending a new message did not help (the original poster).

Risks

When you reconnect GitHub, grant access only to the repositories the agents need (our inference).

Verified scope

Cursor 3.23.23 Projects on macOS and the web, October 2026.

Becomes obsolete when

Cursor shows a specific "GitHub not connected / no default repository" message instead of the generic error.