Title: Codex Extension Cannot Start Its User Interface in Cursor
Hello Cursor Support Team,
I’m experiencing an issue with the official OpenAI Codex extension in Cursor on Windows 11.
The extension installs and activates, but its user interface fails to open.
Errors:
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“Codex could not start. The extension could not start its user interface.”
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“View containers cannot be contributed to the Secondary Side Bar in Cursor. It is reserved for Cursor’s agent UI.”
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“[CodexWebviewProvider] Webview render did not become ready.”
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“Command ‘View: Move View’ resulted in an error.”
Troubleshooting already attempted:
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Updated and reinstalled Cursor.
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Reinstalled Codex.
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Disabled other extensions.
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Reset view locations.
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Reloaded Cursor.
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Attempted to move Codex to the primary sidebar.
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Attempted to install an older Codex version.
None of these resolved the issue.
Could you please confirm whether the official OpenAI Codex extension is compatible with Cursor and whether there is a workaround for this sidebar issue?
Where does the bug appear?
Something else — Third-party extension compatibility (OpenAI Codex).
Describe the Bug
The official OpenAI Codex extension installs and activates successfully in Cursor, but its user interface fails to load.
The following errors appear:
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“Codex could not start. The extension could not start its user interface.”
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“View containers cannot be contributed to the Secondary Side Bar in Cursor.”
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“[CodexWebviewProvider] Webview render did not become ready.”
Moving the Codex view also produces an error.
Steps to Reproduce
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Open Cursor on Windows 11.
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Install the official OpenAI Codex extension.
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Open the Codex sidebar or run “Codex: New Codex Agent.”
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Observe the interface startup error.
Expected Behavior
The Codex interface should open inside Cursor, allowing me to sign in and use the coding agent.
Actual Behavior
The extension activates, but its user interface never loads.
Operating System
Windows 11 Pro, version 25H2.
Version Information
Cursor 3.29.15 (installer version; please verify the installed version).
Codex extension: 26.1021.22027.
Troubleshooting Already Attempted
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Reinstalled and updated Cursor.
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Reinstalled Codex.
-
Disabled other extensions.
-
Reset view locations.
-
Reloaded Cursor.
-
Tried moving Codex to the primary sidebar.
-
Attempted installing an older Codex version, but installation failed.
Additional Information
The issue persists despite multiple troubleshooting attempts. Cursor appears to reject Codex’s secondary sidebar view container.
Does this stop you from using Cursor?
Sometimes — I can use Cursor’s other features, but I cannot use the Codex extension.