Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsIf Cursor 3 shows “Agent Execution Timed Out” and your .ipynb notebook stays blank or loading, start by checking whether Cursor’s extension host has started. Cursor says the message means the extension host did not finish starting within 60 seconds, preventing Agent features from initializing; it does not identify a single cause. Record your Cursor version and operating system, then export logs before clearing caches or changing extensions.
What the error means—and why it may affect Jupyter
Cursor’s Agent troubleshooting documentation defines “Agent Execution Timed Out” as a failure of the extension host to finish starting within 60 seconds. Agent features cannot initialize until that host starts. The timeout is a symptom, not a diagnosis: different problems can prevent startup.
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In an April 10, 2026 Cursor Community Forum report, a user said that after upgrading to Cursor 3, clicking an .ipynb file did nothing or left a blank/loading tab, while the AI panel displayed the timeout. A Cursor staff reply said the screenshots suggested the extension host was not starting and described the notebook and Agent panel as waiting on it. Staff also said they were aware of an extension-host issue after the Cursor 3 update. This connects the symptoms in that report; it does not establish that every notebook-opening failure has the same cause. See the forum report and follow-up.
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- Record your setup. Note the installed Cursor version and operating system. Include whether the notebook fails to open, the Agent panel shows the timeout, or both.
- Export Cursor logs. Open the Command Palette and run
Developer: Export Logs.... Select Main, Window, and Extension Host, then save the exported zip. - Inspect the extension-host output. Open Output and select Extension Host. Cursor says an empty panel is a useful diagnostic clue.
- Share the evidence with Cursor support. Provide the logs zip, a screenshot of the symptoms, and your version and OS details. Cursor recommends collecting logs before changing anything.
These steps help distinguish an extension-host startup failure from a notebook-only issue and give support evidence to investigate. The official troubleshooting page describes the log-export process.
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Try targeted steps for the Cursor 3 notebook-and-Agent symptoms
The suggestions below come from the matching Cursor forum thread. They are troubleshooting options reported or proposed for that case, not guaranteed fixes. If possible, try them after exporting logs so you retain a record of the original failure.
Let startup finish before reopening the notebook
A Mac user in the thread reported that closing notebooks, restarting Cursor, waiting for extensions and the terminal to finish loading, and then reopening a notebook resolved the problem for them. This is a low-impact first attempt if the extension host appears to be starting slowly.
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Clear cached extensions cautiously
For the matching Cursor 3 report, a staff reply suggested closing Cursor and deleting the cached-extension folders for the relevant operating system:
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CachedExtensionsandCachedExtensionVSIXsfolders under~/Library/Application Support/Cursor
Close Cursor before removing these folders. This was a support suggestion in the thread, not a confirmed universal remedy.
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Use a fresh profile only if needed
The same reply suggested renaming the Cursor data folder to create a fresh profile if needed. Back up the profile first: changing the data folder changes the local user-data state. Treat this as a diagnostic reset, not a routine first step.
Check for extension conflicts when logs point that way
A user in the thread reported temporary improvement after disabling the “Python environment” extension. Cursor staff said a conflict was plausible and suggested trying only the Python extension (ms-python.python) and Jupyter (ms-toolsai.jupyter). The thread does not establish that disabling this extension fixes the issue for everyone.
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In a related post, staff connected an ms-toolsai.jupyter error to API proposals absent from Cursor’s current product.json. That is a specific report, not proof that all Jupyter failures in Cursor 3 share that cause. The forum thread contains the reports and staff replies.
Do not apply unrelated Agent-timeout fixes automatically
Cursor forum posts describe other Agent timeouts with different, case-specific explanations. They are useful reminders to match a fix to the installed version and logs, not recipes for the notebook-opening report.
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- In a separate case, staff said Agent sends could time out when Index Repositories for Instant Grep was turned off, and said a fix would ship in Cursor 3.21.18. Details are in the separate Agent timeout thread.
- In another Windows case, staff reproduced an extension-host startup failure caused by a numeric-only environment variable and said a change to ignore such variables would be included in Cursor 3.20. That report is documented in the Windows timeout thread.
Those explanations concern different reports. Do not change a setting or environment variable just because the error text looks similar; first check whether your logs and Cursor version match the case.
When to involve IT on a managed computer
Cursor lists endpoint-security software, including antivirus or endpoint detection and response (EDR) tools, as one possible cause of extension-host startup problems. On a managed or enterprise machine, ask IT to review Cursor’s endpoint-security configuration guidance alongside the exported logs. Security software is a possibility to investigate, not an assumed cause.
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