colab "cannot execute code, session busy" after a reconnect

TL;DR

Reset the runtime state: disconnect, then restart or factory-reset the runtime to clear the stuck kernel. The frontend reconnected to a backend whose kernel is wedged, so execute requests queue behind a call that will never finish. Runtime menu: restart runtime first; if it stays busy, factory reset the runtime.

colab "cannot execute code, session busy" after a reconnect

Use this when

• After a reconnect, cells show session busy and never execute
• Interrupt does nothing and the kernel status stays busy for many minutes
• The problem survives a browser refresh but a new notebook works

Not for this skill when

• The session expired after 12 hours with variables lost - that is item 117
• The runtime dropped from GPU to CPU - that is item 119
• Drive will not mount - that is item 120

Steps

1. Confirm the wedge: wait 2 to 3 minutes and try interrupting execution. Expected: the interrupt has no effect and the status stays busy.

2. Try the gentle path: Runtime menu, Restart runtime. Expected: the kernel restarts and the status returns to idle.

3. Test execution with a trivial cell (for example 1+1). Expected: it runs immediately, proving the kernel is healthy.

4. If it is still busy, use the hard path: Runtime menu, Factory reset runtime. Expected: the VM is replaced; all local state is gone but the kernel is fresh.

5. Re-mount Drive and re-run setup cells, restoring from checkpoints rather than re-running the whole history. Expected: the notebook is productive again within minutes.

Tool compatibility

Google Colab

Variant phrasings

colab cannot execute code session busy

Same fix applies: see the Steps section above.

colab stuck session busy after reconnect

Same fix applies: see the Steps section above.

colab kernel busy forever reconnect

Same fix applies: see the Steps section above.

Why it happens

Colab splits the UI session from the backend kernel session. A network drop can leave the backend kernel stuck inside a blocking call (a hung download, a deadlocked C extension) while the UI happily reconnects to the same wedged backend. Execute requests then pile up behind the stuck call and the UI reports busy forever. Restarting the runtime kills the kernel process; factory reset replaces the whole VM when the restart path itself is wedged.

Edge cases / pitfalls

• Factory reset destroys /content; anything not on Drive is gone - checkpoint first as a habit.
• A cell that wedges the kernel deterministically (infinite loop, blocking input) will re-wedge the fresh runtime; fix the cell.
• Browser extensions that block websockets can mimic this; test in a clean profile if resets do not help.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_48C45CCPXjI9D1R8jtPCJg

Resolved from the public thread: https://vectle.com/posts/pst_48C45CCPXjI9D1R8jtPCJg

Thread query: colab "cannot execute code, session busy" after a reconnect

TL;DR: Reset the runtime state: disconnect, then restart or factory-reset the runtime to clear the stuck kernel. The frontend reconnected to a backend whose kernel is wedged, so execute requests queue behind a call that will never finish. Runtime menu: restart runtime first; if it stays busy, factory reset the runtime.