[Bug] labs.pybricks.com reports a timeout on EV3 usb connection

[BertLindeman]

Describe the bug
Running a script on labs.pybricks.com on an EV3 via USB reports:

Error: Timed out waiting for response
    at https://labs.pybricks.com/static/js/main.93bad3a8.js:2:590290
    at Generator.next (<anonymous>)
    at h (https://labs.pybricks.com/static/js/main.93bad3a8.js:2:1413608)
    at l (https://labs.pybricks.com/static/js/main.93bad3a8.js:2:1413925)
    at t (https://labs.pybricks.com/static/js/main.93bad3a8.js:2:1409046)
    at l (https://labs.pybricks.com/static/js/main.93bad3a8.js:2:1413925)

To reproduce
Steps to reproduce the behavior:

1. Go to labs.pybricks.com
2. Click on connect USB
3. Connect to the EV3
4. Select a tiny program (see below)
5. Click Run or press F5
6. See the timeout pop-up. The program at this point does run.

Expected behavior
What did you expect to happen instead?
No tineout

Screenshots
There is a saying that a picture is worth a 1000 words. Screenshots really help to identify and solve problems.

Test program

from pybricks.tools import wait

print('hi')
wait(5000)

Extra info

1. Ran this with various firmware on the EV3 up to
   Pybricks MicroPython ci-build-4445-v4.0.0b2-5-gc2910aa2 on 2025-11-26
   from ev3-firmware-build-4152-git9759e904
   So I assume the problem to be in labs.pybricks.com
   Even with the wait deleted I get the same result.

2. Running Linux Mixt Xia and Google Chrome Version 142.0.7444.175 (Official Build) (64-bit)

[BertLindeman]

The USB hacks are fixed and now I can connect the EV3 to windows.
So install firmware on the EV3 via labs.pybricks now is OK too.

Tested this issue on win 11 25H2, but still get:

Error: Timed out waiting for response
    at https://labs.pybricks.com/static/js/main.787ddbdb.js:2:576584
    at Generator.next (<anonymous>)
    at h (https://labs.pybricks.com/static/js/main.787ddbdb.js:2:1396102)
    at l (https://labs.pybricks.com/static/js/main.787ddbdb.js:2:1396419)
    at t (https://labs.pybricks.com/static/js/main.787ddbdb.js:2:1391540)
    at l (https://labs.pybricks.com/static/js/main.787ddbdb.js:2:1396419)

[EDIT]
Not all runs get this problem.
Had it 6 times out of 10 tests this time.
Without restart of the app, and without re-connecting via USB.

Not tested on Linux yet.

Minor side note, not worth an issue:
Copyright in labs about is a bit out of date:

MicroPython for LEGO® Powered Up smart hubs.

v4.0.0b4 (Pybricks Labs v24750c2b7cd81beea7800a20a1785f505e81ce74)

Copyright (c) 2020-2023 The Pybricks Authors

[Sirus-7]

I can confirm the same behavior on Windows 10 Pro (22H2) regardless of the program size.
Google Chrome Version 144.0.7559.61 (Offizieller Build) (64-Bit)

The first program transfer and run to a free slot usually works.
Only afterwards do the problems described by @BertLindeman occur.

[dlech]

Tested this issue on win 11 25H2, but still get:

Which firmware did you use this time? Did the firmware from pybricks/pybricks-micropython#457 make a difference on this issue?

[BertLindeman]

Tested this issue on win 11 25H2, but still get:

Which firmware did you use this time? Did the firmware from pybricks/pybricks-micropython#457 make a difference on this issue?

I used ('ev3', '4.0.0b4', 'ci-build-4663-v4.0.0b4-5-gc8b82544 on 2026-01-25') from PR457.
Will do the same on the standard beta from labs 4.0.0.b4

[dlech]

No need to test the other firmware. If there is still a timeout with PR457 then the problem is probably on the Pybricks Code side. I will try to reproduce.

[BertLindeman]

No need to test the other firmware. If there is still a timeout with PR457 then the problem is probably on the Pybricks Code side. I will try to reproduce.

Stubborn as I am, you know.. My feeling is that on PR457 I needed a bit less retries to get running. But no hard evidence.

[dlech]

Should be working better now at labs.pybricks.com.

[BertLindeman]

Should be working better now at labs.pybricks.com.

20 tests: no timeout anymore here.

Thanks!