Hi, seeing an significant increase in timeouts from shopify function delivery customisation runs.
We aren’t exceeding any instruction limits or input limits
Target: purchase.delivery-customization.run
API: delivery_customization • 2025-01
Status: Error
Error type: TimeoutError
Time: Oct 28 22:51:55 UTC
Resource limits
Input size: 3.96 / 128 kB
Instruction count: 4.982 / 11 M
Output size: Not checked
We are experiencing the same thing in our app.
Function
Handle: cart-transformer
Extension type: Cart Transform
Target: cart.transform.run
API: cart_transform • 2025-07
Status: Error
Error type: TimeoutError
Time: Oct 28 23:07:53 UTC
We’ve also seen the same increase in timeouts, starting from roughly 27 Oct 18:00 UTC. We went from successfully processing carts of 50+ items, to failing on carts with a single item.
In our case, we were using a function written in Zig, and narrowed the cause down to slow writes to stdout/stderr - processing time/instruction count played no part. The issue seems to have been resolved by adding a 4KB write buffer (it was previously unbuffered).
Hi @Min_Liu & @stratton.sloane ,
I think this issue is resolved now - huge thanks to @david-shopify. I’ve done some testing on my end and everything seems back to normal with our Functions.
Thanks so much,
Adam
Just so people don’t think Twitter / X is the only way, we also learned about this from this post. Definitely recommend posting issues like this on the Community in the future.
that’s great, thanks everyone!
Hi again @David-Shopify ,
Sorry to bother you with this again. I wanted to let you know that over the last 24 hours, we have continued to get the TimeoutError in our function. Not nearly as many as we were receiving before, but still fairly regularly. Wondering if you would mind checking this again?
Thank you so much for your time and help 
