Environment checks in system progress preperation throws error in Positron

[milesalanmoore]

Hi stan-dev!

I have been trying out the new IDE from Posit, Positron, and have been really loving it. However, when I run a parallelized stan model (by specifying cores in the call to a brms model for example), rstan opens up a port in my browser with the following error message:

Cannot GET /var/folders/lm/4jkt0_jd37j4c_68xmrxp7f00000gn/T//Rtmpp3I1Pz/file5568b9aee29_StanProgress.txt

This behavior is OS agnostic and only applies to the case when a user runs chains in parallel as in:

library(brms)

# This runs without issue
brms::brm(
  mpg ~ hp * cyl, 
  data = mtcars)

# This opens the browser tab
m_mpg = brm(
  mpg ~ hp * cyl, 
  data = mtcars,
chains=4, cores=4)

For reference, see my issue on Positron's repo here.

In it, @juliasilge points out that this behavior is likely a result of rstan's environment checks with Sys.getenv("RSTUDIO"). See the code below, which would not be set to 1 for users working in Positron. Is it possible to generalize these checks a bit to include Positron, @juliasilge provides an example of what she thinks this might look like (linked above).

rstan/rstan/rstan/R/stanmodel-class.R

Line 517 in 9b8d8fe

!identical(Sys.getenv("RSTUDIO"), "1"),

I am happy to take a swing at implementing this if helpful, as well.

[danieltomasz]

Yes, it would be great to have this working on Positron :)

[pkubruce]

It’s now October 23, 2025. I ran the code in the Positron IDE and encountered the same problem. I hope the Stan development team @bgoodri can resolve this issue, as Positron is really an excellent IDE.

brms::brm(mpg ~ hp * cyl, data = mtcars, chains=4, cores=4)

[bgoodri]

I'll look at it. The progress code is very dependent on both the operating system and whether the person is using RStudio. Positron didn't exist at the time and is rather different internally.