Hi,

First, thank you for this amazing package. I would also like to say that I am not used to posting issues so I will try to make my post and questions as reproducible as possible.

I have successfully fitted a mixed model with the custom beta-binomial family on a subset of my data following the example provided in this vignette : Define Custom Response Distributions with brms

The model was run on a laptop under Windows 10 OS.

However, my dataset is very large, and so, I have to run the model on a remote server with multiple cores and within-chain parallelization. The remote server runs on Cent OS Linux 7 and I used brms version 2.14.4. Unfortunately, the data is private so I cannot share it here.

Here is the structure of my model :

```
beta_binomial2 <- custom_family(
"beta_binomial2", dpars = c("mu", "phi"),
links = c("logit", "log"), lb = c(NA, 0),
type = "int", vars = "vint1[n]"
)
stan_funs <- "
real beta_binomial2_lpmf(int y, real mu, real phi, int T) {
return beta_binomial_lpmf(y | T, mu * phi, (1 - mu) * phi);
}
int beta_binomial2_rng(real mu, real phi, int T) {
return beta_binomial_rng(T, mu * phi, (1 - mu) * phi);
}
"
stanvars <- stanvar(scode = stan_funs, block = "functions")
priors <- set_prior("normal(0, 5)", class = "b")
model_formula <- brmsformula(hunting_success | vint(4) ~
Zspeed +
Zspace_covered_rate +
Zprox_mid_guard +
Zhook_start_time +
Zsurv_speed +
Zsurv_space_covered_rate +
(1 | map_name) +
(1 | mirrors_id))
system.time(beta_binom_mod <- brm(formula = model_formula,
family = beta_binomial2,
warmup =3000,
iter = 153000,
thin = 100,
chains = 4,
inits = "0",
threads = threading(10),
backend = "cmdstanr",
seed = 20210414,
stanvars = stanvars,
prior = priors,
control = list(adapt_delta = 0.95),
data = data))
```

For an unknown reason, I get the following error :

```
Compiling Stan program...
Semantic error in
'/localscratch/maxime11.18202117.0/Rtmpzdtb72/model-eede2bbff12.stan', line 34,
column 17 to column 66:
-------------------------------------------------
32: for (n in 1:N) {
33: int nn = n + start - 1;
34: ptarget += beta_binomial2_lpmf(Y[nn] | mu[n], phi, vint1[n]);
^
35: }
36: return ptarget;
-------------------------------------------------
A returning function was expected but an undeclared identifier
'beta_binomial2_lpmf' was supplied.
make: *** [make/program:53:
/localscratch/maxime11.18202117.0/Rtmpzdtb72/model-eede2bbff12.hpp] Error 1
Error: An error occured during compilation! See the message above for more
information.
Timing stopped at: 0.831 1.085 73.23
Execution halted
```

When I run the same model with an observation-level random effect using the native binomial family on the same remote server, I donâ€™t get an error.

I am sorry if this is a simple error, but I am quite new to using Bayesian models with STAN on remote servers, so I am still learning.

Thank you very much for your help.

EDIT** : I realized that without cmdstanr running as a backend, the model runs perfectly. So the error seems to stem from there. However, I need to use within-chain parallelization for the model to run fast.