Backend Portability and Installation

The advanced extension supports the two backends officially exposed by brms: rstan and cmdstanr. The model family, formula, priors, and algorithm remain restricted by gp3bayes. Only the implementation backend is selectable.

Audit installed components

bayesian_backend_capabilities()
#> 
#> Optional Bayesian capabilities
#> 
#>         component installed     version usable
#>              brms      TRUE      2.23.0   TRUE
#>             rstan      TRUE 2.39.0.9000   TRUE
#>          cmdstanr      TRUE       0.9.0   TRUE
#>               loo      TRUE 2.10.1.9000   TRUE
#>        priorsense      TRUE       1.4.0   TRUE
#>  detectseparation      TRUE       0.4.0   TRUE
#>               SBC      TRUE     0.5.0.0   TRUE
#>                                                  detail
#>                                       package available
#>                                       package available
#>  CmdStan 2.39.0 at /github/home/.cmdstan/cmdstan-2.39.0
#>                                       package available
#>                                       package available
#>                                       package available
#>                                       package available

CmdStanR setup

Install CmdStanR from the Stan R-universe repository:

install.packages(
  "cmdstanr",
  repos = c(
    "https://stan-dev.r-universe.dev",
    getOption("repos")
  )
)

Then check the C++ toolchain and install CmdStan:

cmdstanr::check_cmdstan_toolchain()
cmdstanr::install_cmdstan(cores = 2)
check_cmdstan_backend(strict = TRUE)

The gp3bayes installer never installs or repairs CmdStan automatically unless that explicit option is enabled.

Full MCMC only

fit <- fit_duration_model_backend(
  specification = duration_spec,
  backend = "cmdstanr",
  chains = 2,
  iter = 2000,
  warmup = 1000,
  seed = 2026
)

Variational inference, Pathfinder, Laplace approximation, arbitrary Stan code, and arbitrary backend arguments remain outside this wrapper.