To cite the 'sirt' package in publications use:

Robitzsch A (2021). sirt: Supplementary Item Response Theory Models. R package version 3.11-21, https://CRAN.R-project.org/package=sirt.

Corresponding BibTeX entry:

  @Manual{sirt_3.11-21,
    title = {sirt: Supplementary Item Response Theory Models},
    author = {Alexander Robitzsch},
    year = {2021},
    note = {R package version 3.11-21},
    url = {https://CRAN.R-project.org/package=sirt},
  }