Release v4.10.1

An HTTP 1.1 interface to the Stan C++ package, httpstan is a shim that allows users to interact with the Stan C++ library using a REST API. The package is intended for use as a universal backend for frontends which know how to make HTTP requests. The primary audience for this package is developers.

In addition to providing the essential functionality of the command-line interface to Stan (CmdStan) over HTTP, httpstan provides the following features:

  • Automatic caching of compiled Stan models

  • Automatic caching of samples from Stan models

  • Parallel sampling


After installing httpstan, running the module will begin listening on localhost, port 8080:

python3 -m httpstan

An HTTP-based REST API is now available with the endpoint: http://localhost:8080/v1/. The page HTTP-based REST API has a complete description of the resources available.

In a different terminal, make a POST request to http://localhost:8080/v1/models with Stan program code to compile the program:

curl -H "Content-Type: application/json" \
    --data '{"program_code":"parameters {real y;} model {y ~ normal(0,1);}"}' \

This request will return a model name along with all the compiler output:

{"compiler_output": "In file included from …", "stanc_warnings": "", "name": "models/xc2pdjb4"}

(The model name depends on the platform and the version of Stan.)

Drawing samples from this model using default settings requires two steps: (1) launching the sampling operation and (2) retrieving the output of the operation (once it has finished).

First we make a request to launch the sampling operation:

curl -H "Content-Type: application/json" \
    --data '{"function":"stan::services::sample::hmc_nuts_diag_e_adapt"}' \

This request instructs httpstan to draw samples from the normal distribution described in the model. The function name picks out a specific function in the stan::services namespace found in the Stan C++ library (see the Stan C++ documentation for details). This request will return immediately with a reference to a long-running fit operation:

{"name": "operations/gkf54axb", "done": false, "metadata": {"fit": {"name": "models/xc2pdjb4/fits/gkf54axb"}}}

Once the operation is complete, the “fit” can be retrieved. The name of the fit, models/xc2pdjb4/fits/gkf54axb, is included in the metadata field of the operation. The fit is saved as sequence of JSON-encoded messages. These messages are strung together with newlines. To retrieve these messages, saving them locally in the file myfit.jsonlines, make the following request:

curl http://localhost:8080/v1/models/xc2pdjb4/fits/gkf54axb > myfit.jsonlines

The Stan “fit”, saved in myfit.jsonlines, aggregates all messages. By reading them one by one you can recover all messages sent by the Stan C++ library.


We appreciate citations as they let us discover what people have been doing with the software. Citations also provide evidence of use which can help in obtaining grant funding.

To cite httpstan in publications use:

Riddell, A., Hartikainen, A., & Carter, M. (2021). httpstan (4.4.0).

Or use the following BibTeX entry:

  title = {httpstan (4.4.0)},
  author = {Riddell, Allen and Hartikainen, Ari and Carter, Matthew},
  year = {2021},
  month = mar,
  howpublished = {PyPI}

Please also cite Stan.

User Guide