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Record HTTP calls and replay them
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vcr.Rproj first commit Dec 9, 2014


cran checks Project Status: Active – The project has reached a stable, usable state and is being actively developed. Build Status Build status codecov rstudio mirror downloads cran version

An R port of the Ruby gem vcr


Check out the HTTP testing book and the vcr vignettes.

Supported HTTP libraries



cli <- crul::HttpClient$new(url = "")
  use_cassette(name = "helloworld", {
#>    user  system elapsed 
#>   0.182   0.026   1.302

The request gets recorded, and all subsequent requests of the same form used the cached HTTP response, and so are much faster

  use_cassette(name = "helloworld", {
#>    user  system elapsed 
#>   0.079   0.003   0.084

Importantly, your unit test deals with the same inputs and the same outputs - but behind the scenes you use a cached HTTP response - thus, your tests run faster.

The cached response looks something like (condensed for brevity):

- request:
    method: get
      encoding: ''
      string: ''
      User-Agent: libcurl/7.54.0 r-curl/3.2 crul/0.5.2
      status_code: '200'
      message: OK
      explanation: Request fulfilled, document follows
      status: HTTP/1.1 200 OK
      connection: keep-alive
      encoding: UTF-8
      string: "{\n  \"args\": {}, \n  \"headers\": {\n    \"Accept\": \"application/json,
        text/xml, application/xml, */*\", \n    \"Accept-Encoding\": \"gzip, deflate\",
        \n    \"Connection\": \"close\", \n    \"Host\": \"\", \n    \"User-Agent\":
        \"libcurl/7.54.0 r-curl/3.2 crul/0.5.2\"\n  }, \n  \"origin\": \"111.222.333.444\",
        \n  \"url\": \"\"\n}\n"
  recorded_at: 2018-04-03 22:55:02 GMT
  recorded_with: vcr/0.1.0, webmockr/0.2.4, crul/0.5.2

All components of both the request and response are preserved, so that the HTTP client (in this case crul) can reconstruct its own response just as it would if it wasn't using vcr.


  • vcr: the name comes from the idea that we want to record something and play it back later, like a vcr
  • cassette: A thing to record HTTP interactions to. Right now the only option is file system, but in the future could be other things, e.g. a key-value store like Redis
  • Persisters: defines how to save requests - currently only option is the file system
  • Serializers: defines how to serialize the HTTP response - currently only option is YAML; other options in the future could include e.g. JSON
  • insert cassette: create a cassette (all HTTP interactions will be recorded to this cassette)
  • eject cassette: eject the cassette (no longer recording to that cassette)
  • replay: refers to using a cached result of an http request that was recorded earlier

What does vcr do?

The short version is: vcr helps you stub HTTP requests so you don't have to repeat HTTP requests.

The main use case is for unit tests for R packages.

vcr currently works with the crul and httr packages; support for curl is in the works.

How it works in lots of detail

The Steps

  1. Use either vcr::use_cassette or vcr::insert_cassette a. If you use vcr::insert_cassette, make sure to run vcr::eject_cassette when you're done to stop recording
  2. When you first run a request with vcr there's no cached data to use, so we allow HTTP requests until you're request is done.
  3. Before we run the real HTTP request, we "stub" the request with webmockr so that future requests will match the stub. This stub is an R6 class with details of the interaction (request + response), but is not on disk.
  4. After the stub is made, we run the real HTTP request.
  5. We then disallow HTTP requests so that if the request is done again we use the cached response
  6. The last thing we do is write the HTTP interaction to disk in a mostly human readable form.

When you run that request again using vcr::use_cassette or vcr::insert_cassette:

  • We use webmockr to match the request to cached requests, and since we stubbed the request the first time we used the cached response.

Of course if you do a different request, even slightly (but depending on which matching format you decided to use), then the request will have no matching stub and no cached response, and then a real HTTP request is done - we then cache it, then subsequent requests will pull from that cached response.

webmockr has adapters for each R client (again, right now only crul) - so that we actually intercept HTTP requests when webmockr is loaded and the user turns it on. So, webmockr doesn't actually require an internet or localhost connection at all, but can do its thing just fine by matching on whatever the user requests to match on. In fact, webmockr doesn't allow real HTTP requests by default, but can be toggled off of course.

The main use case we are going for in vcr is to deal with real HTTP requests and responses, so we allow real HTTP requests when we need to, and turn it off when we don't.

This gives us a very flexible and powerful framework where we can support webmockr and vcr integration for any number of R clients for HTTP requests and support many different formats serialized to disk.

Just want to mock and not store on disk?

You're looking for webmockr. webmockr only matches requests based on criteria you choose, but does not cache HTTP interactions to disk as vcr does.

Best practices

vcr for tests

  • Add vcr to Suggests in your DESCRIPTION file (optionally add webmockr, but it's not explicitly needed as vcr will pull it in)
  • Make a file in your tests/testthat/ directory called helper-yourpackage.R (or skip if as similar file already exists). In that file use the following lines to setup your path for storing cassettes (change path to whatever you want):
  • In your tests, for whichever tests you want to use vcr, wrap them in a vcr::use_cassette() call like:
vcr::use_cassette("rl_citation", {
  test_that("my test", {
    aa <- rl_citation()

    expect_is(aa, "character")
    expect_match(aa, "IUCN")
    expect_match(aa, "")

OR put the vcr::use_cassette() block on the inside, but put testthat expectations outside of the vcr::use_cassette() block:

test_that("my test", {
  vcr::use_cassette("rl_citation", {
    aa <- rl_citation()

  expect_is(aa, "character")
  expect_match(aa, "IUCN")
  expect_match(aa, "")

Don't wrap the use_cassette() block inside your test_that() block with testthat expectations inside the use_cassette() block, as you'll only get the line number that the use_cassette() block starts on on failures.

  • When running tests or checks of your whole package, note that some users have found different results with devtools::check() vs. devtools::test(). It's not clear why this would make a difference. Do let us know if you run into this problem.

vcr in your R project

You can use vcr in an R project as well.

  • Load vcr in your project
  • Similar to the above example, use use_cassette to run code that does HTTP requests.
  • The first time a real request is done, and after that the cached response will be used.


CRAN version:


Development version:



We set the following defaults:

  • dir = "."
  • record = "once"
  • match_requests_on = c("method", "uri")
  • allow_unused_http_interactions = TRUE
  • serialize_with = "yaml"
  • persist_with = "FileSystem"
  • ignore_hosts = NULL
  • ignore_localhost = FALSE
  • ignore_request = NULL
  • uri_parser = crul::url_parse
  • preserve_exact_body_bytes = FALSE
  • turned_off = FALSE
  • ignore_cassettes = FALSE
  • re_record_interval = NULL
  • clean_outdated_http_interactions = NULL
  • cassettes = list()
  • linked_context = NULL
  • vcr_logging = "vcr.log"
  • vcr_logging_opts = list()

You can get the defaults programmatically with


You can change all the above defaults with vcr_configure():


Calling vcr_configuration() gives you some of the more important defaults in a nice tidy print out



vcr looks for similarity in your HTTP requests to cached requests. You can set what is examined about the request with one or more of the following options:

  • body
  • headers
  • host
  • method
  • path
  • query
  • uri

By default, we use method (HTTP method, e.g., GET) and uri (test for exact match against URI, e.g.,

You can set your own options by tweaking the match_requests_on parameter:

use_cassette(name = "one", {
    cli$post("post", body = list(a = 5))
  match_requests_on = c('method', 'headers', 'body')

vcr in other languages

The canonical vcr (in Ruby) lists ports in other languages at


There's a number of features in this package that are not yet supported, but for which their parameters are found in the package. For example, decode_compressed_response is a parameter in use_cassette() but it is ignored right now.

We've tried to make sure the parameters that are ignored are marked as such. Keep an eye out for package updates for changes in these parameters, and/or let us know you want it and we can move it up in the priority list.

Example packages using vcr


  • Logging
  • Provide toggling a re-record interval so you can say e.g., after 6 hrs, re-record a real response, updating the cached response
  • ...


  • Please report any issues or bugs
  • License: MIT
  • Get citation information for vcr in R doing citation(package = 'vcr')
  • Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms.


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