Use a cassette to record HTTP requests

  record = NULL,
  match_requests_on = NULL,
  update_content_length_header = FALSE,
  allow_playback_repeats = FALSE,
  serialize_with = NULL,
  persist_with = NULL,
  preserve_exact_body_bytes = NULL,
  re_record_interval = NULL,
  clean_outdated_http_interactions = NULL



The name of the cassette. vcr will check this to ensure it is a valid file name. Not allowed: spaces, file extensions, control characters (e.g., \n), illegal characters ('/', '?', '<', '>', '\', ':', '*', '|', and '\"'), dots alone (e.g., '.', '..'), Windows reserved words (e.g., 'com1'), trailing dots (can cause problems on Windows), names longer than 255 characters. See section "Cassette names"


a block of code containing one or more requests (required). Use curly braces to encapsulate multi-line code blocks. If you can't pass a code block use insert_cassette() instead.


The record mode (default: "once"). See recording for a complete list of the different recording modes.


List of request matchers to use to determine what recorded HTTP interaction to replay. Defaults to ["method", "uri"]. The built-in matchers are "method", "uri", "host", "path", "headers", "body" and "query"


(logical) Whether or not to overwrite the Content-Length header of the responses to match the length of the response body. Default: FALSE


(logical) Whether or not to allow a single HTTP interaction to be played back multiple times. Default: FALSE.


(character) Which serializer to use. Valid values are "yaml" (default) and "json". Note that you can have multiple cassettes with the same name as long as they use different serializers; so if you only want one cassette for a given cassette name, make sure to not switch serializers, or clean up files you no longer need.


(character) Which cassette persister to use. Default: "file_system". You can also register and use a custom persister.


(logical) Whether or not to base64 encode the bytes of the requests and responses for this cassette when serializing it. See also preserve_exact_body_bytes in vcr_configure(). Default: FALSE


(integer) How frequently (in seconds) the cassette should be re-recorded. default: NULL (not re-recorded)


(logical) Should outdated interactions be recorded back to file? default: FALSE


an object of class Cassette


A run down of the family of top level vcr functions

  • use_cassette Initializes a cassette. Returns the inserted cassette.

  • insert_cassette Internally used within use_cassette

  • eject_cassette ejects the current cassette. The cassette will no longer be used. In addition, any newly recorded HTTP interactions will be written to disk.

Cassette options

Default values for arguments controlling cassette behavior are inherited from vcr's global configuration. See vcr_configure() for a complete list of options and their default settings. You can override these options for a specific cassette by changing an argument's value to something other than NULL when calling either insert_cassette() or use_cassette().


This function handles a few different scenarios:

  • when everything runs smoothly, and we return a Cassette class object so you can inspect the cassette, and the cassette is ejected

  • when there is an invalid parameter input on cassette creation, we fail with a useful message, we don't return a cassette, and the cassette is ejected

  • when there is an error in calling your passed in code block, we return with a useful message, and since we use on.exit() the cassette is still ejected even though there was an error, but you don't get an object back

  • whenever an empty cassette (a yml/json file) is found, we delete it before returning from the use_cassette() function call. we achieve this via use of on.exit() so an empty cassette is deleted even if there was an error in the code block you passed in

Cassettes on disk

Note that "eject" only means that the R session cassette is no longer in use. If any interactions were recorded to disk, then there is a file on disk with those interactions.

Using with tests (specifically testthat)

There's a few ways to get correct line numbers for failed tests and one way to not get correct line numbers:

Correct: Either wrap your test_that() block inside your use_cassette() block, OR if you put your use_cassette() block inside your test_that() block put your testthat expectations outside of the use_cassette() block.

Incorrect: By wrapping the use_cassette() block inside your test_that() block with your testthat expectations inside the use_cassette() block, you'll only get the line number that the use_cassette() block starts on.

See also


if (FALSE) { library(vcr) library(crul) vcr_configure(dir = tempdir()) use_cassette(name = "apple7", { cli <- HttpClient$new(url = "") resp <- cli$get("get") }) readLines(file.path(tempdir(), "apple7.yml")) # preserve exact body bytes - records in base64 encoding use_cassette("things4", { cli <- crul::HttpClient$new(url = "") bbb <- cli$get("get") }, preserve_exact_body_bytes = TRUE) ## see the body string value in the output here readLines(file.path(tempdir(), "things4.yml")) # cleanup unlink(file.path(tempdir(), c("things4.yml", "apple7.yml"))) # with httr library(vcr) library(httr) vcr_configure(dir = tempdir(), log = TRUE, log_opts = list(file = file.path(tempdir(), "vcr.log"))) use_cassette(name = "stuff350", { res <- GET("") }) readLines(file.path(tempdir(), "stuff350.yml")) use_cassette(name = "catfact456", { res <- GET("") }) # record mode: none library(crul) vcr_configure(dir = tempdir()) ## make a connection first conn <- crul::HttpClient$new("") ## this errors because 'none' disallows any new requests # use_cassette("none_eg", (res2 <- conn$get("get")), record = "none") ## first use record mode 'once' to record to a cassette one <- use_cassette("none_eg", (res <- conn$get("get")), record = "once") one; res ## then use record mode 'none' to see it's behavior two <- use_cassette("none_eg", (res2 <- conn$get("get")), record = "none") two; res2 }