Programming language: Erlang
License: Apache License 2.0
Tags: Testing    
Latest version: v1.6.0-rc

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KATT (Klarna* API Testing Tool) is an HTTP-based API testing tool for Erlang.

* Albeit the "Klarna" mention, this repository is not affiliated with Klarna AB. KATT was indeed born at Klarna, and Klarna AB holds copyright for parts of the code, but it is now being maintained outside the company, by its original authors and new contributors.

Quick start

[An example is worth a 1000 words.](doc/example-httpbin.apib)

Use for shooting HTTP requests in a sequential order and verifying the response. Any relevant difference between expected and actual responses will cause a failure.

The builtin validator supports basic text validation and more advanced validation of HTTP headers, and media-types (application/json, application/*+json, application/x-www-form-urlencoded).

The validator makes use of a few tags with special meaning:

"{{_}}" Match anything including undefined (i.e. no real validation).

"{{expected}}" Match anything but undefined (i.e. no real validation, only check existence).

"{{unexpected}}" Match nothing (i.e. no real validation, only check lack of existence)

"{{>key}}" Store value of the whole string (key must be unique within testcase)

"{{<key}}" Recall stored value.

The "{{_}}" tag can also be used as a JSON object's property in order to validate any other additional properties.

By default, the builtin validator will allow additional properties in an object structure, or additional items in an array structure. To counteract that default, one can do {..., "{{_}}": "{{unexpected}}"} or [..., "{{unexpected}}"], effectively making a rule that no properties/items are expected beyond the ones defined.

NOTE If some values are static (constants) and you want to reuse them across multiple requests, you can add one or more params like below

PARAM a_string="with some text"
PARAM a_boolean=true
PARAM a_null=null
PARAM a_float=1.1
PARAM an_integer=1

For more complex validations, KATT supports extensible validation types. Built-in validation types: set, runtime_value, runtime_validation.

set will ignore the order of an array's items, and just check for existence:

  "some_array": {
    "{{type}}": "set",
    "value": [1, 2, 3]

So the above would validate against JSON instances such as {"some_array": [1, 3, 2]}, or {"some_array": [3, 2, 1]}, or even {"some_array": [4, 3, 2, 1]} unless we add {{unexpected}}.

runtime_value would just run code (only erlang and shell supported for now), while having access to ParentKey, Actual, ItemsMode and Callbacks, and return the expected value and matched against the actual one.

  "rfc1123": {
    "{{type}}": "runtime_value",
    "erlang": "list_to_binary(httpd_util:rfc1123_date(calendar:now_to_datetime(erlang:now())))"

or in array format

  "rfc1123": {
    "{{type}}": "runtime_value",
    "erlang": ["list_to_binary(",
               "  httpd_util:rfc1123_date(",
               "    calendar:now_to_datetime(",
               "      erlang:now()",

runtime_validation would just run code (only erlang and shell supported for now), while having access to ParentKey, Actual, ItemsMode and Callbacks, and return

  • {pass, [{"Key", "Value"}]} i.e. validation passed, store new param "Key" with value "Value"
  • {not_equal, {Key, Expected, Actual}}
  • {not_equal, {Key, Expected, Actual, [{"more", "info"}]}}
  "rfc1123": {
    "{{type}}": "runtime_validation",
    "erlang": "Expected = httpd_util:rfc1123_date(calendar:now_to_datetime(erlang:now())), case Actual =:= Expected of true -> {pass, []}; false -> {not_equal, {ParentKey, Expected, Actual}} end"

or in array format

  "rfc1123": {
    "{{type}}": "runtime_validation",
    "erlang": ["Expected = httpd_util:rfc1123_date(calendar:now_to_datetime(erlang:now())),",
               "case Actual =:= Expected of",
               "  true ->",
               "    {pass, []};",
               "  false ->",
               "    {not_equal, {ParentKey, Expected, Actual}}",


A simple example that will make requests to a third party server:

ERL_LIBS=deps erl -pa ebin -noshell -eval '
  BlueprintFile = "doc/example-httpbin.apib",
  Params = [{base_url, "http://httpbin.org"}, {my_name, "Joe"}, {your_name, "Mike"}],
  io:format("~p~n", [katt:run(BlueprintFile, Params)]).
' -s init stop

... or run the code passed to -eval from the Erlang shell (assuming that you have started the Erlang shell from the repo's root directory with ERL_LIBS=deps erl -pa ebin).


You can also fire up katt from the CLI, with

bin/katt base_url=http://httpbin.org my_name=Joe your_name=Mike -- doc/example-httpbin.apib

If you want non-string params, use := as a separator e.g. my_int:=123.

You can also output the result in JSON format, with --json, and beautify it e.g. with python

bin/katt --json base_url=http://httpbin.org my_name=Joe your_name=Mike -- doc/example-httpbin.apib | python -m json.tool


  • katt:run to be called with
    • filename
    • params (optional)
    • base_url, alternatively you can use the legacy
      • protocol
      • hostname
      • port
      • base_path
    • request_timeout
    • scenario_timeout
    • callbacks (optional)
    • ext to be called with scope (recall_body, parse, validate_body, validate_type)
    • recall to be called with syntax, text, params, callbacks
    • parse to be called with headers, body, params, callbacks
    • request to be called with request, params, callbacks
    • validate to be called with expected, actual, params, callbacks
    • progress to be called with transaction_result
    • text_diff to be called with text, text
    • transform to be called with id, katt_request or {katt_response, actual_response}, params, callbacks

A request can also be configured via HTTP request headers:

  • x-katt-description would take precedence over the transaction's description
  • x-katt-request-timeout would take precedence over the request_timeout param
  • x-katt-request-sleep would delay the request for a specific amount of milliseconds
  • x-katt-transform would call the tranform callback with it's value as id

A response can also be configured via HTTP response headers:

  • x-katt-transform would call the tranform callback with it's value as id

If you would like to convert a HAR file to an APIB file

The HTTP Archive format or HAR, is a JSON-formatted archive file format for logging of a web browser's interaction with a site, standardized by the Web Performance Working Group of the World Wide Web Consortium (W3C).

For example, to convert [doc/example-teapot.har](doc/example-teapot.har) into [doc/example-teapot.apib](doc/example-teapot.apib), run:

bin/katt from-har --apib -- doc/example-teapot.har > doc/example-teapot.apib

If you would like to disable JSON support

OnlyText = fun(_Scope) -> [] end,
katt:run("text_only_scenario.apib", [], [{ext, OnlyText}]).

If you would like to add XML support

PlusXml =
  fun(recall_body) ->
    [ fun custom_callbacks_xml:recall_body/4
    ] ++ katt_callbacks:ext(recall_body);
  fun(parse) ->
    [ fun custom_callbacks_xml:parse/5
    ] ++ katt_callbacks:ext(parse);
  fun(validate_body) ->
    [ fun custom_callbacks_xml:validate_body/3
    ] ++ katt_callbacks:ext(validate_body),
  fun(validate_type) ->
    [ fun custom_callbacks_xml:validate_type/7
    ] ++ katt_callbacks:ext(validate_type),
katt:run("xml_scenario.apib", [], [{ext, PlusXml}]).

See [src/katt_callbacks_json.erl](src/katt_callbacks_json.erl) to see how your custom_callbacks_xml module should be implemented.

If you would like to build KATT with almost no dependencies

export KATT_BARE_MODE=true
# or
touch .rebar/BARE_MODE


A pull-request is most welcome. Please make sure that the following criteria are fulfilled before making your pull-request:

  • Include a description regarding what has been changed and why.
  • Make sure that the changed or added functionality (if you modify code) is covered by unit tests.
  • Make sure that all unit tests pass.


[Apache 2.0](LICENSE)

*Note that all licence references and agreements mentioned in the katt README section above are relevant to that project's source code only.