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Create mock server from Openapi specification


Openapi mock server is one of core components to support contract based development and testing. As part of our journal, several key requirements for such mock server have been identified:

  • Control response generation in non intrusive manner. i.e. without modifying example in contract
  • Randomly generated response to support property based testing
  • Fulfill property reference in response generation to support development against contract. i.e. regarding the following response, guarantee the selectedId property is always a valid id in the items
  "selectedId": 1,
  "items": [
    { "id": 1 },
    { "id": 2 }

After tried several existing options, we cannot find a best solution to meet all the requirements. So we end up with Fakeit.


  • Randomly or statically generated response (application/.*json)
  • Request validation (application/.*json, multipart/form-data)
  • Load specification from local or remote
  • Support hot reload local specification
  • Support specification in yaml or json format
  • [Experimental] Support plain text and binary response, e.g. application/pdf


Install it with:

$ gem install fakeit

Or use the docker image


$ fakeit --spec <Local file or remote url>

Command line options

$ fakeit --help
    --spec               spec file uri (required)
    -p, --port           custom port
    -q, --quiet          mute request and response log
    -l, --log-file       redirect log to a file
    --permissive         log validation error as warning instead of denying request
    --use-example        use example provided in spec if exists
    --static             generate static response
    --static-types       generate static value for specified types, e.g. --static-types integer,string
    --static-properties  generate static value for specified properties, e.g. --static-properties id,uuid
    --base-path          mounts the mock server at the given path, e.g. --base-path /api

other options:
    -v, --version
    -h, --help


  • See here for Openapi properties supported in random response generation
  • See here for default value in static response generation
  • Regarding --use-example mode, property without example specified will still be randomly or statically generated
  • Random response generation can not handle recursive schema reference. If you do need it in your spec file, please provide example property for the recursive part of schema and specify --use-example option.

Configuration endpoint

Mock server behaviour can be changed on the fly

Retrieve current config:

GET /__fakeit_config__

Update config:

PUT /__fakeit_config__

Request and response:

  "permissive": false,
  "use_example": true,
  "static": false,
  "static_types": [
  "static_properties": [
  "base_path": "/api"


After checking out the repo, run bundle install to install dependencies. Then, run rake to run the tests.

To install this gem onto your local machine, run bundle exec rake install.

To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.


Create mock server from Openapi specification








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