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jtd-fuzz generates example data (aka "fuzz tests") from a JSON Typedef schema.

echo '{ "elements": { "type": "string" }}' | jtd-fuzz -n 5


On macOS, you can install jtd-fuzz via Homebrew:

brew install jsontypedef/jsontypedef/jtd-fuzz

For all other platforms, you can download and extract the binary yourself from the latest release. You can also install using cargo by running:

cargo install jtd_fuzz


Basic Usage

To invoke jtd-fuzz, you can either:

  1. Have it read from STDIN. This is the default behavior.
  2. Have it read from a file. To do this, pass a file name as the last argument to jtd-fuzz.

jtd-fuzz reads in a single JSON Typedef schema, and will output, by default, an infinite stream of examples. For example, this will output an infinite sequence of random JSON data:

echo '{}' | jtd-fuzz

If you'd like to have jtd-fuzz output an exact number of results, use -n:

echo '{}' | jtd-fuzz -n 5

Or, to have jtd-fuzz read from a file:

echo '{ "type": "timestamp" }' > foo.jtd.json

jtd-fuzz -n 5 foo.jtd.json

Generating emails, names, etc. with fuzzHint

Oftentimes, it's useful for jtd-fuzz to generate specific sorts of strings, instead of the generic nonsense strings you get by default with schemas whose type is string. You can customize jtd-fuzz's output using the fuzzHint metadata property. For example, this schema:

  "metadata": {
    "fuzzHint": "en_us/internet/email"
  "type": "string"

Would, if you put it in a file called example.jtd.json, generate data like this:

jtd-fuzz -n 5 example.jtd.json

fuzzHint will only work on schemas of {"type": "string"}. Here are some commonly-used values for fuzzHint:

  • en_us/company/company_name generates strings like Hayes, Murray, and Kiehn
  • en_us/internet/email generates strings like
  • en_us/names/full_name generates strings like Alexa Wisozk

A full list of possible values for fuzzHint is available here.

Advanced Usage: Providing a Seed

By default, jtd-fuzz will generate different output every time:

echo '{}' | jtd-fuzz -n 1 ; echo '{}' | jtd-fuzz -n 1

If you'd like to get consistent output from jtd-fuzz, or be able to reproduce its output, you can use the -s option to provide a seed to its internal pseudo-random number generator. For the same seed and schema, jtd-fuzz will output the same data every time:

echo '{}' | jtd-fuzz -n 1 -s 8927 ; echo '{}' | jtd-fuzz -n 1 -s 8927

The -s option takes an integer between 0 and 2^64 - 1.

Seeding jtd-fuzz can be useful if you're using jtd-fuzz to do automated testing against a system. Your automated testing system can pass jtd-fuzz a randomly-generated seed, and if the automated tester finds a seed that reveals a bug, it can output the seed it used. That way, developers can re-use that seed, and try to reproduce the issue locally.

Note that jtd-fuzz is only guaranteed to produce consistent output if you use the same seed, schema, and version of jtd-fuzz. Different versions on jtd-fuzz may output different results, even if you give them the same seed and schema.

Security Considerations

Do not rely on jtd-fuzz as a source of cryptographically secure randomness. jtd-fuzz is meant to be used as a generator of example data, such as for fuzz testing applications. It is not meant to be used for cryptographic purposes.


A CLI tool that generates example data from JSON Typedef schemas







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