Tarn — Built-in Functions Reference

UUIDs

Expression	Output	Deterministic
`{{ $uuid }}`	Random UUID v4	Yes (seeded)
`{{ $uuid_v4 }}`	Same as `$uuid`	Yes (seeded)
`{{ $uuid_v7 }}`	Time-ordered UUID v7 (Unix-ms prefix + random suffix)	Partial (real-time prefix)

Use in request bodies and capture assertions:

body:
  idempotency_key: "{{ $uuid }}"
  event_id: "{{ $uuid_v7 }}"

Timestamps

Expression	Output	Deterministic
`{{ $timestamp }}`	Unix timestamp (seconds since epoch)	No (real-time)
`{{ $now_iso }}`	ISO 8601 datetime string	No (real-time)

Random generators

Expression	Output	Deterministic
`{{ $random_hex(8) }}`	N-character random hex string	Yes (seeded)
`{{ $random_int(1, 100) }}`	Random integer in [MIN, MAX]	Yes (seeded)
`{{ $alpha(8) }}`	N lowercase ASCII letters	Yes (seeded)
`{{ $alnum(8) }}`	N lowercase alphanumeric characters	Yes (seeded)
`{{ $choice(a, b, c) }}`	Pick one option randomly	Yes (seeded)
`{{ $bool }}`	Random `"true"` or `"false"` string	Yes (seeded)

Faker generators (EN locale)

Expression	Output	Example
`{{ $email }}`	Random free email address	`jane.doe@example.com`
`{{ $first_name }}`	Random first name	`Jane`
`{{ $last_name }}`	Random last name	`Doe`
`{{ $name }}`	Random full name	`Jane Doe`
`{{ $username }}`	Random username	`jane_doe_42`
`{{ $phone }}`	Random phone number	`+1-555-123-4567`
`{{ $word }}`	Random single word	`qui`
`{{ $words(3) }}`	N random words joined by space	`lorem ipsum dolor`
`{{ $sentence }}`	Random sentence (4-10 words)	`Lorem ipsum dolor sit amet.`
`{{ $slug }}`	Hyphen-joined lowercase slug (2-4 words)	`lorem-ipsum-dolor`
`{{ $ipv4 }}`	Random IPv4 address	`192.168.1.1`
`{{ $ipv6 }}`	Random IPv6 address	`fe80::1`

Faker example in a request body:

body:
  name: "{{ $name }}"
  email: "{{ $email }}"
  username: "{{ $username }}"
  role: "{{ $choice(admin, user, viewer) }}"
  tags: "{{ $words(3) }}"

Determinism

Set TARN_FAKER_SEED=<u64> environment variable (highest priority) or faker.seed: <u64> in tarn.config.yaml to freeze every RNG-backed built-in for the process:

# tarn.config.yaml
faker:
  seed: 42

With a seed, these become reproducible:

$uuid / $uuid_v4 (v7 is partially real-time)
All $random_* and $choice / $bool
All faker generators ($name, $email, etc.)

These remain real-time even with a seed:

$timestamp — wall clock
$now_iso — wall clock
v7 UUID timestamp prefix — wall clock

Tip

Use a deterministic seed in CI to make test failures reproducible. Use no seed during local development for varied data.