Dynamic Request Bodies

Generate a unique JSON body per request using typed placeholders.

Why use templates?

Static bodies send the same payload on every request. Templates let you generate realistic, varied data — different user IDs, random amounts, unique identifiers — so your load test exercises server-side logic rather than hitting a cache.

Basic example

Create template.json:

{
  "userId": "{{user_id}}",
  "amount": "{{amount}}",
  "_loadtest_metadata_templates": {
    "user_id": {
      "type": "string",
      "details": { "choice": ["user-001", "user-002", "user-003"] }
    },
    "amount": {
      "type": "float",
      "min": 1.0,
      "max": 500.0,
      "details": { "decimals": 2 }
    }
  }
}

Run it:

lmn run -H https://api.example.com/orders -M post -T ./template.json

Each request body will have a randomly chosen userId and a random amount between 1.00 and 500.00.

Placeholder types

String — choice list:

"user_id": { "type": "string", "details": { "choice": ["alice", "bob", "carol"] } }

String — generated:

"session_id": {
  "type": "string",
  "details": {
    "length": { "min": 8, "max": 16 },
    "uppercase": 2,
    "lowercase": 4
  }
}

Float:

"price": { "type": "float", "min": 0.01, "max": 999.99, "details": { "decimals": 2 } }

Once placeholders

Use :once to generate a value once at startup and reuse it across all requests:

{ "session": "{{session_id:once}}" }

Useful for correlation IDs or session tokens that should be consistent across a run.

Environment variable injection

Use {{ENV:VAR_NAME}} to inject environment variables at startup — no definition needed:

{ "apiKey": "{{ENV:API_KEY}}" }

Tip

{{ENV:VAR_NAME}} values are resolved once at template load time and are never logged or included in output.

Tracking response fields

Use a response template (-S) alongside your request template to extract and aggregate fields from response bodies. The most common use: find out what error codes your API returns under load.

Create response.json mirroring the shape of your API's response:

{
  "error": {
    "code": "{{ STRING }}"
  }
}

Run with both:

lmn run -H https://api.example.com/orders -M post \
  -T ./template.json \
  -S ./response.json

After the run, lmn prints a frequency distribution of every distinct value it observed at error.code across all responses. This makes silent application errors — business logic failures that return HTTP 200 — visible under load.

Use {{ FLOAT }} to extract and aggregate numeric fields (e.g. response times, amounts, scores reported by the API itself).

See the Track error codes recipe for a copy-paste example, and Template Placeholders for the full reference.

Full placeholder reference

See Template Placeholders for all types, fields, constraints, and validation rules.