Response Templating

Mock responses don’t have to be fully static JSON. Use {{ }} double-brace syntax inside any string value in response (or a sequence step’s response) to echo back data from the incoming request — no custom code required.

This dramatically cuts down on how many mock files you need. Instead of one file per id, username, or header value, one templated mock covers all of them.

Template sources

Template	Source
{{path.id}}	A named path parameter :id or {id}
{{query.page}}	URL query parameter ?page=2
{{header.x-request-id}}	Request header value (case-insensitive)
{{body.username}}	Top-level JSON (or form) body field
{{body.user.email}}	Nested JSON body field, using dot notation

Example

{
  "method": "POST",
  "path": "/users",
  "status": 201,
  "response": {
    "id": 99,
    "username": "{{body.username}}",
    "email": "{{body.email}}",
    "created_by": "{{header.x-actor}}",
    "self_url": "/users/99"
  }
}

curl -X POST http://localhost:8080/users \
  -H "X-Actor: admin" \
  -H "Content-Type: application/json" \
  -d '{"username":"alice","email":"[email protected]"}'

{
  "id": 99,
  "username": "alice",
  "email": "[email protected]",
  "created_by": "admin",
  "self_url": "/users/99"
}

Combined with path parameters

Templating is what makes path parameters useful for more than routing — the captured value can flow straight into the response:

{
  "method": "GET",
  "path": "/users/:id",
  "status": 200,
  "response": { "id": "{{path.id}}", "name": "Mock User" }
}

curl http://localhost:8080/users/42
# { "id": "42", "name": "Mock User" }

Semantics

• Templates are resolved after the mock (or sequence step) is chosen, so the interpolated value never affects matching itself — you can’t use a template to influence which mock wins.
• An unknown source, a missing key, or an explicit JSON null all resolve to an empty string. Malformed or unresolvable templates never panic and never leak the raw {{ }} text into the response.
• Non-string body values (numbers, booleans, nested objects/arrays) render using their JSON text form — {{body.age}} for {"age": 30} produces 30 (unquoted), not "30".
• A response with no {{ }} expressions is returned unchanged — there’s no templating overhead if you don’t use it.
• Header lookups are case-insensitive: {{header.X-Request-Id}} and {{header.x-request-id}} are equivalent.

Tip

Body templates work with both JSON bodies and application/x-www-form-urlencoded form bodies ({{body.field}} for a top-level form field). Nested dot notation ({{body.a.b.c}}) only applies to JSON bodies, since form data has no nesting.

Common gotchas

• {{path.id}} requires the mock’s path to actually capture that parameter. If your path is /users/:id but you reference {{path.slug}}, it silently renders as an empty string — there’s no error, so double-check the parameter name matches.
• Query values are always strings, same as with query parameter matching. {{query.page}} for ?page=2 renders "2", not 2.
• Templates only work inside response string values. You can’t template into status, method, path, or matcher fields.