Response Templating

Response templating allows you to create dynamic responses that include data from the incoming request, generate random values, or compute values at response time.

Template Syntax

Templates use double curly braces: {{expression}}

{
  "response": {
    "body": {
      "message": "Hello, {{request.query.name}}"
    }
  }
}

Request: GET /api/greet?name=Alice Response: {"message": "Hello, Alice"}

Request Data

Access various parts of the incoming request.

Path Parameters

{
  "matcher": {
    "path": "/api/users/{id}"
  },
  "response": {
    "body": {
      "id": "{{request.pathParam.id}}",
      "url": "/api/users/{{request.pathParam.id}}"
    }
  }
}

Query Parameters

{
  "response": {
    "body": {
      "page": "{{request.query.page}}",
      "limit": "{{request.query.limit}}"
    }
  }
}

Headers

{
  "response": {
    "body": {
      "userAgent": "{{request.header.User-Agent}}",
      "correlationId": "{{request.header.X-Correlation-ID}}"
    }
  }
}

Request Body

Access parsed request body (JSON):

{
  "response": {
    "body": {
      "received": {
        "username": "{{request.body.username}}",
        "email": "{{request.body.email}}"
      },
      "status": "created"
    }
  }
}

Nested access:

{
  "response": {
    "body": {
      "city": "{{request.body.address.city}}",
      "firstItem": "{{request.body.items[0].name}}"
    }
  }
}

Request Metadata

{
  "response": {
    "body": {
      "method": "{{request.method}}",
      "path": "{{request.path}}",
      "fullUrl": "{{request.url}}"
    }
  }
}

Built-in Functions

Timestamps

{
  "response": {
    "body": {
      "timestamp": "{{now}}",
      "isoDate": "{{now.iso}}",
      "unixTimestamp": "{{timestamp}}"
    }
  }
}

Output:

{
  "timestamp": "2024-01-15T10:30:00Z",
  "isoDate": "2024-01-15T10:30:00Z",
  "unixTimestamp": 1705315800
}

Random Values

{
  "response": {
    "body": {
      "id": "{{uuid}}",
      "randomFloat": "{{random.float 0 1}}"
    }
  }
}

String Functions

{
  "response": {
    "body": {
      "upper": "{{upper request.body.name}}",
      "lower": "{{lower request.body.email}}",
      "trim": "{{trim request.body.input}}"
    }
  }
}

Conditional Logic

If/Else

{
  "response": {
    "body": {
      "status": "{{if request.query.premium}}premium{{else}}free{{end}}"
    }
  }
}

Default Values

{
  "response": {
    "body": {
      "page": "{{default request.query.page \"1\"}}",
      "limit": "{{default request.query.limit \"10\"}}"
    }
  }
}

Loops and Arrays

Repeating Elements

{
  "response": {
    "body": {
      "items": "{{repeat 5 '{\"id\": {{index}}, \"name\": \"Item {{index}}\"}'}}"
    }
  }
}

Array Length

{
  "response": {
    "body": {
      "itemCount": "{{len request.body.items}}"
    }
  }
}

Response Headers

Templates work in headers too:

{
  "response": {
    "headers": {
      "X-Request-ID": "{{uuid}}",
      "X-Correlation-ID": "{{request.header.X-Correlation-ID}}",
      "Location": "/api/users/{{request.body.id}}"
    }
  }
}

Escaping

Literal Braces

To output literal {{, escape with backslash:

{
  "body": "Template syntax uses \\{{expression}}"
}

JSON in Templates

When embedding JSON, ensure proper escaping:

{
  "response": {
    "body": "{{jsonEncode request.body}}"
  }
}

Complete Example

{
  "matcher": {
    "method": "POST",
    "path": "/api/orders"
  },
  "response": {
    "statusCode": 201,
    "headers": {
      "Content-Type": "application/json",
      "Location": "/api/orders/{{uuid}}",
      "X-Request-ID": "{{request.header.X-Request-ID}}"
    },
    "body": {
      "id": "{{uuid}}",
      "status": "pending",
      "customer": {
        "name": "{{request.body.customer.name}}",
        "email": "{{lower request.body.customer.email}}"
      },
      "items": "{{request.body.items}}",
      "itemCount": "{{len request.body.items}}",
      "total": "{{request.body.total}}",
      "createdAt": "{{now.iso}}",
      "estimatedDelivery": "{{now.addDays 3}}"
    }
  }
}

Request:

curl -X POST http://localhost:4280/api/orders \
  -H "Content-Type: application/json" \
  -H "X-Request-ID: req-123" \
  -d '{
    "customer": {"name": "Alice", "email": "ALICE@EXAMPLE.COM"},
    "items": [{"sku": "A1", "qty": 2}],
    "total": 49.99
  }'

Response:

{
  "id": "abc123",
  "status": "pending",
  "customer": {
    "name": "Alice",
    "email": "alice@example.com"
  },
  "items": [{"sku": "A1", "qty": 2}],
  "itemCount": 1,
  "total": 49.99,
  "createdAt": "2024-01-15T10:30:00Z",
  "estimatedDelivery": "2024-01-18T10:30:00Z"
}

Template Reference

Expression	Description
{{request.method}}	HTTP method
{{request.path}}	Request path
{{request.pathParam.name}}	Path parameter
{{request.query.name}}	Query parameter
{{request.header.Name}}	Request header
{{request.body}}	Full request body
{{request.body.field}}	Body field
{{now}}	Current ISO timestamp
{{timestamp}}	Unix timestamp (seconds)
{{uuid}}	Random UUID
{{upper value}}	Uppercase string
{{lower value}}	Lowercase string
{{default value fallback}}	Default if empty
{{len array}}	Array length

Next Steps

• Request Matching - Matching rules
• Stateful Mocking - CRUD simulation
• Configuration Reference - Full schema