Parameter Names | Fern Documentation

By default, Fern uses the parameter names from your OpenRPC method definitions as SDK parameter names. You can customize these using the x-fern-parameter-name extension.

Customize parameter names

Use x-fern-parameter-name to specify custom parameter names for method parameters:

openrpc.yml

1 methods:
2   - name: user.create
3     summary: Create a new user
4     params:
5       - name: user_data
6         schema:
7           type: object
8           properties:
9             email_address:
10               type: string
11               format: email
12               x-fern-parameter-name: email
13             full_name:
14               type: string
15               x-fern-parameter-name: name
16             date_of_birth:
17               type: string
18               format: date
19               x-fern-parameter-name: dateOfBirth
20           required:
21             - email_address
22             - full_name
23         x-fern-parameter-name: userData
24         required: true
25     result:
26       name: user
27       schema:
28         $ref: '#/components/schemas/User'

This generates SDK methods with cleaner parameter names:

1 // Instead of user_data with email_address, full_name, date_of_birth
2 await client.user.create({
3   userData: {
4     email: "john@example.com",
5     name: "John Doe",
6     dateOfBirth: "1990-01-15"
7   }
8 });

Language-specific parameter names

You can specify different parameter names for different programming languages:

openrpc.yml

1 methods:
2   - name: order.search
3     summary: Search for orders
4     params:
5       - name: search_criteria
6         schema:
7           type: object
8           properties:
9             customer_id:
10               type: string
11               x-fern-parameter-name:
12                 python: customer_id
13                 typescript: customerId
14                 go: CustomerID
15                 java: customerId
16                 csharp: CustomerId
17             order_status:
18               type: string
19               enum: [pending, processing, shipped, delivered]
20               x-fern-parameter-name:
21                 python: order_status
22                 typescript: orderStatus
23                 go: OrderStatus
24                 java: orderStatus
25                 csharp: OrderStatus
26         x-fern-parameter-name:
27           python: search_criteria
28           typescript: searchCriteria
29           go: SearchCriteria
30           java: searchCriteria
31           csharp: SearchCriteria
32         required: true
33     result:
34       name: orders
35       schema:
36         type: array
37         items:
38           $ref: '#/components/schemas/Order'

Nested parameter naming

Customize parameter names for nested objects:

openrpc.yml

1 methods:
2   - name: payment.process
3     summary: Process a payment
4     params:
5       - name: payment_data
6         schema:
7           type: object
8           properties:
9             payment_method:
10               type: object
11               x-fern-parameter-name: paymentMethod
12               properties:
13                 card_number:
14                   type: string
15                   x-fern-parameter-name: cardNumber
16                 expiry_date:
17                   type: string
18                   x-fern-parameter-name: expiryDate
19                 cvv:
20                   type: string
21                 billing_address:
22                   type: object
23                   x-fern-parameter-name: billingAddress
24                   properties:
25                     street_address:
26                       type: string
27                       x-fern-parameter-name: streetAddress
28                     postal_code:
29                       type: string
30                       x-fern-parameter-name: postalCode
31             amount:
32               type: number
33             currency:
34               type: string
35               enum: [USD, EUR, GBP]
36         x-fern-parameter-name: paymentData
37         required: true
38     result:
39       name: paymentResult
40       schema:
41         $ref: '#/components/schemas/PaymentResult'

Array parameter naming

Handle array parameters with custom naming:

openrpc.yml

1 methods:
2   - name: bulk.createUsers
3     summary: Create multiple users
4     params:
5       - name: user_list
6         schema:
7           type: array
8           items:
9             type: object
10             properties:
11               email_addr:
12                 type: string
13                 format: email
14                 x-fern-parameter-name: email
15               user_name:
16                 type: string
17                 x-fern-parameter-name: username
18             required:
19               - email_addr
20               - user_name
21         x-fern-parameter-name: users
22         minItems: 1
23         maxItems: 100
24         required: true
25     result:
26       name: createdUsers
27       schema:
28         type: array
29         items:
30           $ref: '#/components/schemas/User'

Optional parameter naming

Customize names for optional parameters with defaults:

openrpc.yml

1 methods:
2   - name: report.generate
3     summary: Generate a report
4     params:
5       - name: report_type
6         schema:
7           type: string
8           enum: [daily, weekly, monthly, yearly]
9         x-fern-parameter-name: reportType
10         required: true
11       - name: output_format
12         schema:
13           type: string
14           enum: [pdf, csv, json, excel]
15           default: pdf
16         x-fern-parameter-name: format
17         required: false
18         description: Output format (defaults to PDF)
19       - name: include_charts
20         schema:
21           type: boolean
22           default: true
23         x-fern-parameter-name: includeCharts
24         required: false
25         description: Include charts in the report
26       - name: date_range
27         schema:
28           type: object
29           properties:
30             start_date:
31               type: string
32               format: date
33               x-fern-parameter-name: startDate
34             end_date:
35               type: string
36               format: date
37               x-fern-parameter-name: endDate
38         x-fern-parameter-name: dateRange
39         required: false
40         description: Optional date range filter
41     result:
42       name: report
43       schema:
44         $ref: '#/components/schemas/Report'

Positional parameter naming

For methods using positional parameters, customize the generated parameter names:

openrpc.yml

1 methods:
2   - name: math.calculate
3     summary: Perform mathematical calculation
4     paramStructure: by-position
5     params:
6       - schema:
7           type: number
8         description: First operand
9         x-fern-parameter-name: operandA
10       - schema:
11           type: number
12         description: Second operand
13         x-fern-parameter-name: operandB
14       - schema:
15           type: string
16           enum: [add, subtract, multiply, divide]
17         description: Operation to perform
18         x-fern-parameter-name: operation
19     result:
20       name: result
21       schema:
22         type: number

This generates cleaner SDK interfaces:

1 // Instead of unnamed positional parameters
2 await client.math.calculate(10, 5, "add");
3 
4 // With named parameters
5 await client.math.calculate({
6   operandA: 10,
7   operandB: 5,
8   operation: "add"
9 });

Using custom parameter names ensures consistent naming conventions across your SDK while maintaining compatibility with your existing JSON-RPC method signatures.