TypeScript serde layer | Fern Documentation

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These pages cover SDK generation configured with generators.yml. New projects use sdk-config.yml, so see the Current version of this page for up-to-date guidance. To move an existing project, follow the SDK Config migration guide, which walks through fern sdk migrate, and review the sdk-config.yml reference.

Fern’s TypeScript SDK generator includes an optional serde layer that determines whether the generated SDK matches your original API naming conventions (snake_case vs. camelCase, case-sensitive fields) or follows TypeScript conventions. This serde layer is controlled by the noSerdeLayer option.

By default, the serde layer is disabled (noSerdeLayer: true), meaning field names are preserved exactly as they appear in your API specification.

When to enable the serde layer

generators.yml

groups:
  ts-sdk:
    generators:
    - name: fern-typescript-sdk
      version: 3.107.3
      config:
        noSerdeLayer: false # Enable serde layer

When you turn on the serde layer, (noSerdeLayer: false), the TypeScript SDK generator includes custom serialization code that:

Transforms all property names from your API to camelCase
Validates requests and responses at runtime
Supports complex types like Date and Set.

Enable the serde layer when you want idiomatic TypeScript conventions and don’t have case-sensitive field conflicts. Keep it disabled when your API has fields that differ only by casing or when you need exact field name preservation.

Additional configuration options

Several configuration options work alongside an enabled serde layer:

skipResponseValidation disables the serde layer’s runtime validation while keeping its transformation features
allowExtraFields permits properties not defined in your schema.
useBigInt enables custom JSON handling to preserve large number precision.

These options only take effect when the serde layer is enabled (noSerdeLayer: false), as they require the custom serialization infrastructure.

generators.yml

groups:
  ts-sdk:
    generators:
    - name: fern-typescript-sdk
      version: 3.107.3
      config:
        noSerdeLayer: false           # Enable serde layer
        skipResponseValidation: true  # Disable runtime validation errors
        allowExtraFields: true        # Allow undefined fields in responses
        useBigInt: true               # Preserve precision for large numbers

For the complete list of all TypeScript configuration options, go to TypeScript configuration.