CLI reference

Changelog

February 6, 2026

3.69.1

(internal): Regenerate older IR types with new TypeScript SDK generator.

3.69.0

(feat): [Beta] Add fern docs md generate command for generating MDX documentation from library source code. This feature requires a libraries configuration in docs.yml. The command is in beta and may change in future releases.

3.68.1

(fix): Fix grouped multi-API environment server handling to preserve defaultUrl, urlTemplate, and variables metadata when merging multiple OpenAPI specs with group-multi-api-environments: true. Previously, server URL templating metadata was silently dropped during the merge, causing generated SDKs to use malformed template-substituted URLs as defaults. Also fixes API name extraction for grouped base URL keys to use the clean x-fern-default-url value instead of the variable-substituted URL.

3.68.0

(feat): Add folder-title-source global setting in docs.yml under settings. When set, this value is used as the default title-source for all folder navigations that do not explicitly specify their own title-source. This allows configuring frontmatter or filename as the default title derivation strategy for all folders in one place.

3.67.1

(fix): Fix title-source: frontmatter in folder navigation to correctly pass the setting through to buildNavigationForDirectory. Previously, the title-source option was ignored when expanding folder configurations, so page titles and sidebar titles always derived from file names. Now, when title-source: frontmatter is set, page titles and the folder section title use the frontmatter title field.

3.67.0

(feat): Adds schema name and title collision detection for OpenAPI/AsyncAPI specs. When title-as-schema-name is enabled and multiple schemas share the same title, fern check now reports an error identifying the conflicting schemas. To opt in to automatic resolution, set resolve-schema-collisions: true in your generators.yml settings under api/specs:

1api:
2  specs:
3    - asyncapi: asyncapi.yml
4      settings:
5        title-as-schema-name: true
6        resolve-schema-collisions: true

When enabled, colliding titles are disambiguated with a numeric suffix (e.g. OpenAI2) and a warning is emitted instead of an error.

(fix): Run OSS validation rules (e.g. schema title collision detection) during fern generate, not just during fern check. This ensures validation errors are surfaced before generation proceeds.

3.66.0

(feat): Add support for server URL templating in multi-URL environments. This extends the IR to support OpenAPI server variables for APIs with multiple base URLs.

New Fern definition schema fields for MultipleBaseUrlsEnvironmentSchema:

  • url-templates: URL templates with variable placeholders for each base URL

    1url-templates:
    2  base: "https://api.{region}.example.com/v1"
    3  auth: "https://auth.{region}.example.com"

  • default-urls: Default URLs when no variables are provided (from x-fern-default-url)

    1default-urls:
    2  base: "https://api.us-east-1.example.com/v1"
    3  auth: "https://auth.us-east-1.example.com"

  • variables: Server variables for URL templating, keyed by base URL ID

    1variables:
    2  base:
    3    - id: region
    4      default: us-east-1
    5      values: [us-east-1, us-west-2, eu-west-1]

ServerVariableSchema structure:

  • id: Variable name as it appears in the URL template (e.g., region)
  • default: Default value for the variable
  • values: Optional enum of allowed values

OpenAPI to IR conversion: Server variables from OpenAPI specs are now preserved in the IR rather than being exploded into multiple environment variants. This enables SDKs to expose server variables as constructor parameters with runtime URL interpolation.

x-fern-default-url extension: When present on an OpenAPI server, specifies a separate default URL to use when no variables are provided, allowing the template URL and default URL to differ.

3.65.1

(fix): Fix fern check --strict-broken-links was not validating markdown files referenced by sections with paths.

3.65.0

(feat): Add --generator option to fern generate to run a specific generator by name. Filters within the resolved group (from --group or default-group) to only run the specified generator.