Overview

Project structure

Configuring fern starts with the fern folder, which contains your API definitions, SDK generators, and your CLI version.

Fern recommends a multi-repository structure with your fern folder in a source repository (containing your API definitions and generation configuration) and each generated SDK in its own separate repository.

Directory structure

When you run fern init (for the Fern Definition) or fern init --spec-type path/to/spec (for other specs), your fern folder is initialized with the following files:

$fern/
>  ├─ fern.config.json # Root-level config for entire Fern project
>  ├─ generators.yml # Defines SDKs and docs to generate
>  └─ spec-folder/ # definition, openapi, asyncapi, etc.
>    └─ spec-file.yml # API definition file

Core configuration files

Beyond the core files, you can optionally use an overrides file to customize your API definition without modifying the original spec. See Overrides for instructions.

fern.config.json file

Every fern folder has a single fern.config.json file. This file stores the organization and the version of the Fern CLI that you are using.

fern.config.json
1{
2    "organization": "your-organization",
3    "version": "0.31.2"
4}

Every time you run a fern CLI command, the CLI downloads itself at the correct version to ensure determinism.

generators.yml

The generators.yml file specifies which SDKs to generate. For OpenAPI, it also points to the location of your spec. For Fern Definition, Fern automatically detects your spec when your definition/ folder is at the same level as generators.yml.

generators.yml
1# Your API definition
2api: 
3  specs: 
4    - openapi: ./openapi-1.yml
5
6# SDK generators
7groups:
8  ts-sdk:
9    generators:
10      - name: fernapi/fern-typescript-sdk
11        version: <Markdown src="/snippets/version-number-ts.mdx"/>
12        github:
13          repository: your-organization/typescript-sdk-repo # Location of TypeScript SDK
14  
15  python-sdk:
16    generators:
17      - name: fernapi/fern-python-sdk
18        version: <Markdown src="/snippets/version-number-python.mdx"/>
19        github:
20          repository: your-organization/python-sdk-repo # Location of Python SDK

API definition file

For Fern Definition, your API configuration is split across two files: api.yml for API-wide configuration and separate .yml files for your actual endpoint and type definitions. See What is a Fern Definition? for more information.

For the other specification formats (OpenAPI, AsyncAPI, OpenRPC, and gRPC), you’ll have a single self-contained specification file.

To generate API reference documentation, set the api-name property in your docs.yml file.

Multiple APIs

When working with multiple API definitions, you can organize them in multiple ways depending on your SDK generation needs.

Use this approach when each API should generate its own independent set of SDKs.

$fern/
>  ├─ fern.config.json
>  └─ apis/
>    ├─ user-api/
>    │   ├─ generators.yml        # Configures user-api SDKs
>    │   └─ openapi.yml
>    └─ payments-api/
>        ├─ generators.yml        # Configures payments-api SDKs  
>        └─ openapi.yml

Each API must have its own generators.yml file:

apis/user-api/generators.yml
1api:
2  specs:
3    - openapi: openapi.yml
4groups:
5  ts-sdk:
6    generators:
7      - name: fernapi/fern-typescript-sdk
8        github:
9          repository: company/user-api-typescript
Pro and Enterprise feature

This feature is available only for the Pro and Enterprise plans. You can merge up to five APIs into a single SDK on the Pro plan, and unlimited APIs on the Enterprise plan. To get started, reach out to support@buildwithfern.com.

Use this approach to merge multiple APIs into a single set of SDKs.

$fern/
>  ├─ fern.config.json
>  ├─ generators.yml             # Single config for combined SDKs
>  ├─ user-api/
>  │   └─ openapi.yml
>  └─ payments-api/
>      └─ openapi.yml

List all APIs in a single generators.yml:

generators.yml
1api: 
2  specs: 
3    - openapi: user-api/openapi.yml
4    - openapi: payments-api/openapi.yml

If your APIs have overlapping schema names, add namespaces:

generators.yml
1api: 
2  specs: 
3    - openapi: user-api/openapi.yml
4    - namespace: payments
5      openapi: payments-api/openapi.yml

Use this approach when you need to support multiple API versions simultaneously in your SDK.

Option 1: Branch-based versioning

Each API version is maintained as a separate spec that publishes to a different branch of a single SDK repository. This approach allows you to release updates for older API versions independently and manage each as a different major version of your SDK (v1.x.x, v2.x.x).

$fern/
>  ├─ fern.config.json
>  └─ apis/
>      ├─ v1/
>      │   ├─ generators.yml      # Configures v1 SDK
>      │   └─ openapi.yml
>      └─ v2/
>          ├─ generators.yml      # Configures v2 SDK
>          └─ openapi.yml

Each API version must have its own generators.yml file that targets a specific branch:

apis/v1/generators.yml
1api:
2  specs:
3    - openapi: openapi.yml
4groups:
5  ts-sdk:
6    generators:
7      - name: fernapi/fern-typescript-sdk
8        github:
9          repository: company/sdk
10          mode: push
11          branch: v1              # Targets v1 branch

Option 2: Namespace-based versioning

If you prefer a single package with multiple API versions, use namespaces. This generates clients like client.v1 and client.v2 within the same package. Note that this increases package size as all versions are bundled together.

$fern/
>  ├─ fern.config.json
>  ├─ generators.yml             # Single config for all versions
>  └─ apis/
>      ├─ v1/
>      │   └─ openapi.yml
>      └─ v2/
>          └─ openapi.yml

List all API versions in a single generators.yml with namespaces:

generators.yml
1api: 
2  specs: 
3    - namespace: v1
4      openapi: apis/v1/openapi.yml
5    - namespace: v2
6      openapi: apis/v2/openapi.yml
7groups:
8  ts-sdk:
9    generators:
10      - name: fernapi/fern-typescript-sdk
11        github:
12          repository: company/sdk