V3 is our legacy API that most of our integrations were built on.

For any third-party integrations that require a Kit API key to integrate, you'll need to provide them with V3 API key. You'll find this in your Developer settings under the V3 API key section.

We will be looking to move these integrations into apps to provide a seamless OAuth experience between Kit and any third-party tool you use.

For V3 API documentation, please see here.

V4 API is currently only available to developer in our closed beta.

If you're a developer working on an app, the V4 API keys are for you to test our API for your future app without the need for OAuth authentication. This makes it faster and easier to prototype and test out functionality before fully committing to building your app out.

V4 API keys are only meant for individual use—for testing and for you to automate your own workflows. If you’re creating an app or integration for more customers, you’ll still need to use apps for access to plugins, bulk endpoints, higher rate limits, and more.

For V4 API documentation, please see here.