APIs need documentation, and most of them do have documentation. Sometimes it's pretty good documentation. But the very best documentation is a rich and polished experience, and that's not universal across the industry. One key ingredient in the best API experiences is that they offer more than just a list of fields and a basic description. In this post, take a tour of how Markdown can add the sparkle that will make your API documentation shine.

Markdown is a text-notation dialect

Markdown is possibly the most approachable of the lightweight markup languages, allowing users to add some text-based formatting to plain text content. Using markup in any type of written content can enhance its readability, and in dense reference documentation it definitely brings great value.

OpenAPI supports CommonMark, a formal and well-described version of Markdown. For users who are familiar with GitHub-Flavored Markdown, there are some minor differences to look out for but it's a very transferrable skill. CommonMark doesn't support tables or checklists for example, so look out for those.

Markdown and OpenAPI

OpenAPI supports the use of Markdown in all description fields, anywhere in the API description. In contrast, title and summary fields are plain text only.

How does summary differ from description?

OpenAPI supports both summary and description fields in many contexts. Summary is plain text and is typically used when the item is being shown in a list, so it should be as short as possible - usually just a few words in length. Description fields support Markdown and are used when the user is seeing the detail on a single item, such as an operation, where it makes sense to go into more detail.

By adopting an existing standard, users with Markdown experience can immediately feel familiar with adding Markdown to their OpenAPI descriptions.

Effective use of inline Markdown

Markdown offers multiple formatting options inline to make your descriptions richer. For example, you can add emphasis with bold and italic entries, but formatting is only effective if it is used sparingly. Add formatting to attract users' attention, especially if there's something that might surprise them.

Here's a good example from the GitHub API, under the "Create an Issue" endpoint:

    type: array
    description: 'Labels to associate with this issue. _NOTE: Only users
      with push access can set labels for new issues. Labels are silently
      dropped otherwise._'

If you don't have push access to a repository, you can't set labels. The description makes it clear that no errors will be returned and instead the labels will be ignored. This is very useful context for a user running into this behavior. Another powerful way to add context is to use a hyperlink.

By adding links to our OpenAPI descriptions, we can signpost users to supporting or optional resources that are available if they should need them. Including links is a great way to expand on a key concept without cluttering an API description, or to give a clear indication of what is meant by a particular term in this context.

Still with GitHub's API description handy, here's an example of that in action:

      summary: Create a GitHub App from a manifest
      description: Use this endpoint to complete the handshake necessary when implementing
        the [GitHub App Manifest flow](https://docs.github.com/apps/building-github-apps/creating-github-apps-from-a-manifest/).
        When you create a GitHub App with the manifest flow, you receive a temporary
        `code` used to retrieve the GitHub App's `id`, `pem` (private key), and `webhook_secret`.

Some users know what the GitHub App Manifest flow is, and don't need a lengthy description. Others probably don't, and the information is instantly available in the documentation. The same link is used in a few related endpoints, since they all need the same additional documentation, and it helps the user to find their way around.

I also like this example for its clear use of code markup to indicate field names in the text, making it easier for users to see what those items are.

Expand the experience with multiline descriptions

For a situation where you want to write more than one line of content, or soft wrap a longer line, you can use either folded or wrapped text blocks.

Wrapped blocks use |- as the start of the block, which removes leading and trailing whitespace but discards the indentation and newlines. Using wrapped text means you can make longer sentences easier to read in the source file by wrapping them, but the text will be presented as a single line. Let's look at an example of a wrapped block, taken from the excellent Stripe API.

  description: |-
    The URL the customer will be directed to after the payment or
    subscription creation is successful.

This single-sentence example wouldn't be too hard to manage if it was on one line, but add another sentence or two and this approach becomes even more valuable.

Folded blocks use >-, which preserves the whitespace within the text, but removes leading and trailing space. This type of folding is really valuable when you want to include paragraph breaks, bullet points, or other documentation "furniture" in a description field. Also from Stripe's API, here's an example of a description that includes paragraphs:

  description: >-
    This is an object representing a capability for a Stripe account.

    Related guide: [Account

Being able to use additional formatting can make information easier to digest when there are extensive or complex instructions needed.

Make your API descriptions the best they can be

Using the description fields to their full potential is a great way to give users exactly the information they need, in the context that they need it. By using Markdown features, you can emphasize key details, supply links to additional reading or reference materials that can help the user along the way, or present additional information in a format that can be parsed by humans. The multiline approaches expand these possibilities, and also make our OpenAPI specifications easier to read and maintain.

Add `externalDocs`

Use OpenAPI's externalDocs feature to add documentation links to your API's info section or individual operations. Most documentation tools will render this nicely, and you can supply a description as well as a URL.

Better copy is just one way to improve your API descriptions, but it's well worth your attention. The aim of this post is to give you some tools (and some inspiration) to make your users' API experiences a little more delightful at every step.

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