Writing API Documentation in Plain Language: A Style Guide for Clarity
Plain language API documentation means writing every description, error message, and parameter note so that a developer can understand it on the first read, without re-reading a sentence or looking up an acronym. Apidoke's live preview editor makes it easy to spot unclear prose as you type, but the prose itself still has to be good. The rules below give you a repeatable craft for getting that right.
- Use active voice and short sentences (under 25 words) for all procedural instructions.
- Define every domain term on first use; never assume shared jargon.
- Structure descriptions so the most critical information appears in the first clause, not the last.
- Test your descriptions against a non-native English speaker's reading speed before publishing.
Why writing clarity matters as much as technical accuracy
A parameter table can be technically correct and still confuse a developer if the description is buried in a 50-word sentence. RFC 9110, the HTTP specification, went through repeated plain-language revisions specifically because earlier RFCs were accurate but hard to parse. The same pressure applies to your API docs.
Developers reading your docs are often context-switching from a coding session. They skim, not read. Non-native English speakers (a majority of the global developer audience) lose more time on nominalization, passive constructions, and ambiguous pronouns than native speakers do. Both groups benefit from the same discipline: fewer words, clearer structure, concrete examples.
The six plain-language rules that change doc quality immediately
1. Lead with the verb
Descriptions that open with a noun phrase before the action make readers work harder. Compare these two versions for the same query parameter:
| Unclear | Plain |
|---|---|
| The page_size parameter is a value that controls the number of items that are returned per page in the response. | Sets the number of items returned per page. Accepted values: 10, 25, 50, 100. Default: 25. |
| Authentication is performed via a Bearer token that must be included in the Authorization header of each request. | Pass your Bearer token in the Authorization header on every request. |
| A 401 response is returned when the credentials provided are not valid or have expired. | Returns 401 when your token is invalid or expired. Refresh the token and retry. |
Every plain version is shorter, starts with a verb (sets, pass, returns), and puts the action before any qualification. That order matches how developers read: they want to know what a thing does before they want to know why or under what conditions.
2. Keep sentences under 25 words
A 25-word ceiling is not arbitrary. Research in technical communication consistently shows comprehension drops as sentence length climbs past 20 to 25 words, especially for readers operating in a second language. Count the words in your draft descriptions. If you hit 26, split the sentence or cut a clause.
Subordinate clauses are the main culprit. "The endpoint accepts a JSON body which must include a valid ISO 8601 timestamp in the created_at field, unless the omit_timestamps flag is set to true, in which case the field is ignored" is one sentence and 37 words. Split it:
The endpoint accepts a JSON request body. Include an ISO 8601 timestamp in
created_at. If you setomit_timestampstotrue, the field is ignored.
Three sentences, each under 15 words. A reader scanning fast can process all three.
3. Define every term on first use
"Idempotent", "pagination cursor", "rate limiting window", "content negotiation" all carry meaning that is obvious to a senior backend engineer and opaque to a mobile developer touching a REST API for the first time. Define each term once, in the first place it appears.
In Apidoke, a natural place to do this is in a named data structure or a descriptive section at the top of a Group. In API Blueprint format:
# Group Orders
The Orders endpoints let you create, retrieve, and cancel purchase orders.
All list endpoints are **paginated**: results come back in fixed-size pages.
Pass the `cursor` value from the previous response to fetch the next page.
## Order [/orders/{id}]
"Paginated" is defined in the same breath it is introduced. Developers who already know the term skip the definition in half a second. Developers who do not know it now have context for every cursor parameter that follows.
4. Write for the active subject
Passive voice hides who does what. "The token is validated" does not say whether the client validates the token or the server does. "The server validates the token on every request" is unambiguous. When the subject is your API, name it: "The API returns", "The server checks", "Apidoke renders". When the subject is the developer, say "you": "You must include", "Pass your API key", "Call this endpoint".
5. Put constraints in the description, not just in a type field
A parameter typed as string with no description tells a developer nothing about maximum length, allowed characters, or format. A parameter typed as string with the description "User's full name. Maximum 128 characters. Allowed: letters, spaces, hyphens. Required." tells them everything they need. They will not hit a 422 or a 400 at runtime because your docs were specific.
In API Blueprint MSON (Markdown Syntax for Object Notation, the format used to define data structures inline) this looks like:
## Attributes
+ full_name (string, required) - User's full name. Max 128 characters.
Letters, spaces, and hyphens only. Example: `Ada Lovelace`.
+ status (enum[string], required)
+ Members
+ `active`
+ `inactive`
+ `pending`
The enum member list makes it impossible for a developer to guess allowed values. The description on full_name prevents a 400 Bad Request at runtime.
6. Use parallel structure in lists and tables
Parallel structure means every item in a list or table starts the same grammatical way. If your first parameter description starts with a verb ("Returns the user's ID"), all descriptions in that table should start with a verb. If the first starts with a noun ("The user's ID"), all should. Inconsistency forces the brain to re-orient on every row, which costs reading time.
How to write error descriptions that developers can act on
Error documentation is where plain language pays the highest dividend. A developer who gets a 422 Unprocessable Entity at 11 pm does not want to read "An error has occurred due to invalid input". They want to know which field failed, what the expected format was, and what to send instead.
Write every error description in three parts: what happened, why it happened, how to fix it. Keep it to two or three short sentences.
In API Blueprint, a documented error response looks like this:
### Create User [POST]
+ Request (application/json)
+ Body
{
"email": "ada@example.com",
"full_name": "Ada Lovelace"
}
+ Response 201 (application/json)
+ Body
{
"id": "usr_01HXYZ",
"email": "ada@example.com",
"status": "active"
}
+ Response 422 (application/json)
Returned when required fields are missing or formatted incorrectly.
Check the `errors` array for the specific field and reason.
Correct the value and retry the request.
+ Body
{
"error": "validation_failed",
"errors": [
{
"field": "email",
"message": "Must be a valid email address."
}
]
}
Three sentences in the 422 description, each under 15 words. The error body mirrors the description. A developer who reads the description knows exactly where to look in the response and what to do next. The MDN reference for 422 defines the status code itself if you need a source to link from your own docs.
Scannability: structuring your prose for a developer's eye path
Developers do not read API docs like a book chapter. They land on a specific endpoint, scan for the parameter they need, check the example, and leave. Your layout and sentence structure must support that pattern.
Use short paragraphs and headers generously
Any conceptual explanation longer than four sentences deserves a header. If you are explaining rate limiting, authentication, pagination, and error handling in one block of prose, break them into sections. Each section header is a navigation target for a developer scanning the page.
Put examples before explanations, not after
A concrete example anchors the explanation that follows. If you write two paragraphs about how cursor-based pagination works and then show an example, a developer who goes straight to the example (most of them) will miss your explanation entirely. Show the example first, then annotate the relevant parts.
Use monospace code formatting for every literal value
Values like true, null, application/json, and Authorization should always appear in code formatting, not plain text. This is both a scannability signal (the reader's eye picks out code formatting quickly) and a precision signal (it tells the reader this is a literal string, not a descriptive label).
Writing for non-native English readers
A significant share of your API's developer audience reads English as a second or third language. You cannot know their proficiency. The safest approach is to write at a reading level that costs nothing for fluent readers and prevents comprehension failure for everyone else.
| Pattern to avoid | Why it causes problems | Plain alternative |
|---|---|---|
| Phrasal verbs: "set up", "look up", "hand off" | Phrasal verbs have non-literal meanings that must be memorized per phrase | "configure", "retrieve", "transfer" |
| Idioms: "out of the box", "under the hood" | Idioms are opaque without cultural context | "by default", "internally" |
| Contractions: "won't", "can't", "it's" | Contractions are sometimes parsed incorrectly by non-native readers | "will not", "cannot", "it is" |
| Long relative clauses: "the response which is returned when..." | Nested clauses delay the main verb and increase parsing load | Split into two sentences |
| Ambiguous pronouns: "it", "they", "this" | Unclear referent requires the reader to backtrack | Repeat the noun: "the token", "the endpoint" |
The U.S. Plain Language Guidelines cover these patterns in depth and are directly applicable to technical writing, not just government prose.
How Apidoke supports plain language writing in practice
The discipline of plain language is easier to maintain when you get instant visual feedback on your prose. Apidoke's split-pane CodeMirror editor renders your API Blueprint markup live as you type, so you see exactly how your descriptions will appear in the final three-column layout: navigation on the left, content in the center, and the live try-it console on the right.
That live preview changes how you edit. You can see that a 60-word description collapses into a dense grey block in the rendered view, which prompts you to split it. You can see that a parameter table with inconsistent capitalization looks inconsistent, which prompts you to standardize. The visual feedback loop replaces a separate review step.
Per-project version history means you can review the plain-language changes you made between versions and track whether readability actually improved. And because Apidoke is self-hosted, your draft descriptions never leave your infrastructure before you choose to publish them.
For the structural and naming-convention side of style (resource naming, status code conventions, HTTP method rules), the API style guide for teams covers those decisions in detail. This article focuses on the prose layer that sits on top of that structure.
For a broader look at how writing quality fits into the overall developer experience your API delivers, the developer experience guide for API teams treats documentation as one component of a larger DX system.
A plain-language editing checklist
Run every description through this list before publishing:
- Is the sentence under 25 words? If not, split it.
- Does the sentence start with the action or the subject, not a long noun phrase?
- Is the voice active? Rewrite any passive construction where the actor matters.
- Are all domain terms defined on first use in this document?
- Are all literal values, field names, and status codes in monospace formatting?
- Does every error description include what happened, why, and how to fix it?
- Are all items in each list or table grammatically parallel?
- Have you replaced phrasal verbs, idioms, and ambiguous pronouns with direct alternatives?
- Is the example shown before the explanation, not after?
- Does a developer reading only the first sentence of each section get the core information?
Frequently asked questions
What does plain language mean in API documentation specifically?
In API documentation, plain language means writing descriptions, error messages, and parameter notes in short, active-voice sentences that a developer can understand on the first read. It includes defining technical terms on first use, avoiding idioms and complex relative clauses, and putting actionable information before qualifications.
How do I write API error descriptions that are actually helpful?
Structure each error description in three parts: what happened ("Returns 422 when a required field is missing"), why it happened ("The request body did not include email"), and how to fix it ("Add the missing field and retry"). Keep all three parts to one or two sentences each.
Should I avoid technical jargon entirely in API docs?
No. Technical terms like "idempotent", "pagination", and "Bearer token" are precise and necessary. The rule is to define each term the first time it appears, so developers unfamiliar with it have the context they need for everything that follows.
How do I write API docs for non-native English speakers without dumbing them down?
Replace phrasal verbs with single-word verbs (use "retrieve" instead of "look up"), avoid idioms, repeat nouns instead of using ambiguous pronouns, and keep sentences short. These changes cost nothing for fluent readers and significantly reduce comprehension time for everyone else.
What is the difference between a plain-language style guide and an API style guide?
An API style guide covers structural and naming conventions: how to name resources, which HTTP methods to use, how to format status codes. A plain-language style guide covers writing craft: sentence structure, word choice, and scannability. Both are needed and work best together.
Ready to put these principles into practice? Create your free Apidoke account and start writing API Blueprint docs in the live split-pane editor. Publish a clean, readable three-column reference in minutes, with no toolchain required.
Related reading: How to Pitch Better API Documentation to Your Engineering Team