84% of developers use technical documentation for learning APIs. Yet most API docs are so badly written that developers abandon integrations within an hour. This step-by-step guide teaches you exactly how to write API documentation that works, from the first line to the last error code, with real examples you can copy and adapt today.

Why Most API Documentation Fails

Let me describe a scene every developer has lived through.

You find an API that does exactly what you need. You click the docs link. You land on a page that starts with three paragraphs about the company's history, followed by a list of every endpoint with no indication of which one you need first, followed by code examples using YOUR_API_KEY_HERE and [email protected] and string as placeholder values that tell you absolutely nothing about what real data looks like.

You spend 45 minutes trying to figure out the authentication. You get a 401 error that says Unauthorised. The docs do not mention what causes a 401 or how to fix it. You give up.

Most API docs fall into one of two traps. Either they are auto-generated reference docs with no context, lists of endpoints and parameters without guidance on how to actually use them, or they are written for developers who already understand the system, skipping over the "obvious" stuff that trips up newcomers.

This guide fixes both traps, step by step.

What This Guide Covers

By the end of this article, you will know how to write every section of a complete, professional API documentation set:

1. The Overview: what your API does and who it is for
2. Authentication: how to authenticate, step by step, with real examples
3. Getting Started: your first working API call in under 5 minutes
4. Endpoint Reference: every endpoint documented completely and consistently
5. Error Handling: every error code with the cause and the fix
6. Code Examples: real, working examples in multiple languages
7. Changelog: keeping docs current as the API evolves
8. The OpenAPI Spec, making your docs machine-readable for AI tools

We will build a complete documentation example for a fictional but realistic API: a Task Management API that developers can use to create, read, update, and delete tasks in a project management system.

Before You Write a Single Word: Understand Your Audience

Before typing a single word, think about your audience. Documentation is for people, not machines. External developers need clear instructions and examples they can copy and adapt. Internal teams might understand your domain but still need a reference for endpoints, parameters, and responses.

For the Task Management API, our audience is:

• Primary: Backend developers integrating the API into their applications
• Secondary: Frontend developers building UI on top of the API
• Tertiary: DevOps engineers automating task management workflows

Write this down before you start. Every decision you make what to explain, how much to assume, which examples to choose, should be filtered through "would my primary audience understand this?

Section 1: The Overview

The Overview is the first thing a developer reads. It must answer three questions in 30 seconds:

1. What does this API do?
2. Is this the right API for what I need?
3. What do I need to get started?

❌ What most teams write:

Welcome to the TaskFlow API documentation. TaskFlow is a powerful,
enterprise-grade task management solution built on modern cloud
infrastructure. Our API provides programmatic access to the TaskFlow
platform, enabling developers to leverage our industry-leading
capabilities through a RESTful interface.

Nobody reads this. It says nothing specific and wastes the reader's most valuable attention.

✅ What you should write:

# TaskFlow API

The TaskFlow API lets you create, read, update, and delete tasks,
projects, and team members programmatically.

Use it to:
- Sync tasks between TaskFlow and your own application
- Automate task creation from external events (Jira tickets,
  GitHub issues, Slack messages)
- Build custom dashboards and reports on your task data
- Integrate TaskFlow into your CI/CD pipeline

**Base URL:** https://api.taskflow.io/v1
**Protocol:** HTTPS only
**Format:** JSON request and response bodies
**Auth:** API key (Bearer token)

→ Jump straight to [Quick Start](#quick-start) to make your
  first API call in under 5 minutes.

Why this works:

• Specific use cases instead of vague marketing language
• The developer can immediately tell whether this API solves their problem
• The Quick Start link respects their time; they can start doing it before finishing reading

Section 2: Authentication: The Section That Breaks Most Integrations

Authentication is where most developers get stuck, and most docs fail them. Reduce time-to-first-call from hours to minutes with clear authentication instructions.

Write authentication documentation as if you are teaching someone who has never seen an API key before. Then add the advanced details at the bottom for people who need them.

Step 1: Tell them where to get credentials

## Authentication

The TaskFlow API uses API keys to authenticate requests.

**Get your API key:**
1. Log in to your TaskFlow account at app.taskflow.io
2. Click your avatar → Settings → API Keys
3. Click "Generate New Key"
4. Copy the key immediately — it is only shown once

Your API key looks like this:
tf_live_sk_4xKj9mN2pQrT8vWz3bLhY7cU
(32 characters, starting with tf_live_sk_ for production
or tf_test_sk_ for the sandbox)

Why this matters: "Get your API key from the dashboard" is useless. Show them the exact path. Show them what the key looks like so they know they copied the right thing.

Step 2: Show them exactly how to use it

## How to Authenticate

Include your API key in the Authorization header of every request:

Authorization: Bearer YOUR_API_KEY

**Example — curl:**
```bash
curl https://api.taskflow.io/v1/tasks \
  -H "Authorization: Bearer tf_live_sk_4xKj9mN2pQrT8vWz3bLhY7cU"
```

**Example — JavaScript (fetch):**
```javascript
const response = await fetch('https://api.taskflow.io/v1/tasks', {
  headers: {
    'Authorization': 'Bearer tf_live_sk_4xKj9mN2pQrT8vWz3bLhY7cU',
    'Content-Type': 'application/json'
  }
});
```

**Example — Python (requests):**
```python
import requests

headers = {'Authorization': 'Bearer tf_live_sk_4xKj9mN2pQrT8vWz3bLhY7cU'
}

response = requests.get('https://api.taskflow.io/v1/tasks',headers=headers
)
```

Step 3: Tell them what goes wrong, before it goes wrong

## Authentication Errors

| Error | Cause | Fix |
|-------|-------|-----|
| `401 Unauthorised` | Missing or malformed Authorization header | Check the header name is exactly `Authorization` and value starts with `Bearer ` (note the space) |
| `401 Invalid API key` | API key does not exist or was deleted | Generate a new key at Settings → API Keys |
| `403 Forbidden` | API key exists but lacks permission for this endpoint | Check your API key's permission scopes |
| `429 Too Many Requests` | Rate limit exceeded | Wait for retry-after header value, then retry |

> **Never expose your API key in client-side code, Git repositories,
> or public URLs.** Use environment variables:
>
> ```bash
> export TASKFLOW_API_KEY="tf_live_sk_4xKj9mN2pQrT8vWz3bLhY7cU"
> ```
>
> Then in your code:
> ```javascript
> const apiKey = process.env.TASKFLOW_API_KEY;
> ```

Section 3: Getting Started: The 5-Minute Quick Win

The first thing any developer wants when they open your docs is to make something work. Not to understand your architecture, not to read about your company's vision, just to see a successful response. Your getting started guide should get them there in under five minutes.

Stripe does this exceptionally well. Within minutes of creating an account, you can process a test payment. That quick win builds confidence that the full integration is doable.

Here is how to structure your Quick Start:

## Quick Start — Your First API Call in 5 Minutes

This guide gets you from zero to a working API call in 5 minutes.
You will create your first task and retrieve it.

### What you need
- A TaskFlow account (free at taskflow.io)
- Your API key (see Authentication above)
- curl, Postman, or any HTTP client

---

### Step 1: Create a task (2 minutes)

```bash
curl -X POST https://api.taskflow.io/v1/tasks \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "title": "My first API task","description": "Created using the TaskFlow API","priority": "medium","due_date": "2026-08-01"
  }'
```

**Expected response (201 Created):**
```json
{"id": "task_7Km3pQx9","title": "My first API task","description": "Created using the TaskFlow API","priority": "medium","status": "todo","due_date": "2026-08-01","created_at": "2026-07-17T10:30:00Z","updated_at": "2026-07-17T10:30:00Z","project_id": null,"assignee_id": null
}
```

Copy the `id` from the response. You will need it for the next step.

---

### Step 2: Retrieve your task (1 minute)

```bash
curl https://api.taskflow.io/v1/tasks/task_7Km3pQx9 \
  -H "Authorization: Bearer YOUR_API_KEY"
```

Replace `task_7Km3pQx9` with the `id` you received in Step 1.

**Expected response (200 OK):**
```json
{"id": "task_7Km3pQx9","title": "My first API task","status": "todo",
  ...
}
```

**You just made your first API call.** The task you created is live.

---

### Step 3: Mark the task as complete (2 minutes)

```bash
curl -X PATCH https://api.taskflow.io/v1/tasks/task_7Km3pQx9 \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"status": "done"}'
```

**Expected response (200 OK):**
```json
{"id": "task_7Km3pQx9","title": "My first API task","status": "done","completed_at": "2026-07-17T10:35:00Z",
  ...
}
```

**You now know how to create, retrieve, and update a task.
These three operations cover 80% of typical API usage.**

→ Next: [Full endpoint reference](#endpoints) or
  [Code examples in your language](#code-examples)

The Quick Start rules:

• Real commands the developer can copy and run immediately
• Real response bodies, not {...} or "string"
• Number every step, remove ambiguity about what to do next
• End with what to do next

Section 4: Endpoint Reference : The Most Important Section

The endpoint reference is where developers spend most of their time. Every endpoint needs the same consistent structure. When readers know what to expect, they find information faster.

Here is the template for every single endpoint:

[HTTP METHOD] [Endpoint path]
[One-sentence description]

[When to use this endpoint — optional but valuable]

REQUEST
  Headers     [required headers]
  Path params [if any — with type and description]
  Query params[if any — with type, description, required/optional]
  Body        [if any — with full schema]

REQUEST EXAMPLE
  [Real, working example with real data]

RESPONSE
  Success [status code + full response schema]
  [Full response example with real data]

ERRORS
  [Error codes specific to this endpoint with cause and fix]

Real example: GET /tasks endpoint:

## GET /tasks

Returns a paginated list of tasks. By default, returns the 20 most
recently created tasks in descending order.

**Use this when you need to:**
- Display a list of tasks in your application
- Sync tasks from TaskFlow to another system
- Build reports on task volume or completion rate

---

### Request

**Headers**

| Header | Required | Value |
|--------|----------|-------|
| Authorization | ✅ Yes | `Bearer YOUR_API_KEY` |
| Content-Type | ❌ No | Not needed for GET requests |

**Query Parameters**

| Parameter | Type | Required | Description | Default |
|-----------|------|----------|-------------|---------|
| `status` | string | No | Filter by status. Options: `todo`, `in_progress`, `done`, `cancelled` | All statuses |
| `priority` | string | No | Filter by priority. Options: `low`, `medium`, `high`, `critical` | All priorities |
| `project_id` | string | No | Filter tasks belonging to a specific project | All projects |
| `assignee_id` | string | No | Filter tasks assigned to a specific user | All assignees |
| `due_before` | string | No | ISO 8601 date. Return tasks due before this date | No filter |
| `due_after` | string | No | ISO 8601 date. Return tasks due after this date | No filter |
| `page` | integer | No | Page number for pagination (starts at 1) | `1` |
| `per_page` | integer | No | Tasks per page. Min: 1, Max: 100 | `20` |
| `sort` | string | No | Sort field. Options: `created_at`, `due_date`, `priority`, `title` | `created_at` |
| `order` | string | No | Sort direction. Options: `asc`, `desc` | `desc` |

---

### Request Example

**Get all high-priority tasks due this week, sorted by due date:**

```bash
curl "https://api.taskflow.io/v1/tasks?priority=high&due_before=2026-07-24&sort=due_date&order=asc" \
  -H "Authorization: Bearer YOUR_API_KEY"
```

**In JavaScript:**
```javascript
const params = new URLSearchParams({
  priority: 'high',
  due_before: '2026-07-24',
  sort: 'due_date',
  order: 'asc'
});

const response = await fetch(
  `https://api.taskflow.io/v1/tasks?${params}`,
  {
    headers: {
      'Authorization': `Bearer ${process.env.TASKFLOW_API_KEY}`
    }
  }
);

const data = await response.json();
console.log(`Found ${data.total} high-priority tasks due this week`);
```

---

### Response

**200 OK — Success**

```json
{"data": [{"id": "task_7Km3pQx9","title": "Review Q3 security audit report","description": "Review findings from external security audit and create remediation plan","status": "in_progress","priority": "high","due_date": "2026-07-21","project_id": "proj_4Kn8mRz2","project_name": "Security Compliance Q3","assignee_id": "user_9Lp5qTs7","assignee_name": "Emma Johnson","created_at": "2026-07-10T09:15:00Z","updated_at": "2026-07-15T14:30:00Z","completed_at": null,"tags": ["security", "audit", "Q3"]},{"id": "task_2Hn6kMw8","title": "Update disaster recovery runbook","description": "Incorporate lessons learned from July 5th incident","status": "todo","priority": "high","due_date": "2026-07-22","project_id": "proj_4Kn8mRz2","project_name": "Security Compliance Q3","assignee_id": null,"assignee_name": null,"created_at": "2026-07-08T11:00:00Z","updated_at": "2026-07-08T11:00:00Z","completed_at": null,"tags": ["disaster-recovery", "runbook"]}],"pagination": {"page": 1,"per_page": 20,"total": 7,"total_pages": 1,"has_next": false,"has_prev": false}
}
```

**Response Fields**

| Field | Type | Description |
|-------|------|-------------|
| `data` | array | Array of task objects |
| `data[].id` | string | Unique task identifier. Always starts with `task_` |
| `data[].status` | string | Current task status: `todo`, `in_progress`, `done`, `cancelled` |
| `data[].priority` | string | Task priority: `low`, `medium`, `high`, `critical` |
| `data[].due_date` | string\|null | Due date in ISO 8601 format (`YYYY-MM-DD`), or null if no due date |
| `data[].completed_at` | string\|null | Timestamp when task was marked done, or null if not completed |
| `pagination.total` | integer | Total number of tasks matching your filters |
| `pagination.has_next` | boolean | Whether there is another page of results |

---

### Errors

| Code | Error | Cause | Fix |
|------|-------|-------|-----|
| `400` | `invalid_date_format` | `due_before` or `due_after` is not a valid ISO 8601 date | Use format `YYYY-MM-DD`, e.g. `2026-07-24` |
| `400` | `invalid_per_page` | `per_page` is less than 1 or greater than 100 | Use a value between 1 and 100 |
| `400` | `invalid_sort_field` | `sort` value is not one of the allowed options | Use `created_at`, `due_date`, `priority`, or `title` |
| `401` | `unauthorised` | Missing or invalid API key | Check your Authorization header |
| `429` | `rate_limit_exceeded` | Too many requests | Check `Retry-After` header and wait before retrying |

Section 5: The Error Reference: The Section That Saves the Most Time

Poor error documentation leaves developers unprepared for common failures like 400, 401, 429, and 500 responses. When documentation only covers 200 success cases, developers learn failure modes through trial and error.

Every API needs a dedicated error reference page that covers every error your API can return, not just the endpoint-specific ones.

## Error Reference

All API errors return a consistent JSON structure:

```json
{"error": {"code": "machine_readable_code","message": "Human-readable explanation of what went wrong","details": {},"request_id": "req_8Km3pXz9","docs_url": "https://docs.taskflow.io/errors#invalid_field_value"}
}
```

The `request_id` is unique to every request. Include it when
contacting support — it lets us find your specific request in logs.

---

### HTTP Status Codes

| Code | Name | Meaning |
|------|------|---------|
| `200` | OK | Request succeeded |
| `201` | Created | Resource created successfully |
| `204` | No Content | Request succeeded, no body returned (used for DELETE) |
| `400` | Bad Request | Invalid request — check the `error.message` for specifics |
| `401` | Unauthorised | Invalid or missing API key |
| `403` | Forbidden | Valid API key but insufficient permissions |
| `404` | Not Found | Resource does not exist or you do not have access to it |
| `409` | Conflict | Request conflicts with current state (e.g. duplicate resource) |
| `422` | Unprocessable Entity | Request is valid JSON but contains invalid values |
| `429` | Too Many Requests | Rate limit exceeded |
| `500` | Internal Server Error | Something went wrong on our end |
| `503` | Service Unavailable | API is temporarily unavailable — retry with backoff |

---

### Common Error Codes

**Authentication Errors**

| Error Code | When it happens | How to fix it |
|------------|-----------------|---------------|
| `missing_auth_header` | No Authorization header was sent | Add `Authorization: Bearer YOUR_KEY` to every request |
| `invalid_api_key` | API key format is wrong or key does not exist | Copy your key fresh from Settings → API Keys |
| `expired_api_key` | API key was manually expired | Generate a new key |
| `revoked_api_key` | API key was programmatically revoked | Generate a new key |
| `insufficient_permissions` | Your key cannot perform this action | Check the permission scope assigned to your key |

**Validation Errors**

| Error Code | When it happens | How to fix it |
|------------|-----------------|---------------|
| `missing_required_field` | A required field was not included in the request body | Check the `error.details` object — it names the missing field |
| `invalid_field_value` | A field value is not one of the allowed options | Check `error.details.allowed_values` |
| `field_too_long` | A string field exceeds the maximum length | Check `error.details.max_length` |
| `invalid_date_format` | A date field is not in ISO 8601 format | Use `YYYY-MM-DD` format |

**Rate Limiting**

Rate limits apply per API key:

| Plan | Requests per minute | Requests per day |
|------|--------------------|--------------------|
| Free | 60 | 1,000 |
| Starter | 300 | 10,000 |
| Professional | 1,000 | 100,000 |
| Enterprise | Custom | Custom |

When rate limited, you receive a `429` response with these headers:

X-RateLimit-Limit: 60

X-RateLimit-Remaining: 0

X-RateLimit-Reset: 1721214600

Retry-After: 47

`Retry-After` is the number of seconds to wait before retrying.
Always implement retry logic with exponential backoff:

```javascript
async function callWithRetry(url, options, maxRetries = 3) {
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    const response = await fetch(url, options);

    if (response.status !== 429) return response;

    const retryAfter = parseInt(
      response.headers.get('Retry-After') || '60'
    );

    console.log(`Rate limited. Waiting ${retryAfter}s...`);
    await new Promise(resolve =>
      setTimeout(resolve, retryAfter * 1000)
    );
  }
  throw new Error('Max retries exceeded');
}
```

Section 6: Code Examples: The Section Developers Screenshot and Save

Generic examples with placeholder data ("string", "[email protected]", "12345") are harder to understand than realistic ones. If your API handles e-commerce orders, show an example with actual product names, quantities, and prices. Even better: show complete workflows, not just individual endpoints. Developers rarely need to call just one endpoint.

This is the difference between examples developers screenshot and examples developers ignore:

❌ Generic placeholder example:

import requests

response = requests.post('https://api.taskflow.io/v1/tasks',headers={'Authorization': 'Bearer YOUR_API_KEY'},json={'title': 'string','priority': 'string','due_date': 'string'}
)

✅ Real-world workflow example:

"""
Real-world example: Automatically create TaskFlow tasks from
GitHub issues when they are labelled 'needs-action'

This script runs as a webhook receiver for GitHub issue events.
"""

import requests
import os
from datetime import datetime, timedelta

TASKFLOW_API_KEY = os.environ['TASKFLOW_API_KEY']
TASKFLOW_PROJECT_ID = os.environ['TASKFLOW_PROJECT_ID']
GITHUB_ISSUES_USER_ID = os.environ['TASKFLOW_GITHUB_BOT_USER_ID']

BASE_URL = 'https://api.taskflow.io/v1'

def create_task_from_github_issue(issue: dict) -> dict:"""
    Create a TaskFlow task from a GitHub issue.

    Args:
        issue: GitHub issue payload from webhook

    Returns:
        Created TaskFlow task object
    """
    headers = {'Authorization': f'Bearer {TASKFLOW_API_KEY}','Content-Type': 'application/json'}

    # Map GitHub priority labels to TaskFlow priority values
    priority_map = {'critical': 'critical','bug': 'high','enhancement': 'medium','documentation': 'low'}

    github_labels = [label['name'] for label in issue.get('labels', [])]
    priority = 'medium'  # defaultfor label in github_labels:if label in priority_map:
            priority = priority_map[label]break

    # Set due date to 7 days from now if issue has no milestone
    due_date = Noneif issue.get('milestone') and issue['milestone'].get('due_on'):
        due_date = issue['milestone']['due_on'][:10]  # Extract YYYY-MM-DDelse:
        due_date = (datetime.now() + timedelta(days=7)).strftime('%Y-%m-%d')

    task_data = {'title': f"[GitHub #{issue['number']}] {issue['title']}",'description': (f"Automatically created from GitHub issue.\n\n"f"**Issue URL:** {issue['html_url']}\n"f"**Opened by:** {issue['user']['login']}\n\n"f"**Original description:**\n{issue.get('body', 'No description')}"),'priority': priority,'due_date': due_date,'project_id': TASKFLOW_PROJECT_ID,'assignee_id': GITHUB_ISSUES_USER_ID,'tags': ['github', 'auto-created'] + github_labels
    }

    response = requests.post(f'{BASE_URL}/tasks',headers=headers,json=task_data,timeout=10)

    if response.status_code == 201:
        task = response.json()print(f"Created task {task['id']}: {task['title']}")return task
    else:
        error = response.json().get('error', {})raise Exception(f"Failed to create task: {error.get('message', response.text)}")


# Example usage
if __name__ == '__main__':# Simulated GitHub issue payload
    sample_issue = {'number': 247,'title': 'Login page throws 500 error on Safari','html_url': 'https://github.com/myorg/myrepo/issues/247','user': {'login': 'sarah-dev'},'labels': [{'name': 'bug'}, {'name': 'needs-action'}],'milestone': None,'body': ('When users try to log in using Safari 17+, ''they receive a 500 Internal Server Error. ''Chrome and Firefox work correctly. ''Affects approximately 30% of our users.')}

    task = create_task_from_github_issue(sample_issue)print(f"\nTask created successfully!")print(f"ID: {task['id']}")print(f"Title: {task['title']}")print(f"Priority: {task['priority']}")print(f"Due date: {task['due_date']}")

Why this example works:

• It solves a real, recognisable problem (GitHub → TaskFlow sync)
• Every parameter value makes sense in context
• Comments explain why decisions are made, not just what the code does
• Error handling is included, not left as an exercise
• A developer can run this with minimal changes

Section 7: The OpenAPI Specification: Make Your Docs Machine-Readable

In 2026, your docs are read by AI coding assistants as much as by humans. Gartner predicts over 30% of API demand growth by 2026 will come from AI and LLM tools, requiring machine-readable schemas and structured formats like llms.txt to prevent AI hallucination and enable accurate code generation.

The OpenAPI Specification (OAS) is the standard for machine-readable API documentation. Here is how to write it for the task creation endpoint:

# openapi.yaml
openapi: 3.1.0
info:title: TaskFlow APIversion: 1.0.0description: |
    The TaskFlow API lets you create, read, update, and delete tasks,
    projects, and team members programmatically.contact:email: [email protected]: https://docs.taskflow.io

servers:- url: https://api.taskflow.io/v1description: Production- url: https://sandbox.taskflow.io/v1description: Sandbox (test data, no real effects)

security:- BearerAuth: []

components:securitySchemes:BearerAuth:type: httpscheme: bearerdescription: |
        API key from Settings → API Keys.
        Format: Bearer YOUR_API_KEY

  schemas:Task:type: objectproperties:id:type: stringdescription: Unique task identifierexample: task_7Km3pQx9title:type: stringdescription: Task titlemaxLength: 500example: Review Q3 security audit reportdescription:type: stringnullable: truedescription: Detailed task description. Supports Markdown.example: Review findings from external audit and create planstatus:type: stringenum: [todo, in_progress, done, cancelled]description: Current task statusexample: in_progresspriority:type: stringenum: [low, medium, high, critical]description: Task priority levelexample: highdue_date:type: stringformat: datenullable: truedescription: Due date in YYYY-MM-DD formatexample: "2026-07-21"created_at:type: stringformat: date-timedescription: When the task was created (UTC)example: "2026-07-10T09:15:00Z"

    CreateTaskRequest:type: objectrequired:- titleproperties:title:type: stringdescription: Task title (required)maxLength: 500example: Review Q3 security audit reportdescription:type: stringdescription: Optional task description. Markdown supported.example: Review findings from external auditpriority:type: stringenum: [low, medium, high, critical]default: mediumdescription: Task priority. Defaults to medium if not specified.due_date:type: stringformat: datedescription: Due date in YYYY-MM-DD formatexample: "2026-07-21"project_id:type: stringnullable: truedescription: Project to assign this task toexample: proj_4Kn8mRz2

    Error:type: objectproperties:error:type: objectproperties:code:type: stringexample: missing_required_fieldmessage:type: stringexample: "The 'title' field is required"request_id:type: stringexample: req_8Km3pXz9docs_url:type: stringexample: https://docs.taskflow.io/errors#missing_required_field

paths:/tasks:post:summary: Create a taskdescription: |
        Creates a new task. Only `title` is required — all other fields
        are optional and can be added later with PATCH /tasks/{id}.operationId: createTasktags: [Tasks]requestBody:required: truecontent:application/json:schema:$ref: '#/components/schemas/CreateTaskRequest'examples:minimal:summary: Minimum required fields onlyvalue:title: "Buy groceries"complete:summary: All fields specifiedvalue:title: "Review Q3 security audit report"description: "Review external audit findings"priority: "high"due_date: "2026-07-21"project_id: "proj_4Kn8mRz2"responses:'201':description: Task created successfullycontent:application/json:schema:$ref: '#/components/schemas/Task''400':description: Invalid requestcontent:application/json:schema:$ref: '#/components/schemas/Error''401':description: Unauthorisedcontent:application/json:schema:$ref: '#/components/schemas/Error'

Once you have this YAML file, tools like Swagger UI, Redoc, and Stoplight render it as interactive documentation automatically. Developers can test endpoints directly in the browser without writing code.

Section 8: The Changelog: Building Long-Term Trust

Many API development teams ship code changes several times a week, which puts their documentation at risk of falling out of date. Outdated documentation erodes consumers' trust, especially when updates are not backward compatible.

Your changelog should be the first thing a developer checks when their integration breaks after an API update. Write it for developers, not for management:

## Changelog

### 2026-07-15 — v1.4.0

**New Features**
- `GET /tasks` now supports filtering by `assignee_id`
- Tasks now include a `tags` array field (up to 20 tags per task)
- New endpoint: `POST /tasks/bulk` for creating up to 100 tasks
  in a single request

**Changes**
- `priority` field now accepts `critical` in addition to
  `low`, `medium`, `high`
- `GET /tasks` default `per_page` changed from 50 to 20

  ⚠️ **Migration required if you rely on the default:**
  Add `per_page=50` to maintain previous behaviour

**Bug Fixes**
- Fixed: `due_date` filter was returning tasks due ON the
  filter date when using `due_before`
  The filter now correctly excludes tasks due on the filter date.

  ⚠️ **Behaviour change:** If your integration uses `due_before`,
  check whether this fix affects your results.

---

### 2026-06-01 — v1.3.0

**New Features**
- Webhook support: receive real-time task events via HTTP POST
  See: docs.taskflow.io/webhooks

**Deprecations**
- `GET /tasks/list` is deprecated. Use `GET /tasks` instead.
  `GET /tasks/list` will continue to work until 2026-12-01.

The key rules for changelogs:

• Call out ⚠️ breaking changes explicitly, never bury them
• Give migration instructions, not just "this changed"
• Include dates on every entry
• Never delete old entries, developers search changelogs months after a change

The Documentation Writing Workflow: How to Do This in a Real Team

Here is the exact process professional teams follow:

Step 1: Write the spec before the code

The best time to write API documentation is before the API exists, not after. Writing the spec first forces you to think through the developer experience before you are committed to implementation decisions that would be expensive to change.

Use the OpenAPI spec as your design document. Review it with your team as if it were code. Ask: "Would a developer understand what to do here without asking us?"

Step 2: Use real data everywhere

Before writing a single example, create a test account in your own system and make real API calls. Copy the real request and real response. Every example should be something you can paste into a terminal and run.

Step 3: Get a developer who has never used your API to follow your docs

Before publishing, give your docs to a developer who knows nothing about your system. Watch them try to make their first API call following only the docs, without you explaining anything. Every place they stop and look confused is a documentation gap.

This is the highest-value documentation review you can do, and almost nobody does it.

Step 4: Make docs part of your definition of done

No feature ships without documentation. This is a team commitment enforced in your PR process. If the endpoint changes, the docs change in the same pull request.

Step 5: Set up docs monitoring

Create alerts for:

• 404 errors on doc pages (broken links from outside)
• Docs pages not updated in the last 90 days (stale content)
• Support ticket themes (recurring questions reveal doc gaps)

The Documentation Checklist: Run This Before You Publish

Print this. Check every box before your docs go live:

Overview

• One paragraph that explains what the API does in plain language
• Specific use cases, not vague marketing copy
• Base URL, protocol, and auth method stated upfront
• Link to Quick Start prominently placed

Authentication

• Exactly where to get credentials (specific UI path)
• What the credential looks like (format example)
• How to include it in a request (curl, JS, Python examples)
• What every auth error means and how to fix it
• Security warning about not exposing keys

Quick Start

• Working first call achievable in under 5 minutes
• Commands the developer can copy and run immediately
• Real response body, not placeholders
• What to do next after completing it

Endpoint Reference

• Every endpoint has the same consistent structure
• Every parameter has: type, required/optional, description, example
• Request example with real data
• Response example with real data
• Every field in the response is documented
• Endpoint-specific errors documented

Error Reference

• Every HTTP status code the API returns
• Every error code with cause and fix
• Rate limits documented with retry guidance
• request_id field in error responses for support

Code Examples

• Real-world scenario, not placeholder values
• Multiple languages (minimum: curl, JavaScript, Python)
• Error handling included in examples
• Comments explain why, not just what

OpenAPI Spec

• Valid YAML that passes OpenAPI 3.1 validation
• All schemas documented with examples
• Hosted at a public URL for AI tool discovery

Changelog

• Every release has an entry
• Breaking changes flagged with ⚠️ and migration instructions
• Dates on every entry

What Great API Documentation Looks Like: Learn From the Best

Study these before writing your own:

• Stripe (stripe.com/docs/api): the gold standard. Interactive examples, real code, error reference that actually explains errors.
• Twilio (twilio.com/docs): excellent Quick Start guides. You send your first SMS in under 5 minutes.
• GitHub (docs.github.com/en/rest): comprehensive reference with clear structure. Every endpoint is documented identically.
• Slack (api.slack.com/docs): great at documenting complex workflows, not just individual endpoints.

All four of these share one thing: they get developers to a working result quickly, and they use realistic examples that reflect what developers actually build.

That is the whole job.

References

[1] Fern. API Documentation Best Practices Guide Feb 2026. https://buildwithfern.com/post/api-documentation-best-practices-guide

[2] Theneo. API Documentation Best Practices: How to Simplify Integration for Developers. February 2026. https://www.theneo.io/blog/api-documentation-best-practices-how-to-simplify-integration-for-developers

[3] DreamFactory. The 8 Best API Documentation Examples. May 2026. https://blog.dreamfactory.com/8-api-documentation-examples

[4] Postman. API Documentation: How to Write, Examples and Best Practices. https://www.postman.com/api-platform/api-documentation/

[5] Victor Zion. API Documentation Best Practices for Beginners. Medium, January 2026. https://medium.com/@victorzion1/api-documentation-best-practices-for-beginners-b9307a132f47

[6] Swagger / OpenAPI Initiative. OpenAPI Specification 3.1.0. https://swagger.io/specification/