There are several guides covering various API uses or explanations.
This one gives practical and comprehensive examples on how to use it.
All code examples in this guide aren’t meant to display good practice or to be used as they are.
A lot of checks, error handling and so on are purposely ignored or skipped to focus purely on the API’s usage.
What is the API used for?
Most of your actions in Discourse (posting, liking, editing a setting, etc.) are done using the API by making requests to an endpoint[1].
For example, when you create a topic on meta, a POST request is made to https://meta.discourse.org/post.json. The request contains, among other things, the author, title, category, tags and contents of your post.
Making custom use of the API is usually done to achieve automated tasks and often in conjunction with other services, like webhooks, scripts, third-party software and APIs.
To use the API, it is mandatory to have API credentials. This can be done in a few clicks, by following this guide: Create and configure an API key
Wait no more, and let’s go for a first example of an API use case. 
Creating a monthly topic
In this example, we’ll use curl and cron to create a monthly “free talk” topic on your forum. A popular practice in online communities 
Create the API key
Go through the API key creation guide. Set User Level to Single User.
The chosen user will be the author of the monthly topic.
Then you can either go for a Global scop or a Granular scope, in which case it will need to have at least the Topics → write" access.
Write down your generated API Key. 
Create a curl command
From your server’s command line, run this command to see if it’s working and if a topic is properly created using curl and Discourse’s REST API with your API key:
curl -X POST "https://your-discourse.com/posts.json" -H "Content-Type: application/json" -H "Api-Key: YOUR_API_KEY" -H "Api-Username: YOUR_USERNAME" -d "{\"title\": \"Test topic creation with the API\", \"raw\": \"And here's the topic's content\", \"category\": CATEGORY_ID }"
Replace:
your-discourse.comwith your forum’s domainYOUR_API_KEYwith your API keyYOUR_USERNAMEwith the API key’s chosen usernameCATEGORY_IDwith the ID of the category you want to post your topic in.
If everything’s properly configured, a new test topic should be created on your forum, like:
Most of the work is done at this stage! We now need to change the topic’s title and content for something more appropriate and set up the recurrence of the topic creation.
Start by changing the topic’s title from:
Test topic creation with the API
to:
Free talk of the month - $(date +\%B).
Let’s do the same for the content from:
Test topic creation with the API
to:
What have you appreciated the most and the less during $(date +\%B -d 'last month')?\nFeel free to share your feelings and provide ideas. 🙂
I’m using the date Unix command to insert the current month’s name in the title and the previous month’s name in the contents.
\n in the contents creates a new line.
You can use the command line and run the updated curl request. It should have created a new topic like this on your forum:
Set up the recurring event
We’ll create a cron task that will run the curl command on the first day of every month. 
Edit the cron file with the following command:
crontab -e
Insert this line at the end of the file (replace the request’s content as needed) and save the modification.
0 0 1 * * curl -X POST "https://your-discourse.com/posts.json" -H "Content-Type: application/json" -H "Api-Key: YOUR_API_KEY" -H "Api-Username: YOUR_USERNAME" -d "{\"title\": \"Free talk of the month - $(date +\%B)\", \"raw\": \"What have you appreciated the most and the less during $(date +\%B -d 'last month')?\nFeel free to share your feelings and provide ideas. 🙂\", \"category\": 4 }"
The 0 0 1 * * part defines the interval at which the command will run. You can learn more about the syntax here: https://crontab.guru
It’s done! Your server will now create a new “free talk” topic the first day of each month, using Discourse’s REST API, a curl request, and a cron task. 
Automatically change the color scheme of a theme
Let’s make our default theme use a color scheme matching the current season 
We’ll use Ruby to handle the months checks and a cron task to execute the script.
We could use many other ways than Ruby and cron, but this guide also aims to show how to use the API with various tools.
Prepare the theme and color schemes
Choose the theme to which you want to change the color scheme of, and get its ID. you’ll find the ID in the theme’s URL. For instance, this theme’s ID is 1:
Create 4 color palettes, and write down their ID as well.
Here, the Autumn’s palette’s ID is 17:
Creating the script
Install Ruby on your server.
Create a seasons.rb file. I’ll consider it’s in ~/scripts/ for this example.
Put the following content in this file. We’ll make a POST request to our theme endpoint and send a payload containing the color scheme ID:
require 'net/http'
require 'json'
require 'date'
current_month = Date.today.month
color_scheme_id = case current_month
when 1..3 then 18 # Winter
when 4..6 then 15 # Spring
when 7..9 then 16 # Summer
else 17 # Autumn
end
uri = URI('https://your-discourse.com/admin/themes/THEME_ID.json')
http = Net::HTTP.new(uri.host, uri.port)
http.use_ssl = true
request = Net::HTTP::Put.new(uri, {
'Api-Key' => 'YOUR_API_KEY',
'Api-Username' => 'YOUR_USERNAME',
'Content-Type' => 'application/json'
})
request.body = JSON.generate({
"theme" => {
"color_scheme_id" => color_scheme_id
}
})
response = http.request(request)
Replace:
your-discourse.comwith your forum’s domainYOUR_API_KEYwith your API keyYOUR_USERNAMEwith the API key’s chosen usernameTHEME_IDwith the ID of the Christmas theme component in your instance. You can find it at the end of the URL of the component’s settings page.
For example,https://your-discourse.com/admin/customize/themes/38, the component’s ID is38.
Let’s manually try your script.
First, set it to a non-seasonal color scheme in Discourse’s interface if it’s not already the case.
Then, run the script with the following command:
ruby ~/scripts/seasons.rb
Refresh your browser. The color scheme used by your theme should have changed.
The last thing to do is create the cron job that will run this script on the first day of the month at each season change.
Have a look back at the first example if you don’t remember how to create a cron task.
0 0 1 1,4,7,10 * ruby ~/scripts/seasons.rb
It’s done! Your forum will now wear new colors at the beginning of each new season!
Receive a web request on a web server and use its data to update a Discourse’s topic
This one’s more complex!
Our tool will be PHP, which means we’ll assume you have a working web server somewhere with PHP installed.
In this example, we’ll receive a Ko-Fi (a donations service) webhook payload on a PHP page, which will then use the received data to use Discourse’s API and update a topic’s title and contents.
More specifically, it will update the topic’s title with the donation amount and date, and add a new line to the table that lists previous donations (it will even automatically bump the topic 
):
Each time a user makes a donation, Ko-Fi[2] will send a request to our PHP script.
Configuring Ko-Fi
I set up the configuration on Ko-Fi webhooks page by simply adding my PHP file URL and writing down the verification token hidden in the Advanced section.
On a single donation, Ko-Fi will send a payload like this to our PHP script:
data = {
"verification_token": "d8546b84-c698-4df5-9811-39d35813e2ff",
"message_id": "a499df4c-7bbb-4061-b4a6-8b9d969da689",
"timestamp": "2023-10-19T13:35:06Z",
"type": "Donation",
"is_public": true,
"from_name": "Jo Example",
"message": "Good luck with the integration!",
"amount": "3.00",
"url": "https://ko-fi.com/Home/CoffeeShop?txid=00000000-1111-2222-3333-444444444444",
"email": "jo.example@example.com",
"currency": "USD",
"is_subscription_payment": false,
"is_first_subscription_payment": false,
"kofi_transaction_id": "00000000-1111-2222-3333-444444444444",
"shop_items": null,
"tier_name": null,
"shipping": null
}
We’ll receive this payload, then extract the two information we need:
-
The amount (
3.00), that we will round to an integer. -
The timestamp (
2023-10-19T13:35:06Z), that we’ll format into a more pretty date.
Receive Ko-Fi’s payload
First, we put the following code in our PHP page to receive the request from Ko-Fi, where we ensure that we received a POST request and that the token value matches the one given in the Ko-Fi webhooks page. We also format the amount and date as said earlier.
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
$jsonData = json_decode($_POST['data'], true);
if ($jsonData['verification_token'] === 'YOUR_VERIFICATION_TOKEN') {
$amount = floor(floatval($jsonData['amount']));
$date = (new DateTime($jsonData['timestamp']))->format('d/m/Y');
}
}
Replace:
YOUR_VERIFICATION_TOKENwith the token given by Ko-Fi
Update the topic’s title
The next step is to update our topic’s title. We’ll use curl in our PHP script to make a PUT request to the proper endpoint.
$putData = json_encode(['title' => '🥳 New donation: ' . $amount . '€ on ' . $date]);
$ch = curl_init('https://your-discourse.com/t/test-new-topic/TOPIC_ID');
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PUT');
curl_setopt($ch, CURLOPT_POSTFIELDS, $putData);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, [
'Content-Type: application/json',
'Content-Length: ' . strlen($putData),
'Api-Key: YOUR_API_KEY',
'Api-Username: YOUR_USERNAME'
]);
curl_exec($ch);
Replace:
your-discourse.comwith your forum’s domainTOPIC_IDwith the right topic’s IDYOUR_API_KEYwith your API keyYOUR_USERNAMEwith the API key’s chosen username
Update the post’s content
To be able to append new content to the topic’s first post, we need first to retrieve its current content with a GET request.
$ch_get = curl_init('https://your-discourse.com/posts/POST_ID.json');
curl_setopt($ch_get, CURLOPT_RETURNTRANSFER, true);
$currentContent = json_decode(curl_exec($ch_get), true)['raw'];
Replace:
POST_IDwith the right post’s ID[3]
Finally, we need to update the post content by adding a new line to the table with the amount and date. We’ll do that with a PUT request.
$updatedContent = $currentContent . "\n| " . $amount . "€ | " . $date . " |";
$putPostData = json_encode(['post' => ['raw' => $updatedContent]]);
$ch_put = curl_init('https://your-discourse.com/posts/POST_ID');
curl_setopt($ch_put, CURLOPT_CUSTOMREQUEST, 'PUT');
curl_setopt($ch_put, CURLOPT_POSTFIELDS, $putPostData);
curl_setopt($ch_put, CURLOPT_HTTPHEADER, [
'Content-Type: application/json',
'Content-Length: ' . strlen($putPostData),
'Api-Key: YOUR_API_KEY',
'Api-Username: YOUR_USERNAME'
]);
curl_exec($ch_put);
Replace:
your-discourse.comwith your forum’s domainPOST_IDwith the right post’s IDYOUR_API_KEYwith your API keyYOUR_USERNAMEwith the API key’s chosen username
The final code looks like this:
<?php
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
$jsonData = json_decode($_POST['data'], true);
if ($jsonData['verification_token'] === 'YOUR_VERIFICATION_TOKEN') {
$amount = floor(floatval($jsonData['amount']));
$date = (new DateTime($jsonData['timestamp']))->format('d/m/Y');
$putData = json_encode(['title' => '🥳 New donation: ' . $amount . '€ on ' . $date]);
$ch = curl_init('https://your-discourse.com/t/test-new-topic/TOPIC_ID');
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PUT');
curl_setopt($ch, CURLOPT_POSTFIELDS, $putData);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, [
'Content-Type: application/json',
'Content-Length: ' . strlen($putData),
'Api-Key: YOUR_API_KEY',
'Api-Username: YOUR_USERNAME'
]);
curl_exec($ch);
$ch_get = curl_init('https://your-discourse.com/posts/POST_ID.json');
curl_setopt($ch_get, CURLOPT_RETURNTRANSFER, true);
$currentContent = json_decode(curl_exec($ch_get), true)['raw'];
$updatedContent = $currentContent . "\n| " . $amount . "€ | " . $date . " |";
$putPostData = json_encode(['post' => ['raw' => $updatedContent]]);
$ch_put = curl_init('your-discourse.com/posts/POST_ID');
curl_setopt($ch_put, CURLOPT_CUSTOMREQUEST, 'PUT');
curl_setopt($ch_put, CURLOPT_POSTFIELDS, $putPostData);
curl_setopt($ch_put, CURLOPT_HTTPHEADER, [
'Content-Type: application/json',
'Content-Length: ' . strlen($putPostData),
'Api-Key: YOUR_API_KEY',
'Api-Username: YOUR_USERNAME'
]);
curl_exec($ch_put);
curl_close($ch);
curl_close($ch_get);
curl_close($ch_put);
} else {
http_response_code(403);
echo "Invalid verification token.";
}
} else {
http_response_code(405);
echo "Only POST requests are allowed.";
}
?>
Let me emphasize again the warning at the beginning of this guide 
A lot of checks, error handling and so on are purposely ignored or skipped to focus purely on the API’s usage.
We can now trigger a test request from Ko-Fi, and see how it updates our topic, both the title and content.
That’s all !
You have a topic that will be updated and bumped each time someone makes a donation on Ko-Fi!
This topic is a wiki. Please feel free to correct any error you see and to discuss how this guide could be improved.
A particular URL, in this context. For example,
https://your-discourse.com/posts.json↩︎Bit of info about their API: https://help.ko-fi.com/hc/en-us/articles/360004162298-Does-Ko-fi-Have-an-API-or-Webhook- ↩︎
The post ID can be found in the HTML code. It’s an
<article>element with the following attribute:data-post-id="POST_ID"↩︎