Slack incoming webhook: Request header field Content-type is not allowed by Access-Control-Allow-Headers in preflight response

Question

I try to post a slack message via the fetch API in a browser:

fetch('https://hooks.slack.com/services/xxx/xxx/xx', {
  method: 'post',
  headers: {
    'Accept': 'application/json, text/plain, */*',
    'Content-type': 'application/json'
  },
  body: JSON.stringify({text: 'Hi there'})
})
  .then(response => console.log)
  .catch(error => console.error);
};

I get the following error message:

Fetch API cannot load:
https://hooks.slack.com/services/xxxxxxx/xxxxx. 
Request header field Content-type is not allowed by Access-Control-Allow-Headers in preflight response.

What to do?

Answer 1

That Slack API endpoint unfortunately appears to be broken in its handling of cross-origin requests from frontend JavaScript code—in that it doesn’t handle the CORS preflight OPTIONS request as it should—so the only solution seems to be to omit the Content-Type header.

So it looks like you need to remove the following from the headers part of your request code:

'Content-type': 'application/json'

That part triggers your browser to do a CORS preflight OPTIONS request. So, for your browser to allow your frontend JavaScript code to send the POST request you’re trying to do, the https://hooks.slack.com/services API endpoint must return an Access-Control-Allow-Headers response header that contains Content-Type in its value.

But that endpoint doesn’t return that, so the preflight fails and the browser stops right there.

Normally when posting from frontend JavaScript to an API endpoint that expects JSON, adding that Content-Type: application/json header to the request is exactly what you need to do and should do. But not in this case—because that API endpoint doesn’t handle it properly.