Cannot POST /v1/oauth/token when fetching access token from Notion API

I’m trying to integrate with Notion’s OAuth API, but I’m running into an issue when attempting to fetch the access token using a POST request. My code snippet is as follows:

const authHeader = Buffer.from(`${CLIENT_ID}:${CLIENT_SECRET}`).toString("base64");
const tokenResponse = await fetch("https://api.notion.com/v1/oauth/token", {
  method: "POST",
  headers: {
    Authorization: `Basic ${authHeader}`,
    "Content-Type": "application/json",
    "Notion-Version": "2022-06-28",
    Accept: "application/json",
  },
  body: JSON.stringify({
    grant_type: "authorization_code",
    code: authCode,
    redirect_uri: "http://localhost:3000/api/notion/callback",
  }),
});

if (!tokenResponse.ok) {
  // Log response details to help with debugging
  const text = await tokenResponse.text(); // Read response as plain text
  console.error("Error response: ", text);
  throw new Error(`Request failed with status ${tokenResponse.status}`);
}

However, I’m getting the following error:

Error response:  
<!DOCTYPE html> 
<html lang="en"> 
<head> <meta charset="utf-8"> <title>Error</title> </head> 
<body> <pre>Cannot POST /v1/oauth/token</pre> </body> 
</html>
  • Made sure the URL https://api.notion.com/v1/oauth/token is correct based on Notion’s API documentation.
  • Confirmed that CLIENT_ID and CLIENT_SECRET are correctly populated and encoded in base64.
  • Verified that grant_type, code, and redirect_uri parameters are correctly set in the request body.