DeepL authentication method is deprecated (auth_key no longer supported)

[suicatokyo]

Hello,

It appears that the current implementation of the DeepL API in this project uses a deprecated authentication method.

In functions/translate.ts the request is sent with:

body.append("auth_key", authKey);

However, according to DeepL's official documentation, the legacy authentication method (auth_key in query/body) was deprecated and removed in November 2025.

Now authentication must be provided using the HTTP header:

Authorization: DeepL-Auth-Key <API_KEY>

Because of this change, the current implementation returns:

403 Forbidden
"Please make sure if the DEEPL_AUTH_KEY is correct."

even when the API key is valid.

Suggested fix:

Replace

body.append("auth_key", authKey);

with an Authorization header:

headers: {
"content-type": "application/x-www-form-urlencoded;charset=utf-8",
"Authorization": DeepL-Auth-Key ${authKey}
}

Example:

const deeplResponse = await fetch(url, {
method: "POST",
headers: {
"content-type": "application/x-www-form-urlencoded;charset=utf-8",
"Authorization": DeepL-Auth-Key ${authKey},
},
body,
});

DeepL documentation:
https://developers.deepl.com/docs/resources/breaking-changes-change-notices/november-2025-deprecation-of-legacy-auth-methods

After applying this change, the translation works correctly.

Thank you for maintaining this project.