Spotify Web API returns 405 when adding tracks to playlist via POST /playlists/{id}/items

[borang]

Hello,

I am experiencing a 405 Method Not Allowed error when trying to add tracks to a Spotify playlist through the Web API.

The request is:

POST /v1/playlists/{playlist_id}/items

Request body:
{"uris":["spotify:track:..."]}

The response is:

HTTP 405 Method Not Allowed

Response headers include:

Allow: GET, HEAD, OPTIONS, PUT

Important details:

• The app is in Development Mode.

• OAuth authentication succeeds.

• The access token has these scopes:
  playlist-read-private
  playlist-modify-private
  playlist-modify-public

• The Spotify user is added to the app's User Management.

• GET /v1/playlists/{playlist_id} returns HTTP 200.

• The playlist is owned by the authenticated Spotify user.

• The playlist is a normal private playlist.

• The playlist ID is valid.

• Re-authorizing the Spotify account and generating a new refresh token did not change the result.

• The same POST request consistently returns 405.

• The request follows the current Web API documentation, which specifies POST /playlists/{playlist_id}/items for adding playlist items.

• GitHub Actions is only executing the Python request and is not rewriting the HTTP method or URL.

The unusual part is that the 405 response contains:

Allow: GET, HEAD, OPTIONS, PUT

Could you please confirm whether POST /v1/playlists/{playlist_id}/items is currently working for Development Mode apps, or whether there is a known restriction or server-side issue affecting this endpoint?

Thank you.