[Python] 403 Forbidden Error when updating caption track via captions.update

I’m using the Python client library (google-api-python-client) to call captions.update, but I keep getting a 403 forbidden even though I’ve granted all the obvious scopes. I’d appreciate any insight into what I’m missing.

Environment

• Language: Python 3.10
• Library: google-api-python-client v2.x
• OAuth scopes requested in my code:

SCOPES = [
'https://www.googleapis.com/auth/youtube.upload',
'https://www.googleapis.com/auth/youtube',
'https://www.googleapis.com/auth/youtube.force-ssl',
'https://www.googleapis.com/auth/spreadsheets',
'https://www.googleapis.com/auth/drive'
]

What I’m doing

1. Load saved credentials from token.pickle, refresh if expired.
2. Build YouTube service:

from google.oauth2.credentials import Credentials
from googleapiclient.discovery import build

creds = Credentials.from_authorized_user_file('token.pickle', SCOPES)
youtube = build('youtube', 'v3', credentials=creds)

1. Call captions.update:

request = youtube.captions().update(
part='snippet',
body={
'id': '<CAPTION_TRACK_ID>',
'snippet': {
'language': 'en',
'name': 'English captions',
'isDraft': False
}
},
media_body=MediaFileUpload('captions_en.vtt', mimetype='text/vtt')
)
response = request.execute()
print(response)

Error message


{
"error": {
"code": 403,
"message": "The permissions associated with the request are not sufficient to update the caption track. The request might not be properly authorized.",
"errors": [
{
"domain": "youtube.caption",
"reason": "forbidden",
"message": "The permissions associated with the request are not sufficient to update the caption track. The request might not be properly authorized.",
"location": "id",
"locationType": "parameter"
}
]
}
}


Despite all that, I still get the same 403 error. Has anyone encountered this? Is there another permission or parameter I need to include, especially for non-brand channels? Any help would be greatly appreciated!

• Verified via the tokeninfo endpoint that my access token includes youtube.force-ssl.
• Deleted token.pickle and re-ran OAuth flow to force new token with updated scopes.
• Called youtube.videos().list(mine=True) to confirm the video’s channelId matches my channel.
• Tried uploading a brand-new caption with captions.insert instead of updating the existing track.
• Ensured the video has no auto-generated caption track (since those cannot be updated).

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