How to resolve Gmail API 400 errors when filtering by label: labelIds must use label IDs, not names

Getting 400 Bad Request from Gmail API users.messages.list? Learn why labelIds requires label IDs (not names), see a Python fix, and prevent flaky label filters

Fixing Gmail API 400 errors when filtering by label: why labelIds must be IDs, not names

Problem overview

When calling users.messages.list in the Gmail API, it’s tempting to pass a label’s display name into the labelIds filter. That works only in some cases and will otherwise lead to a 400 error. The method expects label identifiers, not label names, and those two values are not guaranteed to match.

Faulty example

The following snippet iterates over user-defined labels and tries to list messages for each. The issue is that it supplies the label’s name rather than its id:

try:
    # Initialize the Gmail client
    api = build('gmail', 'v1', credentials=auth)
    meta = api.users().labels().list(userId='me').execute()
    user_tags = meta.get('labels', [])

    # Iterate through user-defined labels and fetch messages
    for tag in user_tags:
        if tag['type'] == 'user':
            tag_name = tag['name']  # this is a string, but not the required ID
            out = api.users().messages().list(
                userId='me',
                labelIds=[tag_name],
                maxResults=2
            ).execute()
            break

What’s going on and why it fails

The labelIds parameter takes label identifiers. A label’s name is a human-readable string, while its id is the canonical identifier used internally by the API. Sometimes the name and id look the same, but sometimes they don’t. Passing a name where an id is required causes the 400 error.

Sometimes, these two have the same value; sometimes, they don’t.

Reference: users.messages.list

Solution

Select the id from each label object and pass that to labelIds. Everything else can remain the same:

# Assuming api and user_tags are obtained as above
for tag in user_tags:
    if tag['type'] == 'user':
        tag_id = tag['id']  # Use the label ID, not the name
        out = api.users().messages().list(
            userId='me',
            labelIds=[tag_id],
            maxResults=2
        ).execute()

        # Process the results as needed
        print(f"Messages with label {tag['name']}:", out.get('messages', []))
        break  # Remove this if you want to process all labels

Why this matters

Automating email workflows often depends on label-based filtering. Confusing names with ids leads to brittle logic that intermittently passes or fails depending on how a label was created or renamed. Using the id ensures stable, predictable filtering and avoids 400 responses from the API.

Takeaway

When filtering Gmail messages by label through users.messages.list, always use label['id'] for labelIds. Names are for humans; ids are for the API. If you iterate over labels, extract the id from each label object and you’ll get consistent results without 400 errors.