# 40170: Token callback failed

Identifier: `error_from_client_token_callback`



Authentication could not complete because the client's own token-request mechanism, its authCallback or authUrl, returned an error instead of a token or token request. The failure originates in the application's auth logic or endpoint, not within Ably.

## What you should do

Fix your token-request mechanism. The SDK reached your `authUrl` or `authCallback` but got an error back instead of a token, so start from the accompanying message and check your auth endpoint. Common problems are a slow endpoint that times out, a missing `Content-Type` header, or a response whose content type the SDK can't parse.

## Why it happens

The client's own auth logic failed. The SDK invoked the configured `authUrl` or `authCallback` to obtain a token and got an error, so the cause lies in your application's auth endpoint or callback rather than in Ably. It can be a network error or timeout reaching the endpoint, or a response the SDK can't use.

## What you'll see

The error is reported with code 40170 and HTTP status 401. Messages include `authUrl response is missing a Content-Type header` and `authUrl responded with unacceptable Content-Type ...`. An `authUrl` must return `application/json`, `text/plain`, or `application/jwt`.