# 40012: Invalid client ID

Identifier: `invalid_client_id`



The client ID supplied was not acceptable, for example because it was empty, a wildcard, not a string, or did not match the client ID permitted by the credentials in use.

## What you should do

Supply a valid client ID, and if the client uses token authentication as an [identified client](https://ably.com/docs/auth/identified-clients.md), make sure it matches the client ID its token was issued for.

## Why it happens

Most often the supplied client ID doesn't match the one the token permits: a token authorizes either a single client ID or a wildcard, and a client can't assume an identity outside what its token allows. The value is also rejected if it is empty, not a string, or the reserved wildcard `*` used as an actual identity.

## What you'll see

The error is reported with code 40012 and HTTP status 400. The message names the problem, for example `invalid clientId` or `Invalid token; clientId must be a string`.