> ## Documentation Index
> Fetch the complete documentation index at: https://apidocs.scripe.io/llms.txt
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# Invalid client

# `invalid_client`

| HTTP | When                                                       |
| ---- | ---------------------------------------------------------- |
| 401  | Client authentication failed — the `client_id` is unknown. |

Check that you're presenting the `client_id` exactly as issued by
`/oauth/register`, and that you registered against the same environment
you're calling. Registration is immutable; if you've lost the id,
register again. See [OAuth §2](../oauth.md#2-dynamic-client-registration-rfc-7591).