# 40144: Invalid JWT token

Identifier: `invalid_jwt`



A JWT presented for authentication could not be parsed. The token was not well-formed, so it could not be read as a valid JSON Web Token.

## What you should do

Check how the JWT is generated. It must be well-formed, [signed with a supported algorithm](https://ably.com/docs/auth/token/jwt.md), and carry the required claims such as `iat` and `exp`. Correct the token generation on your auth server and reissue.

## Why it happens

A JWT presented for authentication couldn't be decoded or verified. Causes include a malformed token, an unsupported or deprecated signing algorithm, a missing key identifier (`kid`) in the header, or missing or empty required claims such as `iat` or `exp`.

## What you'll see

The error is reported with code 40144 and HTTP status 401. The message is typically `Unexpected exception decoding token; err = ...`. Some malformed-claim cases instead report status 400 with a message naming the claim, such as `Invalid token; iat must be specified`.