# 40017: Unsupported protocol version

Identifier: `unsupported_protocol_version`



The request specified a protocol version that Ably does not support, or omitted a version where one is required. The connection or request was rejected before processing.

## What you should do

Include a supported protocol version in the request. This normally only arises when calling the Ably API directly rather than through an SDK, since the SDKs set a valid version automatically. If you reach it through an SDK, upgrade to a current version.

## Why it happens

The request specified a protocol version Ably doesn't support, or omitted one where it's required. A stateless (SSE) connection, for example, must specify a version in its query parameters, and a version outside the supported set is rejected.

## What you'll see

The error is reported with code 40017 and HTTP status 400. The messages include `Stateless connections must specify a protocol version (e.g. ?v=1.1)` and `Invalid api version specified: ...`.