# 104001: Session subscription failed

Identifier: `session_subscription_failed`



An AI Transport session could not subscribe to and attach its channel while connecting. The channel may have been rejected, unreachable, or in a failed state, so the session can neither send nor receive.

## What you should do

What to do depends on the `cause`. A transient failure, such as the Ably client being briefly disconnected, clears on a retry: call `connect()` again, which starts a fresh attempt rather than returning the earlier failure. A non-retryable cause, such as a capability or auth rejection, keeps failing until you fix it.

Nothing sends or receives until the attach succeeds, and sends attempted in the meantime are not queued: each rejects with this same code, carrying the connect failure as its `cause` and telling you to connect again.

## Why it happens

Connecting a session subscribes to its channel and attaches it. This error is raised when either step is rejected; common reasons are the Ably client not being connected, the token lacking the channel's [capabilities](https://ably.com/docs/auth/capabilities.md), or the channel being in a failed state. The originating error is preserved as the `cause`.

## What you'll see

The error is reported with code 104001 and status 500. Messages take the form `unable to subscribe and attach channel; ...`. It rejects the `connect()` call and is also delivered to the session's error handler.