Sessions

Why a join failed

Turning a failed join into something you can actually tell the player.

"Could not join" is the least useful thing a game can say. A join has six or seven distinct failure points and Steam reports them through three different mechanisms, so without something structured every one of them reaches the player as the same shrug.

Join Session returns a Steam Join Diagnosis:

Field	Meaning
Success	Whether it worked
Failure	A structured reason you can branch on
Player Message	Plain language, ready to put on screen
Technical Detail	For your log or a bug report
Worth Retrying	Whether trying again might work
Lobby	The session, when the join got that far

The failure reasons

Reason	What it means	Retry?
Steam Unavailable	Steam is not connected	Yes
Session No Longer Exists	The host ended it	No
Session Full	No room	Yes
Not Accepting Players	The match already started	No
Banned From Session	The host banned this account	No
Blocked	One player has the other blocked on Steam	No
Account Limited	Steam requires a purchase to use matchmaking	No
Wrong Password	Joined, then rejected on the password check	Yes
Version Mismatch	The host is on a different build	No
No Server Address	The host has not started the match yet	Yes
Travel Failed	Everything worked until the connection itself	Yes

Using it

The simplest useful handling is two nodes:

Show Player Message in your UI.

If Worth Retrying is true, show a Retry button.

That alone puts you ahead of most shipped games.

For more, branch on Failure:

Wrong Password should clear the field and focus it, not send them back to the browser.

Version Mismatch should point at whatever updates your game.

Session Full should refresh the list.

The post join checks

Some of these can only be decided after the Steam level join succeeds, because none of the session metadata is readable until you are inside the lobby. Password and version are both in that group.

When one of those rejects you, SteamForge leaves the lobby again on your behalf. Without that the player would show as a member of a session they were refused from.