1. Put every player on the same version
Patch 0.05.005 explicitly says players with different game versions cannot connect. Ask each squad member to let Steam finish the update, restart the client if an update is still queued, and verify the installed build. Patch 0.05.007 is the latest official build found in the August 30 check. A single outdated client can produce a session failure even when the host is current.
Do not infer version alignment from the day everybody launched the game. Record the visible build where possible and compare the exact value. If only one player fails, compare that player's installation state with a teammate who connects. This creates a controlled difference and keeps unrelated graphics, account, or router assumptions out of the first pass.
2. Retry Steam invites and lobby searches after 0.05.007
The latest official patch marks Steam invites, lobby searches, and other networking features fixed. After every player updates, create a fresh session and send a new Steam invite rather than relying on an invitation generated before the update. If the group uses the lobby search surface, refresh it after the host has created the current session.
The announcement does not define every discovery rule, visibility delay, region behavior, or session listing state. Treat the current interface as the authority. Note whether the invitation arrives, whether the session appears, whether joining begins, and where it stops. Those checkpoints distinguish discovery, invitation delivery, connection, and in-session failures.
3. Read the disconnect reason literally
Patch 0.05.005 adds different disconnect feedback for a kick, server crash, and game-version mismatch. Capture the exact message instead of replacing it with a general description. A version mismatch returns the group to version alignment. A kick is a different session event. A server crash needs the host's timeline and does not automatically identify the cause.
When the message is missing or unclear, record who hosted, who joined, how the join started, and whether the failure happened before or after entering the bunker. Patch 0.05.007 also fixes a negative session player-count display and adds join or leave notifications, which can make the sequence easier to reconstruct in the current build.
4. Confirm the intended lobby privacy
Patch 0.05.005 fixes a problem that allowed outside players to join Private and Solo lobbies. That establishes an official privacy fix, but it does not document every product choice shown by the current lobby interface. Before inviting the squad, confirm the intended session state in the game and use the controls presented by the live build.
If an unexpected player appears, capture the build, lobby state, host, and join path. If an invited player cannot enter, do not assume the privacy fix is responsible. Return to the version and invite checks, then use the disconnect message. One observation should not be stretched into a universal statement about how all sessions are discovered.
5. Separate host, client, and mission symptoms
Patch 0.05.007 fixes a missing main-quest notification for clients and a death screen that could appear after a completed mission. Version 0.05.006 fixes a Day 3 B.O.S.S. progression case when a player leaves and changes DOUBLE TAP so every player on the server should receive it. These are host-client or mission-state fixes, not proof that a failed invite shares the same cause.
If the session connects but players see different quest, boss, or achievement states, keep the group on the same build and record which role saw each result. If the session never connects, focus on discovery and connection evidence. The co-op guide covers party responsibilities after entry, while this page stays focused on reaching and maintaining the session.
6. Escalate with a minimal network report
After one clean retry on matching versions, prepare a short report for the official Steam Community or developer channels. Include the build for every affected player, host identity, intended lobby state, join method, exact disconnect text, stage of failure, and whether a fresh session changed the outcome. Avoid mixing an old session with results from a new build.
Recent discussions can reveal what the community is asking, but they do not create official product rules. This guide does not make a claim about an undocumented session-discovery feature. It uses only the store's online co-op scope of one to four participants and the named invite, lobby, networking, version, disconnect, and privacy changes published by the developer.