diff --git a/game/glass/btl4lobby.cpp b/game/glass/btl4lobby.cpp
index 2f80f90..482d5b6 100644
--- a/game/glass/btl4lobby.cpp
+++ b/game/glass/btl4lobby.cpp
@@ -96,6 +96,11 @@ static void
 	matchmaking->SetLobbyMemberData(currentLobby, "vh", vehicle);
 	matchmaking->SetLobbyMemberData(currentLobby, "cl", color);
 	matchmaking->SetLobbyMemberData(currentLobby, "xp", experience);
+	// BUILD GATE, member half: publish our exact build so the HOST can
+	// reject mismatches at GO time.  This is what catches OLD exes -- they
+	// predate the joiner-side lobby filter, but they can't fake a "bv" key
+	// they never set.
+	matchmaking->SetLobbyMemberData(currentLobby, "bv", BT_VERSION_STRING);
 }
 
 static int
@@ -187,10 +192,17 @@ static void
 			int n = 0;
 			for (const char *s = member.name; *s && n < 60; ++s) name[n++] = (WCHAR)*s;
 			name[n] = 0;
-			wsprintfW(line, L"  %d.  %-16s  %hs, %hs%s", i + 1, name,
+			// BUILD GATE: flag mismatched members in the roster so the host
+			// sees WHO won't launch before pressing GO (the log alone left
+			// the operator guessing on field nights).
+			const char *bv = matchmaking->GetLobbyMemberData(currentLobby,
+				matchmaking->GetLobbyMemberByIndex(currentLobby, i), "bv");
+			int build_ok = (bv != NULL && strcmp(bv, BT_VERSION_STRING) == 0);
+			wsprintfW(line, L"  %d.  %-16s  %hs, %hs%s%s", i + 1, name,
 				member.vehicle[0] ? member.vehicle : "mech",
 				member.color[0] ? member.color : "-",
-				member.isSelf ? L"   (you)" : L"");
+				member.isSelf ? L"   (you)" : L"",
+				build_ok ? L"" : L"   [WRONG BUILD -- WILL NOT LAUNCH]");
 		}
 		else
 		{
@@ -378,6 +390,25 @@ int
 			{
 				continue;
 			}
+			if (!member.isSelf)
+			{
+				// BUILD GATE, host half: a member on a different zip (or an
+				// exe old enough to have no "bv" key at all) gets NO token.
+				// Omitted from btl4map, their client -- old builds included,
+				// the code predates the gate -- hits its own "the host's map
+				// is missing us" path and fails the join cleanly instead of
+				// desyncing the session (#108's stale-zip confound).
+				const char *bv = matchmaking->GetLobbyMemberData(
+					currentLobby, CSteamID(member.steamID), "bv");
+				if (bv == NULL || strcmp(bv, BT_VERSION_STRING) != 0)
+				{
+					LobbyLog("host: REJECT %s at GO -- their build [%s], "
+						"ours %s (no token minted; their join will fail)",
+						member.name, (bv && *bv) ? bv : "pre-gate/unknown",
+						BT_VERSION_STRING);
+					continue;
+				}
+			}
 			if ((pass == 0) == (member.isSelf != 0))
 			{
 				roster_out->members[roster_out->memberCount++] = member;