diff --git a/runners/agent-runner/src/ipc-mcp-stdio.ts b/runners/agent-runner/src/ipc-mcp-stdio.ts
index 3b3e1f1..0563901 100644
--- a/runners/agent-runner/src/ipc-mcp-stdio.ts
+++ b/runners/agent-runner/src/ipc-mcp-stdio.ts
@@ -765,8 +765,10 @@ If folder is omitted, the host generates one automatically.`,
     name: z.string().describe('Display name for the room'),
     room_mode: z
       .enum(['single', 'tribunal'])
-      .default('single')
-      .describe('single=owner only, tribunal=owner/reviewer roles enabled'),
+      .default('tribunal')
+      .describe(
+        'single=owner only, tribunal=owner/reviewer roles enabled (default)',
+      ),
     owner_agent_type: z
       .enum(['claude-code', 'codex'])
       .optional()
@@ -831,7 +833,7 @@ If folder is omitted, the host generates one automatically.`,
       type: 'assign_room',
       jid: args.jid,
       name: args.name,
-      room_mode: args.room_mode || 'single',
+      room_mode: args.room_mode || 'tribunal',
       owner_agent_type: args.owner_agent_type,
       reviewer_agent_type: args.reviewer_agent_type,
       arbiter_agent_type: args.arbiter_agent_type,
diff --git a/src/db/rooms.ts b/src/db/rooms.ts
index 9cb75e3..189345b 100644
--- a/src/db/rooms.ts
+++ b/src/db/rooms.ts
@@ -194,7 +194,7 @@ export function assignRoomInDatabase(
   input: AssignRoomInput,
 ): (RegisteredGroup & { jid: string }) | undefined {
   const existing = getStoredRoomSettingsRowFromDatabase(database, chatJid);
-  const roomMode = input.roomMode || existing?.roomMode || 'single';
+  const roomMode = input.roomMode || existing?.roomMode || 'tribunal';
   const ownerAgentType =
     input.ownerAgentType || existing?.ownerAgentType || OWNER_AGENT_TYPE;
   const folder = resolveAssignedRoomFolder(
diff --git a/src/ipc-auth.test.ts b/src/ipc-auth.test.ts
index 981d49e..060e720 100644
--- a/src/ipc-auth.test.ts
+++ b/src/ipc-auth.test.ts
@@ -1131,7 +1131,7 @@ describe('assign_room success', () => {
     });
   });
 
-  it('assign_room auto-fills missing folder for single rooms without exposing trigger metadata', async () => {
+  it('assign_room auto-fills missing folder and defaults to tribunal without exposing trigger metadata', async () => {
     await processTaskIpc(
       {
         type: 'assign_room',
@@ -1146,7 +1146,7 @@ describe('assign_room success', () => {
     const group = getRegisteredGroup('partial@g.us');
     expect(group).toBeDefined();
     expect(group!.folder).toMatch(/^grp_whatsapp_/);
-    expect(getStoredRoomSettings('partial@g.us')?.roomMode).toBe('single');
+    expect(getStoredRoomSettings('partial@g.us')?.roomMode).toBe('tribunal');
   });
 
   it('assign_room preserves an existing tribunal room mode when room_mode is omitted', async () => {