diff --git a/chat/state/world.py b/chat/state/world.py
index 5966924..8a9ba89 100644
--- a/chat/state/world.py
+++ b/chat/state/world.py
@@ -29,6 +29,24 @@ def _apply_chat_created(conn: Connection, e: Event) -> None:
     )
 
 
+@on("guest_added")
+def _apply_guest_added(conn: Connection, e: Event) -> None:
+    p = e.payload
+    conn.execute(
+        "UPDATE chats SET guest_bot_id = ? WHERE id = ?",
+        (p["guest_bot_id"], p["chat_id"]),
+    )
+
+
+@on("guest_removed")
+def _apply_guest_removed(conn: Connection, e: Event) -> None:
+    p = e.payload
+    conn.execute(
+        "UPDATE chats SET guest_bot_id = NULL WHERE id = ?",
+        (p["chat_id"],),
+    )
+
+
 @on("container_created")
 def _apply_container_created(conn: Connection, e: Event) -> None:
     p = e.payload
diff --git a/tests/test_guest_events.py b/tests/test_guest_events.py
new file mode 100644
index 0000000..6d6c7e2
--- /dev/null
+++ b/tests/test_guest_events.py
@@ -0,0 +1,96 @@
+from __future__ import annotations
+
+from chat.db.connection import open_db
+from chat.db.migrate import apply_migrations
+from chat.eventlog.log import append_event
+from chat.eventlog.projector import project
+import chat.state.entities  # registers bot_authored handler
+import chat.state.world  # registers chat_created / guest_added / guest_removed
+from chat.state.world import get_chat
+
+
+def _bot_payload(bot_id: str, name: str) -> dict:
+    return {
+        "id": bot_id,
+        "name": name,
+        "persona": "...",
+        "voice_samples": ["sample"],
+        "traits": ["shy"],
+        "backstory": "...",
+        "initial_relationship_to_you": "coworker",
+        "kickoff_prose": "you stay late",
+    }
+
+
+def _chat_payload(**overrides) -> dict:
+    payload = {
+        "id": "chat_bot_a",
+        "host_bot_id": "bot_a",
+        "initial_time": "2026-04-26T20:00:00+00:00",
+        "narrative_anchor": "Day 1 evening",
+        "weather": "clear",
+    }
+    payload.update(overrides)
+    return payload
+
+
+def test_guest_added_sets_guest_bot_id(tmp_path):
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        append_event(conn, kind="bot_authored", payload=_bot_payload("bot_a", "BotA"))
+        append_event(conn, kind="bot_authored", payload=_bot_payload("bot_b", "BotB"))
+        append_event(conn, kind="chat_created", payload=_chat_payload())
+        append_event(conn, kind="guest_added", payload={
+            "chat_id": "chat_bot_a",
+            "guest_bot_id": "bot_b",
+        })
+        project(conn)
+
+        chat = get_chat(conn, "chat_bot_a")
+        assert chat is not None
+        assert chat["guest_bot_id"] == "bot_b"
+
+
+def test_guest_removed_clears_guest_bot_id(tmp_path):
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        append_event(conn, kind="bot_authored", payload=_bot_payload("bot_a", "BotA"))
+        append_event(conn, kind="bot_authored", payload=_bot_payload("bot_b", "BotB"))
+        append_event(conn, kind="chat_created", payload=_chat_payload())
+        append_event(conn, kind="guest_added", payload={
+            "chat_id": "chat_bot_a",
+            "guest_bot_id": "bot_b",
+        })
+        append_event(conn, kind="guest_removed", payload={
+            "chat_id": "chat_bot_a",
+        })
+        project(conn)
+
+        chat = get_chat(conn, "chat_bot_a")
+        assert chat is not None
+        assert chat["guest_bot_id"] is None
+
+
+def test_guest_added_idempotent_overwrite(tmp_path):
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        append_event(conn, kind="bot_authored", payload=_bot_payload("bot_a", "BotA"))
+        append_event(conn, kind="bot_authored", payload=_bot_payload("bot_b", "BotB"))
+        append_event(conn, kind="bot_authored", payload=_bot_payload("bot_c", "BotC"))
+        append_event(conn, kind="chat_created", payload=_chat_payload())
+        append_event(conn, kind="guest_added", payload={
+            "chat_id": "chat_bot_a",
+            "guest_bot_id": "bot_b",
+        })
+        append_event(conn, kind="guest_added", payload={
+            "chat_id": "chat_bot_a",
+            "guest_bot_id": "bot_c",
+        })
+        project(conn)
+
+        chat = get_chat(conn, "chat_bot_a")
+        assert chat is not None
+        assert chat["guest_bot_id"] == "bot_c"