diff --git a/src/google/adk/integrations/firestore/firestore_session_service.py b/src/google/adk/integrations/firestore/firestore_session_service.py
index 83b97c33c2..b2b3a16262 100644
--- a/src/google/adk/integrations/firestore/firestore_session_service.py
+++ b/src/google/adk/integrations/firestore/firestore_session_service.py
@@ -15,6 +15,7 @@
 from __future__ import annotations
 
 import asyncio
+import json
 from contextlib import asynccontextmanager
 from datetime import datetime
 from datetime import timezone
@@ -203,7 +204,7 @@ async def create_session(
         "id": session_id,
         "appName": app_name,
         "userId": user_id,
-        "state": session_state,
+        "state": json.dumps(session_state),
         "createTime": now,
         "updateTime": now,
         "revision": 1,
@@ -314,7 +315,10 @@ async def get_session(
         events.append(Event.model_validate(ed))
 
     # Let's continue getting session.
-    session_state = data.get("state", {})
+    raw_state = data.get("state", {})
+    session_state = (
+        json.loads(raw_state) if isinstance(raw_state, str) else raw_state
+    )
 
     # Fetch shared state
     app_ref = self.client.collection(self.app_state_collection).document(
@@ -407,7 +411,12 @@ async def list_sessions(
       data = doc.to_dict()
       if data:
         u_id = data["userId"]
-        s_state = data.get("state", {})
+        raw_s_state = data.get("state", {})
+        s_state = (
+            json.loads(raw_s_state)
+            if isinstance(raw_s_state, str)
+            else raw_s_state
+        )
         u_state = user_states_map.get(u_id, {})
         merged = self._merge_state(app_state, u_state, s_state)
 
@@ -555,7 +564,7 @@ async def _append_txn(transaction: firestore.AsyncTransaction) -> int:
         transaction.update(
             session_ref,
             {
-                "state": session_only_state,
+                "state": json.dumps(session_only_state),
                 "updateTime": firestore.SERVER_TIMESTAMP,
                 "revision": new_revision,
             },