diff --git a/package-lock.json b/package-lock.json
index ebe19b1..c84db58 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -11,7 +11,7 @@
       "dependencies": {
         "@line/bot-sdk": "^10.0.0",
         "@modelcontextprotocol/sdk": "^1.8.0",
-        "zod": "^4.0.0"
+        "zod": "^3.21.4"
       },
       "bin": {
         "line-bot-mcp-server": "dist/index.js"
@@ -499,6 +499,15 @@
         "url": "https://github.com/sponsors/colinhacks"
       }
     },
+    "node_modules/@modelcontextprotocol/sdk/node_modules/zod-to-json-schema": {
+      "version": "3.24.6",
+      "resolved": "https://registry.npmjs.org/zod-to-json-schema/-/zod-to-json-schema-3.24.6.tgz",
+      "integrity": "sha512-h/z3PKvcTcTetyjl1fkj79MHNEjm+HpD6NXheWjzOekY7kV+lwDYnHw+ivHkijnCSMz1yJaWBD9vu/Fcmk+vEg==",
+      "license": "ISC",
+      "peerDependencies": {
+        "zod": "^3.24.1"
+      }
+    },
     "node_modules/@nodelib/fs.scandir": {
       "version": "2.1.5",
       "resolved": "https://registry.npmjs.org/@nodelib/fs.scandir/-/fs.scandir-2.1.5.tgz",
@@ -2331,22 +2340,13 @@
       "license": "ISC"
     },
     "node_modules/zod": {
-      "version": "4.1.1",
-      "resolved": "https://registry.npmjs.org/zod/-/zod-4.1.1.tgz",
-      "integrity": "sha512-SgMZK/h8Tigt9nnKkfJMvB/mKjiJXaX26xegP4sa+0wHIFVFWVlsQGdhklDmuargBD3Hsi3rsQRIzwJIhTPJHA==",
+      "version": "3.21.4",
+      "resolved": "https://registry.npmjs.org/zod/-/zod-3.21.4.tgz",
+      "integrity": "sha512-m46AKbrzKVzOzs/DZgVnG5H55N1sv1M8qZU3A8RIKbs3mrACDNeIOeilDymVb2HdmP8uwshOCF4uJ8uM9rCqJw==",
       "license": "MIT",
       "funding": {
         "url": "https://github.com/sponsors/colinhacks"
       }
-    },
-    "node_modules/zod-to-json-schema": {
-      "version": "3.24.5",
-      "resolved": "https://registry.npmjs.org/zod-to-json-schema/-/zod-to-json-schema-3.24.5.tgz",
-      "integrity": "sha512-/AuWwMP+YqiPbsJx5D6TfgRTc4kTLjsh5SOcd4bLsfUg2RcEXrFMJl1DGgdHy2aCfsIA/cr/1JM0xcB2GZji8g==",
-      "license": "ISC",
-      "peerDependencies": {
-        "zod": "^3.24.1"
-      }
     }
   }
 }
diff --git a/package.json b/package.json
index 677cde9..fb4f9ef 100644
--- a/package.json
+++ b/package.json
@@ -36,7 +36,7 @@
   "dependencies": {
     "@line/bot-sdk": "^10.0.0",
     "@modelcontextprotocol/sdk": "^1.8.0",
-    "zod": "^4.0.0"
+    "zod": "^3.21.4"
   },
   "devDependencies": {
     "@types/node": "^22",
diff --git a/renovate.json5 b/renovate.json5
index a5a9133..c33b9d5 100644
--- a/renovate.json5
+++ b/renovate.json5
@@ -9,5 +9,8 @@
   platformAutomerge: true,
   labels: [
     "dependency upgrade"
+  ],
+  ignoreDeps: [
+    "zod"
   ]
 }