diff --git a/.changeset/plain-message-identifiers.md b/.changeset/plain-message-identifiers.md
new file mode 100644
index 000000000..5988470e4
--- /dev/null
+++ b/.changeset/plain-message-identifiers.md
@@ -0,0 +1,5 @@
+---
+"@voltagent/core": patch
+---
+
+Use dictionaries without inherited keys for in-memory message storage so user and conversation identifiers such as constructor, toString and __proto__ do not break message operations or user isolation, including after clearing storage.
diff --git a/packages/core/src/memory/adapters/storage/in-memory-identifiers.spec.ts b/packages/core/src/memory/adapters/storage/in-memory-identifiers.spec.ts
new file mode 100644
index 000000000..2beaf3d13
--- /dev/null
+++ b/packages/core/src/memory/adapters/storage/in-memory-identifiers.spec.ts
@@ -0,0 +1,52 @@
+import type { UIMessage } from "ai";
+import { describe, expect, it } from "vitest";
+import { InMemoryStorageAdapter } from "./in-memory";
+
+const message: UIMessage = {
+  id: "message",
+  role: "user",
+  parts: [{ type: "text", text: "Hello" }],
+};
+
+describe("InMemoryStorageAdapter string identifiers", () => {
+  it.each(["constructor", "toString", "__proto__"])(
+    "keeps messages isolated when a user identifier is %s",
+    async (userId) => {
+      const adapter = new InMemoryStorageAdapter();
+      expect(await adapter.getMessages(userId, "constructor")).toEqual([]);
+      await adapter.addMessage(message, userId, "conversation");
+      expect(await adapter.getMessages(userId, "conversation")).toHaveLength(1);
+      expect(await adapter.getMessages("another-user", "conversation")).toEqual([]);
+      expect(adapter.getStats()).toMatchObject({ totalUsers: 1, totalMessages: 1 });
+
+      await adapter.clearMessages(userId);
+      await adapter.addMessage(message, userId, "conversation");
+      expect(await adapter.getMessages(userId, "conversation")).toHaveLength(1);
+      adapter.clear();
+      await adapter.addMessage(message, userId, "conversation");
+      expect(adapter.getStats()).toMatchObject({ totalUsers: 1, totalMessages: 1 });
+    },
+  );
+
+  it.each(["constructor", "toString", "__proto__"])(
+    "supports a conversation identifier named %s through message CRUD",
+    async (conversationId) => {
+      const adapter = new InMemoryStorageAdapter();
+      expect(await adapter.getMessages("user", conversationId)).toEqual([]);
+      await adapter.deleteMessages([message.id], "user", conversationId);
+      await adapter.addMessage(message, "user", conversationId);
+      expect(await adapter.getMessages("user", conversationId)).toHaveLength(1);
+      await adapter.deleteMessages([message.id], "user", conversationId);
+      expect(await adapter.getMessages("user", conversationId)).toEqual([]);
+      await adapter.addMessage(message, "user", conversationId);
+      await adapter.clearMessages("user", conversationId);
+      expect(await adapter.getMessages("user", conversationId)).toEqual([]);
+      await adapter.clearMessages("user");
+      await adapter.addMessage(message, "user", conversationId);
+      expect(await adapter.getMessages("user", conversationId)).toHaveLength(1);
+      adapter.clear();
+      await adapter.addMessage(message, "user", conversationId);
+      expect(await adapter.getMessages("user", conversationId)).toHaveLength(1);
+    },
+  );
+});
diff --git a/packages/core/src/memory/adapters/storage/in-memory.ts b/packages/core/src/memory/adapters/storage/in-memory.ts
index 76dccd800..0fb0281c3 100644
--- a/packages/core/src/memory/adapters/storage/in-memory.ts
+++ b/packages/core/src/memory/adapters/storage/in-memory.ts
@@ -84,7 +84,7 @@ function areMetadataValuesEqual(left: unknown, right: unknown): boolean {
  * Simple implementation for testing and development
  */
 export class InMemoryStorageAdapter implements StorageAdapter {
-  private storage: Record<string, Record<string, StoredUIMessage[]>> = {};
+  private storage: Record<string, Record<string, StoredUIMessage[]>> = Object.create(null);
   private conversations: Map<string, Conversation> = new Map();
   private users: Map<string, UserInfo> = new Map();
   private workflowStates: Map<string, WorkflowStateEntry> = new Map();
@@ -106,7 +106,7 @@ export class InMemoryStorageAdapter implements StorageAdapter {
   ): Promise<void> {
     // Create user's messages container if it doesn't exist
     if (!this.storage[userId]) {
-      this.storage[userId] = {};
+      this.storage[userId] = Object.create(null);
     }
 
     // Create conversation's messages array if it doesn't exist
@@ -165,7 +165,8 @@ export class InMemoryStorageAdapter implements StorageAdapter {
     const { limit, before, after, roles } = options || {};
 
     // Get user's messages or return empty array
-    const userMessages = this.storage[userId] || {};
+    const userMessages: Record<string, StoredUIMessage[]> =
+      this.storage[userId] || Object.create(null);
     let messages = userMessages[conversationId] || [];
 
     // Apply role filter if provided
@@ -299,7 +300,7 @@ export class InMemoryStorageAdapter implements StorageAdapter {
       }
     } else {
       // Clear all messages for the user
-      this.storage[userId] = {};
+      this.storage[userId] = Object.create(null);
       this.conversationSteps.delete(userId);
     }
   }
@@ -724,7 +725,7 @@ export class InMemoryStorageAdapter implements StorageAdapter {
    * Clear all data
    */
   clear(): void {
-    this.storage = {};
+    this.storage = Object.create(null);
     this.conversations.clear();
     this.users.clear();
     this.workflowStates.clear();