diff --git a/package-lock.json b/package-lock.json
index 319e5ef..69e5014 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "chat-state-cloudflare-do",
-  "version": "0.1.8",
+  "version": "0.1.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "chat-state-cloudflare-do",
-      "version": "0.1.8",
+      "version": "0.1.1",
       "license": "MIT",
       "devDependencies": {
         "@cloudflare/workers-types": "^4.20250214.0",
@@ -923,7 +923,6 @@
       "integrity": "sha512-DpzbrH7wIcBaJibpKo9nnSQL0MTRdnWttGyE5haGwK86xgMOkFLp7vEyfQPGLOJh5wNYiJ3V9PmUMDhV9u8kkQ==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "undici-types": "~7.18.0"
       }
@@ -1280,7 +1279,6 @@
       "dev": true,
       "hasInstallScript": true,
       "license": "MIT",
-      "peer": true,
       "bin": {
         "esbuild": "bin/esbuild"
       },
@@ -2369,7 +2367,6 @@
       "integrity": "sha512-5gTmgEY/sqK6gFXLIsQNH19lWb4ebPDLA4SdLP7dsWkIXHWlG66oPuVvXSGFPppYZz8ZDZq0dYYrbHfBCVUb1Q==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "engines": {
         "node": ">=12"
       },
@@ -2419,7 +2416,6 @@
         }
       ],
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "nanoid": "^3.3.11",
         "picocolors": "^1.1.1",
@@ -2808,7 +2804,6 @@
       "integrity": "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw==",
       "dev": true,
       "license": "Apache-2.0",
-      "peer": true,
       "bin": {
         "tsc": "bin/tsc",
         "tsserver": "bin/tsserver"
@@ -2946,7 +2941,6 @@
       "integrity": "sha512-w+N7Hifpc3gRjZ63vYBXA56dvvRlNWRczTdmCBBa+CotUzAPf5b7YMdMR/8CQoeYE5LX3W4wj6RYTgonm1b9DA==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "esbuild": "^0.27.0",
         "fdir": "^6.5.0",
diff --git a/src/adapter.ts b/src/adapter.ts
index 68b2523..c652a66 100644
--- a/src/adapter.ts
+++ b/src/adapter.ts
@@ -128,6 +128,18 @@ export class CloudflareDOStateAdapter implements StateAdapter {
     await this.stub().cacheSet(key, JSON.stringify(value), ttlMs);
   }
 
+  async setIfNotExists<T = unknown>(
+    key: string,
+    value: T,
+    ttlMs?: number
+  ): Promise<boolean> {
+    return this.stub().cacheSetIfNotExists(
+      key,
+      JSON.stringify(value),
+      ttlMs
+    );
+  }
+
   async delete(key: string): Promise<void> {
     await this.stub().cacheDelete(key);
   }
diff --git a/src/durable-object.ts b/src/durable-object.ts
index fa4cc06..ea615b3 100644
--- a/src/durable-object.ts
+++ b/src/durable-object.ts
@@ -210,6 +210,40 @@ export class ChatStateDO<TEnv = unknown> extends DurableObject<TEnv> {
     }
   }
 
+  /**
+   * Set the key only if it does not exist (or is expired). Returns true if
+   * the value was set, false if the key already existed and is not expired.
+   */
+  cacheSetIfNotExists(key: string, value: string, ttlMs?: number): boolean {
+    const now = Date.now();
+    const result = this.ctx.storage.transactionSync(() => {
+      const existing = this.sql
+        .exec(
+          "SELECT 1 FROM cache WHERE key = ? AND (expires_at IS NULL OR expires_at > ?)",
+          key,
+          now
+        )
+        .toArray();
+      if (existing.length > 0) {
+        return { inserted: false, expiresAt: null as number | null };
+      }
+      const expiresAt = ttlMs ? Date.now() + ttlMs : null;
+      this.sql.exec(
+        "INSERT INTO cache (key, value, expires_at) VALUES (?, ?, ?)",
+        key,
+        value,
+        expiresAt
+      );
+      return { inserted: true, expiresAt };
+    });
+
+    // Schedule alarm outside the transaction — same pattern as acquireLock().
+    if (result.inserted && result.expiresAt != null) {
+      this.scheduleCleanupIfNeeded();
+    }
+    return result.inserted;
+  }
+
   cacheDelete(key: string): void {
     this.sql.exec("DELETE FROM cache WHERE key = ?", key);
   }
diff --git a/src/index.test.ts b/src/index.test.ts
index 7c45608..23adfae 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -1,8 +1,13 @@
+/// <reference types="@cloudflare/workers-types" />
 import type { Lock, StateAdapter } from "chat";
 import { beforeEach, describe, expect, it } from "vitest";
 import { CloudflareDOStateAdapter } from "./adapter";
 import { createCloudflareState } from "./index";
 
+// Use concrete adapter type so tests can call methods added in newer Chat SDK
+// (e.g. setIfNotExists in 4.18) even when devDependency is an older version.
+type AdapterUnderTest = CloudflareDOStateAdapter;
+
 // ---------------------------------------------------------------------------
 // Mock DO — mirrors ChatStateDO behavior using in-memory data structures.
 // This lets us test the adapter's delegation logic, sharding, and
@@ -93,6 +98,14 @@ class MockChatStateDO {
     });
   }
 
+  cacheSetIfNotExists(key: string, value: string, ttlMs?: number): boolean {
+    if (this.cacheGet(key) !== null) {
+      return false;
+    }
+    this.cacheSet(key, value, ttlMs);
+    return true;
+  }
+
   cacheDelete(key: string): void {
     this.cache.delete(key);
   }
@@ -132,7 +145,7 @@ function createMockNamespace() {
 // ---------------------------------------------------------------------------
 
 describe("CloudflareDOStateAdapter", () => {
-  let adapter: StateAdapter;
+  let adapter: AdapterUnderTest;
   let mock: ReturnType<typeof createMockNamespace>;
 
   beforeEach(async () => {
@@ -371,6 +384,22 @@ describe("CloudflareDOStateAdapter", () => {
 
       expect(await adapter.get("zero-ttl")).toBe("value");
     });
+
+    it("should setIfNotExists only when key is missing or expired", async () => {
+      const set1 = await adapter.setIfNotExists("nx-key", "first");
+      expect(set1).toBe(true);
+      expect(await adapter.get("nx-key")).toBe("first");
+
+      const set2 = await adapter.setIfNotExists("nx-key", "second");
+      expect(set2).toBe(false);
+      expect(await adapter.get("nx-key")).toBe("first");
+
+      await adapter.set("expiring-nx", "old", 10);
+      await new Promise((resolve) => setTimeout(resolve, 20));
+      const set3 = await adapter.setIfNotExists("expiring-nx", "new");
+      expect(set3).toBe(true);
+      expect(await adapter.get("expiring-nx")).toBe("new");
+    });
   });
 
   // -- Sharding ------------------------------------------------------------