diff --git a/packages/core/lib/v3/agent/AnthropicCUAClient.ts b/packages/core/lib/v3/agent/AnthropicCUAClient.ts
index 9590daccd..b500c3594 100644
--- a/packages/core/lib/v3/agent/AnthropicCUAClient.ts
+++ b/packages/core/lib/v3/agent/AnthropicCUAClient.ts
@@ -554,9 +554,16 @@ export class AnthropicCUAClient extends AgentClient {
         ];
       }
 
-      // Add system parameter if provided
+      // Add system parameter if provided, with cache control so the
+      // tools + system prefix is cached across agent steps
       if (this.userProvidedInstructions) {
-        requestParams.system = this.userProvidedInstructions;
+        requestParams.system = [
+          {
+            type: "text",
+            text: this.userProvidedInstructions,
+            cache_control: { type: "ephemeral" },
+          },
+        ];
       }
 
       // Add thinking parameter if available
diff --git a/packages/core/tests/unit/anthropic-cua-system-prompt-caching.test.ts b/packages/core/tests/unit/anthropic-cua-system-prompt-caching.test.ts
new file mode 100644
index 000000000..48584764a
--- /dev/null
+++ b/packages/core/tests/unit/anthropic-cua-system-prompt-caching.test.ts
@@ -0,0 +1,82 @@
+import { describe, expect, it, vi, beforeEach } from "vitest";
+import { AnthropicCUAClient } from "../../lib/v3/agent/AnthropicCUAClient.js";
+import Anthropic from "@anthropic-ai/sdk";
+
+// Mock the Anthropic SDK's beta.messages.create method
+vi.mock("@anthropic-ai/sdk", () => {
+  const mockCreate = vi.fn().mockResolvedValue({
+    id: "test-id",
+    content: [{ type: "text", text: "test response" }],
+    usage: { input_tokens: 10, output_tokens: 20 },
+  });
+
+  return {
+    default: class MockAnthropic {
+      beta = {
+        messages: {
+          create: mockCreate,
+        },
+      };
+    },
+  };
+});
+
+describe("AnthropicCUAClient system prompt caching", () => {
+  let mockCreate: ReturnType<typeof vi.fn>;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    // Get the mock create function from a new instance
+    const anthropic = new Anthropic({ apiKey: "test" });
+    mockCreate = anthropic.beta.messages.create as ReturnType<typeof vi.fn>;
+    mockCreate.mockResolvedValue({
+      id: "test-id",
+      content: [{ type: "text", text: "test response" }],
+      usage: { input_tokens: 10, output_tokens: 20 },
+    });
+  });
+
+  it("should send the system prompt as a content block array with cache_control", async () => {
+    const instructions = "You are a helpful browser automation assistant.";
+    const client = new AnthropicCUAClient(
+      "anthropic",
+      "claude-opus-4-6",
+      instructions,
+      {
+        apiKey: "test-key",
+      },
+    );
+    client.setViewport(1280, 720);
+
+    await client.getAction([{ role: "user", content: "test" }]);
+
+    expect(mockCreate).toHaveBeenCalledWith(
+      expect.objectContaining({
+        system: [
+          {
+            type: "text",
+            text: instructions,
+            cache_control: { type: "ephemeral" },
+          },
+        ],
+      }),
+    );
+  });
+
+  it("should not set the system parameter when no instructions are provided", async () => {
+    const client = new AnthropicCUAClient(
+      "anthropic",
+      "claude-opus-4-6",
+      undefined,
+      {
+        apiKey: "test-key",
+      },
+    );
+    client.setViewport(1280, 720);
+
+    await client.getAction([{ role: "user", content: "test" }]);
+
+    const callArgs = mockCreate.mock.calls[0][0];
+    expect(callArgs.system).toBeUndefined();
+  });
+});