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fix(agent): enable prompt caching for the Anthropic CUA system prompt #2304

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fix(agent): enable prompt caching for the Anthropic CUA system prompt #2304

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1a2d942
fix(agent): enable prompt caching for the CUA system prompt
aditya-silna Jul 2, 2026
b6feabf
test(agent): cover system prompt cache_control shape in AnthropicCUAC…
aditya-silna Jul 2, 2026
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11 changes: 9 additions & 2 deletions packages/core/lib/v3/agent/AnthropicCUAClient.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -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 = [

@cubic-dev-ai cubic-dev-ai Bot Jul 2, 2026

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P2: The new system content-block array with cache_control is not verified by any unit test, even though the codebase already mocks getAction's Anthropic SDK call and asserts on request parameters. Because the API call is typed as Record<string, unknown> and uses @ts-expect-error, a regression in this shape would compile cleanly but fail at Anthropic at runtime. Adding a focused test that instantiates AnthropicCUAClient with non-empty userProvidedInstructions and asserts on the system field passed to mockCreate would be straightforward given the existing test infrastructure.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At packages/core/lib/v3/agent/AnthropicCUAClient.ts, line 560:

<comment>The new `system` content-block array with `cache_control` is not verified by any unit test, even though the codebase already mocks `getAction`'s Anthropic SDK call and asserts on request parameters. Because the API call is typed as `Record<string, unknown>` and uses `@ts-expect-error`, a regression in this shape would compile cleanly but fail at Anthropic at runtime. Adding a focused test that instantiates `AnthropicCUAClient` with non-empty `userProvidedInstructions` and asserts on the `system` field passed to `mockCreate` would be straightforward given the existing test infrastructure.</comment>

<file context>
@@ -554,9 +554,16 @@ export class AnthropicCUAClient extends AgentClient {
+      // tools + system prefix is cached across agent steps
       if (this.userProvidedInstructions) {
-        requestParams.system = this.userProvidedInstructions;
+        requestParams.system = [
+          {
+            type: "text",
</file context>

{
type: "text",
text: this.userProvidedInstructions,
cache_control: { type: "ephemeral" },
},
];
}

// Add thinking parameter if available
Expand Down
82 changes: 82 additions & 0 deletions packages/core/tests/unit/anthropic-cua-system-prompt-caching.test.ts
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Original file line number Diff line number Diff line change
@@ -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();
});
});
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