diff --git a/packages/pi-ai/src/providers/anthropic-shared.test.ts b/packages/pi-ai/src/providers/anthropic-shared.test.ts
new file mode 100644
index 000000000..279e0afd4
--- /dev/null
+++ b/packages/pi-ai/src/providers/anthropic-shared.test.ts
@@ -0,0 +1,56 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { convertTools } from "./anthropic-shared.js";
+
+const makeTool = (name: string) => ({
+	name,
+	description: `desc for ${name}`,
+	parameters: {
+		type: "object" as const,
+		properties: { arg: { type: "string" } },
+		required: ["arg"],
+	},
+});
+
+describe("convertTools cache_control", () => {
+	it("adds cache_control to the last tool when cacheControl is provided", () => {
+		const tools = [makeTool("Read"), makeTool("Write"), makeTool("Edit")];
+		const cacheControl = { type: "ephemeral" as const };
+		const result = convertTools(tools, false, cacheControl);
+
+		assert.equal(result.length, 3);
+		assert.equal((result[0] as any).cache_control, undefined);
+		assert.equal((result[1] as any).cache_control, undefined);
+		assert.deepEqual((result[2] as any).cache_control, { type: "ephemeral" });
+	});
+
+	it("does not add cache_control when cacheControl is undefined", () => {
+		const tools = [makeTool("Read"), makeTool("Write")];
+		const result = convertTools(tools, false);
+
+		for (const tool of result) {
+			assert.equal((tool as any).cache_control, undefined);
+		}
+	});
+
+	it("handles empty tools array without error", () => {
+		const result = convertTools([], false, { type: "ephemeral" });
+		assert.equal(result.length, 0);
+	});
+
+	it("passes through ttl when provided", () => {
+		const tools = [makeTool("Read")];
+		const cacheControl = { type: "ephemeral" as const, ttl: "1h" as const };
+		const result = convertTools(tools, false, cacheControl);
+
+		assert.deepEqual((result[0] as any).cache_control, { type: "ephemeral", ttl: "1h" });
+	});
+
+	it("single tool gets cache_control", () => {
+		const tools = [makeTool("Read")];
+		const result = convertTools(tools, false, { type: "ephemeral" });
+
+		assert.equal(result.length, 1);
+		assert.deepEqual((result[0] as any).cache_control, { type: "ephemeral" });
+	});
+});