diff --git a/packages/integration-tests/__tests__/workshop-prompt-cache.test.ts b/packages/integration-tests/__tests__/workshop-prompt-cache.test.ts
new file mode 100644
index 000000000..5fe9521de
--- /dev/null
+++ b/packages/integration-tests/__tests__/workshop-prompt-cache.test.ts
@@ -0,0 +1,67 @@
+// Providers cache a prompt by its prefix: a request reads the cache only as far as it matches an
+// earlier request byte for byte. So each request a chat sends must start with the whole of the
+// request before it, or everything after the first difference is paid for again.
+
+import { z } from "zod";
+import { afterAll, beforeAll, expect, it } from "vitest";
+import { openAgentSession } from "../src/agent-session.js";
+import { startTestGatekeeperHarness, TEST_VENDOR_ID, type Harness } from "../src/harness.js";
+import { SCRIPTED_MODEL_ID, scriptedModelRouter } from "../src/mock-model.js";
+import { NetworkInterceptor } from "../src/network-interceptor.js";
+
+let harness: Harness;
+const models = scriptedModelRouter();
+const network = new NetworkInterceptor({ handlers: [models.handler] });
+
+beforeAll(async () => {
+  network.install();
+  harness = await startTestGatekeeperHarness({ enableGadgetExecution: true });
+});
+
+afterAll(async () => {
+  try {
+    await harness?.server.close();
+    expect(network.getUnmockedCalls()).toEqual([]);
+  } finally {
+    network.uninstall();
+  }
+});
+
+// A chat completions request: its messages, and the fields that shape how the provider renders
+// them (tools, model, reasoning options).
+const REQUEST = z.looseObject({ messages: z.array(z.unknown()) });
+
+// Compares serialized JSON, so a reordered key counts as a difference, as it does for the cache.
+function expectEachRequestExtendsThePrevious(requests: readonly unknown[]) {
+  const parsed = requests.map(request => REQUEST.parse(request));
+  for (const [index, { messages, ...fields }] of parsed.entries()) {
+    const previous = parsed[index - 1];
+    if (previous === undefined) continue;
+    const { messages: previousMessages, ...previousFields } = previous;
+    expect(JSON.stringify(fields), `fields of request ${index}`)
+        .toBe(JSON.stringify(previousFields));
+    expect(messages.slice(0, previousMessages.length).map(m => JSON.stringify(m)),
+        `messages of request ${index}`).toEqual(previousMessages.map(m => JSON.stringify(m)));
+  }
+}
+
+const READ_TEST_VALUE =
+    "export default async function(self, env) { console.log(await env.TEST_AMBIENT.readValue()); }";
+
+it.concurrent("each request starts with the whole request before it", async () => {
+  const model = models.script([
+    { toolCall: { id: "read-value", name: "executeCode", arguments: { code: READ_TEST_VALUE } } },
+    { text: "The test value is 42." },
+    { text: "It is still 42." },
+  ]);
+  await using session = await openAgentSession(harness.url, {
+    modelId: SCRIPTED_MODEL_ID,
+    userModel: model.userModel,
+    ambientVendorIds: [TEST_VENDOR_ID],
+  });
+
+  expect((await session.runTurn("Read the test value.")).outcome).toEqual({ status: "completed" });
+  expect((await session.runTurn("Is it the same now?")).outcome).toEqual({ status: "completed" });
+  expect(model.requests).toHaveLength(3);
+  expectEachRequestExtendsThePrevious(model.requests);
+});