test(cursor): reproduce stale Fast controls for drafted and switched models

Preserve a real Cursor CLI reproduction for #5279 without changing production behavior. Draft feature discovery follows the saved CLI model, and a live model switch retains the previous feature list.

Co-authored-by: gengjiawen <technicalcute@gmail.com>

packages/server/src/server/agent/providers/cursor-fast-5279.real.e2e.test.ts

@@ -0,0 +1,67 @@
+import { mkdir, mkdtemp, rm } from "node:fs/promises";
+import path from "node:path";
+import { expect, test } from "vitest";
+
+import { createTestLogger } from "../../../test-utils/test-logger.js";
+import { CursorACPAgentClient } from "./cursor-acp-agent.js";
+
+// Diagnosis-only reproduction for #5279. Requires the installed Cursor CLI;
+// sends no prompts and keeps Cursor's model preferences inside this checkout.
+test("Cursor Fast availability follows the drafted and active model", async () => {
+  const fixtures = path.resolve(".diagnose-5279");
+  await mkdir(fixtures, { recursive: true });
+  const root = await mkdtemp(path.join(fixtures, "cursor-"));
+  const client = new CursorACPAgentClient({
+    logger: createTestLogger(),
+    command: ["cursor-agent", "acp"],
+    env: {
+      CURSOR_CONFIG_DIR: path.join(root, "config"),
+      CURSOR_DATA_DIR: path.join(root, "data"),
+    },
+  });
+  const hasFast = (features: { id: string }[] | undefined) =>
+    features?.some((feature) => feature.id === "fast") ?? false;
+
+  try {
+    const catalog = await client.fetchCatalog({ scope: "workspace", cwd: root, force: true });
+    expect(catalog.models.some((model) => model.id === "composer-2.5")).toBe(true);
+    expect(catalog.models.some((model) => model.id === "kimi-k3")).toBe(true);
+
+    const composer = await client.listFeatures({
+      provider: "acp",
+      cwd: root,
+      model: "composer-2.5",
+    });
+    console.log("Default saved model: Composer draft offers Fast =", hasFast(composer));
+    expect.soft(hasFast(composer), "Composer draft must offer Fast").toBe(true);
+
+    // A real model change persists Composer only in the isolated Cursor config.
+    const seed = await client.createSession({
+      provider: "acp",
+      cwd: root,
+      model: "composer-2.5",
+    });
+    await seed.close();
+
+    for (const model of ["composer-2.5", "kimi-k3"]) {
+      const features = await client.listFeatures({ provider: "acp", cwd: root, model });
+      console.log("Composer saved model:", model, "draft offers Fast =", hasFast(features));
+      expect.soft(hasFast(features), `Draft ${model}`).toBe(model === "composer-2.5");
+    }
+
+    const session = await client.createSession({ provider: "acp", cwd: root });
+    try {
+      expect(hasFast(session.features)).toBe(true);
+      await session.setModel!("kimi-k3");
+      console.log("After Composer -> Kimi: session offers Fast =", hasFast(session.features));
+      expect.soft(hasFast(session.features), "Kimi session must hide Fast").toBe(false);
+      await expect(session.setFeature!("fast", "true")).rejects.toThrow(
+        "Unknown model config option: fast",
+      );
+    } finally {
+      await session.close();
+    }
+  } finally {
+    await rm(root, { recursive: true, force: true });
+  }
+}, 120_000);