diff --git a/api/_lib/function-count.test.ts b/api/_lib/function-count.test.ts
new file mode 100644
index 0000000..186cc9f
--- /dev/null
+++ b/api/_lib/function-count.test.ts
@@ -0,0 +1,27 @@
+import { readdirSync, statSync } from "node:fs";
+import { join, relative } from "node:path";
+import { describe, expect, it } from "vitest";
+
+// Vercel's Hobby plan deploys at most 12 functions; every .ts under api/
+// outside _-prefixed folders counts, tests included, and one more fails the deploy.
+const HOBBY_LIMIT = 12;
+const API_ROOT = join(__dirname, "..");
+
+function functionFiles(dir: string): string[] {
+  return readdirSync(dir).flatMap((name) => {
+    const path = join(dir, name);
+    if (name.startsWith("_") || name.startsWith(".")) return [];
+    if (statSync(path).isDirectory()) return functionFiles(path);
+    return /\.(ts|js|mjs)$/.test(name) && !name.endsWith(".d.ts") ? [relative(API_ROOT, path)] : [];
+  });
+}
+
+describe("api functions", () => {
+  it("stay within the Vercel Hobby limit", () => {
+    expect(functionFiles(API_ROOT).length).toBeLessThanOrEqual(HOBBY_LIMIT);
+  });
+
+  it("do not include tests (they would deploy as functions)", () => {
+    expect(functionFiles(API_ROOT).filter((file) => file.includes(".test."))).toEqual([]);
+  });
+});
diff --git a/api/ranked/challenge.test.ts b/api/_lib/ranked-challenge.test.ts
similarity index 94%
rename from api/ranked/challenge.test.ts
rename to api/_lib/ranked-challenge.test.ts
index cfc0522..df0e695 100644
--- a/api/ranked/challenge.test.ts
+++ b/api/_lib/ranked-challenge.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { selectSnippetCode } from "./challenge";
+import { selectSnippetCode } from "../ranked/challenge";
 import { isWithinLengthSpec } from "../../src/utils/ranking";
 import type { SnippetLength } from "../../src/types";