Basic implementation

Everything seems to be fine...

src/example-vitest.d.ts

@@ -0,0 +1,10 @@
+import "vitest";
+
+interface CustomMatchers {
+    toBeFrob: () => void;
+}
+
+declare module "vitest" {
+    interface Assertion<T = any> extends CustomMatchers {}
+    interface AsymmetricMatchersContaining extends CustomMatchers {}
+}

src/example.test.ts

@@ -1,3 +1,7 @@
-test("example", () => {
+test("baseline", () => {
     expect(1 + 1).toBe(2);
-})
+});
+
+test("toBeFrob", () => {
+    expect("frob").toBeFrob();
+});

src/setupTests.ts

@@ -0,0 +1,19 @@
+expect.extend({
+    toBeFrob(actual) {
+        if (typeof actual !== "string") {
+            throw new Error("Actual value must be a string");
+        }
+
+        const { utils, isNot } = this;
+
+        let pass = actual === "frob";
+        if (isNot) {
+            pass = !pass;
+        }
+
+        return {
+            pass,
+            message: () => `expected ${utils.printReceived(actual)} to be ${utils.printExpected("frob")}`
+        };
+    },
+});