diff --git a/packages/vitest/src/index.ts b/packages/vitest/src/index.ts
index 0801dd93bc38..35db361a9f94 100644
--- a/packages/vitest/src/index.ts
+++ b/packages/vitest/src/index.ts
@@ -122,15 +122,15 @@ declare global {
 
     // eslint-disable-next-line @typescript-eslint/prefer-ts-expect-error
     // @ts-ignore build namspace conflict
-    type VitestAssertion<A> = {
+    type VitestAssertion<A, T> = {
       [K in keyof A]: A[K] extends Chai.Assertion
-        ? Assertion<any>
+        ? Assertion<T>
         : A[K] extends (...args: any[]) => any
           ? A[K] // not converting function since they may contain overload
-          : VitestAssertion<A[K]>
-    }
+          : VitestAssertion<A[K], T>
+    } & ((type: string, message?: string) => Assertion)
 
-    interface Assertion<T = any> extends VitestAssertion<Chai.Assertion>, JestAssertion<T> {
+    interface Assertion<T = any> extends VitestAssertion<Chai.Assertion, T>, JestAssertion<T> {
       resolves: Promisify<Assertion<T>>
       rejects: Promisify<Assertion<T>>
     }
diff --git a/test/core/test/basic.test.ts b/test/core/test/basic.test.ts
index b646ce3220e4..dad3e931880c 100644
--- a/test/core/test/basic.test.ts
+++ b/test/core/test/basic.test.ts
@@ -25,6 +25,12 @@ test('JSON', () => {
   assert.deepEqual(JSON.parse(output), input, 'matches original')
 })
 
+test('assertion is callable', () => {
+  const str = '13'
+  expect(str).to.be.a('string')
+  expect(str).not.to.be.a('number')
+})
+
 const hi = suite('suite')
 
 hi.test('expect truthy', () => {