diff --git a/packages/spy/src/index.ts b/packages/spy/src/index.ts
index e4a3e94c8..5ee596bec 100644
--- a/packages/spy/src/index.ts
+++ b/packages/spy/src/index.ts
@@ -154,7 +154,7 @@ export interface MockInstance<TArgs extends any[] = any[], TReturns = any> {
    * const increment = vi.fn().mockImplementation(count => count + 1);
    * expect(increment(3)).toBe(4);
    */
-  mockImplementation(fn: ((...args: TArgs) => TReturns) | (() => Promise<TReturns>)): this
+  mockImplementation(fn: ((...args: TArgs) => TReturns)): this
   /**
    * Accepts a function that will be used as a mock implementation during the next call. Can be chained so that multiple function calls produce different results.
    * @example
@@ -162,7 +162,7 @@ export interface MockInstance<TArgs extends any[] = any[], TReturns = any> {
    * expect(fn(3)).toBe(4);
    * expect(fn(3)).toBe(3);
    */
-  mockImplementationOnce(fn: ((...args: TArgs) => TReturns) | (() => Promise<TReturns>)): this
+  mockImplementationOnce(fn: ((...args: TArgs) => TReturns)): this
   /**
    * Overrides the original mock implementation temporarily while the callback is being executed.
    * @example
@@ -479,16 +479,16 @@ function enhanceSpy<TArgs extends any[], TReturns>(
   stub.mockReturnValueOnce = (val: TReturns) => stub.mockImplementationOnce(() => val)
 
   stub.mockResolvedValue = (val: Awaited<TReturns>) =>
-    stub.mockImplementation(() => Promise.resolve(val as TReturns))
+    stub.mockImplementation(() => Promise.resolve(val as TReturns) as any)
 
   stub.mockResolvedValueOnce = (val: Awaited<TReturns>) =>
-    stub.mockImplementationOnce(() => Promise.resolve(val as TReturns))
+    stub.mockImplementationOnce(() => Promise.resolve(val as TReturns) as any)
 
   stub.mockRejectedValue = (val: unknown) =>
-    stub.mockImplementation(() => Promise.reject(val))
+    stub.mockImplementation(() => Promise.reject(val) as any)
 
   stub.mockRejectedValueOnce = (val: unknown) =>
-    stub.mockImplementationOnce(() => Promise.reject(val))
+    stub.mockImplementationOnce(() => Promise.reject(val) as any)
 
   Object.defineProperty(stub, 'mock', {
     get: () => mockContext,
diff --git a/test/core/test/jest-mock.test.ts b/test/core/test/jest-mock.test.ts
index e460a1a42..f6c2dd479 100644
--- a/test/core/test/jest-mock.test.ts
+++ b/test/core/test/jest-mock.test.ts
@@ -52,6 +52,27 @@ describe('jest mock compat layer', () => {
     expect(mock2.getMockImplementation()).toBeUndefined()
   })
 
+  it('implementation types allow only function returned types', () => {
+    function fn() {
+      return 1
+    }
+
+    function asyncFn() {
+      return Promise.resolve(1)
+    }
+
+    const mock1 = vi.fn(fn)
+    const mock2 = vi.fn(asyncFn)
+
+    mock1.mockImplementation(() => 2)
+    // @ts-expect-error promise is not allowed
+    mock1.mockImplementation(() => Promise.resolve(2))
+
+    // @ts-expect-error non-promise is not allowed
+    mock2.mockImplementation(() => 2)
+    mock2.mockImplementation(() => Promise.resolve(2))
+  })
+
   it('implementation sync fn', () => {
     const originalFn = function () {
       return 'original'