diff --git a/packages/form-core/tests/mergeForm.spec.ts b/packages/form-core/tests/mergeForm.spec.ts
index 972b6772f..6a572f3f5 100644
--- a/packages/form-core/tests/mergeForm.spec.ts
+++ b/packages/form-core/tests/mergeForm.spec.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'vitest'
-import { mutateMergeDeep } from '../src/mergeForm'
+import { FormApi } from '../src/FormApi'
+import { mergeForm, mutateMergeDeep } from '../src/mergeForm'
 
 type TestObject = Record<string, any>
 
@@ -97,4 +98,163 @@ describe('mutateMergeDeep', () => {
     mutateMergeDeep(a, b)
     expect(a).toStrictEqual({ a: { a: 1, b: 2 } })
   })
+
+  it('should return empty object when target is null', () => {
+    const result = mutateMergeDeep(null, { a: 1 })
+    expect(result).toStrictEqual({})
+  })
+
+  it('should return empty object when target is undefined', () => {
+    const result = mutateMergeDeep(undefined, { a: 1 })
+    expect(result).toStrictEqual({})
+  })
+
+  it('should return empty object when target is not an object', () => {
+    const result = mutateMergeDeep('string' as any, { a: 1 })
+    expect(result).toStrictEqual({})
+  })
+
+  it('should return target unchanged when source is null', () => {
+    const target = { a: 1 }
+    const result = mutateMergeDeep(target, null)
+    expect(result).toBe(target)
+    expect(result).toStrictEqual({ a: 1 })
+  })
+
+  it('should return target unchanged when source is undefined', () => {
+    const target = { a: 1 }
+    const result = mutateMergeDeep(target, undefined)
+    expect(result).toBe(target)
+    expect(result).toStrictEqual({ a: 1 })
+  })
+
+  it('should return target unchanged when source is not an object', () => {
+    const target = { a: 1 }
+    const result = mutateMergeDeep(target, 'string' as any)
+    expect(result).toBe(target)
+    expect(result).toStrictEqual({ a: 1 })
+  })
+
+  it('should replace arrays completely without merging their elements', () => {
+    const target = {
+      users: [
+        { id: 1, name: 'Old User', deleted: true },
+        { id: 2, name: 'Another User' },
+      ],
+    }
+    const source = {
+      users: [{ id: 1, name: 'New User' }],
+    }
+
+    mutateMergeDeep(target, source)
+
+    expect(target.users).toHaveLength(1)
+    expect(target.users[0]).toStrictEqual({ id: 1, name: 'New User' })
+    expect(target.users[0]).not.toHaveProperty('deleted')
+  })
+
+  it('should handle circular references without stack overflow', () => {
+    const target: any = { data: { value: 1 } }
+    target.data.self = target.data
+
+    const source = { data: { newValue: 2 } }
+
+    expect(() => mutateMergeDeep(target, source)).not.toThrow()
+    expect(target.data.newValue).toBe(2)
+    expect(target.data.value).toBe(1)
+    expect(target.data.self).toBe(target.data)
+  })
+
+  it('should not merge symbol keys (Object.keys limitation)', () => {
+    const sym = Symbol('test')
+    const target = { [sym]: 'original', regular: 'value' }
+    const source = { [sym]: 'new', regular: 'updated' }
+
+    mutateMergeDeep(target, source)
+
+    expect(target[sym]).toBe('original')
+
+    expect(target.regular).toBe('updated')
+  })
+})
+
+describe('mergeForm', () => {
+  // `as any` required: FormApi has 12 generic params with TSubmitMeta defaulting to `never`,
+  // incompatible with mergeForm's `any` constraint. Type limitation only - runtime is correct.
+  // Production usage via useTransform receives AnyFormApi, avoiding this.
+
+  it('should merge state into form using mutateMergeDeep', () => {
+    const form = new FormApi({
+      defaultValues: {
+        name: '',
+        age: 0,
+      },
+    })
+
+    form.mount()
+
+    const newState = {
+      values: {
+        name: 'John',
+        age: 30,
+      },
+    }
+
+    const result = mergeForm(form as any, newState)
+
+    expect(result).toBe(form)
+
+    expect(form.state.values).toStrictEqual({
+      name: 'John',
+      age: 30,
+    })
+  })
+
+  it('should handle partial state updates', () => {
+    const form = new FormApi({
+      defaultValues: {
+        firstName: '',
+        lastName: '',
+      },
+    })
+
+    form.mount()
+
+    mergeForm(form as any, {
+      values: {
+        firstName: 'Jane',
+      },
+    })
+
+    expect(form.state.values.firstName).toBe('Jane')
+    expect(form.state.values.lastName).toBe('')
+  })
+
+  it('should handle deeply nested state updates', () => {
+    const form = new FormApi({
+      defaultValues: {
+        user: {
+          profile: {
+            name: '',
+            age: 0,
+          },
+        },
+      },
+    })
+
+    form.mount()
+
+    mergeForm(form as any, {
+      values: {
+        user: {
+          profile: {
+            name: 'Alice',
+          },
+        },
+      },
+    })
+
+    expect(form.state.values.user.profile.name).toBe('Alice')
+    expect(form.state.values.user.profile.age).toBe(0)
+  })
 })