fix(form-core): clear onSubmit error for fields without associated instance

.changeset/salty-carpets-follow.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/form-core': patch
+---
+
+clear onSubmit error for fields without associated instance

packages/form-core/src/FormApi.ts

@@ -1655,6 +1655,23 @@ export class FormApi<
     if (!fieldInstance) {
       const { hasErrored } = this.validateSync(cause)
 
+      // Clear stale onSubmit errors on this field when validation passes,
+      // mirroring what FieldApi.validateSync does for mounted fields.
+      if (!hasErrored && cause !== 'submit') {
+        const submitErrKey = getErrorMapKey('submit')
+        // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+        if (this.getFieldMeta(field)?.errorMap?.[submitErrKey]) {
+          this.setFieldMeta(field, (prev = defaultFieldMeta) => ({
+            ...prev,
+            errorMap: { ...prev.errorMap, [submitErrKey]: undefined },
+            errorSourceMap: {
+              ...prev.errorSourceMap,
+              [submitErrKey]: undefined,
+            },
+          }))
+        }
+      }
+
       if (hasErrored && !this.options.asyncAlways) {
         return this.getFieldMeta(field)?.errors ?? []
       }

packages/form-core/tests/FormApi.spec.ts

@@ -2568,6 +2568,35 @@ describe('form api', () => {
     expect(form.state.errors).toStrictEqual([])
   })
 
+  it('should clear onSubmit field errors via setFieldValue without field instance mounted', async () => {
+    const validateMessage = 'first name is required'
+    const form = new FormApi({
+      defaultValues: { firstName: '' },
+      validators: {
+        onSubmit: ({ value }) => {
+          if (value.firstName.length === 0)
+            return { fields: { firstName: validateMessage } }
+          return null
+        },
+      },
+    })
+
+    form.mount()
+
+    await form.handleSubmit()
+    expect(form.state.isFieldsValid).toBe(false)
+    expect(form.state.canSubmit).toBe(false)
+    expect(form.state.fieldMeta.firstName?.errorMap.onSubmit).toBe(
+      validateMessage,
+    )
+
+    form.setFieldValue('firstName', 'John')
+
+    expect(form.state.fieldMeta.firstName?.errorMap.onSubmit).toBeUndefined()
+    expect(form.state.isFieldsValid).toBe(true)
+    expect(form.state.canSubmit).toBe(true)
+  })
+
   it('should run validators in order form sync -> field sync -> form async -> field async', async () => {
     const order: string[] = []
     const formAsyncChange = vi.fn().mockImplementation(async () => {