FormGroup validator returning sub-field errors for unmounted fields throws "Cannot read properties of undefined (reading 'errorMap')"

[Todomir]

Describe the bug

When a FormGroup validator returns errors for sub-fields using the documented { fields } shape (see the Form Groups guide: "Can set errors on sub-fields"), and one of those sub-fields is not mounted as a field yet (i.e. there is no <form.Field> for it, so it has no entry in fieldMetaBase), group validation throws:

TypeError: Cannot read properties of undefined (reading 'errorMap')
    at FormGroupApi.js:242
    at functionalUpdate (utils.js:4)
    at FormApi.setFieldMeta (FormApi.js:618)
    at FormGroupApi.distributeFieldErrors (FormGroupApi.js:239)
    at validateFieldOrGroupFn (FormGroupApi.js:330)

distributeFieldErrors intentionally continues when the child field has no meta but the validator produced an error for it:

if (!fieldMeta && !newFormValidatorError) continue

However, it then calls setFieldMeta with an updater that unconditionally dereferences the previous meta:

this.form.setFieldMeta(fullName, (prev) => ({
  ...prev,
  errorMap: {
    ...prev.errorMap, // prev is undefined -> TypeError
  },
}))

This is a real-world pattern for multi-step wizards: each step is a FormGroup validated by a step-level schema. Standard Schema validators (e.g. Zod) reach the same code path via remapStandardSchemaResultForGroup, while fields are rendered progressively. As a result, the schema can legitimately report errors for paths that do not yet have mounted fields.

This is the same bug class that was previously fixed for FormApi's field-error fan-out in:

• errorMap can be undefined when using setFieldMeta updater function after a pushFieldValue #1336
• Form-level validation with onChange and arrays throws an undefined errorMap #1709 / fix(form-core): prevent runtime errors when using deleteField #1706 (sync path)
• fix: create fieldMeta on async validation for fields with errors #2056 (async path)

Those fixes used getFieldMeta(field) ?? defaultFieldMeta on reads and (prev = defaultFieldMeta) => for updater writes. FormGroupApi.distributeFieldErrors is the remaining unguarded occurrence.

Your minimal, reproducible example

https://stackblitz.com/edit/vite-react-ts-xewyjxed

Steps to reproduce

npm install

# headless repro: prints "BUG REPRODUCED" plus the TypeError stack
npm run repro:core

# browser repro
npm run dev

For the browser repro: open the app, type one character into the "First name" input, and check the console for the TypeError.

Expected behavior

As a user, I expected the group validator's fields errors to be stored for not-yet-mounted sub-fields (creating their meta from defaults, as FormApi's form-level validators do since #1706/#2056), so the error shows up as soon as the field mounts. Instead, the whole form crashes.

How often does this bug happen?

Every time

Screenshots or Videos

Screen.Recording.2026-07-02.at.18.10.10.mov

Platform

• OS: macOS
• Browser: Chrome (reproduces in any browser; also reproduces in plain Node with @tanstack/form-core only, no browser/DOM - see repro-core.mjs in the linked repro)
• Version: n/a

TanStack Form adapter

react-form

TanStack Form version

1.33.0

TypeScript version

5.8.3

Additional context

Not a duplicate of #1709/#845/#1336. Those issues covered FormApi's and FieldApi's error fan-out and have already been fixed. This is the same unguarded pattern in FormGroupApi.distributeFieldErrors, which none of the prior fixes (#1706, #2056) touched.