fix(form-core): prioritize StandardSchemaV1 check over FieldValidateFn for callable schemas (fixes #2221) by tsushanth · Pull Request #2227 · TanStack/form

tsushanth · 2026-06-30T16:12:51Z

Root cause

UnwrapFieldValidateOrFn checked FieldValidateFn before StandardSchemaV1. For any callable schema (arktype, and potentially other future StandardSchema implementations), this caused the wrong branch to be taken.

Arktype's Type uses Callable<fn extends Fn> from @ark/util, which makes Type structurally extend its call signature (data: unknown) => TOut | ArkErrors. TypeScript's contravariant parameter check then evaluates:

[ArkType] extends [FieldValidateFn<any, any, any>]
// FieldValidateFn = (props: { value: any, fieldApi: ... }) => unknown
// Contravariance: need  { value: any, fieldApi: ... } extends unknown  ← true
// Return:         need  TOut | ArkErrors extends unknown               ← true
// → true

So the StandardSchemaV1 branch (line 457) was unreachable and errors resolved to ReturnType<ArkType> (string | ArkErrors for a string schema) instead of StandardSchemaV1Issue[].

Fix

Swap the check order in both the field and form-group branches of UnwrapFieldValidateOrFn:

// Before
| ([TValidateOrFn] extends [FieldValidateFn<any, any, any>]   // ← arktype hits this
    ? ReturnType<TValidateOrFn>
    : [TValidateOrFn] extends [StandardSchemaV1<any, any>]     // ← never reached
      ? StandardSchemaV1Issue[]
      : undefined)

// After
| ([TValidateOrFn] extends [StandardSchemaV1<any, any>]        // ← arktype now hits this
    ? StandardSchemaV1Issue[]
    : [TValidateOrFn] extends [FieldValidateFn<any, any, any>] // ← plain functions still work
      ? ReturnType<TValidateOrFn>
      : undefined)

Plain function validators (non-schema) are unaffected — they don't implement StandardSchemaV1 so they fall through to FieldValidateFn as before.

Test

Added a type-level test in standardSchemaValidator.test-d.ts asserting that an arktype type('string>0') field validator produces StandardSchemaV1Issue[] | undefined for errorMap.onChange, not an arktype output type.

Fixes #2221

Summary by CodeRabbit

New Features
- Improved type inference for schema-based form validation, so validation errors are recognized more accurately in supported setups.
Bug Fixes
- Fixed an issue where certain validation results could infer the wrong error type, leading to less precise editor feedback.
Tests
- Added coverage for schema validation to ensure the updated inference behavior stays consistent.

…apFieldValidateOrFn Arktype's `Type` implements `Callable<fn>` which makes it structurally extend `(data: unknown) => T`. TypeScript's contravariant parameter check means `ArkType extends FieldValidateFn<any,any,any>` evaluates to `true` (the param check reduces to `{value:any,...} extends unknown`), so the `StandardSchemaV1` branch was never reached and `errors` entries resolved to `ReturnType<ArkType>` instead of `StandardSchemaV1Issue[]`. Fix: check `StandardSchemaV1` first on both the field and form-group branches of `UnwrapFieldValidateOrFn`. Schemas that happen to be callable (arktype, and future StandardSchema implementations) now take the correct path; plain function validators still fall through to the `FieldValidateFn` branch as before. Adds a type-level test asserting that an arktype `type('string>0')` field validator produces `StandardSchemaV1Issue[] | undefined` errors, not the arktype output type. Fixes TanStack#2221

coderabbitai · 2026-06-30T16:13:12Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 57596cfa-d375-4384-9c8a-51ceeef2e801

📥 Commits

Reviewing files that changed from the base of the PR and between 6a73479 and e3af80d.

📒 Files selected for processing (2)

packages/form-core/src/types.ts
packages/form-core/tests/standardSchemaValidator.test-d.ts

📝 Walkthrough

Walkthrough

In UnwrapFieldValidateOrFn, the StandardSchemaV1 branch is moved before the FieldValidateFn/FormGroupValidateFn branches in both union arms, ensuring arktype schemas (which are callable) resolve to StandardSchemaV1Issue[] instead of falling through. A new type-level test confirms errorMap.onChange is inferred as undefined | StandardSchemaV1Issue[] with an arktype validator.

StandardSchemaV1 Type Inference Fix

Layer / File(s)	Summary
UnwrapFieldValidateOrFn branch reordering + type test `packages/form-core/src/types.ts`, `packages/form-core/tests/standardSchemaValidator.test-d.ts`	`StandardSchemaV1` is now checked first in both union arms of `UnwrapFieldValidateOrFn`. A new `vitest` test using `arktype`'s `type('string>0')` asserts that `field.getMeta().errorMap.onChange` resolves to `undefined \| StandardSchemaV1Issue[]`.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐇 A branch out of order caused types to go astray,
The schema came second, but should lead the way!
Now StandardSchema hops first in the queue,
And arktype's errors shine clearly through.
All types resolved — hip hip, hooray! 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately describes the main fix: StandardSchemaV1 is checked before FieldValidateFn for callable schemas.
Description check	✅ Passed	The description covers the root cause, fix, and test, but it omits the template's checklist and release-impact sections.
Linked Issues check	✅ Passed	The change matches `#2221` by fixing callable arktype schema inference and adding a type-level regression test.
Out of Scope Changes check	✅ Passed	Only the targeted type helper and a related test were changed, with no obvious unrelated edits.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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nx-cloud · 2026-07-01T06:32:52Z

View your CI Pipeline Execution ↗ for commit e3af80d

Command	Status	Duration	Result
`nx affected --targets=test:sherif,test:knip,tes...`	✅ Succeeded	3m 27s	View ↗
`nx run-many --target=build --exclude=examples/**`	✅ Succeeded	36s	View ↗

☁️ Nx Cloud last updated this comment at 2026-07-01 06:37:09 UTC

pkg-pr-new · 2026-07-01T06:34:37Z

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commit: e3af80d

codecov-commenter · 2026-07-01T06:37:34Z

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.71%. Comparing base (6892ed0) to head (e3af80d).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2227      +/-   ##
==========================================
- Coverage   90.35%   89.71%   -0.64%     
==========================================
  Files          38       65      +27     
  Lines        1752     3199    +1447     
  Branches      444      803     +359     
==========================================
+ Hits         1583     2870    +1287     
- Misses        149      295     +146     
- Partials       20       34      +14

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fix(form-core): prioritize StandardSchemaV1 check over FieldValidateFn for callable schemas (fixes #2221)#2227

fix(form-core): prioritize StandardSchemaV1 check over FieldValidateFn for callable schemas (fixes #2221)#2227
tsushanth wants to merge 1 commit into
TanStack:mainfrom
tsushanth:fix/arktype-validator-type-inference

tsushanth commented Jun 30, 2026 •

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