@rsc-kit/mcp/guides/validation.md

An MCP server over what an rsc-kit build decided: the routes, why each one is static or not, and what it costs the browser.

# Validation

> Surfacing server-side validation errors in a form.

Validation errors have one shape everywhere: a field name mapped to the
messages for it.

```ts
{ title: ['The title must be at least 3 characters.'], body: ['The body is required.'] }
```

`ServerValidationError` carries that shape, and `<Form>`
catch it and put it in `errors`. Nothing in a component has to know where the
validation ran.

## In a form

```tsx title="src/components/PostForm.tsx"
"use client";

import { Form } from '@rsc-kit/core/Form';
import { createPost } from '../actions';

export function PostForm() {
  return (
    <Form action={createPost}>
      {({ error, pending }) => (
        <>
          <input name="title" />
          {error('title') && <p className="error">{error('title')}</p>}

          <textarea name="body" />
          {error('body') && <p className="error">{error('body')}</p>}

          <button disabled={pending}>Create</button>
        </>
      )}
    </Form>
  );
}
```

No `try`/`catch`, and no state of your own. A successful submit clears
`errors`; a failed one fills them.

## Outside a form

Catch it yourself, wherever the call rejects with one:

```tsx
"use client";

import { ServerValidationError } from '@rsc-kit/core/errors';
import { createPost } from '../actions';

try {
  await createPost(title, body);
} catch (error) {
  if (error instanceof ServerValidationError) {
    setErrors(error.errors);
  }
}
```

## Producing one

<Aside type="caution" title="The server half is the host's">
  There is no validation library here, and no rule syntax to learn. What the
  engine defines is the error the client understands — deciding that input is
  invalid is your code's job, or your host's.
</Aside>

### Before it is submitted

`<Form>` takes any [Standard Schema](https://standardschema.dev) —
Zod, Valibot, ArkType, or anything else implementing it. There is no adapter
per library and no import of one: the schema is asked to validate itself.

```tsx title="src/components/PostForm.tsx"
"use client";

import { z } from 'zod';
import { Form } from '@rsc-kit/core/Form';
import { createPost } from '../actions';

const schema = z.object({
  title: z.string().min(3, 'Too short'),
  body: z.string().min(10, 'Too short'),
});

export function PostForm() {
  return (
    <Form action={createPost} schema={schema}>
      {({ error, pending }) => (
        <>
          <input name="title" />
          {error('title') && <p className="error">{error('title')}</p>}

          <textarea name="body" />
          {error('body') && <p className="error">{error('body')}</p>}

          <button disabled={pending}>Create</button>
        </>
      )}
    </Form>
  );
}
```

A failure fills the same `errors` you were already rendering and the action is
never called — so a mistake costs no round trip, and an optimistic update never
shows a row the server was going to refuse. Async schemas are awaited, so a
uniqueness check works the same way.

Field names follow the schema's paths: a nested field is `address.city`, and an
issue belonging to no field — a cross-field rule like "these must match" — is
under the empty string, so it has somewhere to be shown rather than being
dropped.

<Aside type="danger" title="A schema here is a courtesy, not a control">
  It runs in the browser. The action is a public endpoint reachable without
  this form — with `curl`, with the wrong values, in any order — so the server
  still has to check. See [Authorization](/guides/authorization).

  Use the same schema on both sides if you like; the point is that the one
  running on the server is the one that decides.
</Aside>

### In a JavaScript action

The same schema, on the side that decides. An action **returns** its failure
rather than throwing one, so one line turns that into the error the form
already understands:

```ts title="src/actions.ts"
'use server'

import { schema } from './schema';

export async function createPost(title: string, body: string) {
  const parsed = await schema['~standard'].validate({ title, body });

  if (parsed.issues) {
    return { ok: false as const, issues: parsed.issues };
  }

  return { ok: true as const, post: await savePost(parsed.value) };
}
```

```tsx title="src/components/PostForm.tsx"
import { ServerValidationError } from '@rsc-kit/core/errors';
import { issuesToErrors } from '@rsc-kit/core/standardSchema';

async function submit(formData: FormData) {
  const result = await createPost(
    formData.get('title') as string,
    formData.get('body') as string,
  );

  if (!result.ok) throw new ServerValidationError('Validation failed', issuesToErrors(result.issues));

  return result.post;
}
```

`issuesToErrors` is the same mapping the client-side check uses, so the field
names match whichever side produced them.

## ServerValidationError

| Member | Type | What it holds |
| --- | --- | --- |
| `errors` | `Record<string, string[]>` | Field name to messages. |
| `message` | `string` | A summary, for when there is nowhere to put field errors. |