MCPInstall

safe-action-validation-errors

by next-safe-action

Use when working with validation errors -- returnValidationErrors, formatted vs flattened shapes, custom validation error shapes, throwValidationErrors, or displaying field-level and form-level errors

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Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/next-safe-action/skills --skill safe-action-validation-errors
  1. 01

    Install

    Run the command in your project. The CLI asks which agents to install for.

  2. 02

    Describe the task

    Ask your agent for something this skill covers. It loads safe-action-validation-errors automatically when relevant.

  3. 03

    Review the result

    The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

next-safe-action Validation Errors

Two Sources of Validation Errors

  1. Schema validation — automatic when input doesn't match .inputSchema()
  2. Manual validation — via returnValidationErrors() in server code (e.g., "email already taken")

Both produce the same error structure on the client.

Default Error Shape (Formatted)

Mirrors the schema structure with _errors arrays at each level:

// For schema: z.object({ email: z.string().email(), address: z.object({ city: z.string() }) })
{
  _errors: ["Form-level error"],                    // root errors
  email: { _errors: ["Invalid email address"] },    // field errors
  address: {
    _errors: ["Address section error"],
    city: { _errors: ["City is required"] },        // nested field errors
  },
}

returnValidationErrors

Throws a ActionServerValidationError that the framework catches and returns as result.validationErrors. It never returns — it always throws.

For expected non-validation errors ("out of stock", "not found"), the sibling helper returnServerError(payload) works the same way (throws internally, never returns) but sets result.serverError to the typed payload, bypassing handleServerError. See the safe-action-client skill.

"use server";

import { z } from "zod";
import { returnValidationErrors } from "next-safe-action";
import { actionClient } from "@/lib/safe-action";

const registerSchema = z.object({
  email: z.string().email(),
  username: z.string().min(3),
});

export const register = actionClient
  .inputSchema(registerSchema)
  .action(async ({ parsedInput }) => {
    // Check business rules after schema validation passes
    const existingUser = await db.user.findByEmail(parsedInput.email);
    if (existingUser) {
      returnValidationErrors(registerSchema, {
        email: { _errors: ["This email is already registered"] },
      });
    }

    const existingUsername = await db.user.findByUsername(parsedInput.username);
    if (existingUsername) {
      returnValidationErrors(registerSchema, {
        username: { _errors: ["This username is taken"] },
      });
    }

    // Both checks passed — create the user
    const user = await db.user.create(parsedInput);
    return { id: user.id };
  });

Root-Level Errors

Use _errors at the top level for form-wide errors:

returnValidationErrors(schema, {
  _errors: ["You can only create 5 posts per day"],
});

Supporting Docs

  • Custom validation errors and returnValidationErrors patterns
  • Formatted vs flattened shapes, per-action override

Displaying Validation Errors

// Formatted shape (default)
{result.validationErrors?.email?._errors?.map((error) => (
  <p key={error} className="text-red-500">{error}</p>
))}

// Root-level errors
{result.validationErrors?._errors?.map((error) => (
  <p key={error} className="text-red-500">{error}</p>
))}

// Flattened shape
{result.validationErrors?.fieldErrors?.email?.map((error) => (
  <p key={error} className="text-red-500">{error}</p>
))}

// Form-level errors (flattened)
{result.validationErrors?.formErrors?.map((error) => (
  <p key={error} className="text-red-500">{error}</p>
))}

FAQ

What does the safe-action-validation-errors skill do?

Use when working with validation errors -- returnValidationErrors, formatted vs flattened shapes, custom validation error shapes, throwValidationErrors, or displaying field-level and form-level errors

How do I install safe-action-validation-errors?

Run npx skills add https://github.com/next-safe-action/skills --skill safe-action-validation-errors in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/safe-action-validation-errors/ (personal) or .claude/skills/safe-action-validation-errors/ (project).

Which AI agents can use safe-action-validation-errors?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is safe-action-validation-errors safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains safe-action-validation-errors?

It is published by next-safe-action in next-safe-action/skills. It has 1,202 recorded installs.

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