Overview

External data can’t be trusted. APIs change, users mistype, and JSON drifts from what your code expects. Ack is a schema validation library for Dart and Flutter that puts a guardrail at those boundaries: describe the shape you expect once, then check untrusted data against it. You get a clear error when it doesn’t match — and your data, validated, when it does.

(“Ack” is short for “acknowledgment.”)

import 'package:ack/ack.dart';
 
final userSchema = Ack.object({
  'name': Ack.string().minLength(2).maxLength(50),
  'age': Ack.integer().min(0).max(120),
  'email': Ack.string().email().nullable(),
});
 
final result = userSchema.safeParse({
  'name': 'Ada',
  'age': 36,
  'email': 'ada@example.com',
});
 
if (result.isOk) {
  print('Welcome, ${result.getOrThrow()!['name']}');
} else {
  print(result.getError()); // tells you exactly which field failed, and why
}

On success, getOrThrow() returns your validated data as a Map<String, Object?>. Ack checks and shapes the data — opt into code generation when you’d rather have typed getters than map access.

Why Ack?

• Guard your boundaries. Catch bad API responses, user input, and config before they reach your logic.
• Define each shape once. Reuse one schema for validation, JSON Schema export, and typed models.
• Errors you can act on. Every failure points at the exact field and the reason it failed.
• Just Dart. A fluent API with no required build step — reach for code generation only when you want it.

Go beyond validation

What do you want to do?

• Validate an API response — Quickstart Tutorial, then JSON Serialization
• Show field errors in a Flutter form — Flutter Form Validation
• Make a field optional or nullable — Optional vs nullable
• Parse dates, URIs, and durations — Codecs
• Add custom validation logic — Custom Validation
• Generate typed models (no manual casts) — TypeSafe Schemas
• Reuse and compose schemas — Common Recipes

Install

dart pub add ack

New here? The Quickstart Tutorial takes you from install to handling every validation outcome in a few minutes.

Learn more

• Schema Types — every schema type and how to compose them
• Validation Rules — built-in constraints
• Error Handling — read and display structured errors
• JSON Serialization — validate and encode JSON
• API Reference — the full surface

Building with an AI agent? Start at /ack/llms.txt for a compact, machine-readable index.