schemantic
Instructions and guidelines for using the Schemantic library for type-safe data classes and schemas in Dart.
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/genkit-ai/genkit-dart --skill schemantic- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads schemantic automatically when relevant.
- 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.
Schemantic
Schemantic is a general-purpose Dart library used for defining strongly typed data classes that automatically bind to reusable runtime JSON schemas. It is standard for the genkit-dart framework but works independently as well.
Core Concepts
Always use schemantic when strongly typed JSON parsing or programmatic schema validation is required.
- Annotate your abstract classes with
@Schema(). - Use the
$prefix for abstract schema class names (e.g.,abstract class $User). - Always run
dart run build_runner buildto generate the.g.dartschema files.
Basic Usage
- Defining a schema:
import 'package:schemantic/schemantic.dart';
part 'my_file.g.dart'; // Must match the filename
@Schema()
abstract class $MyObj {
String get name;
$MySubObj get subObj;
}
@Schema()
abstract class $MySubObj {
String get foo;
}
- Using the Generated Class:
The builder creates a concrete class MyObj (no $) with a factory constructor (MyObj.fromJson) and a regular constructor.
// Creating an instance
final obj = MyObj(name: 'test', subObj: MySubObj(foo: 'bar'));
// Serializing to JSON
print(obj.toJson());
// Parsing from JSON
final parsed = MyObj.fromJson({'name': 'test', 'subObj': {'foo': 'bar'}});
- Accessing Schemas at Runtime:
The generated data classes have a static $schema field (of type SchemanticType<T>) which can be used to pass the definition into functions or to extract the raw JSON schema.
// Access JSON schema
final schema = MyObj.$schema.jsonSchema;
print(schema.toJson());
// Validate arbitrary JSON at runtime
final validationErrors = await schema.validate({'invalid': 'data'});
Primitive Schemas
When a full data class is not required, Schemantic provides functions to create schemas dynamically.
final ageSchema = SchemanticType.integer(description: 'Age in years', minimum: 0);
final nameSchema = SchemanticType.string(minLength: 2);
final nothingSchema = SchemanticType.voidSchema();
final anySchema = SchemanticType.dynamicSchema();
final userSchema = SchemanticType.map(.string(), .integer()); // Map<String, int>
final tagsSchema = SchemanticType.list(.string()); // List<String>
Union Types (AnyOf)
To allow a field to accept multiple types, use @AnyOf.
@Schema()
abstract class $Poly {
@AnyOf([int, String, $MyObj])
Object? get id;
}
Schemantic generates a specific helper class (e.g., PolyId) to handle the values:
final poly1 = Poly(id: PolyId.int(123));
final poly2 = Poly(id: PolyId.string('abc'));
Field Annotations
You can use specialized annotations for more validation boundaries:
@Schema()
abstract class $User {
@IntegerField(
name: 'years_old', // Change JSON key
description: 'Age of the user',
minimum: 0,
defaultValue: 18,
)
int? get age;
@StringField(
minLength: 2,
enumValues: ['user', 'admin'],
)
String get role;
}
Recursive Schemas
For recursive structures (like trees), must use useRefs: true inside the generated jsonSchema property. You define it normally:
@Schema()
abstract class $Node {
String get id;
List<$Node>? get children;
}
Note: Node.$schema.jsonSchema(useRefs: true) generates schemas with JSON Schema $ref.
FAQ
What does the schemantic skill do?
Instructions and guidelines for using the Schemantic library for type-safe data classes and schemas in Dart.
How do I install schemantic?
Run npx skills add https://github.com/genkit-ai/genkit-dart --skill schemantic 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/schemantic/ (personal) or .claude/skills/schemantic/ (project).
Which AI agents can use schemantic?
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 schemantic 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 schemantic?
It is published by genkit-ai in genkit-ai/genkit-dart. It has 43 recorded installs.
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