# Add schemas and specifications

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***

Use schemas and specifications to keep implementation contracts close to the architecture resources they describe.

## Add a schema to a message[​](#add-a-schema-to-a-message "Direct link to Add a schema to a message")

Schemas are supported on:

* [Events](/docs/development/guides/messages/events/introduction.md)
* [Commands](/docs/development/guides/messages/commands/introduction.md)
* [Queries](/docs/development/guides/messages/queries/introduction.md)

Open the message resource, then use the **Schema** section to add or edit the schema file.

Supported schema formats include:

* JSON Schema
* Avro
* Protobuf
* Other text-based schema files

When you save a schema, the editor writes the schema file into the resource folder and updates the resource metadata so EventCatalog can render it.

If the schema editor does not expose the exact change you need yet, use source mode or edit the schema file directly.

## Add a specification to a domain or service[​](#add-a-specification-to-a-domain-or-service "Direct link to Add a specification to a domain or service")

Specifications are supported on:

* [Domains](/docs/development/guides/domains/introduction.md)
* [Services](/docs/development/guides/services/introduction.md)

Open the domain or service, then use the specifications area to add:

* OpenAPI
* AsyncAPI
* GraphQL

The editor writes the specification file into the resource folder and updates the resource `specifications` metadata.

![Service resource showing attached API specifications](/assets/images/editor-cb04fe252a011dca91668794595d1dda.png)

## Preview schema and specification changes[​](#preview-schema-and-specification-changes "Direct link to Preview schema and specification changes")

Start EventCatalog locally with:

```
npm run dev
```

Then use **Open Preview** in the editor to check how the schema or specification renders in the catalog.

## When to use source mode[​](#when-to-use-source-mode "Direct link to When to use source mode")

Use source mode when you need to:

* Adjust a custom `SchemaViewer` component
* Add custom MDX around a schema
* Edit specification metadata the visual form does not expose yet

For more on schema documentation in EventCatalog, see the [schema documentation guides](/docs/development/guides/schemas/introduction.md).