> For the complete documentation index, see [llms.txt](https://docs.internetobject.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.internetobject.org/definitions/error-handling.md).

# Error Handling in Definitions

Errors that arise from header definitions and references.

Definitions are resolved after the **entire header** has been read, and references are checked again as data is validated. Two errors are specific to definitions:

| Condition                                | Error code           | Cause                                                            |
| ---------------------------------------- | -------------------- | ---------------------------------------------------------------- |
| Reference to an undefined schema or type | `undefined-schema`   | `$name` is used but no `$name` is defined anywhere in the header |
| Reference to an undefined variable       | `undefined-variable` | `@name` is used but no `@name` is defined anywhere in the header |

> Error codes are stable; messages and positions may vary between implementations. Branch on the code, not the message.

## Undefined schema reference

A `$` reference must name a schema or type defined in the header. An undefined name fails with `undefined-schema`:

```ruby
~ $schema: { name: string, home: $address }
---
~ John, { Main St, NYC }    # ✗ undefined-schema — $address is never defined
```

## Undefined variable reference

A `@` reference must name a variable defined in the header. An undefined name fails with `undefined-variable`:

```ruby
~ $schema: { name: string, isActive: bool }
---
~ John, @active             # ✗ undefined-variable — @active is never defined
```

## Reference order

Because definitions resolve only after the whole header is read, **order within the header is not significant** — a reference MAY appear before the definition it targets. The following resolves even though `$address` is defined after the schema that uses it:

```ruby
~ $schema: { name: string, home: $address }
~ $address: { street, city }
---
~ John, { Main St, NYC }    # ✓
```

For readability you SHOULD still define a reference before you use it; doing so reads top to bottom and makes the dependency obvious.

## See Also

* [Definitions](/definitions/definitions.md) · [Schema References](/definitions/schema-references.md) · [Variables](/definitions/variables.md)
* [Error Model](/parsing-and-errors/error-model.md) — the full error catalogue


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