> 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/schema-definition-language/advanced-schema-concepts/union-types.md).

# Union Types (anyOf)

When a field must accept values of **more than one type**, use the `anyOf` constraint on the `any` type (see [Any](/schema-definition-language/data-types/any.md)). A value is valid if it matches **any one** of the listed alternatives.

```ruby
id: { any, anyOf: [string, int] }
---
~ 42      # ✓ matches int
~ abc     # ✓ matches string
```

A value matching none of the alternatives is rejected:

```ruby
flag: { any, anyOf: [bool, int] }
---
~ T       # ✓
~ hello   # ✗ matches neither
```

## Constrained and structured alternatives

Each alternative may be a full MemberDef (with constraints) or an object shape, not just a bare type name:

```ruby
value: { any, anyOf: [ { int, multipleOf: 5 }, { int, multipleOf: 3 } ] }
---
~ 10      # ✓ multiple of 5
~ 9       # ✓ multiple of 3
```

## Guidance

* Order alternatives from most specific to least specific.
* Prefer `anyOf` over a bare `any` when the set of acceptable types is known — it keeps validation meaningful.

## See Also

* [Any](/schema-definition-language/data-types/any.md) · [MemberDef](/schema-definition-language/advanced-schema-concepts/memberdef.md)
* [Schema References](/definitions/schema-references.md)


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