Meta-schema reference

This page is generated from schema/draft/2026-09/.schema.hcl by hclschema docs — the same command you would run against your own schema.

Meta-schema

Draft 2026-09.

Attribute	Type	Required	Default	Description
__id	string	yes		A stable identity for this document.
__schema	string	yes		The meta-schema this document is written against. Its draft/<id>/ path segment selects the draft.
__schema_sha256	string matching ^[0-9a-fA-F]{64}$	no		Pins the meta-schema to an exact digest.

import

Makes another schema's ids available under an alias, as alias.block_header.<id>.

import "<alias>" { ... }

Labels must be unique among sibling blocks of this type.

Attribute	Type	Required	Default	Description
source	string	yes		A path relative to this document, or an https URL.
sha256	string matching ^[0-9a-fA-F]{64}$	no		Pins the imported document to an exact digest.

body

The root body every instance of this schema must match.

body { ... }

Occurrences: at most 1.

Attribute	Type	Required	Default	Description
open	bool	no		Permit attributes and blocks this body does not declare.

body.attribute

Declares an attribute. Mirrors hcl.AttributeSchema, extended with hcldec.AttrSpec's type and default.

attribute "<attribute_name>" { ... }

Labels must be unique among sibling blocks of this type.

Attribute	Type	Required	Default	Description
at_least_one_of	list(string)	no		This attribute and the named siblings form a group of which at least one must be set.
conflicts_with	list(string)	no		Sibling attributes that must not be set alongside this one.
default	any	no		Substituted when the attribute is absent. Only meaningful when it is optional.
deprecated	string	no		Marks the attribute deprecated, with the reason. Using it produces a warning.
description	string	no		Documentation, surfaced as editor hover text and in generated reference docs.
enum	any	no		The permitted values.
exactly_one_of	list(string)	no		This attribute and the named siblings form a group of which exactly one must be set.
max	number	no		Upper bound on a number's value, a string's length, or a collection's element count.
min	number	no		Lower bound on a number's value, a string's length, or a collection's element count.
pattern	string	no		A regular expression a string value must match.
required	bool	no		The attribute must be present.
required_with	list(string)	no		Sibling attributes that must be set when this one is.
sensitive	bool	no		Keeps the value out of diagnostics.
type	any	no		A type constraint expression, as understood by ext/typeexpr.

body.attribute.validation

A custom predicate over the attribute's value.

validation { ... }

Attribute	Type	Required	Default	Description
condition	any	yes		A boolean expression with self bound to the value.
error_message	string	yes		Reported when the condition is false.

body.block_header

Declares a block. Mirrors hcl.BlockHeaderSchema, extended with hcldec's cardinality.

block_header "<block_header_type>" { ... }

Labels must be unique among sibling blocks of this type.

Attribute	Type	Required	Default	Description
deprecated	string	no		Marks the block deprecated, with the reason.
description	string	no
id	any	no		Registers this block's body so that a ref elsewhere can reuse it.
label_names	list(string)	no		Names for the block's positional labels. Their count is the block's arity.
max_items	number at least 0	no		Most instances permitted. Zero means unbounded.
min_items	number at least 0	no		Fewest instances of this block permitted in its parent body.
open	bool	no		Permit content this block's body does not declare.
ref	any	no		Reuses the body registered under another declaration's id, as block_header.<id>.
required	bool	no		Shorthand for min_items = 1.
unique_labels	bool	no		Sibling blocks of this type must not repeat a label tuple.

body.block_header.label

Constrains one positional label. Mirrors hcldec.BlockLabelSpec, extended with a pattern and an enum.

label "<label_name>" { ... }

Labels must be unique among sibling blocks of this type.

Attribute	Type	Required	Default	Description
description	string	no
enum	list(string)	no		The permitted labels.
pattern	string	no		A regular expression the label must match.

body.block_header.body

body { ... }

Same contents as body.

body.one_of

A group of alternatives, exactly one of which the enclosing body must match.

one_of { ... }

body.one_of.variant

variant "<variant_name>" { ... }

Occurrences: at least 2.

Labels must be unique among sibling blocks of this type.

body.one_of.variant.body

body { ... }

Same contents as body.