Content Engine reference

Query Markdown, MDX, and JSON Assets with the Content Engine.

Assets resources query Markdown, MDX, and JSON files stored in the Assets panel. The Builder and Webstudio MCP use the same structured query contract.

MCP workflow

Use these tools in order when creating or changing an Assets resource:

1. Call get-asset-field-catalog to inspect standard fields and the fields currently observed in Markdown or MDX frontmatter and JSON files.

2. Call validate-asset-query to check the query structure, field paths, operators, and bounded operation counts.

3. Call preview-asset-query with concrete values and inspect its results and diagnostics.

4. Save the query with create-assets-resource or update-assets-resource.

5. Inspect saved queries with list-assets-resources or get-assets-resource. Use delete-resource to remove an obsolete resource.

Omit query when creating a resource to use the default many-result query for asset URLs and image dimensions. Set values.query to null when updating a resource to restore that default.

Fields

Every asset has the standard fields below. Markdown or MDX frontmatter and JSON root fields appear under properties, for example properties.slug or properties.author.name. The field catalog reports their observed types, occurrence counts, optionality, and mixed-type state. A JSON content file must contain an object at its root.

Field	Observed type
id	string
url	string
width	number
height	number
name	string
description	string
path	string
key	string
folderId	string
extension	string
mimeType	string
size	number
createdAt	string
revision	string
excerpt	string

Filters

Put conditions under where.all when every condition must match, or under where.any when at least one condition must match. Groups can be nested. A field path is an array such as ["properties", "slug"].

Operator	Builder label	Compatible observed types
eq	equals	null, boolean, number, string, object, array
ne	does not equal	null, boolean, number, string, object, array
contains	contains	string, array
startsWith	starts with	string
endsWith	ends with	string
gt	greater than	number, string
gte	greater than or equal	number, string
lt	less than	number, string
lte	less than or equal	number, string
in	is one of	null, boolean, number, string, object, array
exists	exists	null, boolean, number, string, object, array
isEmpty	is empty	string, object, array

The field catalog determines which operators fit a schemaless properties field. exists and isEmpty take a boolean. in takes an array. Other operators take one JSON value.

Saved values and preview values

Queries saved with create-assets-resource or update-assets-resource accept expressions for filter values, limits, and offsets. Wrap fixed values as literals. Pass a JavaScript expression string only when the value must be resolved at runtime:

{
  "where": {
    "all": [
      { "field": ["extension"], "operator": "eq", "value": { "type": "literal", "value": "md" } },
      { "field": ["properties", "slug"], "operator": "eq", "value": "system.params.slug" }
    ]
  },
  "limit": { "type": "literal", "value": 1 },
  "offset": { "type": "literal", "value": 0 }
}

validate-asset-query and preview-asset-query execute a concrete query. Pass resolved JSON values such as "hello-world" and 1, not expression wrappers or expression code.

Sorting and pagination

Each sort has a field path and an asc or desc direction. Add id as the final sort when equal values must keep a stable order. limit defaults to 20 and offset defaults to 0. Static filters, limits, and offsets should use literal values. Use expressions only for runtime values such as system.params.slug.

Result modes

Value	Behavior
many	Returns every matching item up to the limit. Use it for listings.
one	Returns one item or null. It fails when more than one document matches.
first	Returns the first sorted item or null. The query must include an explicit sort.
last	Returns the last sorted item or null. The query must include an explicit sort.

Every returned item includes id. In preview-asset-query, a many result has data.items, data.totalCount, and data.hasMore; a single result has data.item and data.totalCount. A saved Assets resource exposes a many result as an ID-keyed map at <dataSource>.data, with totalCount and hasMore at <dataSource>.meta. It exposes a single result as the item or null directly at <dataSource>.data, with totalCount at <dataSource>.meta.

Output modes

Value	Behavior
all	Returns every indexed property and the excerpt. Use selected fields when the page needs only part of a document.
base	Returns no properties or excerpt. Set includeMetadata to include the standard file metadata.
fields	Returns the paths in fields. Set includeMetadata separately when the page also needs standard file metadata.

Choose fields and disable includeMetadata when the page needs only selected values. Fields used only for static filtering or sorting do not need to be returned. When enabled, includeMetadata adds name, description, path, key, folderId, extension, mimeType, size, createdAt, revision. Every result includes id.

Asset reference metadata

A plain local asset path in Markdown or MDX frontmatter, or in JSON properties, keeps the backward-compatible URL string result. Wrap the path in an exact $ref object only when consumers need structured Asset Manager metadata:

featureImage:
  $ref: ./assets/hero.png

The reference declares only the relationship. The query decides which metadata enters its result and published database. A complete properties.featureImage selection returns the Asset Manager asset metadata together with its resolved src. This includes fields such as id, name, description, mimeType, width, and height; new asset metadata becomes available to references without changing the document. Select only the nested fields the page uses:

{
  "output": {
    "mode": "fields",
    "includeMetadata": false,
    "fields": [
      ["properties", "title"],
      ["properties", "featureImage", "src"],
      ["properties", "featureImage", "description"]
    ]
  }
}

The selected value becomes an object. Bind an Image source to post.properties.featureImage.src and its alternative text to post.properties.featureImage.description ?? post.properties.title. A missing description therefore falls back to the article title without duplicating text in frontmatter.

External URLs remain ordinary strings. Existing local path strings also keep their URL string shape, so adopting structured references does not change existing queries or bindings.

Content modes

Value	Behavior
none	Returns no file content. Use this for listings and any query that only needs fields or metadata.
full	Embeds the complete UTF-8 file content in the content database. maxBytes defaults to 1 MiB and cannot be set higher. The query fails if a selected file is larger.
range	Embeds a byte range selected by offset and length in the content database. length cannot exceed 256 KiB.
markdown-body-ref	Stores a reference to a Markdown or MDX body. Webstudio filters and paginates first, then reads only the selected bodies from Assets. maxBytes defaults to 1 MiB and cannot be set higher. The query fails if a selected source file is larger.

Returned content has encoding and text. A range also reports its offset, returned length, and total file size. Use markdown-body-ref for Markdown or MDX article pages. It keeps article bodies out of the published content database and resolves relative links when the selected body is loaded.

Preview diagnostics

preview-asset-query returns renderable results in data and non-bindable statistics in __diagnostics__. The diagnostic scope is always query-preview. Read the two capacity scopes separately:

• query measures the temporary database for the query being previewed.

• database measures the merged database for all reachable Assets resources in the project.

Only database.usedBytes counts toward database.maxBytes. Do not add the query and database sizes together.

Diagnostic	Meaning
usedBytes	Bytes included after applying the database limit.
maxBytes	Maximum bytes allowed for the scope.
unboundedBytes	Bytes the scope would use without the database limit.
includedDocumentCount	Documents included in the compiled database.
omittedDocumentCount	Documents omitted from the compiled database.
omissionReason	Why documents were omitted: size or unavailable.
truncated	Whether the compiled database omitted content.
artifacts	Optional query and merged compiled artifacts used by detailed Builder diagnostics.
unresolved	Optional query result before document references are resolved. It helps inspect the authored $ref values behind resolved output.

If the merged database approaches its limit, remove duplicate reachable resources first. Then remove unused output fields or narrow the candidate documents. Prefer markdown-body-ref over embedded full content for Markdown or MDX articles.

Document references

A document reference is an exact object with one string field:

{ "$ref": "<relative-path>[#<fragment>]" }

Markdown and MDX references can appear in YAML frontmatter. JSON references can appear anywhere in the document. Any format can reference Markdown, MDX, or JSON. References do not run inside a Markdown or MDX body.

Reference	Inserted value
../authors/ada.json	The complete JSON value.
../authors/ada.json#/profile/name	The value at JSON Pointer /profile/name.
../authors/ada.md	The complete Markdown source, including frontmatter.
../authors/ada.md#frontmatter	The Markdown frontmatter object.
../authors/ada.md#body	The Markdown body without frontmatter.

Resolve paths relative to the file containing the reference. JSON Pointer uses ~1 for / and ~0 for ~ in property names. URI-encode filename characters that have URL syntax, such as %23 for #. Missing files, invalid fragments, and reference cycles fail instead of returning partial data.

Query limits

Limit	Value
Query request	512 KiB
Filter conditions	32
Filter nesting depth	8
Sort fields	8
Selected output fields	256
Field path depth	9
Default result count	20
Maximum result count	1000
Candidate documents	1000
Serialized query result	16 MiB
Published content database	500 KiB

Content limits

Limit	Value
Markdown/MDX frontmatter	64 KiB
Frontmatter nesting depth	8
Frontmatter fields	256
Frontmatter string	16 KiB
JSON file	1 MiB
JSON nesting depth	8
JSON fields	256
JSON string	16 KiB
Indexed properties per document	64 KiB
Generated excerpt	2 KiB
MDX nesting depth	100
MDX nodes	20000
MDX properties	4000
Loaded file	1 MiB
Loaded content per query	2 MiB
Loaded files per query	20
Loaded range	256 KiB
Concurrent content reads	8

Related

• Content Engine – Build a file-based blog in the visual editor

• Assets – Create and organize project files

• Webstudio MCP – Let an AI agent build and inspect a project

• Data variables – Define resources and understand their scope

• Collection – Render a query result as repeated content