YAML Language Server

Provides YAML language features over the Language Server Protocol (LSP), including validation, completion, hover, formatting, document symbols, and schema-based intelligence.

Starting from version 1.0.0, the language server uses eemeli/yaml as its YAML parser, which strictly enforces the specified YAML spec version. The default YAML spec version is 1.2. Set yaml.yamlVersion to 1.1 for compatibility with older YAML files.

Schema validation supports JSON Schema draft-04, draft-07, 2019-09, and 2020-12.

Features

1. YAML validation:
   • Detects whether the entire file is valid YAML
   • Reports diagnostics such as:
     • Node is not found
     • Node has an invalid key node type
     • Node has an invalid type
     • Node is not a valid child node
     • Node is an additional property of its parent
2. Document symbols:
   • Provides hierarchical document symbols for YAML nodes
3. Completion:
   • Completes YAML keys, values, and structure based on the associated schema
   • Completes scalar nodes with schema defaults when defaults are available
4. Hover:
   • Shows schema descriptions for YAML nodes when descriptions are available
   • Shows anchor information when yaml.hoverAnchor is enabled
   • Shows schema source information when yaml.hoverSchemaSource is enabled
5. Formatting:
   • Formats YAML documents
   • Supports on-type formatting on newline, including automatic indentation for mappings and array items

Completion and hover content are schema-driven. See Associating schemas for configuration details.

Language server settings

The server supports the following settings supplied by LSP clients:

• yaml.yamlVersion: Set default YAML spec version (1.2 or 1.1). Defaults to 1.2.
• yaml.maxItemsComputed: The maximum number of document symbols and folding regions computed (limited for performance reasons). Defaults to 5000.
• yaml.format.enable: Enable/disable the default YAML formatter. Defaults to true.
• yaml.format.singleQuote: Use single quotes instead of double quotes. Defaults to false.
• yaml.format.bracketSpacing: Print spaces between brackets in objects. Defaults to true.
• yaml.format.proseWrap: Control prose wrapping behavior. always: wrap prose if it exceeds the print width, never: never wrap the prose, preserve: wrap prose as-is. Defaults to preserve.
• yaml.format.printWidth: Specify the line length that the printer will wrap on. Defaults to 80.
• yaml.format.trailingComma: Specify if trailing commas should be used in JSON-like segments of the YAML. Defaults to true.
• yaml.validate: Enable/disable validation feature. Defaults to true.
• yaml.hover: Enable/disable hover. Defaults to true.
• yaml.hoverAnchor: Enable/disable hover feature for anchors. Defaults to true.
• yaml.hoverSchemaSource: Enable/disable showing the schema source in hover tooltips. Defaults to true.
• yaml.completion: Enable/disable autocompletion. Defaults to true.
• yaml.disableDefaultProperties: Disable adding not required properties with default values into completion text. Defaults to false.
• yaml.suggest.parentSkeletonSelectedFirst: If true, the user must select some parent skeleton first before autocompletion starts to suggest the rest of the properties. When the YAML object is not empty, autocompletion ignores this setting and returns all properties and skeletons. Defaults to false.
• yaml.schemas: Associate schemas with files using glob patterns. See Associating schemas for details.
• yaml.disableSchemaDetection: Disable schema detection for YAML files matching the configured glob pattern or list of glob patterns. Modelines still apply.
• yaml.schemaStore.enable: Use the SchemaStore catalog to automatically associate schemas with common YAML file patterns. Defaults to true.
• yaml.schemaStore.url: URL of a schema store catalog to use when downloading schemas. Defaults to https://www.schemastore.org/api/json/catalog.json.
• yaml.customTags: Array of custom tags that the parser will validate against. It has three ways to be used. A tag without a type, such as "!Ref", is treated as a scalar tag. A tag with a node type, such as "!Ref sequence", specifies the YAML node type that the tag is written on. A tag with a node type and return type, such as "!FindInMap sequence:string", also specifies the schema type that the tagged value evaluates to. Supported node types are scalar, sequence, and mapping. Supported return types are string, number, integer, boolean, null, array, and object. The return type aliases scalar, sequence, and mapping are accepted as string, array, and object. See Adding custom tags for usage details.
• yaml.disableAdditionalProperties: Globally set additionalProperties to false for all objects. When enabled, no extra properties are allowed in YAML objects beyond those defined in the schema. Defaults to false.
• yaml.kubernetesCRDStore.enable: Enable/disable validation of Kubernetes custom resources using schemas from well-known Custom Resource Definitions (CRDs). Defaults to true.
• yaml.kubernetesCRDStore.url: The base URL for fetching well-known Custom Resource Definition (CRD) schemas. Defaults to https://raw.githubusercontent.com/datreeio/CRDs-catalog/main.
• yaml.kubernetesVersion: Kubernetes version used to build the schema URL when yaml.schemas maps files to the kubernetes keyword. If omitted, the language server uses its predefined default Kubernetes version.
• yaml.style.flowMapping: Control flow style mappings. Forbids flow style mappings if set to forbid. Defaults to allow.
• yaml.style.flowSequence: Control flow style sequences. Forbids flow style sequences if set to forbid. Defaults to allow.
• yaml.keyOrdering: Enforces alphabetical ordering of keys in mappings when set to true. Defaults to false.
• http.proxy: The URL of the proxy server that will be used when attempting to download a schema. If it is not set or it is undefined, no proxy server will be used.
• http.proxyStrictSSL: If true, the proxy server certificate should be verified against the list of supplied CAs. Defaults to false.
• [yaml].editor.tabSize: Number of spaces to use for YAML indentation. When provided, this setting is used for generated completion text. Defaults to 2.
• editor.tabSize: Fallback indentation size when no YAML-specific tab size is provided. Defaults to 2.
• [yaml].editor.formatOnType: Client/editor setting that controls whether on-type formatting requests are sent for YAML files. When enabled by the client, yaml-language-server can format YAML as the user types, such as adjusting indentation after a newline.

Associating schemas

The language server uses JSON Schema to understand the shape of YAML files. Schema definitions can be written in JSON (.json) or YAML (.yaml or .yml) format.

Schemas can be associated with YAML files using a modeline, an inline $schema property, or the yaml.schemas setting. Integrations can also provide schemas through the registered custom schema provider or the json/schemaAssociations notification.

When multiple schema sources or schema-disabling settings apply to the same file, see Schema resolution priority.

Using a modeline

Specify a schema for a YAML file by adding a modeline comment at the top of the file:

# yaml-language-server: $schema=<schema-url-or-path>

The IntelliJ-compatible $schema comment format is also supported:

# $schema: <schema-url-or-path>

Relative paths in modelines are resolved from the YAML file's location, not the workspace root.

Using an inline $schema property

Specify a schema for a YAML file by adding a top-level $schema property:

$schema: <schema-url-or-path>

Relative paths in inline $schema properties are resolved from the YAML file's location, not the workspace root.

Using yaml.schemas

Configure schema-to-file mappings in your LSP client settings using the yaml.schemas option.

Each entry maps a schema to one or more file patterns:

• Key: Schema URI, local file path, or the kubernetes keyword
• Value: A glob pattern or array of glob patterns

Remote schemas

Use a schema URL as the key:

{
  "yaml.schemas": {
    "https://getcomposer.org/schema.json": "composer.yaml",
    "https://example.com/api-schema.json": ["api/*.yml", "api/*.yaml"]
  }
}

Local schemas

Use an absolute path, file URI, or relative path as the key.

In a single-folder workspace, relative schema paths are resolved from the workspace root.

On macOS or Linux:

{
  "yaml.schemas": {
    "/home/user/custom_schema.json": "someFilePattern.yaml",
    "/home/user/custom_schema.yaml": "anotherPattern.yaml",
    "../relative/path/schema.json": ["filePattern1.yaml", "filePattern2.yaml"]
  }
}

On Windows:

{
  "yaml.schemas": {
    "C:\\Users\\user\\Documents\\custom_schema.json": "someFilePattern.yaml",
    "file:///C:/Users/user/Documents/custom_schema.yaml": "anotherPattern.yaml",
    "../relative/path/schema.json": ["filePattern1.yaml", "filePattern2.yaml"]
  }
}

Multi-root workspaces

In multi-root workspaces, prefix schema paths with the workspace folder name that contains the schema.

Suppose the workspace contains two folders, project-a and project-b:

project-a/
├── test.yaml
└── schema.json
project-b/
├── test.yaml
└── schema.json

Use the workspace folder name at the start of each schema path key:

{
  "yaml.schemas": {
    "project-a/schema.json": "project-a/test.yaml",
    "project-b/schema.json": "project-b/test.yaml"
  }
}

Kubernetes schemas

Use the reserved kubernetes keyword to validate Kubernetes YAML files. The language server resolves the keyword to a versioned Kubernetes schema URL based on yaml.kubernetesVersion.

{
  "yaml.schemas": {
    "kubernetes": "k8s/*.yaml"
  }
}

Specify yaml.kubernetesVersion to choose the Kubernetes schema version:

{
  "yaml.kubernetesVersion": "1.36.1",
  "yaml.schemas": {
    "kubernetes": "k8s/*.yaml"
  }
}

If yaml.kubernetesVersion is not set, the language server uses the default Kubernetes version.

Suppressing diagnostics

To hide diagnostics for a specific YAML line, add a suppression comment immediately before that line. To disable schema validation for an entire file, see Disabling schema validation.

Suppress all diagnostics on a line

Add # yaml-language-server-disable immediately before the line that produces the diagnostic:

# yaml-language-server-disable
version: 123

Suppress matching diagnostics

Add one or more comma-separated diagnostic message substrings after # yaml-language-server-disable. Only diagnostics whose messages contain a matching substring are suppressed; the rest are still reported. Matching is case-insensitive.

Single substring:

# yaml-language-server-disable Incorrect type
version: 123

Multiple substrings:

# yaml-language-server-disable Incorrect type, not accepted
version: 123

The substrings are matched against the diagnostic message text reported by the language server.

Disabling schema validation

Disabling schema validation stops schema-based diagnostics. The file is still parsed as YAML, so YAML syntax errors can still be reported.

Using a modeline

Disable schema validation for the current file by setting $schema to none in a modeline:

# yaml-language-server: $schema=none

The IntelliJ-compatible $schema comment format is also supported:

# $schema: none

Using yaml.disableSchemaDetection

Prevent detected schemas from being applied to specific YAML files by configuring yaml.disableSchemaDetection with one or more glob patterns.

For one file pattern:

{
  "yaml.disableSchemaDetection": "**/.github/workflows/*.yaml"
}

For multiple file patterns:

{
  "yaml.disableSchemaDetection": [
    "some.yaml",
    "**/.github/workflows/*.yaml"
  ]
}

Schema resolution priority

When multiple schema sources or schema-disabling mechanisms apply to the same YAML file, the language server uses the following priority order, from highest to lowest:

1. Modeline
2. Inline $schema property
3. Registered custom schema provider
4. yaml.disableSchemaDetection
5. yaml.schemas
6. json/schemaAssociations notification
7. SchemaStore

Adding custom tags

YAML custom tags extend the language with application-specific syntax. Configure custom tags with the yaml.customTags setting.

Each entry supports one of these formats:

• !Tag: Treats the tag as a scalar tag
• !Tag nodeType: Specifies the YAML node type for the tagged value
• !Tag nodeType:returnType: Specifies the YAML node type and the schema type used during validation

Supported node types are scalar, sequence, and mapping.

Supported return types are string, number, integer, boolean, null, array, and object. The aliases scalar, sequence, and mapping are also accepted as string, array, and object.

For example:

{
  "yaml.customTags": [
    "!Scalar-example",
    "!Seq-example sequence",
    "!Mapping-example mapping",
    "!Seq-as-string-example sequence:string"
  ]
}

These tags can then be used in YAML files:

some_key: !Scalar-example some_value
some_sequence: !Seq-example
  - some_seq_key_1: some_seq_value_1
  - some_seq_key_2: some_seq_value_2
some_mapping: !Mapping-example
  some_mapping_key_1: some_mapping_value_1
  some_mapping_key_2: some_mapping_value_2
some_string: !Seq-as-string-example
  - value_1
  - value_2

In the last example, !Seq-as-string-example is written on a YAML sequence, but schema validation treats the tagged value as a string because its return type is string.

Using the language server

The language server can be used through an existing LSP client or launched directly for integration with an editor, IDE, CLI, or another development tool.

Existing clients

This repository only contains the server implementation. Here are some known clients consuming this server:

• Eclipse Che
• vscode-yaml for VS Code
• coc-yaml for coc.nvim
• Eclipse Wild Web Developer for Eclipse IDE
• lsp-mode for Emacs
• vim-lsp for Vim
• LSP-yaml for Sublime Text
• monaco-yaml for Monaco editor
• Vim-EasyComplete for Vim/NeoVim
• nova-yaml for Nova
• volar-service-yaml for Volar
• Kate
• yaml-schema-lint, a CLI for schema linting YAML files

Building a custom integration

To build a custom integration, run the server using one of the following options.

Using the npm package

Install yaml-language-server globally:

npm install -g yaml-language-server

Start the server with the communication channel required by your client:

yaml-language-server --stdio
yaml-language-server --socket=<port>
yaml-language-server --node-ipc

Using a local build

Clone this repository:

git clone https://github.com/redhat-developer/yaml-language-server.git
cd yaml-language-server

Install dependencies and build the server:

npm install
npm run build

The built server is located at ./out/server/src/server.js.

Run the built server with the communication channel required by your client:

node ./out/server/src/server.js --stdio
node ./out/server/src/server.js --socket=<port>
node ./out/server/src/server.js --node-ipc

Using the container image

The container image is published at quay.io/redhat-developer/yaml-language-server.

To run the server over stdio:

docker run -i --rm quay.io/redhat-developer/yaml-language-server:latest

To run the server on a socket:

docker run --rm -p <port>:<port> quay.io/redhat-developer/yaml-language-server:latest --socket=<port>

LSP extensions

The server uses vscode-languageserver@^9.0.0 and implements LSP 3.17.

The following non-standard LSP extensions support schema association, custom schema providers, and schema selection.

Schema association

json/schemaAssociations notification

Sent from the client to the server to associate schemas with YAML file patterns. This is useful when a client or editor extension owns a YAML file type and needs the server to apply a schema automatically.

Each notification replaces the schema associations previously provided through this method.

Notification:

• method: json/schemaAssociations
• params: ISchemaAssociations | SchemaConfiguration[]

/**
 * Maps schema URIs to file patterns.
 */
type ISchemaAssociations = Record<string, string[]>;

interface SchemaConfiguration {
  /**
   * URI that identifies the schema.
   */
  uri: string;
  /**
   * File patterns associated with the schema.
   */
  fileMatch?: string[];
  /**
   * Optional inline schema content. When omitted, the server attempts to
   * load the schema identified by `uri`.
   */
  schema?: JSONSchema;
}

Custom schema provider

These extensions let a client dynamically select schemas for a YAML document and provide content for schema URI schemes that the server cannot load directly.

yaml/registerCustomSchemaRequest notification

Sent from the client to the server to register the client as a custom schema provider. After registration, the server sends a custom/schema/request request when it needs schemas for a YAML document.

Notification:

• method: yaml/registerCustomSchemaRequest
• params: void

custom/schema/request request

Sent from the server to the registered custom schema provider to retrieve the schema URIs for a YAML document.

Request:

• method: custom/schema/request
• params: URI of the YAML document as a string

Response:

• result: Schema URI as a string, schema URIs as a string[], or no result when the provider has no schema for the document

When the provider returns no schema URI or the request fails, the server continues with lower-priority schema sources.

custom/schema/content request

Sent from the server to the client when it needs to load a schema URI whose scheme it cannot handle directly.

Request:

• method: custom/schema/content
• params: Schema URI as a string

Response:

• result: Schema content as a string

Schema selection

These extensions allow clients to discover schemas known to the server and determine which schemas apply to an open YAML document.

yaml/supportSchemaSelection notification

Sent from the client to the server to opt in to the schema-selection workflow.

Notification:

• method: yaml/supportSchemaSelection
• params: void

yaml/schema/store/initialized notification

Sent from the server to the client after Schema Store initialization finishes. The server sends this notification only after the client has sent yaml/supportSchemaSelection.

After receiving the notification, the client can request schema information from the server.

Notification:

• method: yaml/schema/store/initialized
• params: {}

yaml/get/all/jsonSchemas request

Sent from the client to the server to retrieve all known schemas. The server uses the supplied document URI to indicate which schemas apply to that document.

Request:

• method: yaml/get/all/jsonSchemas
• params: URI of an open YAML document as a string

Response:

• result: JSONSchemaDescriptionExt[]

yaml/get/jsonSchema request

Sent from the client to the server to retrieve the schemas that apply to an open YAML document. Clients can use this request to display the document's active schemas.

Request:

• method: yaml/get/jsonSchema
• params: URI of an open YAML document as a string

Response:

• result: JSONSchemaDescription[]

The schema-selection requests use the following response types:

type SchemaVersions = { [version: string]: string };

interface JSONSchemaDescription {
  /**
   * Schema URI.
   */
  uri: string;
  /**
   * Schema name, when available.
   */
  name?: string;
  /**
   * Schema description, when available.
   */
  description?: string;
  /**
   * Available schema versions, when provided by the schema source.
   */
  versions?: SchemaVersions;
}

interface JSONSchemaDescriptionExt extends JSONSchemaDescription {
  /**
   * Whether the schema applies to the requested document.
   */
  usedForCurrentFile: boolean;
  /**
   * Whether the schema comes from Schema Store.
   */
  fromStore: boolean;
}

Development

Prerequisites

• Node.js v18.18.0 or higher
• npm

Setup

Fork and clone this repository, then install dependencies:

cd yaml-language-server
npm install

Build

Build the language server:

npm run build

The main server output is generated in out/server/src.

Use npm test to run tests.

Module builds

Building YAML Language Server produces CommonJS output in the out/server/src directory. In addition, a build also produces UMD (Universal Module Definition) modules and ES Modules (ESM) in the lib directory. These module formats support different server-side module loaders and browser bundlers such as webpack.

CI

GitHub Actions publish each change in the main branch to the npm registry with the next tag. Use the next version to adopt the latest changes into a project.