Custom Providers¶

Language Check supports external checker binaries that communicate via stdin/stdout JSON.

Note

Vale has built-in first-class support — use engines.vale: true instead of registering it as a custom provider. See Vale Setup.

Configuration¶

engines:
  external:
    - name: custom-checker
      command: ./my-checker
    - name: another-tool
      command: /usr/bin/another-tool
      args: ["--format", "json"]
      extensions: [md, rst]

Protocol¶

Request (stdin)¶

The provider receives a JSON object on stdin:

{
  "text": "The full document text to check.",
  "language_id": "markdown"
}

Response (stdout)¶

The provider must return a JSON array of diagnostics on stdout:

[
  {
    "start_byte": 4,
    "end_byte": 8,
    "message": "Consider using 'complete' instead of 'full'.",
    "suggestions": ["complete", "entire"],
    "rule_id": "style.word-choice",
    "severity": 2,
    "confidence": 0.8
  }
]

Diagnostic Fields¶

Field	Type	Required	Description
`start_byte`	`number`	Yes	Start byte offset (UTF-8)
`end_byte`	`number`	Yes	End byte offset (UTF-8)
`message`	`string`	Yes	Human-readable description
`suggestions`	`string[]`	No	Replacement suggestions (default: `[]`)
`rule_id`	`string`	No	Rule identifier (default: provider name)
`severity`	`number`	No	1=Error, 2=Warning, 3=Info, 4=Hint
`confidence`	`number`	No	0.0–1.0 confidence score (default: 0.7)

Rule IDs are automatically prefixed with external.<provider-name>. for namespacing.

Error Handling¶

If the binary is not found, the provider is silently skipped.
If the binary exits non-zero, the error is logged and no diagnostics are returned.
If the output is not valid JSON, the error is logged and no diagnostics are returned.

High Performance Mode¶

External providers are skipped when performance.high_performance_mode is enabled.