> ## Documentation Index > Fetch the complete documentation index at: https://langwatch.ai/docs/llms.txt > Use this file to discover all available pages before exploring further. # Run an evaluator as a guardrail > Run an evaluator inline and gate on one boolean. Same call as the evaluate path with `as_guardrail` set: every outcome carries `passed`, so an evaluator that skips or fails does not block the request it was guarding. Check `passed` and let the request through when it is true. ## OpenAPI ````yaml POST /api/guardrails/{evaluator}/evaluate openapi: 3.1.0 info: title: LangWatch API version: 1.0.0 description: LangWatch openapi spec servers: - url: https://app.langwatch.ai security: - project_api_key: [] paths: /api/guardrails/{evaluator}/evaluate: post: tags: - Evaluations summary: Run an evaluator as a guardrail description: >- Run an evaluator inline and gate on one boolean. Same call as the evaluate path with `as_guardrail` set: every outcome carries `passed`, so an evaluator that skips or fails does not block the request it was guarding. Check `passed` and let the request through when it is true. operationId: postApiGuardrailsByEvaluatorEvaluate parameters: - in: path name: evaluator required: true schema: type: string description: >- Which evaluator to run. Either a built-in id (`ragas/faithfulness`), the slug of a monitor configured in this project, or `evaluators/{slug|id}` for a saved evaluator. `GET /api/evaluations/list` returns the built-in ids. requestBody: required: true content: application/json: schema: type: object properties: data: type: object additionalProperties: {} description: >- What the evaluator scores. Which fields are required depends on the evaluator; its own entry under Built-in Evaluators lists them. settings: type: object additionalProperties: {} description: >- Per-call overrides of the evaluator's settings. Anything omitted falls back to the saved evaluator or monitor, then to the evaluator's own defaults. trace_id: type: string nullable: true description: Attaches the result to a trace you already sent evaluation_id: type: string nullable: true description: Supply your own id to make the call idempotent evaluator_id: type: string nullable: true name: type: string nullable: true description: Overrides the name the result is recorded under as_guardrail: type: boolean nullable: true description: >- Evaluate as a guardrail: a skipped or failed evaluation answers `passed` rather than an error, so a caller can gate on one field. The /api/guardrails path sets this for you. required: - data additionalProperties: false responses: '200': description: >- The evaluator ran, declined, or failed. Branch on `status`; in guardrail mode `passed` is set on all three. content: application/json: schema: anyOf: - type: object properties: status: type: string const: processed score: type: number passed: type: boolean label: type: string details: type: string cost: type: object properties: currency: type: string amount: type: number required: - currency - amount description: What running the evaluator cost raw_response: description: The evaluator's own output, unprocessed required: - status - type: object properties: status: type: string const: skipped details: type: string description: Why the evaluator declined to score this input passed: type: boolean description: >- Always true in guardrail mode, so a skip does not block required: - status - type: object properties: status: type: string const: error error_type: type: string const: EVALUATOR_ERROR description: 'Constant: the evaluator''s own type is not exposed' details: type: string passed: type: boolean description: >- Always true in guardrail mode, so a failure does not block required: - status - error_type - details '400': description: >- The body was not valid JSON, failed validation, or omitted a field this evaluator requires content: application/json: schema: type: object properties: error: type: string description: The failure, as a sentence kind: type: string description: Stable failure code, on the failures that carry one meta: type: object additionalProperties: {} description: >- What the code needs to be acted on, such as the missing field required: - error '401': description: Missing or invalid API key content: application/json: schema: type: object properties: error: type: string description: The failure, as a sentence kind: type: string description: Stable failure code, on the failures that carry one meta: type: object additionalProperties: {} description: >- What the code needs to be acted on, such as the missing field required: - error '403': description: The API key lacks evaluations:manage content: application/json: schema: type: object properties: error: type: string description: The failure, as a sentence kind: type: string description: Stable failure code, on the failures that carry one meta: type: object additionalProperties: {} description: >- What the code needs to be acted on, such as the missing field required: - error '404': description: No evaluator answers to that id content: application/json: schema: type: object properties: error: type: string description: The failure, as a sentence kind: type: string description: Stable failure code, on the failures that carry one meta: type: object additionalProperties: {} description: >- What the code needs to be acted on, such as the missing field required: - error components: securitySchemes: project_api_key: type: apiKey in: header name: X-Auth-Token description: >- Project API key for sending traces and accessing project-scoped resources. Format: sk-lw-... (no underscore). Obtain one by creating a project via the Admin API or the LangWatch UI. ````