> ## Documentation Index > Fetch the complete documentation index at: https://docs.costgraph.ai/llms.txt > Use this file to discover all available pages before exploring further. # Check that an expression compiles > Compiles an expression without touching cost data, for checking a rule as it is typed. Returns the compiler error when it does not compile. Use preview to find out what it would match. A rule is written one of three ways and exactly one may be sent: expression as CostQL text, clause as a single chip, or node as a tree of chips. A node is rendered server-side by the same renderer the decompose endpoint round-trips through, so a rule saved from chips reads back as the identical tree, and sending node together with expression or clause is a 400 rather than a silent pick. Every leaf of a node is checked like a flat clause, so a tree cannot smuggle in a field or operator the clause path would refuse. Send key as well and the response also reports whether an existing literal rule with no validity window under that tag key (matched case-insensitively) already carries the same predicate, returning its id; the match is on a canonical fingerprint, so a reordered restatement of the same predicate is reported as a duplicate. The response also carries expression_hash, that same canonical fingerprint of the expression that was checked, so a caller can compare two expressions for semantic equality without canonicalising them itself. ## OpenAPI ````yaml /api-reference/costgraph/openapi.json post /api/v1/tenant/virtual-tags/validate openapi: 3.0.0 info: description: Read and manage your CostGraph organization, spend, alerts, and settings. title: CostGraph API contact: {} version: '1.0' servers: - url: https://api.costgraph.ai security: [] tags: - name: ai x-group: AI - name: ai-serving x-group: AI serving - name: anomalies x-group: Anomalies - name: auth x-group: Auth - name: billing x-group: Billing - name: billing-export x-group: Billing export - name: budgets x-group: Budgets - name: ci x-group: CI - name: compute-recommendations x-group: Compute recommendations - name: config x-group: Config - name: cost x-group: Cost - name: gpus x-group: GPUs - name: graphai x-group: Graph AI - name: infracost x-group: Infracost - name: integrations x-group: Integrations - name: invitations x-group: Invitations - name: kubernetes-clusters x-group: Kubernetes clusters - name: marketplace x-group: Marketplace - name: network-requests x-group: Network requests - name: notifications x-group: Notifications - name: oauth x-group: OAuth - name: oauth-clients x-group: OAuth clients - name: opencost x-group: OpenCost - name: organization x-group: Audit log - name: organizations x-group: Organizations - name: placement-alternatives x-group: Placement alternatives - name: reports x-group: Reports - name: service-map x-group: Service map - name: settings x-group: Settings - name: sso x-group: Single sign-on - name: tenants x-group: Tenants - name: user x-group: Users - name: virtual-machines x-group: Virtual machines - name: virtual-tags x-group: Virtual tags - name: workflows x-group: Workflows paths: /api/v1/tenant/virtual-tags/validate: post: tags: - virtual-tags summary: Check that an expression compiles description: >- Compiles an expression without touching cost data, for checking a rule as it is typed. Returns the compiler error when it does not compile. Use preview to find out what it would match. A rule is written one of three ways and exactly one may be sent: expression as CostQL text, clause as a single chip, or node as a tree of chips. A node is rendered server-side by the same renderer the decompose endpoint round-trips through, so a rule saved from chips reads back as the identical tree, and sending node together with expression or clause is a 400 rather than a silent pick. Every leaf of a node is checked like a flat clause, so a tree cannot smuggle in a field or operator the clause path would refuse. Send key as well and the response also reports whether an existing literal rule with no validity window under that tag key (matched case-insensitively) already carries the same predicate, returning its id; the match is on a canonical fingerprint, so a reordered restatement of the same predicate is reported as a duplicate. The response also carries expression_hash, that same canonical fingerprint of the expression that was checked, so a caller can compare two expressions for semantic equality without canonicalising them itself. requestBody: content: application/json: schema: $ref: '#/components/schemas/virtualtag.ValidateRequest' description: >- Expression to check, or a clause or node to build one from; set yields to value for an inherited rule's value expression, and key to also check the predicate is not already taken under that tag key required: true responses: '200': description: OK content: application/json: schema: allOf: - $ref: '#/components/schemas/responses.SuccessResponse' - type: object properties: data: $ref: '#/components/schemas/virtualtag.ValidateResult' '400': description: Bad Request content: application/json: schema: $ref: '#/components/schemas/responses.ErrorResponse' '500': description: Internal Server Error content: application/json: schema: $ref: '#/components/schemas/responses.ErrorResponse' security: - BearerAuth: [] components: schemas: virtualtag.ValidateRequest: type: object properties: clause: $ref: '#/components/schemas/virtualtag.Clause' expression: type: string key: type: string node: $ref: '#/components/schemas/virtualtag.Node' scope: type: string yields: type: string responses.SuccessResponse: type: object required: - message - status properties: data: {} message: type: string example: some message status: type: string example: success virtualtag.ValidateResult: type: object properties: duplicate: type: boolean duplicate_rule_id: type: string expression_hash: type: string responses.ErrorResponse: type: object required: - message - status properties: message: type: string example: some message status: type: string example: error virtualtag.Clause: type: object properties: field: type: string match_value: type: string operator: type: string virtualtag.Node: type: object properties: children: type: array items: $ref: '#/components/schemas/virtualtag.Node' clause: $ref: '#/components/schemas/virtualtag.Clause' kind: type: string securitySchemes: BearerAuth: description: Enter "Bearer {token}" type: apiKey name: Authorization in: header ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.