> ## Documentation Index > Fetch the complete documentation index at: https://docs.closient.com/llms.txt > Use this file to discover all available pages before exploring further. # Verify a GS1 key's check digit and identify its prefix > Check the GS1 mod-10 check digit on any numeric GS1 key (GTIN-8/12/13/14, GLN, SSCC) and — for GTIN-shaped keys — report which GS1 Member Organisation or numbering scheme the prefix belongs to. `expected_check_digit` is returned even when the supplied digit is wrong, because the useful answer to a failed check is which digit it should have been. **Keyless** — no API key, no signup. IP-throttled at 300 requests/minute and cached at the edge for 24h, so repeat calls for the same input do not reach origin. Over-rate callers get a `429` with a `Retry-After` header and a `retry_after` field — it never silently degrades or returns a wrong answer under load. ## OpenAPI ````yaml /openapi/openapi-resolver.json get /resolver/api/v1/public/gs1/analyze openapi: 3.1.0 info: title: Resolver API version: 1.0.0 description: > GS1 Digital Link resolution with content negotiation and linkset support. ## Authentication All endpoints require an API key passed via the `X-API-Key` HTTP header, unless otherwise noted. ``` X-API-Key: csb__ ``` Generate API keys in **Settings > API Keys** in your dashboard, or via the Account API. Session-based (cookie) authentication is also accepted for browser-based access. ## Rate Limits | Tier | Requests / minute | Requests / day | |-------------|-------------------|----------------| | Default | 300 | 10,000 | | Custom | Contact us | Contact us | Rate-limit headers are included on every response so callers can self-throttle without hitting our 429s ("informed governor"): - `RateLimit-Policy` — every active window, e.g. `300;w=60, 10000;w=86400` - `RateLimit-Limit` — quota for the **most-restrictive** currently-active window - `RateLimit-Remaining` — requests left in that window - `RateLimit-Reset` — seconds until that window resets (relative; clock-skew safe) Legacy `X-RateLimit-*` aliases are also emitted for back-compat. `X-RateLimit-Reset` keeps the absolute Unix-timestamp shape to avoid breaking existing consumers. When rate-limited, you receive `429 Too Many Requests` with a `retry_after_seconds` field in the error envelope and a `Retry-After` header. ## Pagination List endpoints return paginated results in this envelope: ```json { "data": [...], "pagination": { "page": 1, "page_size": 25, "total_count": 342, "total_pages": 14, "has_next": true, "has_previous": false } } ``` Use `?page=2&page_size=50` query parameters. Maximum page size is 100. ## Error Responses All errors conform to [RFC 9457 Problem Details](https://www.rfc-editor.org/rfc/rfc9457) with `Content-Type: application/problem+json`: ```json { "type": "https://closient.com/docs/errors/not_found", "title": "Not Found", "status": 404, "detail": "The requested resource was not found.", "error_code": "not_found", "retryable": false, "timestamp": "2026-03-31T12:00:00+00:00" } ``` Common error codes: `unauthorized` (401), `forbidden` (403), `not_found` (404), `validation_error` (422), `rate_limited` (429), `internal_error` (500). termsOfService: https://www.closient.com/terms/ servers: - url: https://www.closient.com security: [] tags: - name: Resolver description: GS1 Digital Link resolution with content negotiation and linkset support. - name: Custom URLs description: >- Reusable custom-URL catalog (C-3339) — create, list, edit, and delete the custom redirect destinations that resolution rules point at. - name: Verification description: >- Serial verification (`gs1:verificationService`) — tiered, rate-limited responses for serialized GTIN scans. Experimental v1; see the verification-service guide. - name: GS1 Primitives (Public) description: >- Keyless GS1 Digital Link primitives (C-4295) — parse, validate, build, decompress, and check-digit analysis. **No API key, no signup.** Pure functions over strings: no database, no network, no catalog data. IP-throttled and edge-cached so repeat calls cost nothing. This is the reference implementation the resolver itself runs on. - name: Resolver Custom Hostnames description: >- BYO domain (C-4058) — register a customer-owned hostname, publish the CNAME records, verify ownership, and serve an org's Digital Link QRs from its own DNS. externalDocs: description: Closient Documentation url: https://docs.closient.com paths: /resolver/api/v1/public/gs1/analyze: get: tags: - GS1 Primitives (Public) summary: Verify a GS1 key's check digit and identify its prefix description: >- Check the GS1 mod-10 check digit on any numeric GS1 key (GTIN-8/12/13/14, GLN, SSCC) and — for GTIN-shaped keys — report which GS1 Member Organisation or numbering scheme the prefix belongs to. `expected_check_digit` is returned even when the supplied digit is wrong, because the useful answer to a failed check is which digit it should have been. **Keyless** — no API key, no signup. IP-throttled at 300 requests/minute and cached at the edge for 24h, so repeat calls for the same input do not reach origin. Over-rate callers get a `429` with a `Retry-After` header and a `retry_after` field — it never silently degrades or returns a wrong answer under load. operationId: apps_resolver_api_public_gs1_analyze_gs1_key_public parameters: - in: query name: key schema: description: >- The GS1 key to analyse, e.g. `09506000164908`. Hyphens and spaces are ignored. maxLength: 64 minLength: 1 title: Key type: string required: true description: >- The GS1 key to analyse, e.g. `09506000164908`. Hyphens and spaces are ignored. responses: '200': description: OK content: application/json: schema: $ref: '#/components/schemas/GS1KeyAnalysisOut' components: schemas: GS1KeyAnalysisOut: description: Check-digit and issuing-organisation analysis of a numeric GS1 key. examples: - check_digit: '8' expected_check_digit: '8' gs1_prefix: '950' is_gtin_length: true is_valid: true issuing_organization: GS1 Global Office length: 14 normalized: '09506000164908' value: '09506000164908' properties: value: description: The input, echoed back unchanged. title: Value type: string is_valid: description: True when the key's trailing GS1 mod-10 check digit is correct. title: Is Valid type: boolean error: anyOf: - type: string - type: 'null' description: Why the key is invalid, or `null` when it checks out. title: Error normalized: anyOf: - type: string - type: 'null' description: >- GTIN-length keys zero-padded to GTIN-14; other keys stripped of separators. `null` when the key could not be read. title: Normalized check_digit: anyOf: - type: string - type: 'null' description: The trailing digit supplied by the caller. examples: - '8' title: Check Digit expected_check_digit: anyOf: - type: string - type: 'null' description: >- The digit the GS1 mod-10 algorithm requires. Populated even when it disagrees with `check_digit`. title: Expected Check Digit length: description: Digit count after stripping hyphens and spaces. examples: - 14 minimum: 0 title: Length type: integer is_gtin_length: description: Whether the length is one of the GTIN-8/12/13/14 lengths. title: Is Gtin Length type: boolean gs1_prefix: anyOf: - type: string - type: 'null' description: >- The three-digit GS1 prefix (digits 2-4 of the GTIN-14). `null` for non-GTIN-length keys. title: Gs1 Prefix issuing_organization: anyOf: - type: string - type: 'null' description: >- Which GS1 Member Organisation or scheme that prefix belongs to. `null` when unassigned or not GTIN-shaped. title: Issuing Organization required: - value - is_valid - length - is_gtin_length title: GS1KeyAnalysisOut type: object ````