> ## Documentation Index > Fetch the complete documentation index at: https://docs.sapiom.ai/llms.txt > Use this file to discover all available pages before exploring further. > ## Agent Instructions > Sapiom Pay is for platforms whose agent shops for many end users. Each end user is a buyer with their own Sapiom key and saved cards. > Use the buyer's key for every call made on that buyer's behalf. Never use one buyer's key for another user, and never give the agent the platform key. > Agents call the hosted MCP tools at https://api.sapiom.ai/v1/mcp. Backends call the REST API at https://api.sapiom.ai/v1. Both take the key in an x-api-key header. > Never ask a buyer for a card number, expiry date or CVV in chat. Buyers enter card details only on Sapiom's hosted pages. > Follow the nextAction in each transaction result. Action links are secrets for the intended buyer only. > Use one idempotencyKey per transaction. After a timeout, retry with the same key. Never start a new transaction to escape an uncertain outcome. > Report an order as placed only when order.outcome is confirmed. If it is unknown, stop and hand off to a person. # Check a merchant > Checks whether a merchant offers a UCP checkout Sapiom can authorize. Buy only from merchants whose `status` is `supported`. Requires a [buyer key](/api-reference/overview#authentication). ## OpenAPI ````yaml /api-reference/openapi.json get /v1/transaction-compatibility openapi: 3.0.0 info: title: Sapiom Pay API version: '1.0' description: >- Create buyers, save their cards and authorize transactions. Send your platform key or a buyer key in the `x-api-key` header. servers: - url: https://api.sapiom.ai description: Production security: - api-key: [] tags: - name: Buyers description: One buyer per end user. Called tenants in the API. - name: Cards description: Hosted card setup links and the buyer's saved cards. - name: Spending limits description: Limits on what each card can spend. - name: Merchants description: Check whether Sapiom can buy from a merchant. - name: Transactions description: Create a transaction and drive its UCP checkout. - name: Browser checkout description: Placeholder cards and the relay Browser Pay uses. paths: /v1/transaction-compatibility: get: tags: - Merchants summary: Check a merchant description: >- Checks whether a merchant offers a UCP checkout Sapiom can authorize. Buy only from merchants whose `status` is `supported`. Requires a [buyer key](/api-reference/overview#authentication). operationId: TransactionCompatibilityController_check parameters: - name: domain required: true in: query description: >- Merchant to check: a bare domain or an https origin with an optional root slash schema: example: shop.example type: string responses: '200': description: Transaction compatibility result. content: application/json: schema: $ref: '#/components/schemas/TransactionCompatibilityResponseDto' '400': description: >- The merchant domain is invalid; a retryable origin may be included as derivedOrigin. content: application/json: schema: type: object required: - code - message properties: code: type: string example: path message: type: string derivedOrigin: type: string example: https://shop.example '401': description: Authentication is missing or invalid. '403': description: The caller lacks org.transactions.read. '429': description: Rate limited to 60 requests per minute per IP. '503': description: >- Transaction compatibility is temporarily unavailable; this is not a merchant verdict. security: - api-key: [] components: schemas: TransactionCompatibilityResponseDto: type: object properties: merchant: description: The merchant that was checked allOf: - $ref: '#/components/schemas/TransactionCompatibilityMerchantDto' status: type: string description: >- supported: structured checkout is available. Anything else: use browser checkout enum: - supported - unsupported - unknown reason: type: string description: Why the merchant has this status enum: - compatible_ucp_profile - no_manifest - no_compatible_version - no_checkout - timeout - fetch_failed - upstream_error - invalid_profile protocol: nullable: true description: The protocol and version negotiated; null unless status is supported allOf: - $ref: '#/components/schemas/TransactionCompatibilityProtocolDto' capabilities: description: >- Allowlisted capabilities the merchant advertises; empty unless status is supported type: array items: $ref: '#/components/schemas/TransactionCompatibilityCapabilityDto' checkedAt: type: string description: When the merchant profile was fetched (ISO 8601) example: '2026-10-16T23:00:00Z' expiresAt: type: string description: When this answer stops being fresh (ISO 8601) example: '2026-10-16T23:05:00Z' retryAfterSeconds: type: number nullable: true description: Whole seconds until expiresAt when status is unknown, otherwise null required: - merchant - status - reason - protocol - capabilities - checkedAt - expiresAt - retryAfterSeconds TransactionCompatibilityMerchantDto: type: object properties: domain: type: string description: The normalized domain from the input example: shop.example origin: type: string description: The https origin that answered the lookup example: https://shop.example required: - domain - origin TransactionCompatibilityProtocolDto: type: object properties: name: type: string description: Commerce protocol the merchant speaks enum: - ucp version: type: string description: The protocol release negotiated with the merchant example: '2026-08-25' required: - name - version TransactionCompatibilityCapabilityDto: type: object properties: name: type: string description: Advertised capability name, from the known-capability allowlist only enum: - dev.ucp.shopping.checkout - dev.ucp.shopping.fulfillment - dev.ucp.shopping.discount - dev.ucp.shopping.cart supported: type: boolean description: True when Sapiom implements this capability required: - name - supported securitySchemes: api-key: type: apiKey in: header name: x-api-key description: >- Your platform key for buyer management, or a buyer key for everything else. ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.