> For clean Markdown of any page, append .md to the page URL.
> For a complete documentation index, see https://amer.developers.trustly.com/llms.txt.
> For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://amer.developers.trustly.com/_mcp/server.

# API fundamentals

Trustly's API uses OAuth for authentication and cryptographic signatures for request integrity. Use the sandbox environment to test your integration before going live.

#### [Authentication and OAuth](/integrate/api-fundamentals/authentication-and-oauth)

OAuth login authorization flows for bank-side authentication.

#### [Secure requests and signature validation](/integrate/api-fundamentals/secure-requests-and-signature-validation)

Generate and validate cryptographic signatures for API requests.

#### [Idempotency](/integrate/api-fundamentals/idempotency)

Prevent duplicate operations with unique merchant references.

#### [Testing](/integrate/api-fundamentals/testing)

Use the sandbox environment and demo banks to simulate transactions.