# WalletD > A wallet operating system: double-entry ledger, top-ups, transfers, > payments with holds, refunds, rewards, subscriptions and credit lines > behind one REST API. Money is integer minor units, every money-moving > call takes an Idempotency-Key, and money only enters through a verified > gateway confirmation. ## Start here - [WalletD for developers](https://docs.walletd.io/index.md): A wallet operating system behind one REST API. Start here whether you are integrating it, reviewing it, operating it, or deciding to buy it. ## Get started - [Quickstart](https://docs.walletd.io/quickstart.md): Your first five API calls, in curl, Go, Python, PHP, JavaScript and Java. - [Authentication](https://docs.walletd.io/authentication.md): API keys, wallet user tokens and IdP tokens: which credential you send decides both who you are and what you may do. - [Testing](https://docs.walletd.io/testing.md): Sandbox keys, processor test cards, and how to provoke every refusal on purpose before your users find them. ## Concepts - [The money model](https://docs.walletd.io/money-model.md): Minor units, balance against available against held, double entry, what a transaction costs, and the float equation that ties a tenant together. - [Idempotency](https://docs.walletd.io/idempotency.md): Every money-moving POST takes an Idempotency-Key, so a timeout is safe to retry and a replay returns the first response byte for byte. - [Errors](https://docs.walletd.io/errors.md): Every failure is an RFC 7807 problem document. Branch on the code field, and know which statuses are worth retrying. ## Guides - [Top-ups: money in](https://docs.walletd.io/guides/topups.md): The only way money enters a wallet: create an intent, let the user pay, and credit only on a verified gateway confirmation. - [Transfers: wallet to wallet](https://docs.walletd.io/guides/transfers.md): Wallet to wallet inside one tenant: synchronous settlement, aliases, a fee charged on top of the sender, and the caps that refuse a send. - [Payments and refunds](https://docs.walletd.io/guides/payments.md): Pay a merchant instantly, or hold the money and capture it later, then refund without unwinding fees or cashback yourself. - [Subscriptions and dunning](https://docs.walletd.io/guides/subscriptions.md): Recurring charges against a wallet, with dunning retries and a billing period that can be charged at most once, structurally. - [Rewards: cashback, points, conversion](https://docs.walletd.io/guides/rewards.md): Cashback, points and conversion as rules the platform evaluates on every capture, with caps enforced under concurrency. - [Limits and credit lines](https://docs.walletd.io/guides/limits.md): Tier caps that refuse money before anything irreversible happens, and credit lines that let a business account spend below zero. - [Payouts: money out](https://docs.walletd.io/guides/payouts.md): Money out of the loop: the ledger moves first, and the payout record is the instruction to make the real transfer. - [The ecosystem: clients, offerings, discovery](https://docs.walletd.io/guides/ecosystem.md): Many businesses in one shared consumer loop: clients, offerings, discovery, what a sale earns, and client-funded rewards. - [Products, stock, orders and bulk import](https://docs.walletd.io/guides/commerce.md): The merchant commerce surface: variants, cost-plus pricing, stock movements, cart orders, and importing a catalog from a spreadsheet. ## Webhooks - [Webhooks](https://docs.walletd.io/webhooks.md): Signed, at-least-once deliveries that tell your backend money moved, and how to verify one in curl, Go, Python, PHP, JavaScript and Java. ## SDKs and tools - [SDKs and tools](https://docs.walletd.io/sdks.md): The Go SDK, the three OpenAPI documents, an importable API collection, and the markdown endpoints coding agents read. - [AI and agent access](https://docs.walletd.io/ai.md): Authored English guides as markdown, the llms.txt index, and machine-readable API contracts. ## API reference - [API reference](https://docs.walletd.io/reference.md): Three contracts, every operation, and the conventions that hold across all of them. ## Ledger API - [Reaching the ledger](https://docs.walletd.io/reference/ledger/reaching-the-ledger.md): ledgerd is not on the internet. Who this section is for, what credential it takes, and what the placeholder host in the contract means. ## Platform - [Architecture overview](https://docs.walletd.io/platform/architecture.md): The four services behind the WalletD API, what is authoritative for money, how credentials are verified, and what one partner deployment contains. - [Environments, limits and quotas](https://docs.walletd.io/platform/environments-and-limits.md): Sandbox and live credentials, the base URL convention, the rate budgets and concurrency compartments the gateway enforces, and what a 429 or a 503 means. - [Going live](https://docs.walletd.io/platform/going-live.md): What to have in place before you point a live key at real money, and how to prove each item rather than assume it. - [Security for integrators](https://docs.walletd.io/platform/security.md): Credential formats and lifecycles, scopes, user tokens, webhook signing, what the platform records and for how long, what is not offered today, and how to report a vulnerability. - [Support, SLAs and status](https://docs.walletd.io/platform/support.md): How WalletD support reaches a partner's data only with a recorded approval, the default severity targets, the escalation path, and where incidents are published.