> ## Documentation Index
> Fetch the complete documentation index at: https://developers.opencard.io/llms.txt
> Use this file to discover all available pages before exploring further.

# How it works

> Connect your card program to OpenCard — push cards and transaction states so connected EMS apps receive real-time spend data and enrichment.

The **Issuer API** lets card issuers integrate directly with OpenCard and distribute transaction data to connected Expense Management Systems (EMS).

In addition to standard transaction data, OpenCard can attach **enriched information** — digital receipts, correct VAT, line items, environmental metrics — through partner networks. EMS users get a fuller picture of each purchase without you building that layer yourself.

<Note>
  This is **not** the digital receipts API on `receipts.opencard.io`. That is a separate product for receipt matching only. See [Digital receipts](/card-issuers/digital-receipts) if you need it.
</Note>

***

## End-to-end flow

```mermaid theme={null}
sequenceDiagram
    participant Customer as Customer signatory
    participant EMS as EMS
    participant OC as OpenCard
    participant Issuer as You (issuer)

    Customer->>EMS: Sign TPA (eID)
    EMS->>OC: TPA signed
    OC->>Issuer: Signed TPA (API / email)
    Issuer->>OC: POST /issuers/{slug}/cards
    Issuer->>OC: POST .../transaction_states
    OC->>EMS: Webhooks (transactions + enrichment)
```

***

## API surface

All endpoints live under:

```
{base_url}/api/v1/issuers/{slug}/
```

Replace `{slug}` with the value OpenCard assigns your integration.

| Area                   | Methods                                                            | Purpose                                           |
| ---------------------- | ------------------------------------------------------------------ | ------------------------------------------------- |
| **Cards**              | `POST` create, `PUT` update, `GET` one, `GET` list, `DELETE` close | Register, maintain, and inspect the card registry |
| **Transaction states** | `POST .../transaction_states`                                      | Deliver transaction lifecycle events              |

***

## Guides

| Guide                                                                     | What it covers                                                       |
| ------------------------------------------------------------------------- | -------------------------------------------------------------------- |
| [Onboarding](/card-issuers/issuer-integration/onboarding)                 | TPA handoff and when to start sending data                           |
| [Authentication](/card-issuers/issuer-integration/authentication)         | OAuth client credentials and scopes                                  |
| [Cards](/card-issuers/issuer-integration/cards)                           | Create, update, list, and delete cards                               |
| [Transaction states](/card-issuers/issuer-integration/transaction-states) | Push purchases through their lifecycle                               |
| [TPA delivery](/card-issuers/issuer-integration/tpa-integration)          | How OpenCard delivers signed / terminated TPAs to you (email or API) |

API reference → [Issuer API](/api-reference/issuers/overview)

***

## Environments

|                 | Sandbox                           | Production                |
| --------------- | --------------------------------- | ------------------------- |
| **Base URL**    | `https://sandbox-api.opencard.io` | `https://api.opencard.io` |
| **OAuth token** | `POST {base_url}/oauth/token`     | Same path                 |

Contact **[support@opencard.io](mailto:support@opencard.io)** to start integration and receive sandbox credentials.