> For the complete documentation index, see [llms.txt](https://docs.payments.thalescloud.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.payments.thalescloud.io/ecom/integrate-the-d1-api/d1-api-overview.md).

# D1 API overview

### API design

#### Data format

Requests and responses use JSON. String encoding is UTF-8 unless stated otherwise.

#### Transport protocol

All interfaces use HTTPS. Mutual TLS (mTLS) and OAuth 2.0 are required.

### Backward compatibility rules

These rules help you build integrations that keep working as the D1 API evolves.

* Accept unknown fields. Thales can add **Optional** parameters in headers, URLs, and bodies. Your integration must not break when new fields appear.

{% hint style="info" %}
“Strict validation” rejects requests or responses that contain fields not in your current spec version.
{% endhint %}

* Treat error message strings as informational. Do not depend on a fixed format.
* Expect requirement changes over time:
  * Parameters in D1 responses can change from **Optional** to **Required**.
  * Parameters in issuer requests can change from **Required** to **Optional**.
* Ignore JSON field order.
* Accept new enum values.
* Expect new API operations (resources and HTTP methods).
* Expect new HTTP status codes in responses.

### API parameter/operation end of life

Thales provides notice at least 9 months before end of life for any deprecated API parameter or API operation.

After the deprecation period ends, you must stop using deprecated parameters and operations in all environments, including Production.

Thales may shorten the deprecation period, without limitation to the following conditions:

* A significant security risk
* A substantial economic or material technical impact
* Requirements from applicable law or payment networks

Deprecated API parameters and operations are marked as deprecated in the documentation.


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