# BR-DE-20 Contract reference must be provided

The contract reference must be provided on the invoice.

## Problem Description

The contract reference must be provided on the invoice.

> **ERROR**: XRechnung 3.0

## Context and Significance

Contract references are often required for public procurement to match invoices to framework agreements or contracts. Use the exact contract identifier provided by the buyer/authority.

## Solution

Add the contract reference to the invoice.

## Valid Examples

- CON-2024-00077
- Framework agreement reference

## Invalid Examples

- Missing contract reference

## Code Examples

### Valid XML
```
<cac:ContractDocumentReference><cbc:ID>CON-2024-00077</cbc:ID></cac:ContractDocumentReference>
```

### Invalid XML
```
<!-- Missing cac:ContractDocumentReference -->
```

## Technical Details

### Affected Fields
Contract Reference

## Related Rules

- [BR-DE-19](/resources/kosit-validator-errors/br-de-19-project-reference-must-be-provided)

## Validation Tools

- [Official KoSIT Validator](https://erechnungsvalidator.service-bw.de/): Validate your XRechnung files against all BR-DE rules.
- [Convert PDF to XRechnung](/pdf-to-xrechnung): Automatic validation error fixes when converting.

## Related Resources

- [Official Specification](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)
- [Complete Error List](/resources/kosit-validator-errors): Overview of all German XRechnung validation rules.