As per Article 26 of the VAT Law, a VAT invoice must be issued upon request for a supply in order to be considered valid. The invoice serves as conclusive evidence for the recipient of the supply to support their input VAT claim, although it is not the sole means of proof that can be utilized.
When requested by the recipient, the supplier is legally obliged to provide a VAT invoice containing all the information specified under the Swiss VAT law. It is worth noting that the law does not prescribe a specific penalty for the failure to produce a VAT invoice.
Learn more about invoice requirements in Switzerland in our comprehensive guide.