Art. 26 CISG

A declaration of avoidance of the contract is effective only if made by notice to the other party.

Secretariat Commentary

The Secretariat Commentary on Art. 24 of the 1978 New York Draft, predecessor to Art. 26 CISG, gives insight into the drafters' unterstanding on the eve of the 1980 Vienna Diplomatic Conference.

Commentary on the Draft Convention on Contracts for the International Sale of Goods, prepared by the Secretariat (UN Doc. A/CONF.97/5), in: United Nations Conference on Contracts for the International Sale of Goods, Vienna, 10 March – 11 April 1980, Official Records: Documents of the Conference and Summary Records of the Plenary Meetings and of the Meetings of the Main Committees, New York: United Nations (1991), pp. 14 et seq., at pp. 26—27

Overview of case law on Art. 26 CISG: UNCITRAL Digest

The UNCITRAL Digest on the CISG can be used as a starting point to discover the relevant case law on this article.

2016 UNCITRAL Digest of case law on Art. 26 CISG

Interpretation of Art. 26 CISG: Selected issues discussed in case law

Are implicit declarations of avoidance possible?

yes:

Mercedes Maybach case
Oberster Gerichtshof (Austrian Supreme Court)
Austria, 05 July 2019 – 4 Ob 110/19h, CISG-online 4509

A declaration of contract avoidance under Art. 49(1) CISG can be made implicitly. The buyer initiating court proceedings directed at repayment of the contract price against the seller constitutes an implicit contract avoidance