Twitter CEO Elon Musk in April accused Microsoft of using the company’s data without permission to train its chatbot. Now the company has sent Microsoft CEO Satya Nadella a letter.

The letter focuses on Microsoft’s unauthorized use of Twitter data collected through an API for use in several products such as the gaming console Xbox, the search engine Bing, and advertising and other tools.

Google and Microsoft for years have used Twitter’s developer API for free in many of its products, including their respective search engines.

Microsoft reportedly declined to pay for continued access to the API last month after Twitter put a price on the service.

The free basic API access that Twitter had long offered to everyone from small indie developers to academic researchers prior to Musk’s acquisition would end. Users would need to pay.

After several delays, Twitter put a price on the API’s value, announcing the price structure in an email to users.

Enterprise API packages moved from free to starting at $42,000 per month for its lowest subscription tier, according to Mashable.

Prior to Musk acquiring the company, Twitter offered a free API tier. Premium and Enterprise API plans were offered.

The highest advertised subscription rate was Premium, costing $2,899 monthly. Large businesses with more than 250 users were previously eligible for the Enterprise API tier, and the cost depended on the organization or company.

Microsoft in April said it would drop Twitter from a Microsoft advertising product, according to a notice on Microsoft’s website.

The notice said as of April 25, 2023, marketers would not have access to their Twitter account through Microsoft’s social management tool. Nor will they have the ability to create and manage drafts or Tweets, view past Tweets and engagement, and schedule Tweets.

A Microsoft representative confirmed the company had received the letter with some questions about its previous use of the free Twitter API.

“We look forward to continuing our long-term partnership with the company,” The Wall Street Journal reported, adding that Microsoft will review the questions and respond.

Read the letter here.