My Company (based in Germany) has been awarded a project (4 weeks) in the United States for an American client.
Three of us will go, all with European passports on the Visa Waiver Program. we can all get ESTAs without any issues, and I actually have one still valid from last year.
However, I cannot find a definite answer whether we are allowed to work for our European company for an American client, on American soil. All references seem to focus on long-term or remote work.
As Europeans, do we need specific visas to be employed by our European company but work short-term in the USA?
As a follow-up question, what should we tell immigration when we land? I know they usually ask questions about purpose of the trip etc.
EDIT: The purpose of the trip is a risk assessment in the Client's plant, which amounts in layman terms to 4 weeks of business meetings.
EDIT 2: I am currently on my trip, and even though some contact information was outdated (e.g. address in the U.S.), at the airport (Pittsburgh) the process was really simple and was let through without any hassle.
I do confirm the issue about "working" vs. "doing business" as we shortly visited the Canadian side of Niagara Falls. On the way back, the officer asked us what we were doing in the U.S. and looked horrified at my colleague saying "working for an American Client". I saved the day, thanks to you guys, explaining that we were just there for business and our salaries were 100% from our company back in Germany.