In the test body, I wrote "foo" to the socket. Copilot immediately filled in the rest of the test: read from the socket, check that the result is "Hello, foo", and fail the test with a descriptive error otherwise.
wtf? How did it figure out that the correct result was "Hello, foo"? I changed "Hello" to "Flarbnax", and sure enough, Copilot suggested "Flarbnax, foo". I added a "!" to the end, and it suggested that too. After pushing a bit further, the suggestions started to lose accuracy: it would check for "???" instead of "??" for example, or it would check for a newline where there was none. But overall I came away very impressed.