と思いきや【とおもいきや】

と思いきや follows a plain-form clause stating what you expected, and the second half says what actually happened instead: "I thought…, but", "just when I thought…". A noun or な-adjective takes だ before it.

The surprise can go either way, good or bad. The second half is something that happened, usually in the past, so it is never a request or your own plan.

と思いきや or と思ったら

とおもったら思ったら covers the same ground in conversation, and it also has a second meaning, "no sooner had…". と思いきや only means the expectation was wrong.

と思いきや is written, and in speech it sounds deliberately dramatic or jokey. In ordinary conversation, say とおもった思ったけど or とおもったら思ったら.