The rules
Seven columns, six rows. Take turns dropping a disc into a column; it falls to the lowest empty slot in that column, so you choose the column and gravity chooses the row. The first player to get four of their own discs in an unbroken line — across, up, or along either diagonal — wins. If all forty-two slots fill with no line of four, the game is drawn. Red moves first. You can also play by pressing keys 1 to 7.
Connect 4 is a solved game, and most sites pretend otherwise
In 1988 James Allen and Victor Allis independently proved that the first player wins with perfect play — but only by opening in the middle column. Opening in either column beside the middle is a draw, and the four outer columns are losses. That is why every strong player fights for the centre: more winning lines pass through it than any other column.
Because it is solved, the honest question for any Connect 4 site is not "is the bot good" but "does it know, or is it guessing". Our hard bot searches to the end of the game and returns the move that is proven best, and the game review marks those moves Best (proven) rather than merely best-looking. Where it can prove the result, it will also tell you plainly that you have a forced win, or that you are already lost.
The honest limitation
An empty Connect 4 board has around 4.5 trillion positions, which is far more than a browser tab can search in the second you are willing to wait for a move. So the solver runs under a budget: it finishes reliably from roughly the sixteenth disc onwards, and before that the bot falls back to a heuristic search nine plies deep. The interface says which one produced the move rather than implying the whole game is played perfectly. Move generation is verified against the published count of legal positions — 7, 49, 238, 1120, 4263, 16422, 54859 and 184275 for one through eight discs — and the solver's answers are checked against a separate unpruned search that uses no shortcuts at all.
Traps, and odd and even threats
Most games are decided by a double threat: two lines of three that can both be completed on the same turn, so blocking one loses to the other. The classic shape is three discs in an L with two separate winning columns. The subtler idea is parity. Because discs stack, a threat only matters if the slot it needs becomes available on your turn — a winning square sitting above an unfilled gap is worthless until the gap fills, and who fills it is often decided fifteen moves earlier. Play the Learn level and the coach will tell you which of your threats are real.
Free review
After every game the review shows what the engine would have played on each of your moves and what the choice cost. Unlimited, no account, no daily cap.