Unexpected Slam

This was a hand I played in a Super Sunday tournament on BBO. It was a daylong tournament. (Partner and opponents were robots.) I had a balanced hand with 19 HCPs. I opened 1♦. My partner responded 1♥. I bid 2NT. He bid 3♠ showing 4 spades. Then I bid 3NT expecting him to pass. Then my partner bid 4♣. I was surprised by his bid. Luckily I had some decent clubs. So I bid 5♣, again thinking partner would pass. But he pushed me into a small slam.

East led 5 of spades. When I saw partner’s hand, I thought I’d probably go down. I had no trump losers. If I could win both finesses in hearts and spades, there would be still 2 losers in each suit. I decided that there would be no point in playing the ace on the first trick. So I played the queen, winning. Then I cashed the ace of hearts and led a heat toward the queen. Lucky! West produced the king. Then he played a spade. I won in dummy and discarded the losing heart on my ace of diamonds. Next I ruffed a diamond and played the queen of hearts discarding my losing spade in dummy. Then I ruffed a spade and cashed the queen of clubs claiming 12 tricks.