[natsu64]

We play Standard Modern Precision (slightly modified). My partner opens 1S (5 cd, 11-15 hcp). RHO passes and I bid 2H establishing a game force. LHO overcalls 3D. My partner jumps to 4H. RHO passes again and I rebid 4S. Is 4S to be read as control cuebid or setting S as trump over H?
We have no formal rule on control cuebidding other than what is standard.
Would appreciate some response and advice.

[mikeh]

Here’s an analogous auction, without interference. 1S 2H 3H 3S.

Traditionally, at least in NA, this was played as responder showing a spade fit. The current trend in expert bidding is for 3S to be a cuebid.

The reasoning is that, when responder is unlimited and thus may be slamming, the ability to start cuebidding as cheaply as 3S is extremely useful while, usually, when responder has 3 card spade support, the heart fit will play as well, or better than, the spade fit.

Now, the analogy isn’t perfect but, on your auction, most of the time hearts should be at least as good a fit as spades.

This argument assumes that responder will rarely bid 2H with 4 card spade support. So 4H will always be at least a 5-3 fit, and 4S would be at most a 5-3 fit, so 4H will rarely, single dummy, be inferior to 4H. And that means that it’s safe to play 4S as a cuebid.

Btw, some pairs play a convention known as kickback and for them 4S would be keycard in hearts.