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What do you think?
The most important thing is to develop and test for your real usage case, because a speech UI is one of those things where the best approach may be very different depending on the purpose of your app. It’s good to use a grammar if your ideal interaction can be described by rules, which might not be the case if it is very simple – if you want to pick up individual words you don’t really get ahead by using a grammar. Two-word phrases are a reasonable use for a grammar, but if you are trying to distinguish between a small number of different phrases and each phrase has the same number of syllables, there is a heightened probability of phrases being mistaken for each other under adverse conditions. Is this the issue? Can we talk more concretely about your actual grammar?