Tone lives between the words. A friend replies to your party invite with a flat 'Yeah... I guess I could come.' What's the real tone?

The real tone behind "Yeah... I guess I could come" is reluctance or a lack of enthusiasm, signaled by the hesitant pause and the hedging phrase "I guess," which softens commitment rather than expressing genuine eagerness. Tone in English frequently lives in these small verbal hedges and pauses rather than in the literal content of the words.

The reply clearly isn't excited, since genuine excitement would use more enthusiastic, committed language rather than trailing off with uncertainty. It also isn't confusion, since the friend understood the invitation perfectly well and is responding to it directly, just half-heartedly.

Recognizing this kind of implied reluctance is a subtle but important social skill in English, since a literal reading of the words alone ("yes, I could come") would completely miss the friend's actual hesitation, something conveyed only through phrasing and delivery.