はず vs わけ — I genuinely cannot tell them apart

Question

彼はもう着いたはずだ vs 彼はもう着いたわけだ — both feel like 'he should have arrived' to me. What's actually different in the logic here?

Accepted Answer

はずだ is an expectation/prediction based on reasoning BEFORE confirming the outcome — 'given the schedule, he should have arrived by now.' わけだ is a conclusion you reach AFTER being given a fact — 'oh, that explains it, no wonder he's here already' (there's a わけだ companion pattern too, worth checking). はず looks forward from logic, わけ looks back and explains.

Answer

Right, and that's also why わけだ often gets translated 'no wonder' / 'so that's why' — it always responds to some piece of info you just received.