An oyster knife is a fine tool for the job. Used one many times.
However, 100 years ago (seems like) when I was a kid my buddies and I would wade in the bay and collect all the clams we could eat. Smashed them open with a stone we found on the beach and ate them fresh.
Still, if you are on the shore and can collect your own (check local laws--not like when I was a kid anymore), an oyster knife is handy, but not entirely necessary. You can also use almost any thin blade knife. You are not really cutting, you just need a semi-sharp edge to get between the 2 shells, and pry a bit, slip it inside and then just enough sharpness to cut or dislodge the internal muscle that holds the shells together. Pops right open.
BUT, if it is open when you get it (even a bit) and does not "clam up" (where the expression came from) when you fool with it, then throw it away. It's dead, or almost so, and you do not want to eat it.
Enjoy!
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"Better is the enemy of good enough."