NPCs only pass aggro if they have had their target in their actual FOV at least once

game/engines/default/engine/interface/ActorAI.lua

@@ -61,12 +61,16 @@ end
 function _M:init(t)
 	self.ai_state = self.ai_state or {}
 	self.ai_target = self.ai_target or {}
+	self.ai_actors_seen = self.ai_actors_seen or {}  -- List of actors the AI has had LOS of at least once (regardless of target)
 	self:autoLoadedAI()
 end
 
 function _M:autoLoadedAI()
 	-- Make the table with weak values, so that threat list does not prevent garbage collection
 	setmetatable(self.ai_target, {__mode='v'})
+
+	self.ai_actors_seen = self.ai_actors_seen or {}
+	setmetatable(self.ai_actors_seen, {__mode='v'})
 end
 
 function _M:aiCanPass(x, y)
@@ -131,6 +135,11 @@ function _M:doAI()
 	-- we forget it
 	self:clearAITarget()
 
+	-- Keep track of actors we've actually seen at least once in our own FOV, NPC calls doFOV right before doAI
+	for i,v in ipairs(self.fov.actors_dist) do
+		self.ai_actors_seen[v] = v
+	end
+
 	-- Update the ai_target table
 	local target_pos = self.ai_target.actor and self.fov and self.fov.actors and self.fov.actors[self.ai_target.actor]
 	if target_pos then

game/modules/tome/class/NPC.lua

@@ -200,8 +200,10 @@ function _M:seen_by(who)
 	if self.dont_pass_target then return end -- This means that ghosts can alert other NPC's but not vice versa ;)
 	local who_target = who.ai_target and who.ai_target.actor
 	if not (who_target and who_target.x) then return end
+	if not (who and who.ai_actors_seen[who_target]) then return end  -- Only pass target if we've seen them via FOV at least once, this limits chain aggro
 	if self.ai_target and self.ai_target.actor == who_target then return end
 	if not rng.percent(who:getRankTalkativeAdjust()) then return end
+
 	-- Only receive (usually) hostile targets from allies
 	if self:reactionToward(who) <= 0 or not who.ai_state._pass_friendly_target and who:reactionToward(who_target) > 0 then return end
 	
@@ -210,7 +212,7 @@ function _M:seen_by(who)
 	-- Check if it's actually a being of cold machinery and not of blood and flesh
 	if not who.aiSeeTargetPos then return end
 	if self.ai_target.actor and not who_target:attr("stealthed_prevents_targetting") then
-		-- Pass last seen coordinates
+		-- Pass last seen coordinates if we already have the same target
 		if self.ai_target.actor == who_target then
 			-- Adding some type-safety checks, but this isn't fixing the source of the errors
 			local last_seen = {turn=0}