I’m working on creating an AI system for my game and need some guidance on behavior trees. I have a few specific questions about implementation:
First question: Should each NPC type get its own behavior tree? Like having guardAI, merchantAI, companionAI, gateAI as separate trees? When I have hundreds of characters on screen, is it better to run complete tree cycles for each unit or just process one node per unit each frame?
Second question: Right now I’m handling AI in my main update loop. I’ve heard some developers use dedicated AI threads instead. What are the pros and cons of this approach?
Third question: For game progression, I’m thinking about using a simple flag system like STAGE=“Level_4” to track where players are in the story. Should this be linear progression, and how would I integrate these flags into my behavior trees?
Any tips on making this whole system more robust would be really helpful. Thanks!
threading ai is honestly a nightmare when ur starting out - just stick with the main thread for now. try batching ur npc updates by distence instead. update close npcs every frame, but far ones only every 3-4 frames. for the tree structure, I’d use one tree per type at first. it’s way easier to debug and you’ll actually understand what’s going wrong. you can optimize later when u hit real performance problems instead of optimizing stuff that doesn’t matter yet.
Been there with my indie project - learned some painful lessons. For NPC types, skip completely separate trees. Use modular subtrees instead - base movement, combat, interaction, etc. Your guardAI gets movement+combat+patrol, merchantAI gets movement+interaction+commerce. Way less duplication but still specialized. Don’t run full cycles every frame for hundreds of units - you’ll tank performance. I use time-slicing where each AI gets a node budget per frame based on player distance and importance. Critical NPCs get full processing, background ones maybe one node every few frames. Tried threading but the complexity wasn’t worth it unless you’re doing heavy pathfinding. Main thread is way easier to debug and sync with game state. Profile your AI costs first before going crazy with threading. Your flag system sounds good for linear progression. I bake similar flags into blackboard variables that behavior trees query directly in condition nodes.
Performance profiling completely changed how I build AI systems. Most developers overthink the architecture when the real bottleneck is usually something basic - pathfinding calcs or collision checks running every single frame. For NPC configs, start with separate trees but share common subtrees. The memory hit is tiny compared to how much clearer your code becomes during development. Once you profile and find actual performance problems, then think about merging trees. Don’t bother with threading unless you’ve got heavy computational work. The sync headaches aren’t worth it. Focus on smart update scheduling instead. I use a simple round-robin that spreads AI updates across multiple frames, prioritizing by game importance rather than just distance. Your flag system’s solid, but add expiration or conditions. I’ve been burned by stale flags that never cleared. Something like STAGE_COMPLETE_TIMESTAMP works way better than basic booleans for tracking progression. Biggest lesson: keep AI logic as stateless as you can. Makes debugging so much easier when NPCs start acting weird.
Shipped a few games with behavior trees and made every mistake possible early on.
Separate trees per NPC type works for prototypes, but you’ll hit a maintenance nightmare fast. Need to update basic movement or add combat mechanics? You’re editing four different trees.
I switched to a hybrid - one master tree with condition branches at the top. Tree checks NPC type first, then branches into specialized chains. Way cleaner for updates.
Performance-wise, don’t run full cycles per frame. Use priority queues instead. Combat NPCs update every frame, idle merchants every 10 frames, background crowd every 30 frames. Player proximity auto-bumps priority.
Your flag system’s good but make it hierarchical. I use CHAPTER.LEVEL.EVENT so trees can check different granularity levels. Guards might care about CHAPTER but ignore specific EVENT flags.
Biggest tip: build debugging tools first. With hundreds of NPCs, you need visual debugging or you’ll go insane tracking down which tree’s broken.
The tree per NPC type approach works great to start. I’ve built systems with hundreds of units - the tree structure never kills performance. It’s always the manual processing logic that does you in.
What actually scales: automate your entire AI coordination system. Skip writing complex frame budgeting or distance batching code yourself. Set up automation that handles NPC behavior triggers, state management, and flag coordination instead.
I’ve watched teams waste months debugging threading issues when they just needed smarter orchestration. Your STAGE flags are perfect here - automate responses to flag changes, trigger different behavior patterns based on game state, coordinate hundreds of NPCs without custom batch processing code.
Here’s the thing: game AI isn’t just behavior trees. It’s the entire system managing when behaviors fire, how they respond to world state, and how they coordinate with game progression.
Automate the coordination layer and your behavior trees become way more powerful. You get dynamic scaling, automatic state management, and way fewer debugging headaches.