Production AI NPC Systems in Unity (Part 2): Structured JSON Function Calling & Behavior Trees

{
  "system_instruction": "You are Sir Gareth, a veteran knight guard in a fantasy RPG. Respond to the player with dialogue, but ALSO decide your physical action in 3D space.",
  "json_schema_format": {
    "dialogue": "String (NPC spoken text)",
    "emotion": "Enum ['Neutral', 'Angry', 'Happy', 'Suspicious']",
    "target_action": "Enum ['Idle', 'WalkToPoint', 'AttackTarget', 'GiveItem']",
    "destination_tag": "String (e.g. 'TownGate', 'Blacksmith', 'Player')",
    "item_id": "String or null"
  }
}

using System;
using UnityEngine;
using UnityEngine.AI;

/// <summary>
/// Production C# Driver for executing LLM-generated JSON actions inside Unity.
/// Connects LLM responses to NavMeshAgent pathfinding and Animator state triggers.
/// </summary>
public class AINPCBehaviorTreeDriver : MonoBehaviour
{
    [Header("Engine Component References")]
    [SerializeField] private NavMeshAgent agent;
    [SerializeField] private Animator animator;

    [Header("Waypoint References")]
    [SerializeField] private Transform townGateTransform;
    [SerializeField] private Transform blacksmithTransform;
    [SerializeField] private Transform playerTransform;

    [System.Serializable]
    public class NPCActionResponse
    {
        public string dialogue;
        public string emotion;
        public string target_action;
        public string destination_tag;
        public string item_id;
    }

    /// <summary>
    /// Parses the raw JSON response from the LLM and executes physical game actions.
    /// </summary>
    public void ProcessAIResponsePayload(string rawJson)
    {
        try
        {
            NPCActionResponse actionPayload = JsonUtility.FromJson<NPCActionResponse>(rawJson);
            Debug.Log($"[AI Action Decoded]: {actionPayload.target_action} -> {actionPayload.destination_tag}");

            // 1. Trigger Animation & Emotion State
            ApplyNPCEmotion(actionPayload.emotion);

            // 2. Drive Physical Movement & Behavior Tree Action
            ExecutePhysicalAction(actionPayload.target_action, actionPayload.destination_tag);
        }
        catch (Exception ex)
        {
            Debug.LogError($"[AI JSON Parsing Error]: Failed to parse payload. Error: {ex.Message}");
        }
    }

    private void ApplyNPCEmotion(string emotion)
    {
        if (animator == null) return;

        switch (emotion.ToLower())
        {
            case "angry":
                animator.SetTrigger("TriggerAngry");
                break;
            case "happy":
                animator.SetTrigger("TriggerHappy");
                break;
            default:
                animator.SetTrigger("TriggerIdle");
                break;
        }
    }

    private void ExecutePhysicalAction(string action, string destinationTag)
    {
        if (agent == null) return;

        if (action == "WalkToPoint")
        {
            Transform targetDestination = ResolveDestinationTransform(destinationTag);
            if (targetDestination != null)
            {
                agent.SetDestination(targetDestination.position);
                if (animator != null) animator.SetBool("IsWalking", true);
            }
        }
        else if (action == "Idle")
        {
            agent.ResetPath();
            if (animator != null) animator.SetBool("IsWalking", false);
        }
    }

    private Transform ResolveDestinationTransform(string tag)
    {
        switch (tag.ToLower())
        {
            case "towngate": return townGateTransform;
            case "blacksmith": return blacksmithTransform;
            case "player": return playerTransform;
            default: return null;
        }
    }
}