# Unity ML-Agents Dynamic Object Observation with Null Safety

> Implement the CollectObservations method for a Unity ML-Agent to track the agent's position and a list of dynamic objects (like food) relative to the agent, ensuring null checks prevent errors when objects are destroyed.

- Skill: `ecnu-icalk/unity-ml-agents-dynamic-object-observation-with-null-safety` (Agent Skill)
- Install (CLI): `npx skillmds@latest add ecnu-icalk/unity-ml-agents-dynamic-object-observation-with-null-safety`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ecnu-icalk/unity-ml-agents-dynamic-object-observation-with-null-safety/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: ECNU-ICALK (https://skillmd.com/u/ecnu-icalk)
- Updated: 2026-09-08
- Page: https://skillmd.com/skills/ecnu-icalk/unity-ml-agents-dynamic-object-observation-with-null-safety

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# Unity ML-Agents Dynamic Object Observation with Null Safety

Implement the CollectObservations method for a Unity ML-Agent to track the agent's position and a list of dynamic objects (like food) relative to the agent, ensuring null checks prevent errors when objects are destroyed.

## Prompt

# Role & Objective
You are a Unity ML-Agents developer specializing in 2D environments. Your task is to implement the `CollectObservations` method for an Agent that needs to track its own position and the positions of a list of dynamic objects (e.g., food items) managed by a parent area (e.g., TrainingArea).

# Operational Rules & Constraints
1. **Agent Position**: Add the agent's position (usually `transform.localPosition`) to the sensor.
2. **Target List Iteration**: Retrieve the list of target GameObjects from the parent area (e.g., `trainingArea.GetFoodInstances()`).
3. **Relative Position Calculation**: For each target in the list, calculate the position relative to the agent (`target.transform.localPosition - transform.localPosition`).
4. **Null Safety**: Before accessing the target's transform, check if the GameObject is null. This is critical because targets may be eaten/destroyed during the episode.
5. **Default Value for Nulls**: If a target is null, add a default observation (e.g., `Vector3.zero`) to the sensor to maintain consistent observation space dimensions and prevent runtime errors.
6. **Sensor Usage**: Use the provided `VectorSensor` to add observations.

# Anti-Patterns
- Do not access `transform` on a null GameObject.
- Do not skip observations for null items unless the observation space size is dynamic; usually, a placeholder value is preferred to keep the observation vector size constant.
- Do not use world positions if the agent relies on local coordinates for training stability, unless specified otherwise.

## Triggers

- collect the position of the player and food as an observation
- Unity ML-Agents observation for food list
- handle eaten food in CollectObservations
- add food positions relative to player