unity-rapid-prototyping

When to Use

• Planning or implementing any feature, mechanic, or system in a Unity game (jam or prototype)
• An agent is about to write C# or build scenes via the Unity MCP
• Deciding between building something vs. using an existing asset, package, or placeholder
• Reviewing scope when time is limited and the game must ship

Prerequisites — validate the environment BEFORE anything

unityMCP

com.coplaydev.unity-mcp

manage_packages(action="list_packages")

com.unity.ai.assistant

manage_packages(action="list_packages")

com.unity.test-framework

manage_packages(action="list_packages")

Install commands (when missing)

# unity-mcp (CoplayDev) — Package Manager → Add from git URL
manage_packages(action="add_package", package="https://github.com/CoplayDev/unity-mcp.git?path=/MCPForUnity#main")

# Unity AI Assistant (Agentes.IA) — requires Unity 6.0+, project linked to Unity Cloud
manage_packages(action="add_package", package="com.unity.ai.assistant")

# Test Framework (for EditMode/PlayMode validation)
manage_packages(action="add_package", package="com.unity.test-framework")

Idea Validator — from theme to ONE playable concept

Collect preferences first — ask, don't assume

• Preferences are a FILTER, not the whole game: genre narrows the field, theme shapes the idea. A shooter preference + theme "growth" → a shooter about growing, not just a shooter.
• If the user does not answer, use sensible defaults (top-down action, 48h, solo, comfortable) and STATE the assumption.
• When generating directions, honor hard preferences (genre, must-avoids) in EVERY pitch; use soft ones (time, team, skill) to tune scope.

Generate 3–5 directions

Validate each direction

Verdicts

• BUILD NOW — passes everything: start the mini-prototype in the first hour.
• VALIDATE FIRST — fun is uncertain: build only the core loop grey-box; fun in that hour decides if it stays.
• CUT — fails core loop, scope, or standout: record it, move to the next direction.

Rules

• The first hour of the jam is a MINI-PROTOTYPE of the chosen idea. If it is not fun after that hour, switch to the #2 direction. Do NOT polish it.
• Do not fall in love with an idea. The validator says NO until an idea survives every criterion.
• Division of labor: the AI proposes directions and enforces constraints (quantity + discipline); the human owns taste and the final pick. The AI is the strict mentor, NOT the creative brain.

Critical Patterns

1. The Decision Gate — run it BEFORE proposing any code

2. Gameplay-first mandate

• Validate the fun BEFORE the architecture. A grey-boxed mechanic in play mode beats a clean-but-unplayable system.
• Minimal architecture: no DI containers, no event buses, no "extensible systems" in a jam. One scene, plain MonoBehaviours, direct references.
• Placeholders are features: cubes, capsules, and white sprites are acceptable stand-ins for art until the loop is fun.

3. Kill-or-simplify timebox

• If a mechanic is not fun after one focused iteration, kill it or simplify it. Do NOT polish a bad idea.
• Prefer deleting code over refactoring it when the feature is not core.

Unity MCP Workflow

1. Idea validated first: ask the user's preferences (genre, time, team, skill, must-haves) → run the Idea Validator with theme + preferences → ONE direction with a core loop stated in one sentence. Do NOT open the editor before this exists.
2. Validate prerequisites first (see Prerequisites section): editor connected via MCP + required packages installed. Then check

   read_console

   for compile errors — never build on a broken compile.
3. Create or modify scripts, then

   read_console

   immediately to catch compilation errors before proceeding.
4. Validate visual/gameplay changes with play mode (

   manage_editor

   play/pause/stop) + screenshots; validate logic with

   run_tests

   (EditMode/PlayMode).
5. Build the scene with the minimum GameObjects needed: camera, light, one playable object, one interactable target.
6. Iterate: change → compile check → play → confirm fun → commit.

Code Examples

// Placeholder player controller — enough to validate the core loop
public class Player : MonoBehaviour
{
    public float speed = 5f;

    void Update()
    {
        var h = Input.GetAxis("Horizontal");
        var v = Input.GetAxis("Vertical");
        transform.position += new Vector3(h, 0, v) * speed * Time.deltaTime;
    }
}

public class Health
{
    public int Current { get; private set; }

    public Health(int start) => Current = start;

    public void TakeDamage(int amount) => Current = Mathf.Max(0, Current - amount);
}

// Assets/Tests/EditMode/HealthTests.cs
public class HealthTests
{
    [Test]
    public void Health_NeverGoesBelowZero()
    {
        var h = new Health(5);
        h.TakeDamage(99);
        Assert.AreEqual(0, h.Current);
    }
}

Commands

# Unity MCP: validate environment first
manage_packages(action="list_packages")   # required: com.coplaydev.unity-mcp, com.unity.ai.assistant, com.unity.test-framework
# Unity MCP: validate logic fast
run_tests(mode="EditMode")          # then poll get_test_job
# Unity MCP: validate feel
manage_editor(action="play")        # play mode
read_console(types=["error"])       # compile + runtime errors

Resources

• Unity MCP tools: full tool list in the session (unityMCP server).
• Decision Gate: keep the 5-question table in front of every proposal.