Unity Csharp Editing

Unity C# Script Editing

A guide for efficient editing, searching, and refactoring of Unity C# scripts.

Quick Start

1. Initialize Code Index

On first use or when the index is outdated, build the index first:

// Check index status
mcp__unity-mcp-server__get_index_status()

// Build index (first time only, may take a few minutes)
mcp__unity-mcp-server__build_index()

2. Basic Reading Flow

// Get symbol list in file
mcp__unity-mcp-server__get_symbols({
  path: "Assets/Scripts/Player.cs"
})

// Get specific symbol details
mcp__unity-mcp-server__find_symbol({
  name: "PlayerController",
  kind: "class",
  exact: true
})

// Read code
mcp__unity-mcp-server__read({
  path: "Assets/Scripts/Player.cs",
  startLine: 10,
  endLine: 50
})

3. Basic Editing Flow

// Small changes (within 80 characters) → snippet
mcp__unity-mcp-server__edit_snippet({
  path: "Assets/Scripts/Player.cs",
  instructions: [{
    operation: "replace",
    anchor: { type: "text", target: "if (health < 0)" },
    newText: "if (health <= 0)"
  }]
})

// Large changes (method body) → structured
mcp__unity-mcp-server__edit_structured({
  path: "Assets/Scripts/Player.cs",
  symbolName: "PlayerController/TakeDamage",
  operation: "replace_body",
  newText: "{\n    health -= damage;\n    OnDamaged?.Invoke(damage);\n}"
})

Core Concepts

Code Index

unity-mcp-server provides a code index via built-in C# LSP. Output is 5x more compact than standard tools, optimizing LLM context.

namePath (Symbol Path)

Symbols are identified by hierarchical path within file:

// Simple class
PlayerController

// Nested member
PlayerController/TakeDamage
PlayerController/health

// Deep nesting
OuterClass/InnerClass/Method

Scope Specification

Narrow search scope for better performance:

• assets - Assets/ only
• packages - Packages/ only
• embedded - Embedded packages only
• all - All scope (default)

Tool Selection Guide

80-Character Rule

The criterion for choosing an editing tool is "whether the diff is within 80 characters"

When to use edit_snippet

Condition: Diff within 80 characters, 1-2 line changes

// ✅ Remove null guard
{ operation: "delete", anchor: { type: "text", target: "if (x == null) return;\n" }}

// ✅ Minor condition fix
{ operation: "replace", anchor: { type: "text", target: "if (x > 10)" }, newText: "if (x > 20)" }

// ✅ Insert log statement
{ operation: "insert", anchor: { type: "text", target: "Process();\n" }, newText: "Debug.Log(\"Processing\");\n" }

When to use edit_structured

Condition: Method body replacement, class member addition

// ✅ Replace method body
{
  symbolName: "Player/Move",
  operation: "replace_body",
  newText: "{\n    transform.position += direction * speed * Time.deltaTime;\n}"
}

// ✅ Add member to class
{
  symbolName: "Player",
  operation: "insert_after",
  newText: "\n    private float _jumpForce = 5f;\n"
}

Decision Flowchart

Is the change within 80 characters?
├─ YES → edit_snippet
└─ NO → Is it entire method/property replacement?
         ├─ YES → edit_structured (replace_body)
         └─ NO → Is it adding to class?
                  ├─ YES → edit_structured (insert_after)
                  └─ NO → Split into multiple snippets

Common Workflows

TDD Cycle (RED-GREEN-REFACTOR)

1. RED: Write the test

// Create test file
mcp__unity-mcp-server__create_class({
  path: "Assets/Tests/PlayerTests.cs",
  className: "PlayerTests",
  namespace: "Tests",
  usings: "NUnit.Framework,UnityEngine"
})

// Add test method
mcp__unity-mcp-server__edit_structured({
  path: "Assets/Tests/PlayerTests.cs",
  symbolName: "PlayerTests",
  operation: "insert_after",
  newText: `
    [Test]
    public void TakeDamage_ReducesHealth()
    {
        var player = new Player(100);
        player.TakeDamage(30);
        Assert.AreEqual(70, player.Health);
    }
`
})

2. GREEN: Minimal implementation

// Add implementation
mcp__unity-mcp-server__edit_structured({
  path: "Assets/Scripts/Player.cs",
  symbolName: "Player",
  operation: "insert_after",
  newText: `
    public void TakeDamage(int damage)
    {
        Health -= damage;
    }
`
})

3. REFACTOR: Improve code

// Check compilation state
mcp__unity-mcp-server__get_compilation_state({ includeMessages: true })

// Check impact before refactoring
mcp__unity-mcp-server__find_refs({
  name: "TakeDamage",
  container: "Player"
})

Create New Class

// 1. Create class file
mcp__unity-mcp-server__create_class({
  path: "Assets/Scripts/Enemies/Enemy.cs",
  className: "Enemy",
  namespace: "Game.Enemies",
  baseType: "MonoBehaviour",
  usings: "UnityEngine,System"
})

// 2. Add fields and methods
mcp__unity-mcp-server__edit_structured({
  path: "Assets/Scripts/Enemies/Enemy.cs",
  symbolName: "Enemy",
  operation: "insert_after",
  newText: `
    [SerializeField] private int _health = 100;
    [SerializeField] private float _speed = 3f;

    public int Health => _health;

    public void TakeDamage(int damage)
    {
        _health = Mathf.Max(0, _health - damage);
        if (_health == 0) Die();
    }

    private void Die()
    {
        Destroy(gameObject);
    }
`
})

// 3. Update index
mcp__unity-mcp-server__update_index({
  paths: ["Assets/Scripts/Enemies/Enemy.cs"]
})

Refactoring (Symbol Rename)

// 1. Check impact
mcp__unity-mcp-server__find_refs({
  name: "oldMethodName",
  container: "ClassName",
  scope: "all"
})

// 2. Execute rename (applies to entire project)
mcp__unity-mcp-server__rename_symbol({
  relative: "Assets/Scripts/Player.cs",
  namePath: "Player/oldMethodName",
  newName: "newMethodName",
  preview: true  // Preview first
})

// 3. Apply if no issues
mcp__unity-mcp-server__rename_symbol({
  relative: "Assets/Scripts/Player.cs",
  namePath: "Player/oldMethodName",
  newName: "newMethodName",
  preview: false
})

Code Search

// Pattern search (regex)
mcp__unity-mcp-server__search({
  pattern: "GetComponent<.*>",
  patternType: "regex",
  scope: "assets",
  returnMode: "snippets",
  snippetContext: 2
})

// Search usages of specific class
mcp__unity-mcp-server__find_refs({
  name: "PlayerController",
  kind: "class",
  scope: "all"
})

Advanced Patterns

Batch Editing (Multiple locations at once)

// Edit up to 10 locations at once
mcp__unity-mcp-server__edit_snippet({
  path: "Assets/Scripts/GameManager.cs",
  instructions: [
    { operation: "replace", anchor: { type: "text", target: "Debug.Log" }, newText: "Logger.Info" },
    { operation: "delete", anchor: { type: "text", target: "// TODO: remove\n" }},
    { operation: "insert", anchor: { type: "text", target: "void Start()\n    {" }, newText: "\n        Initialize();" }
  ]
})

Preview Mode

For large or uncertain edits, verify with preview first:

// Validate in preview mode
mcp__unity-mcp-server__edit_structured({
  path: "Assets/Scripts/Player.cs",
  symbolName: "Player/Update",
  operation: "replace_body",
  newText: "{ /* new implementation */ }",
  preview: true  // Don't write to file
})

// Apply if no LSP diagnostic errors
mcp__unity-mcp-server__edit_structured({
  // Same parameters with preview: false
})

Symbol Removal

// Confirm no references
mcp__unity-mcp-server__find_refs({
  name: "UnusedMethod",
  container: "Player"
})

// Remove if no references
mcp__unity-mcp-server__remove_symbol({
  path: "Assets/Scripts/Player.cs",
  namePath: "Player/UnusedMethod",
  failOnReferences: true,  // Error if referenced
  apply: true
})

Common Mistakes

1. Anchor Mismatch

Problem: anchor_not_found error

// ❌ Incorrect whitespace/newlines
anchor: { type: "text", target: "if(x>10)" }

// ✅ Exact match with actual code
anchor: { type: "text", target: "if (x > 10)" }

Solution: Use read to check file contents, copy exact string including whitespace/newlines

2. 80-Character Limit Exceeded

Problem: diff exceeds 80 characters error

// ❌ Editing long code with snippet
{ operation: "replace", anchor: {...}, newText: "very long code..." }

// ✅ Use structured for method body replacement
{ symbolName: "Class/Method", operation: "replace_body", newText: "..." }

3. Code Index Not Built

Problem: LSP timeout (60 seconds)

// ✅ Always check index status
mcp__unity-mcp-server__get_index_status()

// Build if coverage is low
mcp__unity-mcp-server__build_index()

4. Multiple Matches

Problem: anchor_not_unique error

// ❌ Anchor too generic
anchor: { type: "text", target: "return;" }

// ✅ Include context for uniqueness
anchor: { type: "text", target: "        return health;\n    }" }

5. Index Not Updated After Edit

Problem: Outdated symbol information returned after editing

// ✅ Always update index after editing
mcp__unity-mcp-server__update_index({
  paths: ["Assets/Scripts/Player.cs"]
})

Tool Reference

Note: For architecture patterns (Fail-Fast, UniTask, VContainer, etc.), see parent skill unity-development.