# Refactor a legacy module in Unity with Atlas in 2026

> Atlas helps Unity developers restructure legacy C# modules by mapping public surfaces, pinning behavior with Unity Test Framework (NUnit), and applying verified changes.

To restructure an old Unity C# module without changing its behavior or breaking its callers, Atlas provides a robust workflow that integrates directly with your existing Unity toolchain, including the Unity Test Framework (NUnit) and Unity Package Manager (UPM). Atlas maps the module's public surface, pins behavior with existing tests, applies structural changes with context-aware patches, and verifies each step, ensuring your game's framerate and functionality remain intact.

## Key takeaways

- Atlas uses `lsp` for precise symbol mapping and `findReferences` in Unity C# projects, identifying all callers.
- `Unity Test Framework (NUnit)` integration allows Atlas to pin and verify module behavior throughout refactoring.
- `apply_patch` provides safe, context-aware modifications to Unity scripts, failing if files have drifted.
- Atlas ensures iterative verification by re-running `Unity Test Framework (NUnit)` tests after each change.
- Atlas integrates with `git` for transparent change tracking and rollback, and explicitly surfaces `.meta` file changes for review.
- Permission-gated tool calls and read-only planning agents provide robust safety for Unity codebase modifications.

## How Atlas maps Unity module dependencies and callers

Atlas precisely maps a Unity module's public surface and its 0-N callers by leveraging the `lsp` tool's `documentSymbol` and `findReferences` operations. This initial step is crucial in 2026 for understanding the full impact of any refactoring, especially in complex Unity projects with many interconnected MonoBehaviour scripts and ScriptableObjects.

Before any changes are made, Atlas uses its `lsp` tool to interact with the language server protocol, providing a comprehensive view of your Unity C# codebase. First, `lsp documentSymbol` identifies all exported symbols within the legacy module. Then, for each identified symbol, `lsp findReferences` enumerates every single callsite across your project, including those within `Assets/` and packages resolved by the Unity Package Manager (UPM). This detailed mapping ensures that Atlas has a complete understanding of the module's external dependencies, preventing silent breakage at unforeseen callsites. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, ensuring accuracy even in large Unity projects.

## Pinning Unity module behavior with Unity Test Framework (NUnit)

Pinning the existing behavior of a Unity module is a critical first step, ensuring that any refactoring does not introduce regressions. Atlas achieves this by running your `Unity Test Framework (NUnit)` tests with the `bash` tool, recording a green baseline before any code modifications begin. This process establishes a clear, verifiable standard for the module's functionality in 2026.

Atlas integrates directly with the `Unity Test Framework (NUnit)` to establish a behavioral baseline. Using the `bash` tool, Atlas executes your existing EditMode and PlayMode tests located under the `Tests/` folder. For example, Atlas might run a command like `bash -c "/Applications/Unity/Hub/Editor/2023.3.0f1/Unity.app/Contents/MacOS/Unity -runTests -testPlatform EditMode -projectPath . -logFile -"` to get the initial test results. This baseline is essential; any subsequent refactoring steps must maintain this 'green' state. Atlas's ability to run these tests repeatedly and efficiently is fundamental to its safe refactoring workflow, especially when dealing with MonoBehaviour lifecycle order and per-frame allocations that are critical in Unity gameplay code.

## Applying structural changes safely to Unity C# scripts

Atlas applies structural changes to Unity C# scripts using the `apply_patch` tool, which anchors on context lines and refuses to apply against a drifted file, ensuring safety. This method is vital for refactoring in 2026, as it prevents unintended modifications and maintains the integrity of your codebase, especially when dealing with Unity's specific file structures and `.meta` files.

The `apply_patch` tool is central to Atlas's refactoring process. When Atlas proposes a change, it generates a unified diff. `apply_patch` then attempts to apply this diff, but it's designed with a crucial safety mechanism: it seeks each hunk's context and old_lines. If the target file, such as a C# script in `Assets/Scripts/LegacyModule/OldComponent.cs`, has drifted from the expected state, `apply_patch` will fail with 'Failed to find context'. This prevents applying patches to an outdated or modified file, significantly reducing the risk of introducing errors. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, giving you full control. Importantly, Atlas never touches `.meta` files or scene YAML without showing you first, allowing you to review these critical Unity-specific files in your editor before committing.

## Iterative testing and tracking for Unity refactors

Atlas promotes an iterative approach to refactoring Unity modules, re-running `Unity Test Framework (NUnit)` tests with `bash` after each hunk lands, not just once at the end. This ensures immediate feedback on behavior preservation. Additionally, Atlas uses the `todowrite` tool to track remaining callsites, preventing a partially migrated module from being mistaken for a finished one in 2026.

After each successful application of a patch hunk to a Unity C# script, Atlas immediately re-runs the `Unity Test Framework (NUnit)` tests using the `bash` tool. This rapid feedback loop is crucial for catching regressions early, especially in Unity where subtle changes can impact performance or gameplay. If tests remain green, Atlas proceeds. If they fail, the change can be rolled back using Atlas's git integration, which snapshots file changes as git patches. Furthermore, Atlas uses the `todowrite` tool to maintain a list of remaining callsites that still need migration. This ensures that the refactoring process is transparent and that the project state is always clear, preventing the deployment of an incomplete module. Atlas reads git branches, status, and diffs, and can stage and create commits on your behalf, streamlining the entire workflow.

## Ensuring safety and review in Unity refactoring with Atlas

Atlas prioritizes safety and developer review throughout the refactoring process in Unity, offering multiple layers of protection. Every Atlas tool call is permission-gated against allow, ask, and deny rules, and Atlas drafts a plan in a read-only agent before execution. This ensures that in 2026, no changes are made to your Unity project without explicit approval and understanding.

Atlas provides robust safety mechanisms tailored for Unity development. Before any tool, such as `lsp` or `apply_patch`, is executed, Atlas checks against permission-gated rules (allow, ask, deny). This means you control what Atlas can do. Atlas also drafts a comprehensive plan in a read-only plan agent and asks for your approval before switching to a build agent to execute changes. For every file edit, Atlas computes a unified diff and surfaces it for your approval, allowing you to review changes to C# scripts, `Packages/manifest.json`, or other project files. This is particularly important for Unity projects, where `.meta` files and scene YAML are sensitive; Atlas explicitly shows you these changes for review in your editor, ensuring you maintain full control over your project's integrity and asset serialization.

## Steps

1. Run Atlas in your Unity project, ensuring it can read your `Assets/` folder and `Packages/manifest.json`.
2. Ask Atlas to map the legacy module's public surface using `atlas lsp documentSymbol` and then enumerate all callers with `atlas lsp findReferences` for each exported symbol.
3. Pin the module's current behavior by running existing `Unity Test Framework (NUnit)` tests with `atlas bash -c "/Applications/Unity/Hub/Editor/2023.3.0f1/Unity.app/Contents/MacOS/Unity -runTests -testPlatform EditMode -projectPath . -logFile -"` and record the green baseline.
4. Instruct Atlas to restructure the module using `atlas apply_patch`, reviewing each proposed change and its unified diff before approval. Pay close attention to any `.meta` file changes Atlas surfaces for review.
5. After each successful patch, re-run the `Unity Test Framework (NUnit)` tests with `atlas bash` to verify behavior preservation immediately.
6. Use `atlas todowrite` to track any remaining callsites that need migration, ensuring a clear path to completion.
7. Review the final diffs in your editor, especially for any Unity-specific files, and then use Atlas to stage and create a commit for the refactored module.

## FAQ

### How does Atlas handle Unity's `.meta` files during refactoring?

Atlas never touches `.meta` files or scene YAML without explicitly showing you the proposed changes first. It computes a unified diff for these files and surfaces it for your approval, allowing you to review them in your editor before any write operation, ensuring asset integrity.

### Can Atlas refactor MonoBehaviour scripts and their lifecycle methods?

Yes, Atlas is designed to work with Unity C# gameplay code, including MonoBehaviour scripts. It can help hoist `GetComponent` calls out of `Update` and cache them in `Awake`, for example, to optimize per-frame allocations and improve performance.

### What Unity testing tools does Atlas integrate with?

Atlas integrates directly with the `Unity Test Framework (NUnit)`. It uses the `bash` tool to run your existing EditMode and PlayMode tests, establishing a behavioral baseline and verifying changes throughout the refactoring process.

### How does Atlas ensure no breaking changes to callers in a Unity project?

Atlas ensures no breaking changes by first mapping all callers using `lsp findReferences` before any modifications. It then applies changes with `apply_patch`, which fails if context is lost, and re-runs `Unity Test Framework (NUnit)` tests after each step to immediately detect regressions.

### Does Atlas support Unity Package Manager (UPM) dependencies?

Yes, Atlas is designed to operate within a Unity project that uses the Unity Package Manager (UPM). It can read your `Packages/manifest.json` and understand code within packages resolved by UPM, ensuring comprehensive refactoring across your entire project.

### How does Atlas prevent accidental changes to Unity scene files?

Atlas's safety mechanisms, including permission-gated tool calls and explicit diff review, extend to Unity scene files. Atlas will never modify scene YAML without first showing you the proposed changes for approval, giving you full control over these critical assets.

### Can Atlas help optimize Unity C# code, like `GetComponent` calls?

Yes, Atlas can assist with common Unity C# optimizations. For instance, you can ask Atlas to hoist a `GetComponent` call out of `Update` and cache it in `Awake`, preventing unnecessary per-frame allocations and improving game performance.

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Canonical HTML: https://runatlas.sh/resources/stacks/refactor-a-legacy-module-in-unity
Source of truth: aeo_pages row `/resources/stacks/refactor-a-legacy-module-in-unity` (segment: Stacks) (this file is generated from it, never hand-edited).
Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.