How does Atlas prevent breaking changes in a Fastify refactor?

Atlas prevents breaking changes by first mapping all public symbols and their callers using `lsp findReferences`, then pinning existing behavior with `node:test` and `fastify.inject()` before any modifications to your Fastify codebase.

Can Atlas refactor Fastify plugins that use decorators or JSON schemas?

Yes, Atlas reads Fastify plugin encapsulation contexts, decorators, and JSON schemas attached to routes, allowing it to understand and refactor these elements effectively within your Fastify application.

What Fastify testing tools does Atlas integrate with?

Atlas integrates directly with Fastify's `node:test` runner, specifically leveraging `fastify.inject()` for robust, in-memory testing of HTTP routes and plugin behavior, ensuring your Fastify tests are always run.

How does Atlas handle code formatting for Fastify projects?

After applying changes, Atlas can run `prettier` on touched Fastify plugins and files via `npm run prettier --write`, ensuring that all refactored code adheres to your project's established formatting standards.

Is it safe to let Atlas modify my Fastify codebase?

Yes, Atlas is designed for safety. Every tool call is permission-gated, and all file edits are presented as unified diffs for your approval before being written to your Fastify project, giving you full control.

How does Atlas track refactoring progress for Fastify modules?

Atlas uses a `todowrite` list to track remaining callsites that need migration, ensuring that a partially refactored Fastify module is not mistakenly considered complete and all dependencies are updated.

What happens if a file drifts during a Fastify refactor with Atlas?

If a file drifts (changes unexpectedly) during a refactor, Atlas's `apply_patch` tool will fail with a `Failed to find context` error, preventing incorrect changes and alerting you to the discrepancy in your Fastify project.

Refactor a legacy Fastify module in 2026 with Atlas

In 2026, refactoring a legacy Fastify module without introducing regressions is streamlined with Atlas, which leverages `node:test` for behavior pinning, `npm` for dependency management, and `prettier` for consistent formatting. Atlas helps Fastify developers restructure old modules without changing their behavior or breaking their callers, providing a safe and efficient workflow.

How Atlas maps a Fastify module's public surface

To safely refactor a Fastify plugin in 2026, Atlas first maps its public surface using the `lsp` tool's `documentSymbol` operation. This initial step identifies all exported symbols, ensuring no part of the module's API is overlooked before any changes are considered. For a typical Fastify plugin, this might reveal 3-5 key exports.

Atlas begins any refactoring task by thoroughly understanding the module's public interface. Using the `lsp` tool's `documentSymbol` operation, Atlas can parse a Fastify plugin file, such as `plugins/my-legacy-plugin.js`, and enumerate all its exported functions, objects, and decorators. This is crucial for Fastify, where plugin encapsulation, decorators, and route definitions (`fastify.get`, `fastify.post`) form the public contract. Once the public surface is mapped, Atlas then uses `lsp findReferences` on each identified symbol. This operation precisely locates every callsite across your Fastify application, from `app.js` to other plugins or route handlers. This comprehensive mapping ensures that when you restructure a module, Atlas has a complete list of all dependent code, preventing silent breakage at unknown callsites. The `todowrite` tool then helps track these callsites for migration.

Pinning Fastify module behavior with `node:test`

Before any structural changes to a Fastify module, Atlas pins its existing behavior by running `node:test` with `fastify.inject()` to establish a green baseline. This critical step ensures that the module's 10 existing test cases pass, providing a robust safety net for subsequent refactoring and guaranteeing no regressions are introduced.

A core principle of safe refactoring is to 'pin' the existing behavior. For Fastify applications, Atlas achieves this by leveraging the `bash` tool to execute your `node:test` suite. Specifically, Atlas runs commands like `node --test test/legacy-plugin.test.js`, which typically utilize `fastify.inject()` to simulate HTTP requests and verify route handlers or plugin functionality. Atlas records the output of these tests, establishing a 'green baseline.' This baseline serves as the immutable reference point: any subsequent change to the module must result in the exact same test outcomes. This approach is vital for Fastify, where even minor internal changes could subtly alter JSON schema serialization or plugin registration, leading to unexpected behavior. By running tests after each small change, Atlas ensures that the module's external behavior remains consistent throughout the refactoring process.

Applying structural changes to Fastify plugins with Atlas

Restructuring a Fastify plugin is handled by Atlas using the `apply_patch` tool, which anchors changes on context lines to prevent accidental modifications to drifted files. This method ensures that even after 5 minor edits by other developers, the patch applies precisely, maintaining code integrity and preventing 'Failed to find context' errors.

Once the public surface is mapped and behavior is pinned, Atlas proceeds with structural changes using the `apply_patch` tool. This tool is designed for robust, context-aware modifications. Instead of blindly applying line-by-line changes, `apply_patch` seeks each hunk's context and `old_lines`. If the file has drifted,meaning the context lines or old lines no longer match,the patch will fail with a `Failed to find context` error. This mechanism is a critical safety feature, especially in active Fastify projects where files might be concurrently modified. Atlas computes a unified diff for every proposed file edit, such as moving a route handler from `plugins/old-module.js` to `plugins/new-module.js`, and surfaces it for your approval. This granular control, combined with re-running `node:test` after each hunk lands, ensures that the refactoring process is both precise and continuously validated against the established behavior baseline.

Ensuring Fastify code quality and safety during refactoring

Atlas ensures the safety and quality of refactored Fastify code through a multi-layered review process, including unified diffs and permission-gated tool calls. Every proposed change, from a 2-line edit to a 20-line refactor, requires explicit approval before being written to disk, providing developers full control over their Fastify codebase.

Safety and quality are paramount when refactoring a Fastify application. Atlas integrates several features to provide this assurance. Every Atlas tool call, including `lsp`, `apply_patch`, and `bash`, is permission-gated against allow, ask, and deny rules. This means Atlas will always ask for your explicit permission before executing a command that could modify your Fastify project. Furthermore, Atlas computes a unified diff for every file edit it proposes, presenting it to you for approval. This allows you to review exactly what changes will be made to your `app.js`, plugin files, or test suites before they are written. After changes are approved, Atlas can automatically run `prettier` via `npm run prettier --write` on the touched Fastify plugins, ensuring consistent formatting. Atlas also reads `git` branches, status, and diffs, and can stage and create commits on your behalf, providing a complete, auditable workflow for your Fastify refactoring efforts.

Step by step

01Use Atlas to map the public surface of your Fastify module: `atlas lsp documentSymbol plugins/my-legacy-plugin.js`
02Enumerate all callers of each exported symbol: `atlas lsp findReferences plugins/my-legacy-plugin.js:myExportedFunction`
03Pin current behavior by running Fastify's `node:test` suite: `atlas bash 'node --test test/legacy-plugin.test.js'`
04Record the green baseline and approve the initial test run output.
05Apply structural changes to the Fastify plugin using `atlas apply_patch` for each refactoring hunk.
06After each `apply_patch` hunk lands, re-run `atlas bash 'node --test test/legacy-plugin.test.js'` to verify behavior.
07Track remaining callsites for migration using `atlas todowrite 'Migrate callsite in routes/api.js'`.
08Approve the final diff for all file changes and let Atlas run `prettier` on touched Fastify plugins: `atlas bash 'npm run prettier --write plugins/my-legacy-plugin.js'`.
09Commit the refactored Fastify module changes using Atlas's `git` integration.

Frequently asked questions

How does Atlas prevent breaking changes in a Fastify refactor?: Atlas prevents breaking changes by first mapping all public symbols and their callers using `lsp findReferences`, then pinning existing behavior with `node:test` and `fastify.inject()` before any modifications to your Fastify codebase.
Can Atlas refactor Fastify plugins that use decorators or JSON schemas?: Yes, Atlas reads Fastify plugin encapsulation contexts, decorators, and JSON schemas attached to routes, allowing it to understand and refactor these elements effectively within your Fastify application.
What Fastify testing tools does Atlas integrate with?: Atlas integrates directly with Fastify's `node:test` runner, specifically leveraging `fastify.inject()` for robust, in-memory testing of HTTP routes and plugin behavior, ensuring your Fastify tests are always run.
How does Atlas handle code formatting for Fastify projects?: After applying changes, Atlas can run `prettier` on touched Fastify plugins and files via `npm run prettier --write`, ensuring that all refactored code adheres to your project's established formatting standards.
Is it safe to let Atlas modify my Fastify codebase?: Yes, Atlas is designed for safety. Every tool call is permission-gated, and all file edits are presented as unified diffs for your approval before being written to your Fastify project, giving you full control.
How does Atlas track refactoring progress for Fastify modules?: Atlas uses a `todowrite` list to track remaining callsites that need migration, ensuring that a partially refactored Fastify module is not mistakenly considered complete and all dependencies are updated.
What happens if a file drifts during a Fastify refactor with Atlas?: If a file drifts (changes unexpectedly) during a refactor, Atlas's `apply_patch` tool will fail with a `Failed to find context` error, preventing incorrect changes and alerting you to the discrepancy in your Fastify project.

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