How does Atlas ensure my Elixir refactor doesn't break existing code?

Atlas ensures safety by first mapping all callsites with `lsp findReferences` and then pinning behavior by running `ExUnit via mix test` with `bash`. All code changes are applied via `apply_patch` with context validation, and tests are re-run after each modification, ensuring no regressions in your Elixir project.

Can Atlas refactor a `GenServer` or `OTP application` in Elixir?

Yes, Atlas works across `mix` projects and `OTP applications`. It can read your supervision tree, contexts, and dependencies. You can ask Atlas to restructure a `GenServer` or any other Elixir module, reviewing the proposed diffs before changes are applied.

What Elixir tools does Atlas integrate with for refactoring?

Atlas integrates deeply with the Elixir toolchain. It uses `ExUnit via mix test` for behavior verification, `Mix and Hex` for project context, and `mix format` for code style. It also leverages LSP capabilities for code analysis and `bash` for executing commands.

How does Atlas handle concurrent changes to Elixir files during a refactor?

Atlas's `apply_patch` tool is designed to handle this. It anchors changes on context lines. If a target Elixir file, like `lib/my_app/module.ex`, has drifted due to concurrent edits, `apply_patch` will fail with 'Failed to find context', preventing the application of an outdated or incorrect patch.

Does Atlas keep my Elixir code local or send it to third-party servers?

Atlas can build its code index with local Ollama embeddings, keeping your Elixir code off third-party servers. All interactions and code analysis happen within your terminal environment, ensuring your intellectual property remains secure.

How do I review changes Atlas proposes for my Elixir codebase?

Atlas provides a comprehensive review process. It drafts a plan in a read-only agent for your approval. Every proposed file edit, whether to `mix.exs` or an Elixir module, is presented as a unified diff for your explicit approval before Atlas writes any changes to disk.

Can Atlas help me manage `git` commits for Elixir refactors?

Yes, Atlas reads `git` branches, status, and diffs. It can stage and create commits on your behalf, integrating direct into your existing `git` workflow for Elixir projects. Atlas also snapshots file changes as `git` patches for easy diffing and rollback.

Refactor a legacy module in Elixir with Atlas in 2026

Refactoring a legacy Elixir module without introducing regressions is a critical task in 2026, and Atlas, the terminal-native AI coding agent, streamlines this process by leveraging the Elixir toolchain. Atlas uses `ExUnit via mix test` to pin existing behavior, `lsp` to map module surfaces and callers, and `apply_patch` for precise, context-aware code modifications, ensuring your Elixir codebase remains stable.

How Atlas maps Elixir module surfaces and finds callers

Atlas maps an Elixir module's public surface and identifies all its callers by leveraging the `lsp` tool's `documentSymbol` and `findReferences` operations. This process ensures that in 2026, no callsite is overlooked during a refactor, providing a comprehensive understanding of the module's dependencies before any code changes.

Before Atlas touches any Elixir code, it first builds a complete understanding of the module you intend to refactor. Using the `lsp` tool, Atlas executes `documentSymbol` to enumerate all exported functions and macros within the target module, such as `lib/my_app/legacy_module.ex`. For each identified public symbol, Atlas then runs `findReferences` to locate every single callsite across your entire `mix` project and any linked `OTP applications`. This meticulous mapping prevents silent breakage, a common risk in refactoring, by giving Atlas a full list of all locations that depend on the legacy module. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, ensuring accurate symbol resolution even in complex Elixir codebases.

How Atlas pins Elixir module behavior with `mix test`

Atlas pins the existing behavior of an Elixir module by executing its `ExUnit via mix test` suite with the `bash` tool, establishing a green baseline before any refactoring begins. This crucial step, performed in 2026, ensures that all subsequent code changes can be verified against the module's original, expected functionality, preventing regressions.

A core principle of safe refactoring in Elixir is to first establish a reliable behavioral baseline. Atlas achieves this by using its `bash` tool to run your project's `ExUnit via mix test` suite. For example, Atlas might execute `bash -c "mix test test/my_app/legacy_module_test.exs"` or `bash -c "mix test"` to run all tests. The output of these tests is recorded, and Atlas verifies that all tests pass, confirming a 'green' baseline. This initial test run is critical because it provides a concrete, verifiable contract for the module's behavior. After every subsequent structural change, Atlas will re-run these tests, ensuring that the refactor does not inadvertently alter the module's functionality or break existing `ExUnit` assertions.

How Atlas applies structural changes to Elixir code safely

Atlas applies structural changes to Elixir code using the `apply_patch` tool, which anchors on context lines and refuses to apply against a drifted file, ensuring atomic and safe modifications. This method, vital for 2026 Elixir development, minimizes the risk of introducing errors by verifying the patch's applicability before writing to disk.

When it's time to modify the Elixir module, Atlas employs the `apply_patch` tool for precise and safe structural changes. Unlike simple find-and-replace, `apply_patch` operates on unified diffs, which include context lines around each change. If the target file, such as `lib/my_app/legacy_module.ex`, has drifted since the patch was generated (e.g., due to concurrent edits), `apply_patch` will fail with a 'Failed to find context' error, preventing a malformed patch from being applied. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, giving you full control. Furthermore, Atlas snapshots file changes as git patches, allowing edits to be easily diffed and rolled back if necessary, providing robust safety for your `mix` project.

How Atlas tracks refactoring progress in Elixir projects

Atlas tracks the remaining callsites and refactoring progress in Elixir projects using the `todowrite` tool, ensuring that a partially migrated module is never mistaken for a finished one. This systematic approach, essential for complex refactors in 2026, provides a clear, actionable list of outstanding tasks.

Refactoring a large Elixir module often involves migrating numerous callsites. Atlas manages this complexity with the `todowrite` tool. After initially mapping all references with `lsp findReferences`, Atlas creates a `todowrite` list containing each callsite that needs to be updated. As Atlas successfully migrates a callsite and verifies the change (e.g., by re-running `ExUnit via mix test`), it marks that item off the `todowrite` list. This transparent tracking mechanism ensures that you always have a clear overview of the remaining work. It prevents the common pitfall of thinking a refactor is complete when some callers are still using the old, deprecated interface, maintaining the integrity of your `mix` project and `OTP applications`.

Atlas's safety and review mechanisms for Elixir refactors

Atlas incorporates multiple safety and review mechanisms for Elixir refactors, including permission-gated tool calls and a read-only plan agent, ensuring developer oversight in 2026. Every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, providing granular control over agent actions.

Atlas is designed with developer control and safety at its forefront for Elixir refactoring. Before any tool, such as `lsp`, `bash`, or `apply_patch`, is executed, Atlas checks against user-defined allow, ask, and deny rules. This means you explicitly grant permission for Atlas to interact with your `mix` project. Atlas drafts a plan in a read-only plan agent and asks for approval before switching to a build agent, giving you a chance to review the strategy. Every proposed file edit, including changes to `mix.exs` or `lib/my_app/module.ex`, is presented as a unified diff for your approval before it is written to disk. Atlas also reads git branches, status, and diffs, and can stage and create commits on your behalf, integrating direct into your existing Elixir development workflow and providing robust rollback capabilities via git patches.

Step by step

01Run Atlas in your Elixir project with a `mix.exs` file to allow it to index your supervision tree, contexts, and dependencies.
02Ask Atlas to map the public surface of the legacy Elixir module using `lsp documentSymbol` to identify all exported functions and macros.
03Instruct Atlas to find all references to each public symbol using `lsp findReferences` across your `mix` project, enumerating every callsite.
04Pin the module's current behavior by having Atlas run existing `ExUnit via mix test` suites with `bash -c "mix test test/path/to/legacy_module_test.exs"` and record the green baseline.
05Direct Atlas to restructure a hunk of the Elixir module using `apply_patch`, reviewing the unified diff for approval before writing.
06After each `apply_patch` operation, have Atlas re-run the `ExUnit via mix test` suite with `bash` to immediately verify that behavior remains unchanged.
07Use Atlas's `todowrite` tool to track remaining callsites that need migration, ensuring a clear list of outstanding tasks.
08Repeat the restructuring, testing, and tracking steps until all callsites are migrated and the legacy Elixir module is fully refactored.

Frequently asked questions

How does Atlas ensure my Elixir refactor doesn't break existing code?: Atlas ensures safety by first mapping all callsites with `lsp findReferences` and then pinning behavior by running `ExUnit via mix test` with `bash`. All code changes are applied via `apply_patch` with context validation, and tests are re-run after each modification, ensuring no regressions in your Elixir project.
Can Atlas refactor a `GenServer` or `OTP application` in Elixir?: Yes, Atlas works across `mix` projects and `OTP applications`. It can read your supervision tree, contexts, and dependencies. You can ask Atlas to restructure a `GenServer` or any other Elixir module, reviewing the proposed diffs before changes are applied.
What Elixir tools does Atlas integrate with for refactoring?: Atlas integrates deeply with the Elixir toolchain. It uses `ExUnit via mix test` for behavior verification, `Mix and Hex` for project context, and `mix format` for code style. It also leverages LSP capabilities for code analysis and `bash` for executing commands.
How does Atlas handle concurrent changes to Elixir files during a refactor?: Atlas's `apply_patch` tool is designed to handle this. It anchors changes on context lines. If a target Elixir file, like `lib/my_app/module.ex`, has drifted due to concurrent edits, `apply_patch` will fail with 'Failed to find context', preventing the application of an outdated or incorrect patch.
Does Atlas keep my Elixir code local or send it to third-party servers?: Atlas can build its code index with local Ollama embeddings, keeping your Elixir code off third-party servers. All interactions and code analysis happen within your terminal environment, ensuring your intellectual property remains secure.
How do I review changes Atlas proposes for my Elixir codebase?: Atlas provides a comprehensive review process. It drafts a plan in a read-only agent for your approval. Every proposed file edit, whether to `mix.exs` or an Elixir module, is presented as a unified diff for your explicit approval before Atlas writes any changes to disk.
Can Atlas help me manage `git` commits for Elixir refactors?: Yes, Atlas reads `git` branches, status, and diffs. It can stage and create commits on your behalf, integrating direct into your existing `git` workflow for Elixir projects. Atlas also snapshots file changes as `git` patches for easy diffing and rollback.

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