# Locate Where a Behavior Is Implemented in Ruby with Atlas (2026)

> Atlas locates behavior in a Ruby codebase with codebase_search for meaning, grep for exact text, and the lsp tool for the symbol graph.

To locate where a behavior is implemented in Ruby, describe the behavior to Atlas and let codebase_search query the semantic index, then confirm the hit with grep and open it with read. Atlas is the terminal-native AI coding agent, and it runs directly in a project with a Gemfile, so it can follow a Ruby mixin from lib into app/services, list every callsite with the lsp tool, and hand you a file and line reference. Bundler resolves the gems, RSpec proves the behavior, and RuboCop keeps the eventual edit inside your .rubocop.yml style.

## Key takeaways

- Atlas attacks Ruby retrieval from three angles at once: codebase_search for meaning, grep for exact text, and the lsp tool for the symbol graph.
- Atlas indexes code by AST declarations using tree-sitter, so a hit is a Ruby class, module, or def, not a blind line window.
- grep runs through ripgrep with real regex plus include and path filters, which pins a Ruby definition after codebase_search suggests it.
- The lsp tool's findReferences operation finds callsites through Ruby mixins and aliases that plain text search would miss.
- Bundler, RSpec, and RuboCop stay in charge: run `bundle exec rspec` to verify and `bundle exec rubocop` against .rubocop.yml before review.
- Every Atlas tool call is permission-gated against allow, ask, and deny rules, and local Ollama embeddings keep a private Ruby repo off third-party servers.

## How do I find where a behavior is implemented in a Ruby codebase?

Atlas locates behavior in a Ruby codebase with 3 tools working together: codebase_search for meaning, grep for exact text, and the lsp tool for the symbol graph. Describe the behavior in plain language and the semantic index returns candidate declarations from app/services and lib, even when your words never appear in the source.

The three tools are complementary, which is why Atlas ships all three rather than one fuzzy search box. Ruby makes the case sharply. A behavior like "subscriptions stop billing after cancellation" might live in an ActiveSupport concern in app/models/concerns, in a plain service object under app/services, or in a Rakefile task, and none of those files is guaranteed to contain the word "cancel". Start Atlas in the directory that holds your Gemfile so the whole project is in scope, including gems that Bundler has vendored. Ask codebase_search the question the way you would ask a teammate. Atlas returns ranked candidate declarations with file paths, and from there grep and read narrow the answer down to a single method.

## What does codebase_search find in Ruby that grep alone misses?

Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, and indexes code by AST declarations using tree-sitter rather than blind line windows. In a Ruby project in 2026 that means a query about expiring a subscription surfaces a service class in app/services even though the word expire appears nowhere in its body.

Ruby is a language of indirection. Behavior arrives through modules included at runtime, through method_missing, through metaprogrammed accessors, and through gem code that Bundler pulled in from your Gemfile.lock. A pure text search only finds the literal string you typed. Because Atlas indexes by AST declarations using tree-sitter, the unit of retrieval is the declaration itself: a class, a module, a def. So a ranked hit points at `def self.expire_overdue` in app/services rather than at a random 40-line window that happens to straddle two unrelated methods. That declaration boundary is what makes the follow-up read useful instead of noisy.

## How do I confirm the Ruby method with grep and read?

Confirming a Ruby candidate takes 2 Atlas tools. grep takes a real regex plus include and path filters and runs through ripgrep, so a pattern like `def self.expire` scoped to app/**/*.rb is a single call. read then opens the file, and a wrong guess fails loudly with File not found plus a Did you mean list.

The loud failure matters in Ruby, where autoloading conventions mean a guessed path such as app/models/subscription_expirer.rb is frequently almost right and occasionally exactly wrong. Atlas does not silently return empty content for a path that does not exist. It reports File not found and offers a Did you mean list, so a bad path is a signal rather than a dead end. Use grep to pin the definition and every literal mention of the constant, then use read on the two or three files that survive. At that point you are looking at real Ruby, not at a summary of it, and the call path back to config/routes.rb or a Rakefile task is usually two hops away.

## How do I find every callsite of a Ruby method with Atlas?

Atlas uses the lsp tool's findReferences operation to list every callsite of a Ruby method, and workspaceSymbol to jump to a declaration by name. For Ruby that matters more than in most languages, because a single module mixed into 6 classes produces one definition in lib and many callers scattered across app and spec.

Once codebase_search and grep agree on the definition, the remaining question is who reaches it. findReferences answers that from the symbol graph rather than from string matching, so a call through an aliased method or an included module is still a reference. workspaceSymbol goes the other direction: you know the constant name, such as a class in lib, and you want its declaration without guessing the file. Atlas then summarizes the call path back to you with concrete file and line references, which is the artifact you actually paste into a pull request description or a Slack thread. RSpec files under spec/ show up in the reference list too, which tells you immediately whether the behavior is already covered.

## Is Atlas safe to run on a private Ruby repository?

Atlas is safe on a private Ruby repository because every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs. Atlas can also build its code index with local Ollama embeddings, so in 2026 a proprietary Gemfile and the app tree under it never leave the machine.

Locating a behavior is a read-only job, and Atlas treats it that way by default. codebase_search, grep, read, and the lsp tool all inspect. None of them writes. If the investigation turns into a change, Atlas computes a unified diff for every file edit and surfaces it for approval before writing, and Atlas snapshots file changes as git patches so edits can be diffed and rolled back. Atlas also reads git branches, status, and diffs, so the agent knows whether the file it just found is already dirty in your working tree before it proposes touching it.

## What do I run after Atlas finds the Ruby code?

After Atlas locates the Ruby method, verify the behavior with RSpec by running `bundle exec rspec spec/services/subscription_expirer_spec.rb`, then run `bundle exec rubocop` so any change matches your .rubocop.yml. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so nothing lands unreviewed in 2026.

The point of locating a behavior is usually to change or test it. Bundler is the package manager, so every command goes through `bundle exec` to pin the gem versions in Gemfile.lock. RSpec is the test runner, and a focused example file is faster feedback than the whole spec/ directory. RuboCop is the formatter, and running it before you open the pull request keeps the diff about behavior instead of about whitespace. Atlas can also write the RSpec example or extract a module for you, and because the unified diff appears in the approval prompt, you read the change before it exists on disk.

## Steps

1. Run atlas in a project with a Gemfile so the Ruby app tree, lib, and spec directories are all in scope.
2. Describe the behavior to codebase_search in plain language; the semantic index returns candidate declarations from app/services, app/models/concerns, and lib even when your words do not appear in the source.
3. Confirm the hit with grep, which takes a real regex plus include and path filters and runs through ripgrep, scoping a pattern like `def self.expire` to app/**/*.rb.
4. Open the best candidate with read; a wrong path fails loudly with File not found plus a Did you mean list, so a mistaken Ruby autoload path does not go unnoticed.
5. Use the lsp tool's findReferences operation to list every callsite across app and spec, and workspaceSymbol to jump straight to a declaration by constant name.
6. Have Atlas summarize the call path back to you with concrete file and line references, from config/routes.rb or a Rakefile task down to the method.
7. Prove the behavior with RSpec: `bundle exec rspec spec/services/subscription_expirer_spec.rb`, using Bundler so the gems in Gemfile.lock are the ones under test.
8. If you change the code, review the unified diff Atlas surfaces before it writes, then run `bundle exec rubocop` to satisfy .rubocop.yml.

## FAQ

### how to find where a method is defined in a large rails app

Ask Atlas codebase_search a plain-language question about the behavior, then use the lsp tool's workspaceSymbol operation to jump to the declaration by constant name. grep with an include filter on app/**/*.rb confirms the exact `def` line.

### ruby find all callers of a method

Use the lsp tool's findReferences operation in Atlas. It reads the symbol graph, so callsites that reach the method through an included module or an alias still appear, including RSpec examples under spec/.

### can atlas search a ruby codebase without sending code to a cloud provider

Yes. Atlas can build its code index with local Ollama embeddings, keeping code off third-party servers, so the Ruby files under your Gemfile stay on your machine.

### why does codebase_search find ruby code that grep cannot

Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, and indexes by AST declarations using tree-sitter. A Ruby service class matches the concept you described even when the literal word never appears in the file.

### atlas says file not found when reading a ruby file

Atlas fails loudly on purpose. A wrong path returns File not found plus a Did you mean list rather than empty content, which catches guessed Rails autoload paths like app/models/subscription_expirer.rb before you act on them.

### does atlas run rspec and rubocop for me

Atlas can write RSpec examples and run `bundle exec rspec`, and it can apply RuboCop fixes against your .rubocop.yml. Atlas computes a unified diff for every file edit and surfaces it for approval before writing.

### how do i use an ai coding agent on a bundler project

Run atlas in the directory containing your Gemfile. Atlas reads your modules, gems, and Rakefile tasks, and Bundler still resolves dependencies, so `bundle exec` commands behave exactly as they do without an agent.

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