Flay review
A focused, open-source Ruby tool for locating duplicated code across files.
Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026
Flay is an open-source command-line tool for developers and teams that want to identify duplicated code locally. Distributed as a Ruby gem, it analyzes Ruby and ERB while comparing structure rather than literal values, variable names, method names, whitespace, or formatting style. Its scope fits solo developers through enterprise teams that need a focused way to measure duplication in source code, rather than a broader static-analysis suite.
The core analysis offers several ways to inspect duplication. Flay can report differences at multiple code levels, compare code across files, and display N-way diffs for matching sections. Conservative and liberal pruning options let teams adjust how aggressively matches are reduced, while fuzzy duplication detection addresses less exact similarities. Ruby and ERB support is included, plugins can extend analysis to other languages, and a FlayTask integrates the tool with Rakefiles. Projects can also skip files through .flayignore patterns, which helps keep targeted scans aligned with repository boundaries.
Flay is free and open source, with self-hosted deployment across Linux, macOS, and Windows. Its focused design is a strength for teams specifically tracking structural duplication, but it does not provide security analysis, type checking, autofix, or custom rules. There is no hosted option, so teams need to incorporate its command-line workflow into their own development environment. Choose Flay when duplication measurement and code comparison are the priority; choose a broader analysis tool when you need security checks, type diagnostics, automated fixes, or centrally hosted service features.
Flay pros and cons
- Where it wins
- Finds structural similarities while ignoring names, literals, and formatting
- Compares matches across files with multi-level and N-way diffs
- Supports Ruby, ERB, Rake, fuzzy detection, and .flayignore patterns
- Where it doesn't
- Focuses on duplication rather than security or type analysis
- Provides no autofix or custom-rule capabilities
- Uses a command-line, self-hosted workflow without a hosted option
Flay fact sheet, pricing and score →
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