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Customize review instructions for GitLab Duo

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Create custom merge request review instructions to ensure that GitLab Duo applies consistent and specific code review standards to your project.

Both GitLab Duo Code Review (Classic) and Code Review Flow support custom review instructions.

For example, you can enforce Ruby style conventions only on Ruby files, and Go style conventions on Go files.

GitLab Duo appends your custom review instructions to its standard review criteria, instead of replacing them.

Configure custom review instructions

To configure custom merge request review instructions:

  1. In the root of your repository, create a .gitlab/duo directory if one doesn’t already exist.

  2. In the .gitlab/duo directory, create a file named mr-review-instructions.yaml.

  3. Optional. Ask GitLab Duo Chat (Agentic) to analyze the codebase and documentation, and generate custom review instructions.

    Example prompt:

    I need to create custom rules for GitLab Duo Code Review. When you look at the source code,
    which languages are missing and need to be added to the mr-review-instructions.yaml
    file?
  4. Add your custom instructions using the following format:

    instructions:
      - name: <instruction_group_name>
        fileFilters:
          - <glob_pattern_1>
          - <glob_pattern_2>
          - !<exclude_pattern>  # Exclude files matching this pattern
        instructions: |
          <your_custom_review_instructions>

    Use glob patterns in the fileFilters section to target specific files for the custom review rules.

    For example:

    instructions:
      - name: Ruby Style Guide
        fileFilters:
          - "*.rb"           # Ruby files in the root directory
          - "lib/**/*.rb"    # Ruby files in lib and its subdirectories
          - "!spec/**/*.rb"  # Exclude test files
        instructions: |
          1. Ensure all methods have proper documentation
          2. Follow Ruby style guide conventions
          3. Prefer symbols over strings for hash keys
    
      - name: TypeScript Source Files
        fileFilters:
          - "**/*.ts"        # Typescript files in any directory
          - "!**/*.test.ts"  # Exclude test files
          - "!**/*.spec.ts"  # Exclude spec files
        instructions: |
          1. Ensure proper TypeScript types (avoid 'any')
          2. Follow naming conventions
          3. Document complex functions
    
      - name: All Files Except Tests
        fileFilters:
          - "!**/*.test.*"   # Exclude all test files
          - "!**/*.spec.*"   # Exclude all spec files
          - "!test/**/*"     # Exclude test directories
          - "!spec/**/*"     # Exclude spec directories
        instructions: |
          1. Follow consistent code style
          2. Add meaningful comments for complex logic
          3. Ensure proper error handling
    
      - name: Test Coverage
        fileFilters:
          - "spec/**/*_spec.rb" # Ruby test files in spec directory
        instructions: |
          1. Test both happy paths and edge cases
          2. Include error scenarios
          3. Use shared examples to reduce duplication

    For glob syntax examples, see the file pattern reference.

  5. Optional: Add a Code Owners entry to protect changes to the mr-review-instructions.yaml file.

    [GitLab Duo]
    .gitlab/duo @default-owner @tech-lead
  6. Create a merge request to review and merge the changes:

    • GitLab Duo automatically applies your custom instructions when the file patterns match.

    • Multiple instruction groups can apply to a single file.

    • For review comments triggered by your custom instructions, GitLab Duo uses this format:

      According to custom instructions in '[instruction_name]': [feedback comments]

      The instruction_name value corresponds to the name property from your .gitlab/duo/mr-review-instructions.yaml file. Standard GitLab Duo comments do not use this format.

  7. Optional:

    • Review the feedback and refine your instructions as needed.
    • Test the patterns to ensure they match the intended files.

Best practices

When writing custom review instructions:

  • Be specific and actionable.
  • Number your instructions for clarity.
  • Focus on the most important standards.
  • Explain the “why” when helpful.
  • Start with straightforward instructions, and add complexity as needed.

For example:

instructions: |
  1. All public functions must include docstrings with parameter descriptions
  2. Use parameterized queries to prevent SQL injection
  3. Validate user input before processing (check type, length, format)
  4. Include error handling for all external API calls
  5. Avoid hardcoded credentials - use environment variables

For language-specific examples, see the use case examples.

File pattern reference

Use glob patterns in fileFilters to target specific files.

For example, for a project that contains Ruby files:

PatternMatch
**/*.rbAll Ruby files in any directory
*.rbRuby files in root directory only
lib/**/*.rbRuby files in the lib directory and its subdirectories
!**/*.test.rbExclude all Ruby test files
!spec/**/*.rbExclude all Ruby files in the spec directory and its subdirectories
!tests/**/*Exclude all files in the tests directory and its subdirectories
**/*.{js,jsx}JavaScript and JSX files in all directories

The following example shows the difference between **/*.rb and *.rb:

project/
├── app.rb              ← matched by both *.rb and **/*.rb
├── lib/
│   └── helper.rb       ← matched only by **/*.rb
└── app/
    └── models/
        └── user.rb     ← matched only by **/*.rb
  • *.rb would only match app.rb
  • **/*.rb would match all three files

For the mr-review-instructions.yaml file, **/*.rb ensures that review instructions apply to Ruby files anywhere in the project structure, not just the root directory.

Use case examples

instructions:
  - name: Assembly Style Guide
    fileFilters:
      - "**/*.asm"
      - "**/*.s"
      - "**/*.S"
    instructions: |
      1. Document the target architecture (x86-64, ARM, RISC-V, AVR, etc.) at the top
      2. Use meaningful labels and comment all non-obvious instructions
      3. Document register usage and calling conventions
      4. Align code sections properly for readability
      5. Include memory layout and stack usage documentation
instructions:
  - name: C Style Guide
    fileFilters:
      - "**/*.c"
      - "**/*.h"
    instructions: |
      1. goto is not allowed
      2. Avoid using global variables
      3. Use meaningful variable names
      4. Add comments for complex logic
instructions:
  - name: C++ Style Guide
    fileFilters:
      - "**/*.cpp"
      - "**/*.{h,hpp}"
    instructions: |
      1. Ensure all methods have proper documentation
      2. Use smart pointers for dynamic memory management
      3. Avoid raw pointers
instructions:
  - name: C# Style Guide
    fileFilters:
      - "**/*.cs"
    instructions: |
      1. Follow Microsoft C# coding conventions
      2. Use XML documentation comments for public APIs
      3. Prefer async/await for asynchronous operations
      4. Use nullable reference types appropriately
      5. Follow .NET naming conventions (PascalCase for public members)
instructions:
  - name: COBOL Style Guide
    fileFilters:
      - "**/*.CBL"
      - "**/*.cbl"
      - "**/*.COB"
      - "**/*.cob"
    instructions: |
      1. Use clear and meaningful names for variables and procedures
      2. Prefer COBOL-85 syntax where possible
      3. Use proper division structure (IDENTIFICATION, ENVIRONMENT, DATA, PROCEDURE)
      4. Document all paragraphs and sections with meaningful comments
      5. Use 88-level condition names for boolean flags and status codes
      6. Avoid GO TO statements, prefer PERFORM for structured programming
      7. Use proper error handling with declaratives or status code checking
      8. Define working storage variables with appropriate PICTURE clauses
      9. Use meaningful paragraph names that describe the operation
      10. For mainframe integration, document JCL dependencies and file layouts
instructions:
  - name: Go Style Guide
    fileFilters:
      - "**/*.go"
    instructions: |
      1. Use idiomatic Go practices
      2. Ensure all public functions and types have documentation
      3. Prefer standard library packages over third-party ones when possible
instructions:
  - name: Java Style Guide
    fileFilters:
      - "**/*.java"
    instructions: |
      1. Do not modernize Java 8 code to Java 11+ features, unless there is a GitLab issue or task specifically requesting modernization
      2. All public classes must have Javadoc describing purpose and usage
      3. All public methods must have Javadoc with @param and @return tags
      4. Include code examples in main class Javadoc
      5. All public methods must have at least one test case
instructions:
  - name: JavaScript/TypeScript Files
    fileFilters:
      - "src/**/*.js"
      - "src/**/*.jsx"
      - "src/**/*.ts"
      - "src/**/*.tsx"
      - "!**/*.test.js"
      - "!**/*.test.ts"
      - "!**/*.spec.js"
      - "!**/*.spec.ts"
    instructions: |
      1. Use const/let instead of var
      2. Prefer async/await over promise chains
      3. Add JSDoc comments for complex functions
      4. Ensure proper error handling in async code
      5. Avoid any 'any' types in TypeScript
instructions:
  - name: Kotlin Style Guide
    fileFilters:
      - "**/*.kt"
      - "**/*.kts"
    instructions: |
      1. Follow Kotlin coding conventions
      2. Prefer immutability (val over var)
      3. Use coroutines for asynchronous operations
      4. Leverage Kotlin's null safety features
      5. Document public APIs with KDoc
instructions:
  - name: MATLAB Style Guide
    fileFilters:
      - "**/*.m"
    instructions: |
      1. Use descriptive variable and function names with camelCase convention
      2. Vectorize operations instead of using loops where possible
      3. Document functions with H1 line and help text comments
      4. Preallocate arrays before loops to improve performance
      5. Use proper error handling with try-catch blocks and error() function
instructions:
  - name: Perl Style Guide
    fileFilters:
      - "**/*.pl"
      - "**/*.pm"
    instructions: |
      1. Follow idiomatic Perl practices
      2. Ensure proper module documentation
      3. Use strict and warnings pragmas
instructions:
  - name: PHP Style Guide
    fileFilters:
      - "**/*.php"
    instructions: |
      1. Follow PSR-12 coding standard
      2. Use type declarations for function parameters and return types
      3. Ensure compatibility with PHP 8+
      4. Use proper error handling and exceptions
      5. Document classes and methods with PHPDoc
instructions:
  - name: Python Source Files
    fileFilters:
      - "**/*.py"
      - "!tests/**/*.py"
      - "!test_*.py"
    instructions: |
      1. All functions must have docstrings with parameters and return types
      2. Use type hints for function signatures
      3. Follow PEP 8 style conventions
      4. Ensure proper exception handling
      5. Avoid using bare 'except' clauses

  - name: Python Tests
    fileFilters:
      - "tests/**/*.py"
      - "test_*.py"
    instructions: |
      1. Use pytest fixtures for common setup
      2. Test names should clearly describe the scenario being tested
      3. Include assertions for both expected outcomes and edge cases
      4. Mock external dependencies appropriately
instructions:
  - name: Ruby Style Guide
    fileFilters:
      - "*.rb"
      - "lib/**/*.rb"
      - "!spec/**/*.rb"  # Exclude test files
    instructions: |
      1. Follow Ruby style guide conventions
      2. Prefer symbols over strings for hash keys
      3. Use snake_case for methods/variables, SCREAMING_SNAKE_CASE for constants, CamelCase for classes
      4. Prefer Ruby 3.0+ features (pattern matching, endless methods) where appropriate
      5. Use proper error handling - raise exceptions over returning nil for errors
      6. Write idiomatic Ruby - use blocks, enumerables, and Ruby idioms over procedural patterns
      7. Use meaningful method names - use ? for predicates, ! for dangerous methods
      8. Prefer keyword arguments for methods with multiple parameters
      9. All public methods should have corresponding RSpec/Minitest tests
      10. Manage dependencies with Gemfile and ensure version compatibility
      11. Document thread-safe code and use proper synchronization for concurrent operations
      12. Handle signals (SIGTERM, SIGINT) properly for daemon processes
instructions:
  - name: R Style Guide
    fileFilters:
      - "**/*.r"
      - "**/*.R"
    instructions: |
      1. Follow tidyverse style guide conventions
      2. Use snake_case for variable and function names
      3. Document functions with roxygen2 comments
      4. Prefer vectorized operations over loops
      5. Use proper error handling with tryCatch and stop()
instructions:
  - name: Rust Style Guide
    fileFilters:
      - "**/*.rs"
    instructions: |
      1. Follow Rust idioms and conventions
      2. Use proper error handling with Result and Option types
      3. Avoid unsafe code unless absolutely necessary and well-documented
      4. Ensure all public items have documentation comments
instructions:
  - name: Scala Style Guide
    fileFilters:
      - "**/*.scala"
    instructions: |
      1. Follow Scala style guide conventions
      2. Prefer immutable data structures (val over var)
      3. Use pattern matching effectively for control flow
      4. Document public APIs with ScalaDoc
      5. Use proper error handling with Try, Either, or Option types
instructions:
  - name: Shell Script Style Guide
    fileFilters:
      - "**/*.sh"
      - "**/*.bash"
      - "**/*.zsh"
      - "**/*.ksh"
    instructions: |
      1. Always quote variables to prevent word splitting ("$var" not $var)
      2. Use proper error handling with set -euo pipefail at script start
      3. Document script purpose, parameters, and exit codes in header comments
      4. Prefer [[ ]] over [ ] for conditional tests
      5. Use meaningful function names and avoid complex one-liners
instructions:
  - name: SQL Style Guide
    fileFilters:
      - "**/*.sql"
    instructions: |
      1. Use uppercase for SQL keywords (SELECT, FROM, WHERE, JOIN)
      2. Always specify column names explicitly instead of using SELECT *
      3. For PostgreSQL use SERIAL/RETURNING, for MySQL use AUTO_INCREMENT, for Oracle use SEQUENCE
      4. For NoSQL (MongoDB) use proper indexing and aggregation pipelines to avoid N+1 queries
      5. Document database-specific features and expected performance characteristics
      6. Use proper indentation for complex queries and subqueries
instructions:
  - name: VHDL Style Guide
    fileFilters:
      - "**/*.vhd"
      - "**/*.vhdl"
    instructions: |
      1. Follow IEEE VHDL coding standards
      2. Use meaningful signal and entity names with clear prefixes
      3. Document all entities, architectures, and processes with comments
      4. Use synchronous design practices with proper clock and reset handling
      5. Avoid combinational loops and ensure proper timing constraints
instructions:
  - name: Configuration Files
    fileFilters:
      - "*.yaml"
      - "*.yml"
      - "*.json"
      - "config/**/*"
      - "!.gitlab/**/*"
    instructions: |
      1. Do not include sensitive data (passwords, API keys)
      2. Use environment variables for environment-specific values
      3. Document all configuration options
      4. Validate configuration schema if possible
instructions:
  - name: Ansible Style Guide
    fileFilters:
      - "*.yaml"
      - "*.yml"
      - "playbooks/**/*.yaml"
      - "roles/**/*.yaml"
    instructions: |
      1. Use meaningful play and task names that describe the action
      2. Prefer modules over shell/command tasks when possible
      3. Use variables and defaults for reusability across environments
      4. Implement idempotency - tasks should be safe to run multiple times
      5. Use handlers for service restarts and notifications
      6. Document playbook purpose, required variables, and dependencies

  - name: Dockerfile Style Guide
    fileFilters:
      - "Dockerfile"
      - "*.dockerfile"
      - "Dockerfile.*"
    instructions: |
      1. Use specific base image tags, avoid 'latest'
      2. Minimize layers by combining RUN commands with && where logical
      3. Use multi-stage builds to reduce final image size
      4. Run containers as non-root user for security
      5. Use .dockerignore to exclude unnecessary files
      6. Document exposed ports, volumes, and environment variables

  - name: GitLab CI/CD Style Guide
    fileFilters:
      - ".gitlab-ci.yml"
      - "**/.gitlab-ci.yml"
    instructions: |
      1. Use job extends instead of YAML anchors for reusability
      2. Always use rules instead of only/except for job conditions
      3. Define appropriate caching strategies for dependencies
      4. Use stages to organize pipeline workflow logically
      5. Include security scanning templates (SAST, dependency scanning, secret detection)
      6. Document job purpose, required variables, and dependencies in comments

  - name: Helm Chart Style Guide
    fileFilters:
      - "Chart.yaml"
      - "values.yaml"
      - "templates/**/*.yaml"
    instructions: |
      1. Use semantic versioning for chart versions
      2. Provide sensible defaults in values.yaml with comments
      3. Use template functions for conditional logic and loops
      4. Include NOTES.txt with post-installation instructions
      5. Validate charts with helm lint before committing
      6. Document all configurable values and their purpose

  - name: Kubernetes Style Guide
    fileFilters:
      - "*.yaml"
      - "*.yml"
      - "k8s/**/*.yaml"
      - "kubernetes/**/*.yaml"
    instructions: |
      1. Use explicit API versions and avoid deprecated APIs
      2. Always define resource limits and requests for containers
      3. Use namespaces to organize resources logically
      4. Define liveness and readiness probes for all deployments
      5. Use ConfigMaps and Secrets instead of hardcoded values
      6. Document resource purpose and dependencies in metadata annotations

  - name: Terraform/OpenTofu Style Guide
    fileFilters:
      - "*.tf"
      - "*.tfvars"
    instructions: |
      1. Use consistent naming conventions for resources (environment_service_resource)
      2. Organize code into modules for reusability
      3. Use variables with descriptions and validation rules
      4. Define outputs for important resource attributes
      5. Use remote state with locking for team collaboration
      6. Document module purpose, inputs, outputs, and provider requirements

Example projects

For more custom review instructions use cases, see the following production examples: