simplecov-mcp

MCP server + CLI + Ruby library for inspecting SimpleCov coverage data

Gem Version

What is simplecov-mcp?

simplecov-mcp makes SimpleCov coverage data queryable and actionable through three interfaces:

  • MCP server - Lets AI assistants analyze your coverage
  • CLI - Fast command-line coverage reports and queries
  • Ruby library - Programmatic API for custom tooling

Works with any SimpleCov-generated .resultset.json file—no runtime dependency on your test suite.

Key Features

  • ✅ Multiple interfaces - MCP server, CLI, and Ruby API
  • Annotated source code - --source full|uncovered with --context-lines N for context lines
  • ✅ Staleness detection - Identify outdated coverage (missing files, timestamp mismatches, line count changes)
  • ✅ Multi-suite support - Automatic merging of multiple test suites (RSpec + Cucumber, etc.)
  • ✅ Flexible path resolution - Works with absolute or relative paths
  • ✅ Comprehensive error handling - Context-aware messages for each mode
  • ⚠️ Branch coverage limitation - Branch-level metrics are collapsed to per-line totals. Use native SimpleCov reports for branch-by-branch analysis.

Practical Use Cases

  • Query coverage data from AI assistants, e.g.:
    • "Using simplecov-mcp, analyze test coverage data and write a report to a markdown file containing a free text analysis of each issue and then two tables, one sorted in descending order of urgency, the other in ascending order of level of effort."
    • "Using simplecov-mcp, generate a table of directories and their average coverage rates, in ascending order of coverage."
  • Find files with the lowest coverage
  • Investigate specific files or directories
  • Generate CI/CD coverage reports
  • Create custom pass/fail predicates for scripts and CI - use the library API or CLI JSON output to implement arbitrarily complex coverage rules beyond simple thresholds (e.g., require higher coverage for critical paths, exempt test utilities, track coverage trends)

Quick Start

Installation

gem install simplecov-mcp

Generate Coverage Data

# Run your tests with SimpleCov enabled
bundle exec rspec  # or your test command

# Verify coverage was generated
ls -l coverage/.resultset.json

Basic Usage

CLI - View Coverage Table:

simplecov-mcp

CLI - Check Specific File:

simplecov-mcp summary lib/simplecov_mcp/model.rb
simplecov-mcp uncovered lib/simplecov_mcp/cli.rb

CLI - Find the Project Homepage Fast: Run simplecov-mcp -h and the banner's second line shows the repository URL. Some terminal applications (e.g. iTerm2) will enable direct clicking the link using modifier keys such as Cmd or Alt.

Usage:      simplecov-mcp [options] [subcommand] [args]
Repository: https://github.com/keithrbennett/simplecov-mcp  # <--- Project URL ---

Ruby Library:

require "simplecov_mcp"

model = SimpleCovMcp::CoverageModel.new
files = model.all_files
# => [{ "file" => "lib/simplecov_mcp/model.rb", "covered" => 114, "total" => 118, "percentage" => 96.61, "stale" => false }, ...]

summary = model.summary_for("lib/simplecov_mcp/model.rb")
# => { "file" => "lib/simplecov_mcp/model.rb", "summary" => { "covered" => 114, "total" => 118, "percentage" => 96.61 }, "stale" => false }

MCP Server: See MCP Integration Guide for AI assistant setup.

Multi-Suite Coverage Merging

How It Works

When a .resultset.json file contains multiple test suites (e.g., RSpec + Cucumber), simplecov-mcp automatically merges them using SimpleCov's combine logic. All covered files from every suite become available to the CLI, library, and MCP tools.

Performance: Single-suite projects avoid loading SimpleCov at runtime. Multi-suite resultsets trigger a lazy SimpleCov load only when needed, keeping the tool fast for the simpler coverage configurations.

Current Limitations

Staleness checks: When suites are merged, we keep a single "latest suite" timestamp. This matches prior behavior but may under-report stale files if only some suites were re-run after a change. A per-file timestamp refinement is planned. Until then, consider multi-suite staleness checks advisory rather than definitive.

Multiple resultset files: Only suites stored inside a single .resultset.json are merged automatically. If your project produces separate resultset files (e.g., different CI jobs writing coverage/job1/.resultset.json, coverage/job2/.resultset.json), you must merge them yourself before pointing simplecov-mcp at the combined file.

Documentation

Getting Started:

Advanced Usage:

Reference:

Requirements

  • Ruby >= 3.2 (required by mcp gem dependency)
  • SimpleCov-generated .resultset.json file
  • simplecov gem >= 0.21

Configuring the Resultset

simplecov-mcp automatically searches for .resultset.json in standard locations (coverage/.resultset.json, .resultset.json, tmp/.resultset.json). For non-standard locations:

# Command-line option (highest priority)
simplecov-mcp --resultset /path/to/your/coverage

# Environment variable (project-wide default)
export SIMPLECOV_MCP_OPTS="--resultset /path/to/your/coverage"

# MCP server configuration
# Add to your MCP client config:
# "args": ["--resultset", "/path/to/your/coverage"]

See CLI Usage Guide for complete details.

Common Workflows

Find Coverage Gaps

# Files with worst coverage
simplecov-mcp -o d list           # -o = --sort-order, d = descending (worst at end)
simplecov-mcp list | less         # display table in pager, worst files first
simplecov-mcp list | head -10     # truncate the table

# Specific directory
simplecov-mcp -g "lib/simplecov_mcp/tools/**/*.rb" list  # -g = --tracked-globs

# Export for analysis
simplecov-mcp -fJ list > coverage-report.json

Working with JSON Output

The -fJ flag enables programmatic processing of coverage data using command-line JSON tools.

Using jq:

# Filter files below 80% coverage
simplecov-mcp -fJ list | jq '.files[] | select(.percentage < 80)'

Using Ruby one-liners:

# Count files below threshold
simplecov-mcp -fJ list | ruby -r json -e '
  puts JSON.parse($stdin.read)["files"].count { |f| f["percentage"] < 80 }
'

Using rexe:

rexe is a Ruby gem that enables shorter Ruby command lines by providing command-line options for input and output formats, plus other conveniences. It eliminates the need for explicit JSON parsing and formatting code.

Install: gem install rexe

# Filter files below 80% coverage with pretty-printed JSON output
simplecov-mcp -fJ list | rexe -ij -mb -oJ 'self["files"].select { |f| f["percentage"] < 80 }'

# Count files below threshold
simplecov-mcp -fJ list | rexe -ij -mb -op 'self["files"].count { |f| f["percentage"] < 80 }'

# Human-readable output with AwesomePrint
simplecov-mcp -fJ list | rexe -ij -mb -oa 'self["files"].first(3)'

With rexe's -ij -mb options, self automatically becomes the parsed JSON object. The same holds true for JSON output -- using -oJ produces pretty-printed JSON without explicit formatting calls. Rexe also supports YAML input/output (-iy, -oy) and AwesomePrint output (-oa) for human consumption.

Run rexe -h to see all available options, or visit the rexe project page for more examples.

For comprehensive JSON processing examples, see docs/user/EXAMPLES.md.

CI/CD Integration

# Fail build if coverage is stale
simplecov-mcp --staleness error || exit 1

# Generate coverage report artifact
simplecov-mcp -fJ list > artifacts/coverage.json

Investigate Specific Files

# Quick summary
simplecov-mcp summary lib/simplecov_mcp/model.rb

# See uncovered lines
simplecov-mcp uncovered lib/simplecov_mcp/cli.rb

# View in context
simplecov-mcp -s u -c 3 uncovered lib/simplecov_mcp/cli.rb  # -s = --source (u = uncovered), -c = --context-lines

# Detailed hit counts
simplecov-mcp detailed lib/simplecov_mcp/util.rb

# Project totals
simplecov-mcp totals
simplecov-mcp -fJ totals

Commands and Tools

CLI Subcommands: list, summary, uncovered, detailed, raw, totals, validate, version

MCP Tools: coverage_summary_tool, coverage_detailed_tool, coverage_raw_tool, uncovered_lines_tool, all_files_coverage_tool, coverage_totals_tool, coverage_table_tool, validate_tool, help_tool, version_tool

📖 See also:

Troubleshooting

  • "command not found" - See Installation Guide
  • "cannot load such file -- mcp" - Requires Ruby >= 3.2. Verify: ruby -v
  • "Could not find .resultset.json" - Ensure SimpleCov is configured in your test suite, then run tests to generate coverage. See the Configuring the Resultset section for more details.
  • MCP server won't connect - Check PATH and Ruby version in MCP Troubleshooting
  • RVM in sandboxed environments (macOS) - RVM requires /bin/ps which may be blocked by sandbox restrictions. Use rbenv or chruby instead.

For more detailed help, see the full Troubleshooting Guide.

Development

# Clone and setup
git clone https://github.com/keithrbennett/simplecov-mcp.git
cd simplecov-mcp
bundle install

# Run tests
bundle exec rspec

# Test locally
bundle exec exe/simplecov-mcp

# Build and install
gem build simplecov-mcp.gemspec
gem install simplecov-mcp-*.gem

See docs/dev/DEVELOPMENT.md for more.

SimpleCov Dependency

simplecov-mcp declares a runtime dependency on simplecov (>= 0.21) to support multi-suite merging using SimpleCov's combine helpers. The dependency is lazy-loaded only when needed, ensuring fast startup for single-suite projects.

Contributing

Contributions are welcome! Please:

  1. Fork the repository
  2. Create a feature branch
  3. Add tests for new functionality
  4. Ensure all tests pass (bundle exec rspec)
  5. Submit a pull request

License

MIT License - see LICENSE file for details.

Next Steps

📦 Install: gem install simplecov-mcp

📖 Read: CLI Usage Guide | MCP Integration

🐛 Report issues: GitHub Issues

⭐ Star the repo if you find it useful!