unclebob/mutate4go

Latest recorded source activity: May 22, 2026 at 15:42

SkillsMP catalog refreshed: November 2, 2026 at 18:02

skills collected: 1

GitHub stars: 23
GitHub forks: 3

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# mutate4go Mutation testing for Go. Discovers mutation sites, applies each one, runs your tests, and reports killed, survived, and uncovered mutations. ## Setup Run from the root of a Go module: ```bash go run github.com/unclebob/mutate4go/cmd/mutate4go@latest path/to/file.go ``` Or install it: ```bash go install github.com/unclebob/mutate4go/cmd/mutate4go@latest mutate4go path/to/file.go ``` ## Usage ```bash # Mutate-test a source file. # If the file already has a footer manifest, this defaults to changed functions only. mutate4go internal/foo/foo.go # Scan a file for mutation counts without running coverage or tests mutate4go internal/foo/foo.go --scan # Rewrite the embedded manifest without running coverage or mutations mutate4go internal/foo/foo.go --update-manifest # Retest only specific lines mutate4go internal/foo/foo.go --lines 45,67,89 # Force differential mutation explicitly mutate4go internal/foo/foo.go --since-last-run # Override differential behavior and mutate all covered sites mutate4go internal/foo/foo.go --mutate-all # Reuse existing coverage data without refreshing coverage mutate4go internal/foo/foo.go --reuse-coverage # Warn when a module exceeds a mutation-count threshold mutate4go internal/foo/foo.go --mutation-warning 75 # Use a custom timeout multiplier mutate4go internal/foo/foo.go --timeout-factor 15 # Use a custom test command mutate4go internal/foo/foo.go --test-command "go test ./internal/foo" # Run mutations in parallel with isolated workers mutate4go internal/foo/foo.go --max-workers 4 # Log major actions before and after they run mutate4go internal/foo/foo.go --verbose # Show help mutate4go --help ``` The tool automatically: - Runs coverage with the configured test command plus `-coverprofile=target/coverage/coverage.out` - Runs a baseline test command, defaulting to `go test ./...` - Applies each covered mutation, runs tests with a timeout, and restores the original file - Runs mutations in parallel with isolated worker directories when `--max-workers` is greater than `1` - Writes an embedded footer manifest with the last test date and function hashes - Defaults to differential mutation when that footer manifest already exists - Prints a warning when mutation count exceeds `--mutation-warning` (default `50`) - Can reuse existing coverage data with `--reuse-coverage` - Can log major actions, arguments, and results to stderr with `--verbose` `--scan` is the fast structural mode. It skips coverage, skips test execution, and reports: - total mutation sites - changed mutation sites relative to the embedded manifest - the standard mutation-count warning `--update-manifest` rewrites the embedded footer manifest for the file's current contents without running coverage, baseline tests, or mutation workers. ## Recommended Workflow Run mutation testing one file at a time. ```bash go test ./... mutate4go internal/foo/foo.go --scan mutate4go internal/foo/foo.go ``` Recommended loop for each file: 1. Run `mutate4go path/to/file.go --max-workers 3`. 2. If any mutations are uncovered, add or fix tests until they are covered. 3. If any mutations survive, change code or tests until they are killed. 4. Rerun the same single-file mutation command. 5. Only start the next file when the current file has no uncovered mutations and no survivors. For local incremental work, once a file has a footer manifest the default run is differential. You can still be explicit: ```bash mutate4go internal/foo/foo.go --since-last-run ``` To force a full rerun on a file with a manifest: ```bash mutate4go internal/foo/foo.go --mutate-all ``` ## Mutation Rules | Category | Mutations | |----------|-----------| | Arithmetic | `+` -> `-`, `-` -> `+`, `*` -> `/` | | Comparison | `>` <-> `>=`, `<` <-> `<=` | | Equality | `==` <-> `!=` | | Boolean | `true` <-> `false` | | Logical | `&&` <-> `||` | | Constant | `0` <-> `1` | ## Coverage Integration mutate4go reads Go coverage profiles from: ```bash target/coverage/coverage.out ``` Coverage freshness is regenerated automatically unless `--reuse-coverage` is supplied. If a mutation site is not covered by the profile, mutate4go reports it as uncovered and does not spend time running the mutation. ## Manifest The footer manifest is embedded at the end of the source file and records: - the last successful mutation test date - each function's id - its line span - a hash of its normalized source text Differential mutation runs update the footer manifest after completion, so the next differential run compares against the latest mutation baseline. ## Claude Code Skill This repo includes a Claude Code skill at `skills/using-mutate4go/SKILL.md`. ## Development ```bash go test ./... go run ./cmd/mutate4go --help go run ./cmd/mutate4go internal/mutations/mutations.go --scan ``` ## License Copyright (c) Robert C. Martin. All rights reserved.

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Software Quality Assurance Analysts & Testers

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using-mutate4go

Software Quality Assurance Analysts & Testers

Use when mutation-testing Go code, assessing test quality beyond coverage, or investigating surviving mutations to strengthen tests

2026-05-22

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