JSON Output Schemas

go-mutesting can write three JSON files after each run.

Full report (json_output)

Set json_output: true in the configuration file to write report.json. Its top-level sources object maps each source-file path to its original text. Mutants refer to that path through mutator.originalFilePath, so consumers can look up the source once instead of receiving a duplicate copy for every mutant. Each mutant also includes a checksum field containing the 32-character lowercase checksum accepted by --blacklist, and an id field containing the stable ID used by --baseline and --run-mutant-id.

The legacy mutator.originalSourceCode and mutator.mutatedSourceCode fields remain accepted by the Go model for compatibility, but new reports omit them.

--logger-summary-json

Writes go-mutesting-summary.json. Useful for CI badges, dashboards, and downstream scripts.

{
  "totalMutantsCount": 42,
  "killedCount": 35,
  "escapedCount": 5,
  "errorCount": 0,
  "skippedCount": 2,
  "notCoveredCount": 0,
  "timeOutCount": 0,
  "msi": 0.8333,
  "mutationCodeCoverage": 0,
  "coveredCodeMsi": 0.9211
}

Field	Type	Description
totalMutantsCount	int	Total mutations generated
killedCount	int	Mutations caught by tests
escapedCount	int	Mutations not caught (test gaps)
errorCount	int	Mutations that caused a build or test error
skippedCount	int	Mutations skipped (blacklisted or annotated)
notCoveredCount	int	Mutations on lines with no coverage (requires --coverage)
timeOutCount	int	Mutations that timed out during testing
msi	float	Mutation Score Indicator: killed / total, range 0–1
mutationCodeCoverage	int	Lines covered by the coverage profile
coveredCodeMsi	float	MSI restricted to covered lines only, range 0–1

msi and coveredCodeMsi are in the 0–1 range (not 0–100). Both the summary JSON report and the agentic JSON report (--logger-agentic-json) use a 0–1 ratio for msi.

--logger-agentic-json

Writes go-mutesting-agentic.json. A richer payload designed for LLM consumption. Each survived mutant gets a stable ID, the unified diff, surrounding context lines, nearby test file paths, a plain-English description of the mutation, and a hint for writing a killing test.

{
  "generated_at": "2026-05-19T08:13:38Z",
  "msi": 0.5857,
  "escaped_count": 5,
  "reminder": "A mutant is an example of how this code could be wrong...",
  "mutants": [
    {
      "id": "abc123",
      "checksum": "a1b2c3d4e5f60123456789abcdef0123",
      "file": "pkg/foo/foo.go",
      "line": 42,
      "mutator": "branch/if",
      "diff": "--- Original\n+++ Mutated\n...",
      "context_start_line": 39,
      "context_lines": ["func Foo() {", "  if x > 0 {", "  }"],
      "test_files": ["pkg/foo/foo_test.go"],
      "description": "Removes an if-block body so the condition becomes a no-op",
      "kill_hint": "Write a test that enters this branch and asserts the output or side effect it produces"
    }
  ]
}

Field	Type	Description
generated_at	string	RFC 3339 timestamp of the run
msi	float	Overall MSI as a 0–1 ratio
escaped_count	int	Number of survived mutants
reminder	string	A plain-English reminder about how to interpret mutants — useful context when feeding the file to an LLM
mutants[].id	string	Stable hash of file + mutator + diff — survives refactors that shift line numbers, and is the same however the file target is spelled on the command line
mutants[].checksum	string	32-character lowercase checksum accepted by --blacklist
mutants[].file	string	Path to the mutated file, relative to the module root
mutants[].line	int	Line number of the mutation
mutants[].mutator	string	Mutator name (e.g. branch/if, statement/return)
mutants[].diff	string	Unified diff of original vs mutated code
mutants[].context_start_line	int	1-based source line number of context_lines[0]; use this to anchor the context snippet without guessing offsets
mutants[].context_lines	[]string	Surrounding source lines for orientation
mutants[].test_files	[]string	Test files in the same package
mutants[].description	string	Human-readable description of what the mutator changed
mutants[].kill_hint	string	A concrete suggestion for a test that would kill this mutant

Usage with an LLM

go-mutesting --logger-agentic-json --quiet ./...
# Then feed go-mutesting-agentic.json to an LLM:
# "Here are the mutants my tests didn't catch. For each one,
#  write a Go test that would kill it."