Use JSON when another tool or agent needs to consume state without parsing terminal text:

fixture3 check --feature linting --json
fixture3 status --all --json
fixture3 diff --suite lint-rules --json
fixture3 explain --suite lint-rules --json
fixture3 doctor --json

check --json writes one record per selected suite with status, exit code, fixture count, received path, diff path, and error text. status --json writes approved, received, and diff booleans. diff --json writes diff status and text. doctor --json writes setup findings. reduce always writes JSON.