Run tests and return structured health — fix_first, broken_areas, failure_clusters, coverage_by_area, blind_spots, deploy probes. Not for finding which files to edit (find_code). Workflow: task=detect (0 credits — framework, command, test file count, missing-test gaps) → task=run once (1 credit hosted on success) → task=failures|missing|why|status|fix_prompt on session_id (0 credits, cached session, no re-run). task=missing scans git diff for source files without tests (0 credits, no suite run). meta.credits and meta.charges_usage show billing. Read summary and fix_first first; detail_level=brief on PASS. Call after every substantive edit, when user asks if tests pass or if tests are missing, or before push. Pass diff_base: main for failures_in_diff. area/file_filter scopes re-runs after a full run. Local stdio: absolute project path (runs on your machine). Hosted: public GitHub URL or inline_files — not local disk paths. NOT for symbols (find_code), packages (check_package), stack brief (get_project_context), URL audit (audit_headers). Example: check_test({ task: "detect", path: "/abs/my-app" }) then check_test({ task: "missing", path: "/abs/my-app" }) then check_test({ task: "run", path: "/abs/my-app" }). Does not modify source.