diff --git a/package.json b/package.json
index 470f78efa4..20fce8d615 100644
--- a/package.json
+++ b/package.json
@@ -146,6 +146,7 @@
     "check:lockfile": "node scripts/check/check-lockfile.mjs",
     "check:bundle-size": "node scripts/check/check-bundle-size.mjs",
     "check:circular-deps": "node scripts/check/check-circular-deps.mjs",
+    "check:mutation-ratchet": "node scripts/check/check-mutation-ratchet.mjs",
     "check:licenses": "node scripts/check/check-licenses.mjs",
     "check:pr-evidence": "node scripts/check/check-pr-evidence.mjs",
     "check:vuln-ratchet": "node scripts/check/check-vuln-ratchet.mjs",
diff --git a/scripts/check/check-mutation-ratchet.mjs b/scripts/check/check-mutation-ratchet.mjs
new file mode 100644
index 0000000000..09ea607125
--- /dev/null
+++ b/scripts/check/check-mutation-ratchet.mjs
@@ -0,0 +1,157 @@
+#!/usr/bin/env node
+// scripts/check/check-mutation-ratchet.mjs
+// Catraca de mutationScore (Quality Gate v2 / Fase 9 T5 — Onda 2, Task 3).
+//
+// Mirrors check-bundle-size.mjs: ADVISORY by default (always exit 0), BLOCKING only
+// with --ratchet — and even then exits 1 SE — E SOMENTE SE — um módulo medido REGREDIU
+// vs o baseline (direction: UP, o score só pode subir). Skip gracioso (exit 0) quando
+// não há mutation.json (ex.: o nightly não rodou) ou não há baseline para o módulo —
+// falta de dados NUNCA bloqueia, só uma regressão medida bloqueia.
+//
+// Score por módulo = COVERED score = detected / (detected + survived), onde
+// detected = Killed + Timeout. NoCoverage é EXCLUÍDO do denominador (é uma lacuna de
+// cobertura, não um sinal de qualidade-de-teste) — mesmo denominador que a radiografia
+// (scripts/quality/mutation-radiography.mjs).
+//
+// O nightly divide o `mutate` em batches paralelos (um reports/mutation/mutation.json
+// por job). Este script roda DENTRO de cada job sobre o report daquele batch e compara
+// só os módulos presentes nele. Aceita vários paths para uso local/agregado.
+//
+// Uso:
+//   node scripts/check/check-mutation-ratchet.mjs                 (advisory; report default)
+//   node scripts/check/check-mutation-ratchet.mjs reports/mutation/mutation.json
+//   node scripts/check/check-mutation-ratchet.mjs <a.json> <b.json> ... --ratchet
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const ROOT = process.cwd();
+const BASELINE_PATH = path.join(ROOT, "config/quality/quality-baseline.json");
+const DEFAULT_REPORT = path.join(ROOT, "reports/mutation/mutation.json");
+const BASELINE_PREFIX = "mutationScore.";
+const RATCHET = process.argv.includes("--ratchet");
+
+const DETECTED = new Set(["Killed", "Timeout"]);
+const SURVIVED = new Set(["Survived"]);
+
+/**
+ * Avalia o score MEDIDO de um módulo contra o baseline. Direction: UP (o score só
+ * pode subir — menor = regressão).
+ * @param {number} current
+ * @param {number} baseline
+ * @returns {{ regressed: boolean, improved: boolean }}
+ */
+export function evaluateMutationRatchet(current, baseline) {
+  return {
+    regressed: current < baseline,
+    improved: current > baseline,
+  };
+}
+
+/**
+ * Covered mutation score de um arquivo: detected/(detected+survived)*100.
+ * NoCoverage/Ignored/RuntimeError/CompileError ficam fora do denominador.
+ * @param {{mutants?: Array<{status: string}>}} fileData
+ * @returns {number|null} score em %, ou null se não houver mutantes cobertos
+ */
+export function mutationScoreForFile(fileData) {
+  let detected = 0;
+  let survived = 0;
+  for (const m of fileData?.mutants || []) {
+    if (DETECTED.has(m.status)) detected += 1;
+    else if (SURVIVED.has(m.status)) survived += 1;
+  }
+  const denom = detected + survived;
+  return denom === 0 ? null : (detected / denom) * 100;
+}
+
+/**
+ * Score por arquivo a partir de um ou mais reports (batches). Arquivos sem mutante
+ * coberto (score null) são omitidos.
+ * @param {object|object[]} reportOrReports parsed mutation.json (ou array)
+ * @returns {Record<string, number>}
+ */
+export function measureMutationScores(reportOrReports) {
+  const reports = Array.isArray(reportOrReports) ? reportOrReports : [reportOrReports];
+  const out = {};
+  for (const report of reports) {
+    for (const [file, data] of Object.entries(report?.files || {})) {
+      const score = mutationScoreForFile(data);
+      if (score !== null) out[file] = score;
+    }
+  }
+  return out;
+}
+
+/**
+ * Lê metrics["mutationScore.<path>"].value do quality-baseline.json.
+ * Retorna {} se o arquivo ou as chaves estiverem ausentes (sem baseline não há
+ * ratchet possível — o caller trata como SKIP gracioso, exit 0).
+ * @param {string} baselinePath
+ * @returns {Record<string, number>}
+ */
+export function readBaselineMutationScores(baselinePath = BASELINE_PATH) {
+  if (!fs.existsSync(baselinePath)) return {};
+  let baselineJson;
+  try {
+    baselineJson = JSON.parse(fs.readFileSync(baselinePath, "utf8"));
+  } catch {
+    return {};
+  }
+  const metrics = baselineJson?.metrics || {};
+  const out = {};
+  for (const [key, val] of Object.entries(metrics)) {
+    if (key.startsWith(BASELINE_PREFIX) && val && typeof val.value === "number") {
+      out[key.slice(BASELINE_PREFIX.length)] = val.value;
+    }
+  }
+  return out;
+}
+
+function loadReport(p) {
+  return JSON.parse(fs.readFileSync(p, "utf8"));
+}
+
+function main(argv) {
+  const paths = argv.slice(2).filter((a) => !a.startsWith("--"));
+  const reportPaths = paths.length > 0 ? paths : [DEFAULT_REPORT];
+  const existing = reportPaths.filter((p) => fs.existsSync(p));
+  if (existing.length === 0) {
+    process.stdout.write("mutationScore=SKIP reason=no-report\n");
+    process.exit(0);
+  }
+
+  const measured = measureMutationScores(existing.map(loadReport));
+  const baseline = readBaselineMutationScores();
+
+  const regressions = [];
+  const modules = Object.keys(measured).sort();
+  for (const mod of modules) {
+    const current = measured[mod];
+    if (!(mod in baseline)) {
+      process.stdout.write(`mutationScore.${mod}=${current.toFixed(2)} (no baseline — advisory)\n`);
+      continue;
+    }
+    const { regressed } = evaluateMutationRatchet(current, baseline[mod]);
+    const tag = regressed ? "REGRESSED" : "ok";
+    process.stdout.write(
+      `mutationScore.${mod}=${current.toFixed(2)} baseline=${baseline[mod].toFixed(2)} ${tag}\n`
+    );
+    if (regressed) regressions.push({ mod, current, baseline: baseline[mod] });
+  }
+
+  if (regressions.length > 0 && RATCHET) {
+    process.stderr.write(
+      `\nMutation ratchet FAILED — ${regressions.length} module(s) dropped below baseline:\n`
+    );
+    for (const r of regressions) {
+      process.stderr.write(`  ${r.mod}: ${r.current.toFixed(2)} < ${r.baseline.toFixed(2)}\n`);
+    }
+    process.exit(1);
+  }
+  process.exit(0);
+}
+
+if (import.meta.url === `file://${process.argv[1]}` || process.argv[1] === fileURLToPath(import.meta.url)) {
+  main(process.argv);
+}
diff --git a/scripts/quality/mutation-radiography.mjs b/scripts/quality/mutation-radiography.mjs
new file mode 100644
index 0000000000..9ba7954c85
--- /dev/null
+++ b/scripts/quality/mutation-radiography.mjs
@@ -0,0 +1,218 @@
+#!/usr/bin/env node
+/**
+ * Mutation radiography (Quality Gate v2 / Fase 9 T5 — Onda 2, Task 1).
+ *
+ * Classifies every COVERING test file by its mutation-kill contribution, using the
+ * `killedBy` attribution that the Stryker tap-runner emits per mutant
+ * (`coverageAnalysis: perTest`, validated by the Task 12 spike — see
+ * docs/ops/MUTATION_GATE_SPIKE_VERDICT.md):
+ *
+ *   🔴 empty       — the test file never appears in any `killedBy` (kills no mutant
+ *                    of the mutated modules). Prime R1-prune candidate (Task 2).
+ *   🟠 redundant   — every mutant it kills is ALSO killed by ≥1 other test file
+ *                    (zero unique kills).
+ *   🟡 overlapping — kills ≥1 unique mutant, but the MAJORITY of its kills are shared.
+ *   🟢 unique      — kills ≥1 mutant that NO other test file kills (and unique kills
+ *                    are not outnumbered by shared kills).
+ *
+ * CAVEAT — bail-on-first-kill: Stryker bails after the first test kills a mutant
+ * (we do NOT set `disableBail`), so `killedBy` lists the FIRST killer, not every
+ * killer. Consequence: 🔴 empty is RELIABLE (a sole killer is always recorded, so a
+ * file that never appears in killedBy is never the sole killer of any mutant → safe
+ * R1-prune candidate w.r.t. mutationScore), but 🟢/🟠/🟡 are OPTIMISTIC — "unique" is
+ * overstated and "redundant" understated, because a non-first coverer that WOULD also
+ * kill is never recorded. Use 🟢/🟠/🟡 as advisory only; an accurate redundancy split
+ * (for R2) needs a `disableBail: true` run. R1 (Task 2) acts on 🔴 alone + a line-
+ * coverage cross-check + human review, so bail-on-first is sufficient there.
+ *
+ * IMPORTANT — multi-batch merge: the nightly splits `mutate` across parallel batches
+ * (one mutation.json per batch). Stryker assigns numeric test ids PER RUN, so id "12"
+ * in batch c is unrelated to id "12" in batch d. Each report is therefore resolved
+ * (id -> file name, via its own `testFiles` section) and classified independently;
+ * `aggregateRadiography` then sums the per-FILE kill counts across batches and
+ * reclassifies. A file empty in one batch but unique in another is unique overall.
+ *
+ * Usage:
+ *   node scripts/quality/mutation-radiography.mjs <mutation-c.json> [<mutation-d.json> ...]
+ * The universe of test files (so 🔴 empty files are detectable) defaults to
+ * `stryker.conf.json:tap.testFiles`; pass --no-conf-universe to use only the union
+ * of the reports' own `testFiles` sections instead.
+ */
+
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const SCRIPT_DIR = path.dirname(fileURLToPath(import.meta.url));
+const REPO_ROOT = path.resolve(SCRIPT_DIR, "..", "..");
+
+export function loadMutationReport(reportPath) {
+  return JSON.parse(fs.readFileSync(reportPath, "utf8"));
+}
+
+/**
+ * Threshold rules shared by single-report and aggregated classification.
+ * @param {number} uniqueKills mutants this file kills ALONE
+ * @param {number} sharedKills mutants this file kills together with others
+ */
+export function classifyFromCounts(uniqueKills, sharedKills) {
+  if (uniqueKills === 0 && sharedKills === 0) return "empty";
+  if (uniqueKills === 0) return "redundant";
+  if (sharedKills > uniqueKills) return "overlapping";
+  return "unique";
+}
+
+// Map each numeric test id to its file name via the report's `testFiles` section.
+// Real tap-runner reports key killedBy by id; the synthetic test fixtures key it by
+// file name directly (no testFiles section) — those pass through unchanged.
+function buildIdToFile(report) {
+  const map = new Map();
+  for (const [file, data] of Object.entries(report.testFiles || {})) {
+    for (const t of data.tests || []) {
+      map.set(String(t.id), t.name || file);
+    }
+  }
+  return map;
+}
+
+// Raw per-file kill counts for ONE report (no universe, no classification).
+function countKills(report) {
+  const idToFile = buildIdToFile(report);
+  const counts = new Map();
+  const bump = (file, key) => {
+    const c = counts.get(file) || { uniqueKills: 0, sharedKills: 0 };
+    c[key] += 1;
+    counts.set(file, c);
+  };
+  for (const data of Object.values(report.files || {})) {
+    for (const m of data.mutants || []) {
+      if (m.status !== "Killed") continue;
+      const killers = [...new Set((m.killedBy || []).map((id) => idToFile.get(String(id)) ?? id))];
+      if (killers.length === 0) continue;
+      if (killers.length === 1) bump(killers[0], "uniqueKills");
+      else for (const k of killers) bump(k, "sharedKills");
+    }
+  }
+  return counts;
+}
+
+function materialize(counts, universe) {
+  const files = new Set(universe || []);
+  for (const f of counts.keys()) files.add(f);
+  const out = {};
+  for (const file of files) {
+    const { uniqueKills = 0, sharedKills = 0 } = counts.get(file) || {};
+    out[file] = { class: classifyFromCounts(uniqueKills, sharedKills), uniqueKills, sharedKills };
+  }
+  return out;
+}
+
+/**
+ * Classify the test files of a SINGLE mutation report.
+ * @param {object} report parsed mutation.json
+ * @param {string[]} [allTestFiles] universe; defaults to the report's testFiles keys
+ */
+export function classifyTestFiles(report, allTestFiles) {
+  const universe = allTestFiles || Object.keys(report.testFiles || {});
+  return materialize(countKills(report), universe);
+}
+
+/**
+ * Merge several per-batch reports at the file level, then classify.
+ * @param {object[]} reports parsed mutation.json objects (one per batch)
+ * @param {string[]} [allTestFiles] universe; defaults to the union of testFiles keys
+ */
+export function aggregateRadiography(reports, allTestFiles) {
+  const total = new Map();
+  const universe = new Set(allTestFiles || []);
+  for (const report of reports) {
+    if (!allTestFiles) for (const f of Object.keys(report.testFiles || {})) universe.add(f);
+    for (const [file, c] of countKills(report)) {
+      const acc = total.get(file) || { uniqueKills: 0, sharedKills: 0 };
+      acc.uniqueKills += c.uniqueKills;
+      acc.sharedKills += c.sharedKills;
+      total.set(file, acc);
+    }
+  }
+  return materialize(total, [...universe]);
+}
+
+// ── CLI ──────────────────────────────────────────────────────────────────────
+
+function tapTestFilesUniverse() {
+  try {
+    const conf = JSON.parse(fs.readFileSync(path.join(REPO_ROOT, "stryker.conf.json"), "utf8"));
+    return conf?.tap?.testFiles || null;
+  } catch {
+    return null;
+  }
+}
+
+const CLASS_LABEL = {
+  empty: "🔴 empty",
+  redundant: "🟠 redundant",
+  overlapping: "🟡 overlapping",
+  unique: "🟢 unique",
+};
+const CLASS_ORDER = ["empty", "redundant", "overlapping", "unique"];
+
+function renderMarkdown(classification) {
+  const byClass = { empty: [], redundant: [], overlapping: [], unique: [] };
+  for (const [file, info] of Object.entries(classification)) byClass[info.class].push({ file, ...info });
+  for (const k of CLASS_ORDER) byClass[k].sort((a, b) => a.file.localeCompare(b.file));
+
+  const total = Object.keys(classification).length;
+  const lines = [];
+  lines.push("# Mutation Radiography");
+  lines.push("");
+  lines.push(`Test files classified by mutation-kill contribution (\`killedBy\`). Total: **${total}**.`);
+  lines.push("");
+  lines.push("| Class | Count | Meaning |");
+  lines.push("| --- | --- | --- |");
+  lines.push(`| 🔴 empty | ${byClass.empty.length} | kills no mutant of the mutated modules (R1-prune candidate) |`);
+  lines.push(`| 🟠 redundant | ${byClass.redundant.length} | every kill is shared with another file |`);
+  lines.push(`| 🟡 overlapping | ${byClass.overlapping.length} | kills ≥1 unique but mostly shared |`);
+  lines.push(`| 🟢 unique | ${byClass.unique.length} | kills ≥1 mutant no other file kills |`);
+  lines.push("");
+  lines.push(
+    "> **Bail caveat:** Stryker bails on the first kill (no `disableBail`), so `killedBy` is the " +
+      "FIRST killer only. 🔴 empty is reliable (safe R1-prune candidate w.r.t. mutationScore); " +
+      "🟢/🟠/🟡 are optimistic (unique overstated, redundant understated) — advisory until a " +
+      "`disableBail` run. R1 prunes 🔴 only, with a line-coverage cross-check + human review."
+  );
+  lines.push("");
+  for (const k of CLASS_ORDER) {
+    const rows = byClass[k];
+    lines.push(`## ${CLASS_LABEL[k]} (${rows.length})`);
+    lines.push("");
+    if (rows.length === 0) {
+      lines.push("_none_");
+    } else {
+      lines.push("| Test file | unique | shared |");
+      lines.push("| --- | --- | --- |");
+      for (const r of rows) lines.push(`| ${r.file} | ${r.uniqueKills} | ${r.sharedKills} |`);
+    }
+    lines.push("");
+  }
+  return lines.join("\n");
+}
+
+function main(argv) {
+  const args = argv.filter((a) => a !== "--no-conf-universe");
+  const useConfUniverse = !argv.includes("--no-conf-universe");
+  const paths = args.slice(2);
+  if (paths.length === 0) {
+    process.stderr.write(
+      "usage: mutation-radiography.mjs <mutation-1.json> [<mutation-2.json> ...] [--no-conf-universe]\n"
+    );
+    process.exit(2);
+  }
+  const reports = paths.map(loadMutationReport);
+  const universe = useConfUniverse ? tapTestFilesUniverse() : null;
+  const classification = aggregateRadiography(reports, universe || undefined);
+  process.stdout.write(renderMarkdown(classification) + "\n");
+}
+
+if (import.meta.url === `file://${process.argv[1]}`) {
+  main(process.argv);
+}
diff --git a/tests/unit/build/check-mutation-ratchet.test.ts b/tests/unit/build/check-mutation-ratchet.test.ts
new file mode 100644
index 0000000000..f7870a8193
--- /dev/null
+++ b/tests/unit/build/check-mutation-ratchet.test.ts
@@ -0,0 +1,91 @@
+import { test } from "node:test";
+import assert from "node:assert";
+import os from "node:os";
+import fs from "node:fs";
+import path from "node:path";
+import {
+  evaluateMutationRatchet,
+  mutationScoreForFile,
+  measureMutationScores,
+  readBaselineMutationScores,
+} from "../../../scripts/check/check-mutation-ratchet.mjs";
+
+// ── evaluateMutationRatchet: direction UP (score can only improve) ───────────
+test("mutation ratchet flags a drop (direction up)", () => {
+  assert.equal(evaluateMutationRatchet(72.0, 75.0).regressed, true);
+  assert.equal(evaluateMutationRatchet(76.0, 75.0).regressed, false);
+  assert.equal(evaluateMutationRatchet(76.0, 75.0).improved, true);
+  assert.equal(evaluateMutationRatchet(75.0, 75.0).regressed, false); // equal holds
+  assert.equal(evaluateMutationRatchet(75.0, 75.0).improved, false);
+});
+
+// ── mutationScoreForFile: covered score = detected/(detected+survived),
+// NoCoverage EXCLUDED (it is a coverage gap, not a test-quality signal). ─────
+test("mutationScoreForFile computes the covered score and excludes NoCoverage", () => {
+  const fileData = {
+    mutants: [
+      { status: "Killed" },
+      { status: "Killed" },
+      { status: "Killed" },
+      { status: "Timeout" }, // Timeout counts as detected
+      { status: "Survived" },
+      { status: "Survived" },
+      { status: "NoCoverage" }, // excluded from denominator
+      { status: "NoCoverage" },
+      { status: "Ignored" }, // excluded (not a valid mutant)
+    ],
+  };
+  // detected = 4 (3 Killed + 1 Timeout); denom = 6 (+2 Survived); NoCoverage/Ignored out.
+  assert.ok(Math.abs(mutationScoreForFile(fileData) - (4 / 6) * 100) < 1e-9);
+});
+
+test("mutationScoreForFile returns null when there are no covered mutants", () => {
+  assert.equal(mutationScoreForFile({ mutants: [{ status: "NoCoverage" }] }), null);
+  assert.equal(mutationScoreForFile({ mutants: [] }), null);
+});
+
+// ── measureMutationScores: per-file scores from a report (and merges batches) ─
+test("measureMutationScores maps each mutated file to its score", () => {
+  const report = {
+    files: {
+      "src/a.ts": { mutants: [{ status: "Killed" }, { status: "Survived" }] }, // 50
+      "src/b.ts": { mutants: [{ status: "Killed" }, { status: "Killed" }] }, // 100
+      "src/empty.ts": { mutants: [{ status: "NoCoverage" }] }, // null -> omitted
+    },
+  };
+  const scores = measureMutationScores(report);
+  assert.equal(scores["src/a.ts"], 50);
+  assert.equal(scores["src/b.ts"], 100);
+  assert.equal("src/empty.ts" in scores, false);
+});
+
+test("measureMutationScores accepts several reports (per-batch) and unions them", () => {
+  const c = { files: { "src/a.ts": { mutants: [{ status: "Killed" }, { status: "Survived" }] } } };
+  const g = { files: { "src/b.ts": { mutants: [{ status: "Killed" }] } } };
+  const scores = measureMutationScores([c, g]);
+  assert.equal(scores["src/a.ts"], 50);
+  assert.equal(scores["src/b.ts"], 100);
+});
+
+// ── readBaselineMutationScores: graceful skip when the file/keys are absent ──
+test("readBaselineMutationScores returns {} when the baseline file is missing", () => {
+  assert.deepEqual(readBaselineMutationScores("/no/such/baseline.json"), {});
+});
+
+test("readBaselineMutationScores extracts mutationScore.<path> metric values", () => {
+  // Write a tiny baseline to a temp file and read it back.
+  const tmp = path.join(os.tmpdir(), `mut-baseline-${process.pid}.json`);
+  fs.writeFileSync(
+    tmp,
+    JSON.stringify({
+      metrics: {
+        eslintWarnings: { value: 10, direction: "down" },
+        "mutationScore.src/a.ts": { value: 70, direction: "up", dedicatedGate: true },
+        "mutationScore.src/b.ts": { value: 80, direction: "up", dedicatedGate: true },
+      },
+    })
+  );
+  const base = readBaselineMutationScores(tmp);
+  assert.deepEqual(base, { "src/a.ts": 70, "src/b.ts": 80 });
+  fs.rmSync(tmp, { force: true });
+});
diff --git a/tests/unit/build/mutation-radiography.test.ts b/tests/unit/build/mutation-radiography.test.ts
new file mode 100644
index 0000000000..102fdaa81c
--- /dev/null
+++ b/tests/unit/build/mutation-radiography.test.ts
@@ -0,0 +1,119 @@
+import { test } from "node:test";
+import assert from "node:assert";
+import {
+  classifyTestFiles,
+  aggregateRadiography,
+  classifyFromCounts,
+} from "../../../scripts/quality/mutation-radiography.mjs";
+
+// ── classifyTestFiles: the plan's canonical fixture ──────────────────────────
+// 2 testFiles that kill mutants + 1 that kills nothing.
+//   m1 killed ONLY by A   -> A gets a unique kill
+//   m2 killed by A AND B  -> both get a shared kill
+//   m3 survived           -> nobody
+// Universe = [A, B, C]; C never appears in any killedBy -> empty.
+test("classifies unique / redundant / empty test files from killedBy (file-name killedBy)", () => {
+  const report = {
+    files: {
+      "open-sse/utils/error.ts": {
+        mutants: [
+          { id: "m1", status: "Killed", killedBy: ["A.test.ts"] },
+          { id: "m2", status: "Killed", killedBy: ["A.test.ts", "B.test.ts"] },
+          { id: "m3", status: "Survived", killedBy: [] },
+        ],
+      },
+    },
+  };
+  const allTestFiles = ["A.test.ts", "B.test.ts", "C.test.ts"];
+  const r = classifyTestFiles(report, allTestFiles);
+  assert.equal(r["A.test.ts"].class, "unique"); // mata m1 sozinho
+  assert.equal(r["A.test.ts"].uniqueKills, 1);
+  assert.equal(r["A.test.ts"].sharedKills, 1);
+  assert.equal(r["B.test.ts"].class, "redundant"); // só m2 (compartilhado)
+  assert.equal(r["B.test.ts"].uniqueKills, 0);
+  assert.equal(r["C.test.ts"].class, "empty"); // não mata nada
+});
+
+// ── id resolution: real Stryker tap-runner reports use numeric test ids in
+// killedBy and a testFiles{} section mapping id -> file name. ────────────────
+test("resolves numeric killedBy ids to file names via the testFiles section", () => {
+  const report = {
+    testFiles: {
+      "tests/unit/x.test.ts": { tests: [{ id: "0", name: "tests/unit/x.test.ts" }] },
+      "tests/unit/y.test.ts": { tests: [{ id: "1", name: "tests/unit/y.test.ts" }] },
+      "tests/unit/z.test.ts": { tests: [{ id: "2", name: "tests/unit/z.test.ts" }] },
+    },
+    files: {
+      "src/m.ts": {
+        mutants: [
+          { id: "m1", status: "Killed", killedBy: ["0"] }, // x alone -> unique
+          { id: "m2", status: "Killed", killedBy: ["0", "1"] }, // x + y shared
+        ],
+      },
+    },
+  };
+  // allTestFiles defaults to the testFiles keys when omitted.
+  const r = classifyTestFiles(report);
+  assert.equal(r["tests/unit/x.test.ts"].class, "unique");
+  assert.equal(r["tests/unit/y.test.ts"].class, "redundant");
+  assert.equal(r["tests/unit/z.test.ts"].class, "empty");
+});
+
+// ── overlapping (🟡): kills ≥1 unique but the majority of its kills are shared.
+test("classifies overlapping when shared kills outnumber unique kills", () => {
+  const report = {
+    files: {
+      "src/m.ts": {
+        mutants: [
+          { id: "m1", status: "Killed", killedBy: ["D"] }, // D unique
+          { id: "m2", status: "Killed", killedBy: ["D", "E"] }, // D shared
+          { id: "m3", status: "Killed", killedBy: ["D", "E", "F"] }, // D shared
+        ],
+      },
+    },
+  };
+  const r = classifyTestFiles(report, ["D", "E", "F"]);
+  assert.equal(r["D"].uniqueKills, 1);
+  assert.equal(r["D"].sharedKills, 2);
+  assert.equal(r["D"].class, "overlapping"); // 2 shared > 1 unique
+  assert.equal(r["E"].class, "redundant");
+  assert.equal(r["F"].class, "redundant");
+});
+
+// ── classifyFromCounts: the pure threshold helper. ───────────────────────────
+test("classifyFromCounts applies the threshold rules", () => {
+  assert.equal(classifyFromCounts(0, 0), "empty");
+  assert.equal(classifyFromCounts(0, 3), "redundant");
+  assert.equal(classifyFromCounts(2, 1), "unique"); // shared not > unique
+  assert.equal(classifyFromCounts(1, 1), "unique"); // tie -> unique
+  assert.equal(classifyFromCounts(1, 5), "overlapping"); // shared > unique
+});
+
+// ── aggregateRadiography: merge per-batch reports at the FILE level (ids are
+// per-run, so each report is classified independently then summed). A file can
+// be empty in one batch but unique in another -> unique overall. ─────────────
+test("aggregateRadiography sums per-file kills across batches and reclassifies", () => {
+  const batchC = {
+    files: {
+      "src/routeGuard.ts": {
+        mutants: [{ id: "c1", status: "Killed", killedBy: ["A"] }], // A unique here
+      },
+    },
+  };
+  const batchG = {
+    files: {
+      "src/chatCore/x.ts": {
+        mutants: [
+          { id: "g1", status: "Killed", killedBy: ["A", "B"] }, // A shared, B shared
+        ],
+      },
+    },
+  };
+  // B only ever shares; A is unique in C and shared in G -> A unique overall.
+  const agg = aggregateRadiography([batchC, batchG], ["A", "B", "C"]);
+  assert.equal(agg["A"].uniqueKills, 1);
+  assert.equal(agg["A"].sharedKills, 1);
+  assert.equal(agg["A"].class, "unique");
+  assert.equal(agg["B"].class, "redundant");
+  assert.equal(agg["C"].class, "empty");
+});