diff --git a/tools/brain-retro-analyzer.mjs b/tools/brain-retro-analyzer.mjs
index dc2588a3..457933cb 100644
--- a/tools/brain-retro-analyzer.mjs
+++ b/tools/brain-retro-analyzer.mjs
@@ -219,6 +219,43 @@ export function analyze(episodes, options = {}) {
   const disciplineByClassification = disciplinePercentByClassification(normal, classificationMap);
   const routerStep = routerStepReached(normal);
   const boundariesRate = boundariesAppliedRate(normal);
+
+  // Phase 3 Task 20 — v4 aggregation: inheritance count + reviewer outcome
+  // distribution + cost totals. Reads schema_version >=4 fields gracefully.
+  let inheritanceCount = 0;
+  const reviewQuality = { correct: 0, wrong_node: 0, overkill: 0, underkill: 0, disputable: 0 };
+  const reviewerCoverage = { reviewed: 0, pending: 0, errored: 0 };
+  let degradedCount = 0;
+  const costTotals = {
+    classifier_input_tokens: 0,
+    classifier_output_tokens: 0,
+    self_assessment_input_tokens: 0,
+    self_assessment_output_tokens: 0,
+    reviewer_input_tokens: 0,
+    reviewer_output_tokens: 0,
+  };
+  for (const e of normal) {
+    if (e?.inheritance?.inherited_from_task_id) inheritanceCount += 1;
+    if (e?.degraded_mode === true) degradedCount += 1;
+    const r = e?.review;
+    if (r && typeof r === 'object') {
+      if (r.reviewer_error) reviewerCoverage.errored += 1;
+      else if (typeof r.node_quality === 'string') {
+        reviewerCoverage.reviewed += 1;
+        if (reviewQuality[r.node_quality] !== undefined) reviewQuality[r.node_quality] += 1;
+      }
+    } else if (e?.schema_version >= 4) {
+      reviewerCoverage.pending += 1;
+    }
+    const tc = e?.task_cost;
+    if (tc && typeof tc === 'object') {
+      for (const k of Object.keys(costTotals)) {
+        const v = tc[k];
+        if (typeof v === 'number' && Number.isFinite(v)) costTotals[k] += v;
+      }
+    }
+  }
+
   return {
     episodeCount: normal.length,
     v1SkippedCount,
@@ -230,6 +267,11 @@ export function analyze(episodes, options = {}) {
     disciplineByClassification,
     routerStep,
     boundariesRate,
+    inheritanceCount,
+    reviewQuality,
+    reviewerCoverage,
+    degradedCount,
+    costTotals,
   };
 }
 
diff --git a/tools/brain-retro-analyzer.test.mjs b/tools/brain-retro-analyzer.test.mjs
index 48f19f9c..ffd63b6c 100644
--- a/tools/brain-retro-analyzer.test.mjs
+++ b/tools/brain-retro-analyzer.test.mjs
@@ -357,3 +357,55 @@ describe('analyze — discipline metrics (stage 2)', () => {
     expect(res.boundariesRate.rate).toBeCloseTo(0.5);
   });
 });
+
+describe('analyze — v4 aggregations (Phase 3 Task 20)', () => {
+  it('aggregates inheritanceCount across v4 episodes', () => {
+    const eps = [
+      ep({ schema_version: 4, inheritance: { inherited_from_task_id: 'x' } }),
+      ep({ schema_version: 4, timestamps: { started_at: '2026-05-19T10:01:00Z', ended_at: '2026-05-19T10:02:00Z' }, inheritance: { inherited_from_task_id: 'y' } }),
+      ep({ schema_version: 4, timestamps: { started_at: '2026-05-19T10:02:00Z', ended_at: '2026-05-19T10:03:00Z' } }),
+    ];
+    expect(analyze(eps).inheritanceCount).toBe(2);
+  });
+
+  it('aggregates reviewQuality distribution from review.node_quality', () => {
+    const eps = [
+      ep({ schema_version: 4, review: { node_quality: 'correct' } }),
+      ep({ schema_version: 4, timestamps: { started_at: '2026-05-19T10:01:00Z', ended_at: '2026-05-19T10:02:00Z' }, review: { node_quality: 'correct' } }),
+      ep({ schema_version: 4, timestamps: { started_at: '2026-05-19T10:02:00Z', ended_at: '2026-05-19T10:03:00Z' }, review: { node_quality: 'wrong_node' } }),
+    ];
+    const res = analyze(eps);
+    expect(res.reviewQuality.correct).toBe(2);
+    expect(res.reviewQuality.wrong_node).toBe(1);
+    expect(res.reviewerCoverage.reviewed).toBe(3);
+  });
+
+  it('counts review pending for v4 episodes without a review block', () => {
+    const eps = [ep({ schema_version: 4 })];
+    expect(analyze(eps).reviewerCoverage.pending).toBe(1);
+  });
+
+  it('counts reviewer_error escalations under reviewerCoverage.errored', () => {
+    const eps = [ep({ schema_version: 4, review: { reviewer_error: 'malformed episode' } })];
+    expect(analyze(eps).reviewerCoverage.errored).toBe(1);
+  });
+
+  it('aggregates degradedCount on degraded_mode=true', () => {
+    const eps = [
+      ep({ schema_version: 4, degraded_mode: true }),
+      ep({ schema_version: 4, timestamps: { started_at: '2026-05-19T10:01:00Z', ended_at: '2026-05-19T10:02:00Z' }, degraded_mode: false }),
+    ];
+    expect(analyze(eps).degradedCount).toBe(1);
+  });
+
+  it('sums task_cost tokens into costTotals', () => {
+    const eps = [
+      ep({ schema_version: 4, task_cost: { classifier_input_tokens: 100, classifier_output_tokens: 30 } }),
+      ep({ schema_version: 4, timestamps: { started_at: '2026-05-19T10:01:00Z', ended_at: '2026-05-19T10:02:00Z' }, task_cost: { classifier_input_tokens: 200, reviewer_input_tokens: 500 } }),
+    ];
+    const ct = analyze(eps).costTotals;
+    expect(ct.classifier_input_tokens).toBe(300);
+    expect(ct.classifier_output_tokens).toBe(30);
+    expect(ct.reviewer_input_tokens).toBe(500);
+  });
+});
diff --git a/tools/observer-stop-hook.mjs b/tools/observer-stop-hook.mjs
index 46911d4a..295fc8b5 100644
--- a/tools/observer-stop-hook.mjs
+++ b/tools/observer-stop-hook.mjs
@@ -192,7 +192,7 @@ export function buildExecutionTrace({ recommended_chain = [], invoked = [] } = {
  */
 export function buildEpisode({ state = null, transcriptText = null, ctx = {} } = {}) {
   const base = buildEpisodeFromContext(ctx, transcriptText);
-  base.schema_minor = 2; // Task 17 bump (self_assessment surface added)
+  base.schema_minor = 3; // Task 20 bump (cost totals + reviewer distribution surface)
   if (state?.inheritance) {
     base.inheritance = { ...state.inheritance };
   }
diff --git a/tools/observer-stop-hook.test.mjs b/tools/observer-stop-hook.test.mjs
index fa34904b..fbcc09dd 100644
--- a/tools/observer-stop-hook.test.mjs
+++ b/tools/observer-stop-hook.test.mjs
@@ -206,10 +206,10 @@ describe('buildExecutionTrace + buildEpisode — Phase 3 Task 16 (spec §5)', ()
     expect(ep.inheritance).toBeUndefined();
   });
 
-  it('buildEpisode marks schema_minor=2 (Task 17 bump)', () => {
+  it('buildEpisode marks schema_minor=3 (Task 20 bump)', () => {
     const ep = buildEpisode({ state: {}, ctx: { session_id: 'sess-x' } });
     expect(ep.schema_version).toBe(4);
-    expect(ep.schema_minor).toBe(2);
+    expect(ep.schema_minor).toBe(3);
   });
 });