diff --git a/packages/loopover-engine/src/finding-severity-calibration.ts b/packages/loopover-engine/src/finding-severity-calibration.ts
index 5b292eb771..d5fa55487f 100644
--- a/packages/loopover-engine/src/finding-severity-calibration.ts
+++ b/packages/loopover-engine/src/finding-severity-calibration.ts
@@ -148,6 +148,10 @@ function finiteNonNegativeInt(value: number | undefined): number {
   return Math.floor(value);
 }
 
+function isInvalidWeight(value: number | undefined): boolean {
+  return value !== undefined && (!Number.isFinite(value) || value < 0);
+}
+
 function roundScore(value: number): number {
   return Math.round(Math.min(1, Math.max(0, value)) * 1_000_000) / 1_000_000;
 }
@@ -366,7 +370,19 @@ function normalizeCompositeWeights(weights: FindingSeverityCalibrationWeights |
   // Preserve explicitly-zeroed weights rather than substituting the defaults: a caller that zeroes every component
   // must reach the objective-only fallback in the composite scorer, not silently get the default 45/35/20 blend
   // (converges with reviewer-consensus-calibration.ts's already-correct behavior; #6170).
-  if (total <= 0) return { objectiveAnchor: 0, pairwiseJudge: 0, structuredFindingSeverity: 0 };
+  // NaN/negative inputs still recover to DEFAULT_COMPOSITE_WEIGHTS when the clamped total is empty rather than
+  // collapsing to a 100%-objective-anchor composite, mirroring pairwise-calibration.ts's invalid-weight
+  // distinction (#7443; #8643).
+  if (total <= 0) {
+    if (
+      isInvalidWeight(weights?.objectiveAnchor) ||
+      isInvalidWeight(weights?.pairwiseJudge) ||
+      isInvalidWeight(weights?.structuredFindingSeverity)
+    ) {
+      return DEFAULT_COMPOSITE_WEIGHTS;
+    }
+    return { objectiveAnchor: 0, pairwiseJudge: 0, structuredFindingSeverity: 0 };
+  }
   return {
     objectiveAnchor: raw.objectiveAnchor / total,
     pairwiseJudge: raw.pairwiseJudge / total,
diff --git a/packages/loopover-engine/src/gate-verdict-calibration.ts b/packages/loopover-engine/src/gate-verdict-calibration.ts
index c0569ebe68..3be844481c 100644
--- a/packages/loopover-engine/src/gate-verdict-calibration.ts
+++ b/packages/loopover-engine/src/gate-verdict-calibration.ts
@@ -147,6 +147,10 @@ function finiteNonNegative(value: number | undefined, fallback: number): number
   return value;
 }
 
+function isInvalidWeight(value: number | undefined): boolean {
+  return value !== undefined && (!Number.isFinite(value) || value < 0);
+}
+
 function roundScore(value: number): number {
   return Math.round(Math.min(1, Math.max(0, value)) * 1_000_000) / 1_000_000;
 }
@@ -319,7 +323,19 @@ function normalizeCompositeWeights(weights: GateVerdictCalibrationWeights | unde
   // Preserve explicitly-zeroed weights rather than substituting the defaults: a caller that zeroes every component
   // must reach the objective-only fallback in the composite scorer, not silently get the default 45/35/20 blend
   // (converges with reviewer-consensus-calibration.ts's already-correct behavior; #6170).
-  if (total <= 0) return { objectiveAnchor: 0, pairwiseJudge: 0, structuredGateVerdict: 0 };
+  // NaN/negative inputs still recover to DEFAULT_COMPOSITE_WEIGHTS when the clamped total is empty rather than
+  // collapsing to a 100%-objective-anchor composite, mirroring pairwise-calibration.ts's invalid-weight
+  // distinction (#7443; #8643).
+  if (total <= 0) {
+    if (
+      isInvalidWeight(weights?.objectiveAnchor) ||
+      isInvalidWeight(weights?.pairwiseJudge) ||
+      isInvalidWeight(weights?.structuredGateVerdict)
+    ) {
+      return DEFAULT_COMPOSITE_WEIGHTS;
+    }
+    return { objectiveAnchor: 0, pairwiseJudge: 0, structuredGateVerdict: 0 };
+  }
   return {
     objectiveAnchor: raw.objectiveAnchor / total,
     pairwiseJudge: raw.pairwiseJudge / total,
diff --git a/packages/loopover-engine/src/reviewer-consensus-calibration.ts b/packages/loopover-engine/src/reviewer-consensus-calibration.ts
index 105f58e9f3..7b70162b1b 100644
--- a/packages/loopover-engine/src/reviewer-consensus-calibration.ts
+++ b/packages/loopover-engine/src/reviewer-consensus-calibration.ts
@@ -154,6 +154,10 @@ function finiteNonNegative(value: number | undefined, fallback: number): number
   return value;
 }
 
+function isInvalidWeight(value: number | undefined): boolean {
+  return value !== undefined && (!Number.isFinite(value) || value < 0);
+}
+
 function roundScore(value: number): number {
   return Math.round(Math.min(1, Math.max(0, value)) * 1_000_000) / 1_000_000;
 }
@@ -402,7 +406,19 @@ function normalizeCompositeWeights(weights: ReviewerConsensusCalibrationWeights
   const total = raw.objectiveAnchor + raw.pairwiseJudge + raw.structuredReviewerConsensus;
   // Preserve explicitly-zeroed weights rather than substituting the defaults: a caller that zeroes every component
   // must reach the objective-only fallback in the composite scorer, not silently get the default 45/35/20 blend.
-  if (total <= 0) return { objectiveAnchor: 0, pairwiseJudge: 0, structuredReviewerConsensus: 0 };
+  // NaN/negative inputs still recover to DEFAULT_COMPOSITE_WEIGHTS when the clamped total is empty rather than
+  // collapsing to a 100%-objective-anchor composite, mirroring pairwise-calibration.ts's invalid-weight
+  // distinction (#7443; #8643).
+  if (total <= 0) {
+    if (
+      isInvalidWeight(weights?.objectiveAnchor) ||
+      isInvalidWeight(weights?.pairwiseJudge) ||
+      isInvalidWeight(weights?.structuredReviewerConsensus)
+    ) {
+      return DEFAULT_COMPOSITE_WEIGHTS;
+    }
+    return { objectiveAnchor: 0, pairwiseJudge: 0, structuredReviewerConsensus: 0 };
+  }
   return {
     objectiveAnchor: raw.objectiveAnchor / total,
     pairwiseJudge: raw.pairwiseJudge / total,
diff --git a/packages/loopover-engine/test/finding-severity-calibration.test.ts b/packages/loopover-engine/test/finding-severity-calibration.test.ts
index ee5d1f07c5..653eb238c2 100644
--- a/packages/loopover-engine/test/finding-severity-calibration.test.ts
+++ b/packages/loopover-engine/test/finding-severity-calibration.test.ts
@@ -483,3 +483,15 @@ test("computeFindingSeverityCompositeCalibrationScore falls back to objective-on
   assert.deepEqual(result.weights, { objectiveAnchor: 1, pairwiseJudge: 0, structuredFindingSeverity: 0 });
   assert.equal(result.compositeScore, 0.4);
 });
+
+test("computeFindingSeverityCompositeCalibrationScore normalizes invalid weights to the default blend, not an objective-only collapse (#8643)", () => {
+  const result = computeFindingSeverityCompositeCalibrationScore({
+    objectiveAnchor: 0.4,
+    pairwise: 0.4,
+    findingSeverity: [signal()],
+    weights: { objectiveAnchor: Number.NaN, pairwiseJudge: -1, structuredFindingSeverity: -1 },
+  });
+  // NaN/negative weights (with real pairwise + structured scores present) recover to the documented default
+  // 45/35/20 blend -- NOT a 100%-objective-anchor collapse, matching pairwise-calibration.ts's distinction.
+  assert.deepEqual(result.weights, { objectiveAnchor: 0.45, pairwiseJudge: 0.35, structuredFindingSeverity: 0.2 });
+});
diff --git a/packages/loopover-engine/test/gate-verdict-calibration.test.ts b/packages/loopover-engine/test/gate-verdict-calibration.test.ts
index 60c995e75b..403bee2f36 100644
--- a/packages/loopover-engine/test/gate-verdict-calibration.test.ts
+++ b/packages/loopover-engine/test/gate-verdict-calibration.test.ts
@@ -602,6 +602,40 @@ test("computeGateVerdictCompositeCalibrationScore falls back to objective-only w
   assert.equal(result.compositeScore, 0.4);
 });
 
+test("computeGateVerdictCompositeCalibrationScore normalizes invalid weights to the default blend, not an objective-only collapse (#8643)", () => {
+  const objectiveAnchor = scoreObjectiveAnchor({
+    replayed: { paths: ["src/review/a.ts"], labels: ["feature"] },
+    revealed: { paths: ["src/review/b.ts"], labels: ["feature"] },
+  });
+  const pairwise = computePairwiseCalibrationScore({
+    objectiveAnchor,
+    samples: [{ attempts: [{ replayFirst: "replay_better", revealedFirst: "revealed_better" }] }],
+  });
+  const result = computeGateVerdictCompositeCalibrationScore({
+    objectiveAnchor,
+    pairwise,
+    gateVerdicts: [
+      {
+        repoFullName: "JSONbored/Loopover",
+        replayRunId: "replay-7",
+        gateRunId: "gate-7",
+        optedIn: true,
+        dimensions: [
+          { dimension: "correctness", outcome: "pass" },
+          { dimension: "tests", outcome: "warn" },
+        ],
+      },
+    ],
+    weights: { objectiveAnchor: Number.NaN, pairwiseJudge: -1, structuredGateVerdict: -1 },
+  });
+
+  // NaN/negative weights (with real pairwise + structured scores present) recover to the documented default
+  // 45/35/20 blend -- NOT a 100%-objective-anchor collapse, matching pairwise-calibration.ts's distinction.
+  assert.equal(pairwise.pairwiseJudgeScore, 1);
+  assert.equal(result.structuredGateVerdictScore, 0.75);
+  assert.deepEqual(result.weights, { objectiveAnchor: 0.45, pairwiseJudge: 0.35, structuredGateVerdict: 0.2 });
+});
+
 test("computeGateVerdictCompositeCalibrationScore preserves a malformed-repo (invalid_repo) rejected row instead of dropping it (#6170)", () => {
   const result = computeGateVerdictCompositeCalibrationScore({
     objectiveAnchor: 0.5,
diff --git a/packages/loopover-engine/test/reviewer-consensus-calibration.test.ts b/packages/loopover-engine/test/reviewer-consensus-calibration.test.ts
index 0e86892882..7733af1640 100644
--- a/packages/loopover-engine/test/reviewer-consensus-calibration.test.ts
+++ b/packages/loopover-engine/test/reviewer-consensus-calibration.test.ts
@@ -309,6 +309,23 @@ test("composite honors custom weights and falls back to objective-only when all
   assert.equal(allZero.compositeScore, 0.4);
 });
 
+test("composite normalizes invalid weights to the default blend, not an objective-only collapse (#8643)", () => {
+  const ingestion = ingestReviewerConsensusCalibrationSignals([signal()]);
+  const result = computeReviewerConsensusCompositeCalibrationScore({
+    objectiveAnchor: 0.4,
+    pairwise: 0.4,
+    reviewerConsensus: ingestion,
+    weights: { objectiveAnchor: Number.NaN, pairwiseJudge: -1, structuredReviewerConsensus: -1 },
+  });
+  // NaN/negative weights (with real pairwise + structured scores present) recover to the documented default
+  // 45/35/20 blend -- NOT a 100%-objective-anchor collapse, matching pairwise-calibration.ts's distinction.
+  assert.deepEqual(result.weights, {
+    objectiveAnchor: 0.45,
+    pairwiseJudge: 0.35,
+    structuredReviewerConsensus: 0.2,
+  });
+});
+
 test("composite sanitizes pre-ingested reviewer-consensus rows before auditing", () => {
   const poisoned = {
     accepted: [
diff --git a/test/unit/engine-calibration-convergence.test.ts b/test/unit/engine-calibration-convergence.test.ts
index e2dd190520..d45dda45d4 100644
--- a/test/unit/engine-calibration-convergence.test.ts
+++ b/test/unit/engine-calibration-convergence.test.ts
@@ -3,6 +3,8 @@ import {
   computeGateVerdictCompositeCalibrationScore,
   computeFindingSeverityCompositeCalibrationScore,
   computePairwiseCalibrationScore,
+  computeReviewerConsensusCompositeCalibrationScore,
+  ingestReviewerConsensusCalibrationSignals,
 } from "../../packages/loopover-engine/src/index";
 
 // Converges gate-verdict + finding-severity calibration with reviewer-consensus-calibration.ts's already-correct
@@ -54,6 +56,64 @@ describe("gate-verdict/finding-severity calibration convergence (#6170)", () =>
     expect(result.weights.structuredFindingSeverity).toBeGreaterThan(0);
   });
 
+  it("gate-verdict: NaN/negative weights recover to the default 45/35/20 blend, NOT a 100%-objective-anchor collapse (#8643)", () => {
+    const result = computeGateVerdictCompositeCalibrationScore({
+      objectiveAnchor: 0.4,
+      pairwise: 0.4,
+      gateVerdicts: [
+        {
+          repoFullName: "acme/widgets",
+          replayRunId: "replay-1",
+          gateRunId: "gate-1",
+          optedIn: true,
+          dimensions: [{ dimension: "correctness", outcome: "pass" }],
+        },
+      ],
+      weights: { objectiveAnchor: Number.NaN, pairwiseJudge: -1, structuredGateVerdict: -1 },
+    });
+    // Real pairwise + structured scores are present, so the recovered default blend survives unrenormalized.
+    expect(result.weights).toEqual({ objectiveAnchor: 0.45, pairwiseJudge: 0.35, structuredGateVerdict: 0.2 });
+  });
+
+  it("finding-severity: NaN/negative weights recover to the default blend, NOT the objective-only collapse (#8643)", () => {
+    const result = computeFindingSeverityCompositeCalibrationScore({
+      objectiveAnchor: 0.4,
+      pairwise: 0.4,
+      findingSeverity: [
+        { repoFullName: "acme/widgets", replayRunId: "replay-1", reviewRunId: "review-1", optedIn: true, tiers: [{ tier: "blocker", total: 2, confirmed: 2 }] },
+      ],
+      weights: { objectiveAnchor: Number.NaN, pairwiseJudge: -1, structuredFindingSeverity: -1 },
+    });
+    expect(result.weights).toEqual({ objectiveAnchor: 0.45, pairwiseJudge: 0.35, structuredFindingSeverity: 0.2 });
+  });
+
+  it("reviewer-consensus: explicit all-zero weights fall back to objective-only (covers the total<=0 real-zeros arm)", () => {
+    const ingestion = ingestReviewerConsensusCalibrationSignals([
+      { repoFullName: "acme/widgets", replayRunId: "replay-1", reviewRunId: "review-1", optedIn: true, dimensions: [{ dimension: "correctness", votes: ["pass", "pass"] }] },
+    ]);
+    const result = computeReviewerConsensusCompositeCalibrationScore({
+      objectiveAnchor: 0.4,
+      pairwise: 0.4,
+      reviewerConsensus: ingestion,
+      weights: { objectiveAnchor: 0, pairwiseJudge: 0, structuredReviewerConsensus: 0 },
+    });
+    expect(result.weights).toEqual({ objectiveAnchor: 1, pairwiseJudge: 0, structuredReviewerConsensus: 0 });
+    expect(result.compositeScore).toBe(0.4);
+  });
+
+  it("reviewer-consensus: NaN/negative weights recover to the default blend, NOT the objective-only collapse (#8643)", () => {
+    const ingestion = ingestReviewerConsensusCalibrationSignals([
+      { repoFullName: "acme/widgets", replayRunId: "replay-1", reviewRunId: "review-1", optedIn: true, dimensions: [{ dimension: "correctness", votes: ["pass", "pass"] }] },
+    ]);
+    const result = computeReviewerConsensusCompositeCalibrationScore({
+      objectiveAnchor: 0.4,
+      pairwise: 0.4,
+      reviewerConsensus: ingestion,
+      weights: { objectiveAnchor: Number.NaN, pairwiseJudge: -1, structuredReviewerConsensus: -1 },
+    });
+    expect(result.weights).toEqual({ objectiveAnchor: 0.45, pairwiseJudge: 0.35, structuredReviewerConsensus: 0.2 });
+  });
+
   it("gate-verdict: preserves a malformed-repo (invalid_repo) rejected row instead of dropping it; keeps a valid repo and drops a non-string one", () => {
     const result = computeGateVerdictCompositeCalibrationScore({
       objectiveAnchor: 0.5,