diff --git a/packages/loopover-engine/src/phase7-calibration-loop.ts b/packages/loopover-engine/src/phase7-calibration-loop.ts
index 3309e9b83d..20ea6d896e 100644
--- a/packages/loopover-engine/src/phase7-calibration-loop.ts
+++ b/packages/loopover-engine/src/phase7-calibration-loop.ts
@@ -177,7 +177,14 @@ function normalizeCompositeWeights(config: Phase7CalibrationConfig): { historica
   };
   const total = raw.historicalReplay + raw.prOutcome;
   if (total <= 0) {
-    return { historicalReplay: DEFAULT_CONFIG.historicalReplayWeight, prOutcome: DEFAULT_CONFIG.prOutcomeWeight };
+    // #8644: PRESERVE an explicit all-zero weighting instead of silently substituting the defaults. This
+    // branch is reachable ONLY when both weights were explicitly 0 -- `finiteNonNegative` floors any invalid
+    // input to a positive default, which cannot sum to 0 -- so a 0 total is a deliberate "disable composite
+    // weighting" signal from the operator, not a degraded input. Returning {0,0} leaves weightTotal 0
+    // downstream, which the combinedAccuracy guard already renders as a null composite (the same state reached
+    // when no source contributes), matching the "preserve explicit zero" principle the sibling calibration
+    // composers follow -- rather than overriding the operator's intent back to a 50/50 default.
+    return { historicalReplay: 0, prOutcome: 0 };
   }
   return {
     historicalReplay: raw.historicalReplay / total,
diff --git a/packages/loopover-engine/test/phase7-calibration-loop.test.ts b/packages/loopover-engine/test/phase7-calibration-loop.test.ts
index 1751e8a90d..951e59d3d1 100644
--- a/packages/loopover-engine/test/phase7-calibration-loop.test.ts
+++ b/packages/loopover-engine/test/phase7-calibration-loop.test.ts
@@ -493,3 +493,21 @@ test("REGRESSION (#3014): combined metric stays anchored to the documented 62% b
   assert.equal(result.combinedAccuracy, 0.62);
   assert.equal(result.deltaFromBaseline, 0);
 });
+
+test("normalizeCompositeWeights preserves an explicit all-zero weighting rather than silently defaulting (#8644)", () => {
+  const config = enabledConfig({ historicalReplayWeight: 0, prOutcomeWeight: 0 });
+  assert.equal(config.historicalReplayWeight, 0);
+  assert.equal(config.prOutcomeWeight, 0);
+
+  const result = computePhase7CalibrationLoop({
+    config,
+    prOutcome: sufficientPrOutcome(0.75),
+    historicalReplay: healthyReplay(0.82),
+    now: NOW,
+  });
+
+  // Preserved, not reverted to the 50/50 default; {0,0} yields no composite (null), matching the
+  // "preserve explicit zero" principle rather than overriding the operator's intent.
+  assert.deepEqual(result.weights, { historicalReplay: 0, prOutcome: 0 });
+  assert.equal(result.combinedAccuracy, null);
+});
diff --git a/test/unit/phase7-calibration-loop-weights.test.ts b/test/unit/phase7-calibration-loop-weights.test.ts
new file mode 100644
index 0000000000..c9abd74cd8
--- /dev/null
+++ b/test/unit/phase7-calibration-loop-weights.test.ts
@@ -0,0 +1,33 @@
+import { describe, expect, it } from "vitest";
+
+import { computePhase7CalibrationLoop, resolvePhase7CalibrationConfig } from "../../packages/loopover-engine/src/index";
+
+// #8644: root-vitest mirror so `normalizeCompositeWeights`'s explicit-all-zero branch in
+// phase7-calibration-loop.ts is Codecov-measured. The package-native node:test suite
+// (packages/loopover-engine/test/) runs under the engine workspace, not this root run that Codecov grades --
+// same reasoning as #8438's signal-tracking root mirror. Imports the engine SOURCE (src/index) so coverage
+// attributes to the .ts rather than the built dist.
+describe("phase7 composite weights — explicit all-zero is preserved (#8644)", () => {
+  it("preserves both weights as 0 instead of silently reverting to the 50/50 default", () => {
+    const config = resolvePhase7CalibrationConfig({
+      miner: { calibration: { phase7LoopEnabled: true, historicalReplayWeight: 0, prOutcomeWeight: 0 } },
+    });
+    // Precondition: an explicit 0 is stored as 0, not defaulted — otherwise the branch under test is unreachable.
+    expect(config.historicalReplayWeight).toBe(0);
+    expect(config.prOutcomeWeight).toBe(0);
+
+    const result = computePhase7CalibrationLoop({ config });
+    expect(result.weights).toEqual({ historicalReplay: 0, prOutcome: 0 });
+    // {0,0} leaves no composite weighting, so combinedAccuracy is null — the same state reached when no
+    // source contributes, not a silently-defaulted weighted average.
+    expect(result.combinedAccuracy).toBeNull();
+  });
+
+  it("still normalizes a normal positive config to sum 1 (the unchanged happy path, both branches covered)", () => {
+    const config = resolvePhase7CalibrationConfig({
+      miner: { calibration: { phase7LoopEnabled: true, historicalReplayWeight: 3, prOutcomeWeight: 1 } },
+    });
+    const result = computePhase7CalibrationLoop({ config });
+    expect(result.weights).toEqual({ historicalReplay: 0.75, prOutcome: 0.25 });
+  });
+});