diff --git a/src/topic/__tests__/TopicMolecule.setProchiralHydrogenLabels.test.js b/src/topic/__tests__/TopicMolecule.setProchiralHydrogenLabels.test.js
index e346f40..5591b14 100644
--- a/src/topic/__tests__/TopicMolecule.setProchiralHydrogenLabels.test.js
+++ b/src/topic/__tests__/TopicMolecule.setProchiralHydrogenLabels.test.js
@@ -1,5 +1,5 @@
 import { Molecule } from 'openchemlib';
-import { expect, test } from 'vitest';
+import { expect, onTestFinished, test, vi } from 'vitest';
 
 import { TopicMolecule } from '../TopicMolecule';
 
@@ -204,30 +204,35 @@ test('canonizedProchiralities is transferred across TopicMolecule instances with
   // (e.g. after expanding hydrogens) should not recompute CIP.
   const molecule = Molecule.fromSmiles('CC(Cl)CC');
   const original = new TopicMolecule(molecule);
-  const labels = original.prochiralities;
-  const labelledCount = labels.filter((l) => l !== undefined).length;
 
-  expect(labelledCount).toBe(2);
+  // Compute the original labels once, populating its prochirality cache.
+  expect(
+    original.prochiralities.filter((l) => l !== undefined).toSorted(),
+  ).toStrictEqual(['r', 's']);
 
-  const oclModule = molecule.getOCL().Molecule;
-  let cipCalls = 0;
-  const originalEnsure = oclModule.prototype.ensureHelperArrays;
-  oclModule.prototype.ensureHelperArrays = function spy(bits) {
-    if (bits === oclModule.cHelperCIP) cipCalls++;
-    return originalEnsure.call(this, bits);
-  };
+  // Count only the dedicated CIP computation (the exact cHelperCIP bit);
+  // composite requests that merely include that bit are unrelated.
+  const oclMolecule = molecule.getOCL().Molecule;
+  const spy = vi.spyOn(oclMolecule.prototype, 'ensureHelperArrays');
+  onTestFinished(() => spy.mockRestore());
 
-  try {
-    const reused = original.fromMolecule(molecule);
-    const reusedLabels = reused.prochiralities;
+  const reused = original.fromMolecule(molecule);
 
-    expect(
-      reusedLabels.filter((l) => l !== undefined).toSorted(),
-    ).toStrictEqual(['r', 's']);
-    expect(cipCalls).toBe(0);
-  } finally {
-    oclModule.prototype.ensureHelperArrays = originalEnsure;
-  }
+  // The transferred entry is the very same object: that shared reference is the
+  // whole optimisation — there is nothing to recompute.
+  expect(reused.cache.prochiralityByEnantioID).toBe(
+    original.cache.prochiralityByEnantioID,
+  );
+
+  expect(
+    reused.prochiralities.filter((l) => l !== undefined).toSorted(),
+  ).toStrictEqual(['r', 's']);
+
+  const cipCalls = spy.mock.calls.filter(
+    ([bits]) => bits === oclMolecule.cHelperCIP,
+  ).length;
+
+  expect(cipCalls).toBe(0);
 });
 
 test('CC(Cl)CC: implicit-H molecule gets no labels, only moleculeWithH does', () => {
diff --git a/src/topic/__tests__/getCanonizedDiaIDs.test.js b/src/topic/__tests__/getCanonizedDiaIDs.test.js
index 5eee4de..215e641 100644
--- a/src/topic/__tests__/getCanonizedDiaIDs.test.js
+++ b/src/topic/__tests__/getCanonizedDiaIDs.test.js
@@ -1,5 +1,5 @@
 import { Molecule } from 'openchemlib';
-import { expect, test } from 'vitest';
+import { expect, onTestFinished, test, vi } from 'vitest';
 
 import { TopicMolecule } from '../TopicMolecule';
 
@@ -10,22 +10,14 @@ test('symmetric atoms with the same heterotopic rank share a cached diaID (no re
   const topicMolecule = new TopicMolecule(molecule);
 
   const oclModule = molecule.getOCL().Molecule;
-  let canonizationCalls = 0;
-  const originalGetCanonizedIDCode = oclModule.prototype.getCanonizedIDCode;
-  oclModule.prototype.getCanonizedIDCode = function spy(...args) {
-    canonizationCalls++;
-    return originalGetCanonizedIDCode.apply(this, args);
-  };
-
-  try {
-    const diaIDs = topicMolecule.diaIDs;
-
-    expect(diaIDs).toHaveLength(12);
-    expect(canonizationCalls).toBe(2);
-    expect(new Set(diaIDs).size).toBe(2);
-  } finally {
-    oclModule.prototype.getCanonizedIDCode = originalGetCanonizedIDCode;
-  }
+  const spy = vi.spyOn(oclModule.prototype, 'getCanonizedIDCode');
+  onTestFinished(() => spy.mockRestore());
+
+  const diaIDs = topicMolecule.diaIDs;
+
+  expect(diaIDs).toHaveLength(12);
+  expect(spy).toHaveBeenCalledTimes(2);
+  expect(new Set(diaIDs).size).toBe(2);
 });
 
 test('a molecule with all-distinct atoms still gets one canonization per atom', () => {