diff --git a/plugins/ultraplan-local/lib/parsers/jaccard.mjs b/plugins/ultraplan-local/lib/parsers/jaccard.mjs
new file mode 100644
index 0000000..c5dc8c4
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+++ b/plugins/ultraplan-local/lib/parsers/jaccard.mjs
@@ -0,0 +1,41 @@
+// lib/parsers/jaccard.mjs
+// Jaccard similarity for SC4 determinism floor.
+//
+// jaccard(A, B) = |A ∩ B| / |A ∪ B|
+// Inputs are arrays of strings; deduplicated internally.
+// Both empty → 1.0 (vacuously identical). One empty → 0.0.
+
+/**
+ * Compute Jaccard similarity between two string sets.
+ * @param {string[]} setA
+ * @param {string[]} setB
+ * @returns {number} similarity in [0, 1]
+ */
+export function jaccardSimilarity(setA, setB) {
+  if (!Array.isArray(setA) || !Array.isArray(setB)) {
+    throw new TypeError('jaccardSimilarity: both inputs must be arrays');
+  }
+  const a = new Set(setA);
+  const b = new Set(setB);
+  if (a.size === 0 && b.size === 0) return 1.0;
+  if (a.size === 0 || b.size === 0) return 0.0;
+
+  let intersection = 0;
+  for (const x of a) {
+    if (b.has(x)) intersection += 1;
+  }
+  const union = a.size + b.size - intersection;
+  return intersection / union;
+}
+
+/**
+ * Check whether a similarity meets a threshold.
+ * @param {number} similarity
+ * @param {number} threshold
+ * @returns {boolean}
+ */
+export function meetsThreshold(similarity, threshold) {
+  if (typeof similarity !== 'number' || typeof threshold !== 'number') return false;
+  if (!Number.isFinite(similarity) || !Number.isFinite(threshold)) return false;
+  return similarity >= threshold;
+}
diff --git a/plugins/ultraplan-local/tests/lib/jaccard.test.mjs b/plugins/ultraplan-local/tests/lib/jaccard.test.mjs
new file mode 100644
index 0000000..5f4c9cc
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+++ b/plugins/ultraplan-local/tests/lib/jaccard.test.mjs
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+import { test } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { jaccardSimilarity, meetsThreshold } from '../../lib/parsers/jaccard.mjs';
+
+test('jaccardSimilarity — identical sets → 1.0', () => {
+  assert.equal(jaccardSimilarity(['a', 'b', 'c'], ['a', 'b', 'c']), 1.0);
+});
+
+test('jaccardSimilarity — disjoint sets → 0.0', () => {
+  assert.equal(jaccardSimilarity(['a', 'b'], ['c', 'd']), 0.0);
+});
+
+test('jaccardSimilarity — partial overlap [a,b,c] vs [b,c,d] → 0.5', () => {
+  assert.equal(jaccardSimilarity(['a', 'b', 'c'], ['b', 'c', 'd']), 0.5);
+});
+
+test('jaccardSimilarity — both empty → 1.0', () => {
+  assert.equal(jaccardSimilarity([], []), 1.0);
+});
+
+test('jaccardSimilarity — one empty → 0.0', () => {
+  assert.equal(jaccardSimilarity([], ['a']), 0.0);
+  assert.equal(jaccardSimilarity(['a'], []), 0.0);
+});
+
+test('jaccardSimilarity — duplicates deduplicated within each set', () => {
+  // [a,a,b] dedup → {a,b}; [a,b,b] dedup → {a,b}; identical → 1.0
+  assert.equal(jaccardSimilarity(['a', 'a', 'b'], ['a', 'b', 'b']), 1.0);
+});
+
+test('jaccardSimilarity — fixture sets {α..ε} vs {α..ζ} → 0.833 (SC4 anchor)', () => {
+  // SC4 fixture math: A=5 IDs, B=A∪{ζ}=6 IDs, intersection=5, union=6 → 5/6
+  const A = ['α', 'β', 'γ', 'δ', 'ε'];
+  const B = ['α', 'β', 'γ', 'δ', 'ε', 'ζ'];
+  const sim = jaccardSimilarity(A, B);
+  assert.ok(Math.abs(sim - 5 / 6) < 1e-9);
+  assert.ok(sim >= 0.70); // SC4 threshold
+});
+
+test('jaccardSimilarity — non-array input throws TypeError', () => {
+  assert.throws(() => jaccardSimilarity('a', ['b']), TypeError);
+  assert.throws(() => jaccardSimilarity(['a'], null), TypeError);
+});
+
+test('meetsThreshold — boundary 0.699 → false, 0.700 → true', () => {
+  assert.equal(meetsThreshold(0.699, 0.7), false);
+  assert.equal(meetsThreshold(0.7, 0.7), true);
+  assert.equal(meetsThreshold(0.71, 0.7), true);
+});
+
+test('meetsThreshold — non-finite or non-number → false', () => {
+  assert.equal(meetsThreshold(NaN, 0.7), false);
+  assert.equal(meetsThreshold(Infinity, 0.7), false);
+  assert.equal(meetsThreshold('0.8', 0.7), false);
+  assert.equal(meetsThreshold(0.8, null), false);
+});