diff --git a/hailbytes-vuln-calculator.js b/hailbytes-vuln-calculator.js
index 1d2c2aa..729c308 100644
--- a/hailbytes-vuln-calculator.js
+++ b/hailbytes-vuln-calculator.js
@@ -48,7 +48,36 @@ const TIME_MULT      = { light: 0.5, medium: 1.0, aggressive: 2.0, continuous: 0
 
 // ─── Calculation engine ────────────────────────────────────────────────────────
 
-function calculateResources(data) {
+/**
+ * Normalize raw inputs so the pure calculate() API tolerates omitted optional
+ * fields the same way the DOM component does. `scan_window` and
+ * `compliance_needs` are declared optional in the public types, so callers may
+ * omit them; without this, calculate() throws on a missing `compliance_needs`
+ * and yields NaN window-utilization on a missing `scan_window`.
+ */
+function normalizeInputs(data) {
+  const raw = data ?? {};
+  const hosts = Number(raw.target_hosts);
+  // An empty tools array is a valid, zero-cost state for the pure API (the DOM
+  // component injects its own UI fallback); only a missing/invalid value needs
+  // a default to avoid a crash on `.includes`.
+  const tools = Array.isArray(raw.scanning_tools) ? raw.scanning_tools.filter(Boolean) : [];
+
+  const window = Number(raw.scan_window);
+
+  return {
+    ...raw,
+    target_hosts:     Number.isFinite(hosts) && hosts > 0 ? hosts : 1,
+    scan_intensity:   raw.scan_intensity ?? 'medium',
+    scan_frequency:   raw.scan_frequency ?? 'weekly',
+    scan_window:      Number.isFinite(window) && window > 0 ? window : 8,
+    scanning_tools:   tools,
+    compliance_needs: Array.isArray(raw.compliance_needs) ? raw.compliance_needs : [],
+  };
+}
+
+function calculateResources(rawData) {
+  const data = normalizeInputs(rawData);
   const hasAsm = data.scanning_tools.includes('hailbytes_asm');
 
   const vmResources   = hasAsm
diff --git a/test/calculate.test.mjs b/test/calculate.test.mjs
index 0bab1d1..4fe4831 100644
--- a/test/calculate.test.mjs
+++ b/test/calculate.test.mjs
@@ -780,6 +780,80 @@ describe('Edge Cases', () => {
   });
 });
 
+// ═════════════════════════════════════════════════════════════════════════════
+// Optional input handling — fields the public types mark as optional
+// (scan_window?, compliance_needs?) must not crash or produce NaN when omitted.
+// ═════════════════════════════════════════════════════════════════════════════
+
+describe('Optional input handling', () => {
+  test('omitting compliance_needs does not throw and defaults to no compliance', () => {
+    const r = calculate({
+      target_hosts: 1000,
+      scan_intensity: 'medium',
+      scan_frequency: 'weekly',
+      scan_window: 8,
+      scanning_tools: ['hailbytes_asm'],
+      // compliance_needs intentionally omitted
+    });
+    assert.ok(r.vm_resources.cpu_cores >= 2);
+    // complianceFactor should behave like an empty list (1.0 multiplier)
+    assert.deepEqual(r.inputs.compliance_needs, []);
+    assert.ok(!r.recommendations.some(rec => rec.includes('PCI DSS requires')));
+  });
+
+  test('omitting scan_window yields a finite utilization, not NaN', () => {
+    const r = calculate({
+      target_hosts: 1000,
+      scan_intensity: 'medium',
+      scan_frequency: 'weekly',
+      scanning_tools: ['hailbytes_asm'],
+      compliance_needs: [],
+      // scan_window intentionally omitted
+    });
+    assert.equal(typeof r.timing.scan_window_utilization, 'number');
+    assert.ok(Number.isFinite(r.timing.scan_window_utilization));
+    assert.ok(!Number.isNaN(r.timing.scan_window_utilization));
+    // efficiency rating should reflect the real number, not the NaN→'poor' fallthrough
+    assert.ok(['excellent', 'good', 'acceptable', 'poor'].includes(
+      r.timing.performance_metrics.efficiency_rating));
+  });
+
+  test('omitting scanning_tools does not throw and produces zero tool cost', () => {
+    const r = calculate({
+      target_hosts: 1000,
+      scan_intensity: 'medium',
+      scan_frequency: 'weekly',
+      // scanning_tools intentionally omitted
+    });
+    assert.ok(r.vm_resources);
+    assert.equal(r.has_asm, false);
+    assert.equal(r.costs.tool_licensing_annual, 0);
+    assert.equal(r.costs.tool_management_monthly, 0);
+    assert.equal(r.costs.tool_setup_cost, 0);
+  });
+
+  test('zero / invalid scan_window falls back to a safe default instead of dividing by zero', () => {
+    const r = calculate({
+      target_hosts: 1000,
+      scan_intensity: 'medium',
+      scan_frequency: 'weekly',
+      scan_window: 0,
+      scanning_tools: ['hailbytes_asm'],
+      compliance_needs: [],
+    });
+    assert.ok(Number.isFinite(r.timing.scan_window_utilization));
+  });
+
+  test('only the minimal required fields are needed to get a full result', () => {
+    const r = calculate({ target_hosts: 500 });
+    assert.ok(r.vm_resources);
+    assert.ok(r.timing);
+    assert.ok(r.costs);
+    assert.ok(Array.isArray(r.recommendations));
+    assert.equal(typeof r.timestamp, 'string');
+  });
+});
+
 // ═════════════════════════════════════════════════════════════════════════════
 // Additional Accuracy Tests - Cross-validate full scenario
 // ═════════════════════════════════════════════════════════════════════════════
diff --git a/test/smoke.test.mjs b/test/smoke.test.mjs
index 5573ccf..9463bd1 100644
--- a/test/smoke.test.mjs
+++ b/test/smoke.test.mjs
@@ -78,6 +78,19 @@ test('calculate() recognises HailBytes ASM tool', () => {
   assert.equal(r.has_asm, true);
 });
 
+test('calculate() tolerates omitted optional inputs (scan_window, compliance_needs)', () => {
+  // index.d.ts marks scan_window and compliance_needs as optional, so omitting
+  // them must not throw or yield NaN.
+  const r = mod.calculate({
+    target_hosts: 1000,
+    scan_intensity: 'medium',
+    scan_frequency: 'weekly',
+    scanning_tools: ['hailbytes_asm'],
+  });
+  assert.ok(r.vm_resources);
+  assert.ok(Number.isFinite(r.timing.scan_window_utilization));
+});
+
 test('class declares observedAttributes including theme and branding', () => {
   const observed = mod.default.observedAttributes;
   assert.ok(Array.isArray(observed));