diff --git a/tests/build.test.js b/tests/build.test.js
index 4c5b53b..370ab99 100644
--- a/tests/build.test.js
+++ b/tests/build.test.js
@@ -25,3 +25,49 @@ test('userscript self-test block is wired to #tat-test', () => {
   assert.ok(bundle.includes("location.hash === '#tat-test'"), 'self-test guard missing');
   assert.ok(bundle.includes('runSelfTest'), 'runSelfTest function missing');
 });
+
+test('bundle summary behavior matches src/pure.js (catches Math.floor-style regressions)', () => {
+  // Read both the source and the bundle.
+  const src = readFileSync(join(root, 'src/pure.js'), 'utf8');
+  const bundle = readFileSync(join(root, 'torn-attribute-tracker.user.js'), 'utf8');
+
+  // Extract the body of `summary` from src/pure.js between `function summary(` and the next `}`.
+  // Use a simple state machine: find the function header, then count braces.
+  function extractBody(source, fnName) {
+    const start = source.indexOf('function ' + fnName + '(');
+    if (start === -1) throw new Error('could not find ' + fnName + ' in source');
+    // Find the opening brace of the function body.
+    let i = source.indexOf('{', start);
+    if (i === -1) throw new Error('no opening brace for ' + fnName);
+    let depth = 1;
+    i++;
+    while (i < source.length && depth > 0) {
+      const c = source[i];
+      if (c === '{') depth++;
+      else if (c === '}') depth--;
+      i++;
+    }
+    return source.slice(start, i);
+  }
+
+  // Eval-extract the `summary` function from both sources. The src/pure.js
+  // version references the module-level `MS_PER_DAY` constant, so inject
+  // it into the eval scope; the bundle inlines the literal value, so the
+  // injected binding is a harmless no-op there.
+  const srcSummary = eval('(function() { const MS_PER_DAY = 86_400_000; ' + extractBody(src, 'summary') + ' ; return summary; })()');
+  const bundleSummary = eval('(function() { const MS_PER_DAY = 86_400_000; ' + extractBody(bundle, 'summary') + ' ; return summary; })()');
+
+  // Run both with a known input that exercises perDay flooring.
+  const NOW = 1_700_000_000_000;
+  const entries = [
+    { ts: NOW - 1000, delta: 247 },
+    { ts: NOW - 2000, delta: 247 },
+  ];
+  const srcResult = srcSummary(entries, NOW);
+  const bundleResult = bundleSummary(entries, NOW);
+
+  // The bundle MUST match the source exactly.
+  assert.deepEqual(bundleResult, srcResult, 'bundle summary differs from src/pure.js summary');
+  // And specifically: perDay must be the floored integer 70, not the float 70.57.
+  assert.equal(bundleResult.perDay, 70, 'perDay should be floored to 70');
+});