diff --git a/scripts/benchmark-live-control.mjs b/scripts/benchmark-live-control.mjs
index 86d705cce..ef6f779c5 100644
--- a/scripts/benchmark-live-control.mjs
+++ b/scripts/benchmark-live-control.mjs
@@ -6,9 +6,12 @@ import os from 'node:os';
 import path from 'node:path';
 import { fileURLToPath } from 'node:url';
 
+import { parseArgs, positiveIntFlag } from './lib/cli-args.mjs';
+
 const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
-const iterations = Math.max(1, Number(arg('--iterations') || 5));
-const fixture = arg('--fixture') || 'vite8-react-plain';
+const args = parseArgs(process.argv.slice(2));
+const iterations = positiveIntFlag(args.iterations, 5);
+const fixture = args.fixture ? String(args.fixture) : 'vite8-react-plain';
 const metricsFile = path.join(os.tmpdir(), 'impeccable-live-control-' + process.pid + '.jsonl');
 
 try {
@@ -26,7 +29,7 @@ try {
     });
   }
 
-  const rows = fs.readFileSync(metricsFile, 'utf-8').trim().split('\n').filter(Boolean).map(JSON.parse);
+  const rows = readMetrics(metricsFile);
   console.log(JSON.stringify({
     fixture,
     iterations: rows.length,
@@ -39,9 +42,39 @@ try {
   try { fs.unlinkSync(metricsFile); } catch {}
 }
 
+/**
+ * Read the metrics the e2e run appended. Fail loudly rather than reporting a
+ * summary of nothing: an absent file means the run never produced a sample, and
+ * an ENOENT stack or a `{"medianMs": null}` report both read as "measured" when
+ * nothing was measured at all.
+ */
+function readMetrics(file) {
+  let raw;
+  try {
+    raw = fs.readFileSync(file, 'utf-8');
+  } catch (error) {
+    if (error.code !== 'ENOENT') throw error;
+    throw new Error(`no metrics were recorded at ${file}. Did the e2e run emit IMPECCABLE_E2E_METRICS_FILE rows?`);
+  }
+  const rows = raw.trim().split('\n').filter(Boolean).map((line, index) => {
+    try {
+      return JSON.parse(line);
+    } catch (error) {
+      throw new Error(`metrics line ${index + 1} is not valid JSON: ${error.message}`);
+    }
+  });
+  if (rows.length === 0) throw new Error(`metrics file ${file} is empty; nothing to summarize`);
+  return rows;
+}
+
 function summarize(values) {
-  const sorted = [...values].sort((a, b) => a - b);
+  const sorted = values.filter((value) => Number.isFinite(value)).sort((a, b) => a - b);
+  // Distinguish "every sample was missing this metric" from a real measurement.
+  // percentile() on an empty array reads sorted[-1] and yields NaN, which
+  // JSON.stringify turns into null and silently passes for a result.
+  if (sorted.length === 0) return { samples: 0, medianMs: null, p95Ms: null, minMs: null, maxMs: null };
   return {
+    samples: sorted.length,
     medianMs: percentile(sorted, 0.5),
     p95Ms: percentile(sorted, 0.95),
     minMs: sorted[0],
@@ -55,8 +88,3 @@ function percentile(sorted, p) {
   const upper = Math.ceil(index);
   return Math.round((sorted[lower] * (1 - (index - lower)) + sorted[upper] * (index - lower)) * 100) / 100;
 }
-
-function arg(name) {
-  const index = process.argv.indexOf(name);
-  return index >= 0 ? process.argv[index + 1] : undefined;
-}
diff --git a/scripts/benchmark-live-init.mjs b/scripts/benchmark-live-init.mjs
index 5998b887d..336d12af4 100644
--- a/scripts/benchmark-live-init.mjs
+++ b/scripts/benchmark-live-init.mjs
@@ -6,11 +6,14 @@ import os from 'node:os';
 import path from 'node:path';
 import { fileURLToPath } from 'node:url';
 
+import { parseArgs, positiveIntFlag } from './lib/cli-args.mjs';
+
 const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
 const liveScript = path.join(root, 'skill/scripts/live.mjs');
 const serverScript = path.join(root, 'skill/scripts/live-server.mjs');
-const iterations = Math.max(1, Number(arg('--iterations') || 10));
-const fixture = arg('--fixture') || 'vite8-react-plain';
+const args = parseArgs(process.argv.slice(2));
+const iterations = positiveIntFlag(args.iterations, 10);
+const fixture = args.fixture ? String(args.fixture) : 'vite8-react-plain';
 const fixtureDir = path.join(root, 'tests/framework-fixtures', fixture, 'files');
 const tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'impeccable-live-init-'));
 
@@ -95,8 +98,3 @@ function percentile(sorted, value) {
 function round(value) {
   return Math.round(value * 100) / 100;
 }
-
-function arg(name) {
-  const index = process.argv.indexOf(name);
-  return index >= 0 ? process.argv[index + 1] : undefined;
-}