diff --git a/scripts/lib/github-coordination/gh-api.js b/scripts/lib/github-coordination/gh-api.js
index dd2cb4397..874c247f2 100644
--- a/scripts/lib/github-coordination/gh-api.js
+++ b/scripts/lib/github-coordination/gh-api.js
@@ -12,8 +12,9 @@ function normalizeRepo(repo) {
 }
 
 function normalizeIssueNumber(value) {
-  const parsed = Number.parseInt(String(value), 10);
-  if (!Number.isFinite(parsed) || parsed <= 0) {
+  const text = String(value).trim();
+  const parsed = Number(text);
+  if (!/^\+?\d+$/.test(text) || !Number.isSafeInteger(parsed) || parsed <= 0) {
     throw new Error(`Invalid issue number: ${value}`);
   }
   return parsed;
diff --git a/tests/lib/github-coordination-gh-api.test.js b/tests/lib/github-coordination-gh-api.test.js
new file mode 100644
index 000000000..558d93f17
--- /dev/null
+++ b/tests/lib/github-coordination-gh-api.test.js
@@ -0,0 +1,96 @@
+/**
+ * Tests for scripts/lib/github-coordination/gh-api.js — normalizeIssueNumber
+ *
+ * Guards against silent truncation of malformed issue numbers
+ * (https://github.com/affaan-m/ECC/issues/3514): parseInt-style prefix
+ * parsing accepted "42oops" as 42, "42.9" as 42, and "4e2" as 4, while
+ * integers beyond 2^53 lost precision (9007199254740993 became ...992).
+ * The same helper also parses --limit, so malformed values there must be
+ * rejected too. Whitespace, leading plus, and leading zero tolerance is
+ * preserved.
+ *
+ * Run with: node tests/lib/github-coordination-gh-api.test.js
+ */
+
+'use strict';
+
+const assert = require('assert');
+
+const {
+  normalizeIssueNumber,
+} = require('../../scripts/lib/github-coordination/gh-api');
+const { test, banner, section, summary } = require('./helpers/mini-test-runner');
+
+let passed = 0;
+let failed = 0;
+
+banner('normalizeIssueNumber — accepted forms:');
+
+if (test('accepts a plain positive integer', () => {
+  assert.strictEqual(normalizeIssueNumber('42'), 42);
+})) passed++; else failed++;
+
+if (test('preserves surrounding whitespace tolerance', () => {
+  assert.strictEqual(normalizeIssueNumber(' 7 '), 7);
+})) passed++; else failed++;
+
+if (test('preserves leading plus tolerance', () => {
+  assert.strictEqual(normalizeIssueNumber('+7'), 7);
+})) passed++; else failed++;
+
+if (test('preserves leading zero tolerance', () => {
+  assert.strictEqual(normalizeIssueNumber('007'), 7);
+})) passed++; else failed++;
+
+if (test('accepts MAX_SAFE_INTEGER exactly', () => {
+  assert.strictEqual(normalizeIssueNumber('9007199254740991'), 9007199254740991);
+})) passed++; else failed++;
+
+if (test('accepts numeric (non-string) input', () => {
+  assert.strictEqual(normalizeIssueNumber(42), 42);
+})) passed++; else failed++;
+
+section('normalizeIssueNumber — malformed tokens are rejected, not truncated:');
+
+for (const [value, label] of [
+  ['42oops', 'trailing garbage after digits'],
+  ['42.9', 'decimal fraction'],
+  ['4e2', 'exponent notation'],
+  ['0x10', 'hex notation'],
+  ['1e999', 'exponent beyond Number range'],
+  ['12 34', 'internal whitespace'],
+  ['-.', 'punctuation only'],
+]) {
+  if (test(`rejects ${JSON.stringify(value)} (${label})`, () => {
+    assert.throws(() => normalizeIssueNumber(value), /Invalid issue number/);
+  })) passed++; else failed++;
+}
+
+section('normalizeIssueNumber — unsafe integers are rejected:');
+
+for (const value of [
+  '9007199254740992', // 2^53
+  '9007199254740993', // 2^53 + 1 (rounds to 2^53 as a Number)
+  '999999999999999999999999',
+]) {
+  if (test(`rejects ${value}`, () => {
+    assert.throws(() => normalizeIssueNumber(value), /Invalid issue number/);
+  })) passed++; else failed++;
+}
+
+section('normalizeIssueNumber — non-positive and empty inputs:');
+
+for (const value of ['0', '-3', '', '   ', 'abc']) {
+  if (test(`rejects ${JSON.stringify(value)}`, () => {
+    assert.throws(() => normalizeIssueNumber(value), /Invalid issue number/);
+  })) passed++; else failed++;
+}
+
+if (test('error message preserves the original input', () => {
+  assert.throws(
+    () => normalizeIssueNumber('42oops'),
+    (error) => error.message === 'Invalid issue number: 42oops'
+  );
+})) passed++; else failed++;
+
+summary(passed, failed);
diff --git a/tests/scripts/github-coordination.test.js b/tests/scripts/github-coordination.test.js
index 73394a1ef..006406658 100644
--- a/tests/scripts/github-coordination.test.js
+++ b/tests/scripts/github-coordination.test.js
@@ -93,6 +93,28 @@ async function runTests() {
   let passed = 0;
   let failed = 0;
 
+  if (
+    await test('rejects malformed issue numbers during argument parsing', async () => {
+      const { parseArgs } = require(SCRIPT);
+
+      for (const argv of [
+        ['node', 'github-coordination.js', 'claim', '42oops', '--repo', 'example/project'],
+        ['node', 'github-coordination.js', 'claim', '--issue', '42.9', '--repo', 'example/project'],
+        ['node', 'github-coordination.js', 'claim', '--issue', '4e2', '--repo', 'example/project'],
+        ['node', 'github-coordination.js', 'claim', '--issue', '9007199254740993', '--repo', 'example/project'],
+        ['node', 'github-coordination.js', 'claim', '--limit', '42.9', '--repo', 'example/project'],
+      ]) {
+        assert.throws(() => parseArgs(argv), /Invalid issue number/);
+      }
+
+      const valid = parseArgs(['node', 'github-coordination.js', 'claim', '7', '--repo', 'example/project']);
+      assert.strictEqual(valid.issueNumber, 7);
+      assert.strictEqual(valid.limit, 100);
+    })
+  )
+    passed++;
+  else failed++;
+
   if (
     await test('claims an epic issue, updates GitHub state, and caches a work item', async () => {
       const rootDir = createTempDir('github-coordination-claim-');