diff --git a/scripts/lib/upstream-drift.js b/scripts/lib/upstream-drift.js
index fede71a..a9513c1 100644
--- a/scripts/lib/upstream-drift.js
+++ b/scripts/lib/upstream-drift.js
@@ -97,48 +97,63 @@ function formatIssueTitle({ count, lastSyncedShaShort }) {
  * Body for the rolling tracking issue. Truncates the commit list at
  * COMMIT_BODY_LIMIT entries and links to the compare URL for the full diff.
  *
+ * `totalCount` is the true number of commits upstream is ahead by
+ * (i.e. `compare.ahead_by` from the GitHub API). `commits.length`
+ * can be smaller because the compare endpoint caps responses at 250
+ * commits regardless of the true delta. Using `commits.length` in
+ * the intro would silently understate the drift when the API caps
+ * (e.g. show "250 commits ahead" when the truth is 1521).
+ *
  * @param {{
  *   commits: Array<{ sha: string, author: string, message: string }>,
  *   lastSyncedSha: string,
  *   upstreamHeadSha: string,
  *   upstreamRepo: string,
+ *   totalCount: number,
  * }} args
  * @returns {string}
  */
-function formatIssueBody({ commits, lastSyncedSha, upstreamHeadSha, upstreamRepo }) {
+function formatIssueBody({ commits, lastSyncedSha, upstreamHeadSha, upstreamRepo, totalCount }) {
   if (!Array.isArray(commits)) {
     throw new Error('formatIssueBody: commits must be an array');
   }
   if (!REPO_RE.test(upstreamRepo)) {
     throw new Error(`formatIssueBody: upstreamRepo must be "owner/repo" (got: ${upstreamRepo})`);
   }
+  if (typeof totalCount !== 'number' || totalCount < 0 || !Number.isInteger(totalCount)) {
+    throw new Error(`formatIssueBody: totalCount must be a non-negative integer (got: ${totalCount})`);
+  }
 
   const compareUrl = `https://github.com/${upstreamRepo}/compare/${lastSyncedSha}...${upstreamHeadSha}`;
   const lines = [];
 
-  lines.push(`Upstream \`${upstreamRepo}\` has **${commits.length}** commit(s) ahead of the recorded baseline.`);
+  lines.push(`Upstream \`${upstreamRepo}\` has **${totalCount}** commit(s) ahead of the recorded baseline.`);
   lines.push('');
   lines.push(`- **Baseline**: \`${shortSha(lastSyncedSha)}\` (recorded in [\`upstream/.upstream-sync.json\`](../blob/main/upstream/.upstream-sync.json))`);
   lines.push(`- **Upstream HEAD**: \`${shortSha(upstreamHeadSha)}\``);
   lines.push(`- **Full diff**: ${compareUrl}`);
   lines.push('');
 
-  if (commits.length === 0) {
+  if (totalCount === 0) {
     lines.push('_No commits to show._');
     lines.push('');
   } else {
     const visible = commits.slice(0, COMMIT_BODY_LIMIT);
-    const truncated = commits.length - visible.length;
+    const omitted = totalCount - visible.length;
 
     lines.push('<details>');
-    lines.push(`<summary>Commits (${commits.length} total${truncated > 0 ? `, showing first ${visible.length}` : ''})</summary>`);
+    if (omitted === 0) {
+      lines.push(`<summary>Commits (${totalCount})</summary>`);
+    } else {
+      lines.push(`<summary>Commits (showing first ${visible.length} of ${totalCount})</summary>`);
+    }
     lines.push('');
     for (const c of visible) {
       const firstLine = (c.message || '').split('\n')[0].trim();
       lines.push(`- \`${shortSha(c.sha)}\` ${c.author ? `(@${c.author}) ` : ''}${firstLine}`);
     }
-    if (truncated > 0) {
-      lines.push(`- _… and ${truncated} more. See the [full diff](${compareUrl})._`);
+    if (omitted > 0) {
+      lines.push(`- _… and ${omitted} more. See the [full diff](${compareUrl})._`);
     }
     lines.push('');
     lines.push('</details>');
diff --git a/scripts/upstream/check-upstream-drift.js b/scripts/upstream/check-upstream-drift.js
index 6baabce..31aa2ae 100644
--- a/scripts/upstream/check-upstream-drift.js
+++ b/scripts/upstream/check-upstream-drift.js
@@ -179,6 +179,9 @@ function buildIssueContent(state, compare) {
     lastSyncedSha: state.lastSyncedSha,
     upstreamHeadSha: headSha,
     upstreamRepo: state.upstream,
+    // compare.ahead_by is the true delta even when commits[] is
+    // capped at 250 by the GitHub compare endpoint.
+    totalCount: compare.ahead_by,
   });
   return { title, body };
 }
diff --git a/tests/lib/upstream-drift.test.js b/tests/lib/upstream-drift.test.js
index bd0ba5d..4684078 100644
--- a/tests/lib/upstream-drift.test.js
+++ b/tests/lib/upstream-drift.test.js
@@ -151,6 +151,7 @@ test('formatIssueBody includes baseline, head, compare URL', () => {
     lastSyncedSha: '9db98673d054f5ed0991ba9d67ff4c883c81a42f',
     upstreamHeadSha: 'bbbb2222bbbb2222bbbb2222bbbb2222bbbb2222',
     upstreamRepo: 'affaan-m/everything-claude-code',
+    totalCount: 1,
   });
   assert.ok(body.includes('9db9867'), 'should contain baseline short sha');
   assert.ok(body.includes('bbbb222'), 'should contain head short sha');
@@ -160,19 +161,22 @@ test('formatIssueBody includes baseline, head, compare URL', () => {
   assert.ok(!body.includes('body'), 'should not include commit message body, only first line');
 });
 
-test('formatIssueBody handles empty commit list', () => {
+test('formatIssueBody handles empty commit list (totalCount=0)', () => {
   const body = lib.formatIssueBody({
     commits: [],
     lastSyncedSha: '0'.repeat(40),
     upstreamHeadSha: '1'.repeat(40),
     upstreamRepo: 'a/b',
+    totalCount: 0,
   });
   assert.ok(body.includes('_No commits to show._'));
+  assert.ok(body.includes('**0**'));
 });
 
-test('formatIssueBody truncates beyond COMMIT_BODY_LIMIT', () => {
+test('formatIssueBody truncates display beyond COMMIT_BODY_LIMIT (totalCount === commits.length)', () => {
   const many = [];
-  for (let i = 0; i < lib.COMMIT_BODY_LIMIT + 25; i += 1) {
+  const N = lib.COMMIT_BODY_LIMIT + 25;
+  for (let i = 0; i < N; i += 1) {
     many.push({
       sha: i.toString(16).padStart(40, '0'),
       author: 'a',
@@ -184,26 +188,78 @@ test('formatIssueBody truncates beyond COMMIT_BODY_LIMIT', () => {
     lastSyncedSha: '0'.repeat(40),
     upstreamHeadSha: '1'.repeat(40),
     upstreamRepo: 'a/b',
+    totalCount: N,
   });
   assert.ok(body.includes('… and 25 more'));
-  assert.ok(body.includes(`(${lib.COMMIT_BODY_LIMIT + 25} total`));
-  assert.ok(!body.includes(`msg ${lib.COMMIT_BODY_LIMIT + 24}`), 'last commit should not be rendered');
+  assert.ok(body.includes(`showing first ${lib.COMMIT_BODY_LIMIT} of ${N}`));
+  assert.ok(!body.includes(`msg ${N - 1}`), 'last commit should not be rendered');
+});
+
+test('formatIssueBody uses totalCount when API caps commits (totalCount > commits.length)', () => {
+  // Real-world case: ahead_by=1521 but compare endpoint capped commits[] at 250.
+  // Intro and summary must report 1521, not 250.
+  const commits = [];
+  for (let i = 0; i < 250; i += 1) {
+    commits.push({
+      sha: i.toString(16).padStart(40, '0'),
+      author: 'a',
+      message: `msg ${i}`,
+    });
+  }
+  const body = lib.formatIssueBody({
+    commits,
+    lastSyncedSha: '0'.repeat(40),
+    upstreamHeadSha: '1'.repeat(40),
+    upstreamRepo: 'a/b',
+    totalCount: 1521,
+  });
+  assert.ok(body.includes('**1521** commit(s) ahead'), 'intro should use totalCount, not commits.length');
+  assert.ok(body.includes(`showing first ${lib.COMMIT_BODY_LIMIT} of 1521`), 'summary should reference totalCount');
+  assert.ok(body.includes(`… and ${1521 - lib.COMMIT_BODY_LIMIT} more`), 'truncation must reflect totalCount');
+  assert.ok(!body.includes('250'), 'API window size 250 should never appear in the body');
+});
+
+test('formatIssueBody omits the truncation line when total fits in display', () => {
+  const commits = [
+    { sha: 'a'.repeat(40), author: 'x', message: 'm1' },
+    { sha: 'b'.repeat(40), author: 'y', message: 'm2' },
+  ];
+  const body = lib.formatIssueBody({
+    commits,
+    lastSyncedSha: '0'.repeat(40),
+    upstreamHeadSha: '1'.repeat(40),
+    upstreamRepo: 'a/b',
+    totalCount: 2,
+  });
+  assert.ok(body.includes('Commits (2)'), 'summary should be the simple form when nothing is omitted');
+  assert.ok(!body.includes('more.'), 'no truncation note when omitted === 0');
 });
 
 test('formatIssueBody rejects non-array commits', () => {
   assert.throws(
-    () => lib.formatIssueBody({ commits: null, lastSyncedSha: '0', upstreamHeadSha: '1', upstreamRepo: 'a/b' }),
+    () => lib.formatIssueBody({ commits: null, lastSyncedSha: '0', upstreamHeadSha: '1', upstreamRepo: 'a/b', totalCount: 0 }),
     /must be an array/,
   );
 });
 
 test('formatIssueBody rejects malformed upstreamRepo', () => {
   assert.throws(
-    () => lib.formatIssueBody({ commits: [], lastSyncedSha: '0', upstreamHeadSha: '1', upstreamRepo: 'no-slash' }),
+    () => lib.formatIssueBody({ commits: [], lastSyncedSha: '0', upstreamHeadSha: '1', upstreamRepo: 'no-slash', totalCount: 0 }),
     /owner\/repo/,
   );
 });
 
+test('formatIssueBody rejects non-integer or negative totalCount', () => {
+  assert.throws(
+    () => lib.formatIssueBody({ commits: [], lastSyncedSha: '0', upstreamHeadSha: '1', upstreamRepo: 'a/b', totalCount: -1 }),
+    /non-negative integer/,
+  );
+  assert.throws(
+    () => lib.formatIssueBody({ commits: [], lastSyncedSha: '0', upstreamHeadSha: '1', upstreamRepo: 'a/b', totalCount: 1.5 }),
+    /non-negative integer/,
+  );
+});
+
 // extractCommitsForBody
 
 test('extractCommitsForBody flattens the compare API shape', () => {