diff --git a/apps/desktop/electron/main.cjs b/apps/desktop/electron/main.cjs
index 4ecefa4604b..ce42e3474dc 100644
--- a/apps/desktop/electron/main.cjs
+++ b/apps/desktop/electron/main.cjs
@@ -57,6 +57,7 @@ const {
 const { gitRootForIpc } = require('./git-root.cjs')
 const { worktreesForIpc } = require('./git-worktrees.cjs')
 const { OFFICIAL_REPO_HTTPS_URL, isOfficialSshRemote } = require('./update-remote.cjs')
+const { resolveBehindCount, shouldCountCommits } = require('./update-count.cjs')
 const { runRebuildWithRetry } = require('./update-rebuild.cjs')
 const {
   buildPosixCleanupScript,
@@ -1706,15 +1707,34 @@ async function checkUpdates() {
   }
 
   const git = args => runGit(args, { cwd: updateRoot }).then(r => r.stdout.trim())
-  const [currentSha, targetSha, countStr, dirtyStr, currentBranch] = await Promise.all([
+  const [currentSha, targetSha, dirtyStr, currentBranch, shallowStr, mergeBaseStr] = await Promise.all([
     git(['rev-parse', 'HEAD']),
     git(['rev-parse', `origin/${branch}`]),
-    git(['rev-list', `HEAD..origin/${branch}`, '--count']),
     git(['status', '--porcelain']),
-    git(['rev-parse', '--abbrev-ref', 'HEAD'])
+    git(['rev-parse', '--abbrev-ref', 'HEAD']),
+    git(['rev-parse', '--is-shallow-repository']),
+    // merge-base exits non-zero with empty stdout when HEAD shares no common
+    // ancestor with the freshly fetched tip — exactly the shallow-clone case.
+    git(['merge-base', 'HEAD', `origin/${branch}`])
   ])
 
-  const behind = Number.parseInt(countStr, 10) || 0
+  const isShallow = shallowStr === 'true'
+  const hasMergeBase = Boolean(mergeBaseStr)
+  // Only enumerate the commit count when it is meaningful. On a shallow checkout
+  // with no merge-base, `rev-list --count` walks the entire remote ancestry
+  // (thousands of commits, see #51922) and resolveBehindCount discards the
+  // result anyway in favour of a SHA compare — so skip the expensive query.
+  const countStr = shouldCountCommits({ isShallow, hasMergeBase })
+    ? await git(['rev-list', `HEAD..origin/${branch}`, '--count'])
+    : ''
+
+  const behind = resolveBehindCount({
+    countStr,
+    currentSha,
+    targetSha,
+    isShallow,
+    hasMergeBase
+  })
   const commits = behind > 0 ? await readCommitLog(updateRoot, branch) : []
 
   return {
diff --git a/apps/desktop/electron/update-count.cjs b/apps/desktop/electron/update-count.cjs
new file mode 100644
index 00000000000..de8d57c4ee6
--- /dev/null
+++ b/apps/desktop/electron/update-count.cjs
@@ -0,0 +1,28 @@
+'use strict'
+
+// Whether `git rev-list HEAD..origin/<branch> --count` produces a meaningful
+// number worth computing. On a SHALLOW checkout (installer clones with
+// --depth 1) the local history often shares no merge-base with the freshly
+// fetched origin tip, so the count enumerates the entire remote ancestry and
+// returns a bogus huge number (e.g. 12104) — see #51922. resolveBehindCount
+// discards that bogus count in favour of a SHA compare, so the caller should
+// SKIP the expensive rev-list entirely in that case rather than run it and
+// throw the result away.
+function shouldCountCommits({ isShallow, hasMergeBase }) {
+  return !(isShallow && !hasMergeBase)
+}
+
+// Resolve how many commits the local checkout is behind origin for the desktop
+// update indicator. When the count isn't meaningful (shallow + no merge-base)
+// fall back to a binary up-to-date check by SHA, exactly like the official-SSH
+// path in checkUpdates() and the CLI guard in hermes_cli/banner.py. Full clones
+// (developers / Docker dev images) keep the exact count path unchanged.
+function resolveBehindCount({ countStr, currentSha, targetSha, isShallow, hasMergeBase }) {
+  if (!shouldCountCommits({ isShallow, hasMergeBase })) {
+    if (currentSha && targetSha && currentSha === targetSha) return 0
+    return 1 // behind by an unknown amount — show a generic "update available"
+  }
+  return Number.parseInt(countStr, 10) || 0
+}
+
+module.exports = { resolveBehindCount, shouldCountCommits }
diff --git a/apps/desktop/electron/update-count.test.cjs b/apps/desktop/electron/update-count.test.cjs
new file mode 100644
index 00000000000..69ee99aa616
--- /dev/null
+++ b/apps/desktop/electron/update-count.test.cjs
@@ -0,0 +1,79 @@
+'use strict'
+const test = require('node:test')
+const assert = require('node:assert/strict')
+const { resolveBehindCount, shouldCountCommits } = require('./update-count.cjs')
+
+// FAIL-BEFORE: pre-fix the function did `Number.parseInt(countStr) || 0`
+// unconditionally, so a shallow checkout with no merge-base surfaced the bogus
+// rev-list count (e.g. 12104). This asserts the new shallow/no-merge-base branch.
+test('shallow checkout with no merge-base does NOT trust the bogus rev-list count', () => {
+  assert.equal(resolveBehindCount({
+    countStr: '12104', currentSha: 'aaa', targetSha: 'bbb',
+    isShallow: true, hasMergeBase: false,
+  }), 1)
+})
+
+test('shallow checkout with no merge-base but identical SHA reports up-to-date', () => {
+  assert.equal(resolveBehindCount({
+    countStr: '12104', currentSha: 'abc', targetSha: 'abc',
+    isShallow: true, hasMergeBase: false,
+  }), 0)
+})
+
+test('shallow checkout WITH a merge-base keeps the exact count (reliable)', () => {
+  assert.equal(resolveBehindCount({
+    countStr: '3', currentSha: 'aaa', targetSha: 'bbb',
+    isShallow: true, hasMergeBase: true,
+  }), 3)
+})
+
+test('full (non-shallow) clone keeps the exact count path unchanged', () => {
+  assert.equal(resolveBehindCount({
+    countStr: '7', currentSha: 'aaa', targetSha: 'bbb',
+    isShallow: false, hasMergeBase: true,
+  }), 7)
+})
+
+test('up-to-date full clone reports 0', () => {
+  assert.equal(resolveBehindCount({
+    countStr: '0', currentSha: 'x', targetSha: 'x',
+    isShallow: false, hasMergeBase: true,
+  }), 0)
+})
+
+test('non-numeric count falls back to 0 (defensive, unchanged behaviour)', () => {
+  assert.equal(resolveBehindCount({
+    countStr: '', currentSha: 'aaa', targetSha: 'bbb',
+    isShallow: false, hasMergeBase: true,
+  }), 0)
+})
+
+// shouldCountCommits gates the expensive `rev-list --count` in checkUpdates().
+// FAIL-BEFORE: in the shallow + no-merge-base case the caller ran rev-list
+// unconditionally and discarded the bogus result; this predicate lets the
+// caller SKIP the whole-ancestry enumeration in exactly that case (#51922).
+test('shallow checkout with no merge-base SKIPS the rev-list count', () => {
+  assert.equal(shouldCountCommits({ isShallow: true, hasMergeBase: false }), false)
+})
+
+test('shallow checkout WITH a merge-base still runs the count', () => {
+  assert.equal(shouldCountCommits({ isShallow: true, hasMergeBase: true }), true)
+})
+
+test('full (non-shallow) clone always runs the count', () => {
+  assert.equal(shouldCountCommits({ isShallow: false, hasMergeBase: true }), true)
+  assert.equal(shouldCountCommits({ isShallow: false, hasMergeBase: false }), true)
+})
+
+// The skip path produces an empty countStr; resolveBehindCount must NOT trust
+// it and must fall through to the SHA compare (mirrors the live call site).
+test('skipped-count path resolves via SHA compare, never via empty countStr', () => {
+  assert.equal(resolveBehindCount({
+    countStr: '', currentSha: 'aaa', targetSha: 'bbb',
+    isShallow: true, hasMergeBase: false,
+  }), 1)
+  assert.equal(resolveBehindCount({
+    countStr: '', currentSha: 'same', targetSha: 'same',
+    isShallow: true, hasMergeBase: false,
+  }), 0)
+})
diff --git a/apps/desktop/package.json b/apps/desktop/package.json
index 2661f8af18b..5ab50597139 100644
--- a/apps/desktop/package.json
+++ b/apps/desktop/package.json
@@ -37,7 +37,7 @@
     "test:desktop:nsis": "node scripts/test-desktop.mjs nsis",
     "test:desktop:existing": "node scripts/test-desktop.mjs existing",
     "test:desktop:fresh": "node scripts/test-desktop.mjs fresh",
-    "test:desktop:platforms": "node --test electron/bootstrap-platform.test.cjs electron/hardening.test.cjs electron/backend-env.test.cjs electron/backend-probes.test.cjs electron/backend-ready.test.cjs electron/bootstrap-runner.test.cjs electron/connection-config.test.cjs electron/dashboard-token.test.cjs electron/gateway-ws-probe.test.cjs electron/oauth-net-request.test.cjs electron/desktop-uninstall.test.cjs electron/session-windows.test.cjs electron/link-title-window.test.cjs electron/workspace-cwd.test.cjs electron/fs-read-dir.test.cjs electron/git-root.test.cjs electron/windows-child-process.test.cjs electron/update-remote.test.cjs electron/update-rebuild.test.cjs electron/update-marker.test.cjs electron/update-relaunch.test.cjs electron/windows-user-env.test.cjs electron/window-state.test.cjs",
+    "test:desktop:platforms": "node --test electron/bootstrap-platform.test.cjs electron/hardening.test.cjs electron/backend-env.test.cjs electron/backend-probes.test.cjs electron/backend-ready.test.cjs electron/bootstrap-runner.test.cjs electron/connection-config.test.cjs electron/dashboard-token.test.cjs electron/gateway-ws-probe.test.cjs electron/oauth-net-request.test.cjs electron/desktop-uninstall.test.cjs electron/session-windows.test.cjs electron/link-title-window.test.cjs electron/workspace-cwd.test.cjs electron/fs-read-dir.test.cjs electron/git-root.test.cjs electron/windows-child-process.test.cjs electron/update-remote.test.cjs electron/update-count.test.cjs electron/update-rebuild.test.cjs electron/update-marker.test.cjs electron/update-relaunch.test.cjs electron/windows-user-env.test.cjs electron/window-state.test.cjs",
     "typecheck": "tsc -p . --noEmit",
     "lint": "eslint src/ electron/",
     "lint:fix": "eslint src/ electron/ --fix",