diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 2f3ce49e15ec..454fc692ae3b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -365,6 +365,9 @@ jobs:
       - name: Validate package test selection
         run: python3 tests/test_ci_select_package_tests.py
 
+      - name: Validate shallow complexity comparison fetching
+        run: python3 tests/test_ci_fetch_complexity_base.py
+
       - name: Validate Ghostty helper cache failure handling
         run: bash tests/test_ghostty_cli_helper_cache.sh
 
diff --git a/.github/workflows/web-complexity.yml b/.github/workflows/web-complexity.yml
index 80edebbc9345..37ef381a1692 100644
--- a/.github/workflows/web-complexity.yml
+++ b/.github/workflows/web-complexity.yml
@@ -29,9 +29,14 @@ jobs:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
-          fetch-depth: 0
+          fetch-depth: 1
           persist-credentials: false
 
+      - name: Fetch comparison revision
+        env:
+          COMPARISON_SHA: ${{ github.event.pull_request.base.sha || github.event.before }}
+        run: bash scripts/ci/fetch-complexity-base.sh "$COMPARISON_SHA"
+
       - name: Detect web changes
         id: scope
         shell: bash
diff --git a/scripts/ci/fetch-complexity-base.sh b/scripts/ci/fetch-complexity-base.sh
new file mode 100644
index 000000000000..9b4188fa7574
--- /dev/null
+++ b/scripts/ci/fetch-complexity-base.sh
@@ -0,0 +1,17 @@
+#!/bin/bash
+# Complexity compares trees and baseline files, not ancestry. Fetch the exact
+# comparison commit without downloading the repository's entire history.
+set -euo pipefail
+
+revision="${1:-}"
+if [ -z "$revision" ] || [ "$revision" = "0000000000000000000000000000000000000000" ]; then
+  exit 0
+fi
+if [[ ! "$revision" =~ ^[0-9a-fA-F]{40}$ ]]; then
+  echo "Expected a full comparison commit SHA" >&2
+  exit 2
+fi
+if ! git cat-file -e "${revision}^{commit}" 2>/dev/null; then
+  git -c protocol.version=2 fetch --no-tags --depth=1 origin "$revision"
+fi
+git cat-file -e "${revision}^{commit}"
diff --git a/tests/test_ci_fetch_complexity_base.py b/tests/test_ci_fetch_complexity_base.py
new file mode 100644
index 000000000000..4a2fc32101d0
--- /dev/null
+++ b/tests/test_ci_fetch_complexity_base.py
@@ -0,0 +1,73 @@
+#!/usr/bin/env python3
+"""Exercise comparison-commit fetching against actual shallow Git repositories."""
+
+from pathlib import Path
+import subprocess
+import tempfile
+import unittest
+
+ROOT = Path(__file__).resolve().parents[1]
+HELPER = ROOT / "scripts/ci/fetch-complexity-base.sh"
+
+
+class ComparisonBaseTests(unittest.TestCase):
+    def setUp(self):
+        self.temp = tempfile.TemporaryDirectory()
+        self.addCleanup(self.temp.cleanup)
+        root = Path(self.temp.name)
+        self.origin = root / "origin"
+        self.origin.mkdir()
+        self.git(self.origin, "init", "-q")
+        (self.origin / "web").mkdir()
+        (self.origin / "web/oxlint-complexity-baseline.txt").write_text("original baseline\n")
+        self.git(self.origin, "add", ".")
+        self.git(self.origin, "commit", "-qm", "base")
+        self.base = self.git(self.origin, "rev-parse", "HEAD")
+        for number in range(8):
+            (self.origin / "web/change.ts").write_text(f"export const value = {number};\n")
+            self.git(self.origin, "add", ".")
+            self.git(self.origin, "commit", "-qm", f"change {number}")
+        self.head = self.git(self.origin, "rev-parse", "HEAD")
+        self.clone = root / "checkout"
+        self.git(root, "clone", "-q", "--depth=1", self.origin.as_uri(), str(self.clone))
+
+    def git(self, cwd, *args):
+        return subprocess.check_output([
+            "git", "-c", "user.name=CI", "-c", "user.email=ci@example.test",
+            "-c", "core.hooksPath=/dev/null", *args,
+        ], cwd=cwd, text=True, stderr=subprocess.PIPE).strip()
+
+    def fetch(self, revision):
+        return subprocess.run(["bash", str(HELPER), revision], cwd=self.clone,
+                              text=True, capture_output=True)
+
+    def test_old_base_is_available_without_unshallowing_or_moving_head(self):
+        self.assertEqual(self.git(self.clone, "rev-list", "--count", "HEAD"), "1")
+        result = self.fetch(self.base)
+        self.assertEqual(result.returncode, 0, result.stderr)
+        self.assertEqual(self.git(self.clone, "show", f"{self.base}:web/oxlint-complexity-baseline.txt"),
+                         "original baseline")
+        self.assertEqual(self.git(self.clone, "diff", "--name-only", self.base, self.head), "web/change.ts")
+        self.assertEqual(self.git(self.clone, "rev-parse", "HEAD"), self.head)
+        self.assertEqual(self.git(self.clone, "rev-parse", "--is-shallow-repository"), "true")
+        self.assertEqual(self.git(self.clone, "rev-list", "--count", self.head, self.base), "2")
+
+    def test_available_commit_needs_no_network(self):
+        self.git(self.clone, "remote", "set-url", "origin", "/nonexistent/cmux-comparison-test")
+        result = self.fetch(self.head)
+        self.assertEqual(result.returncode, 0, result.stderr)
+
+    def test_initial_push_without_a_base_needs_no_network(self):
+        self.git(self.clone, "remote", "set-url", "origin", "/nonexistent/cmux-comparison-test")
+        for revision in ("", "0" * 40):
+            result = self.fetch(revision)
+            self.assertEqual(result.returncode, 0, result.stderr)
+
+    def test_invalid_or_unavailable_base_fails(self):
+        for revision in ("main", "--upload-pack=anything", "a" * 40):
+            result = self.fetch(revision)
+            self.assertNotEqual(result.returncode, 0)
+
+
+if __name__ == "__main__":
+    unittest.main()