diff --git a/.github/workflows/ocr-review.yml b/.github/workflows/ocr-review.yml
index 6c0806d2..1933c0fe 100644
--- a/.github/workflows/ocr-review.yml
+++ b/.github/workflows/ocr-review.yml
@@ -23,8 +23,9 @@ concurrency:
 
 on:
   # Use pull_request_target instead of pull_request so that secrets are
-  # available even for PRs from forks. This is safe because OCR only reads
-  # the diff and does not execute any code from the PR.
+  # available even for PRs from forks. Security: no code is checked out;
+  # file contents are fetched via the GitHub REST API and assembled into
+  # a synthetic repo for diff computation only.
   pull_request_target:
     types: [opened]
 
@@ -39,17 +40,105 @@ jobs:
       image: node:20
     if: github.event_name == 'pull_request_target'
     steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
+      - name: Download base repository snapshot
+        run: |
+          mkdir -p /tmp/review-repo
+          curl -sL \
+            -H "Authorization: Bearer ${{ github.token }}" \
+            "https://api.github.com/repos/${{ github.repository }}/tarball/${{ github.event.pull_request.base.sha }}" \
+            | tar xz --strip-components=1 -C /tmp/review-repo
+          cd /tmp/review-repo
+          git init
+          git config user.email "ocr@ci"
+          git config user.name "OCR CI"
+          git add -A
+          git commit -m "base"
+
+      - name: Apply PR changes to review repository
+        uses: actions/github-script@v7
         with:
-          fetch-depth: 0  # Full history needed for merge-base diff
-          ref: ${{ github.event.pull_request.head.sha }}
+          script: |
+            const fs = require('fs');
+            const path = require('path');
+            const { execSync } = require('child_process');
 
-      - name: Mark repository as safe directory
-        run: git config --global --add safe.directory '*'
+            const repoDir = '/tmp/review-repo';
+            const run = (cmd) => execSync(cmd, { cwd: repoDir, encoding: 'utf8' });
+
+            const owner = context.repo.owner;
+            const repo = context.repo.repo;
+            const prNumber = context.issue.number;
+            const headSha = context.payload.pull_request.head.sha;
+            const headOwner = context.payload.pull_request.head.repo.owner.login;
+            const headRepo = context.payload.pull_request.head.repo.name;
+
+            const files = [];
+            for (let page = 1; ; page++) {
+              const resp = await github.rest.pulls.listFiles({
+                owner, repo, pull_number: prNumber, per_page: 100, page
+              });
+              files.push(...resp.data);
+              if (resp.data.length < 100) break;
+            }
+            console.log(`PR has ${files.length} changed file(s)`);
 
-      - name: Fetch PR head ref (ensures fork commits are available)
-        run: git fetch origin pull/${{ github.event.pull_request.number }}/head
+            function safePath(base, rel) {
+              const resolved = path.resolve(base, rel);
+              if (!resolved.startsWith(base + '/') && resolved !== base) return null;
+              return resolved;
+            }
+
+            async function fetchContent(fileOwner, fileRepo, filePath, ref) {
+              try {
+                const resp = await github.rest.repos.getContent({
+                  owner: fileOwner, repo: fileRepo, path: filePath, ref
+                });
+                if (resp.data.encoding === 'base64' && resp.data.content) {
+                  return Buffer.from(resp.data.content, 'base64');
+                }
+                if (resp.data.download_url) {
+                  const raw = await fetch(resp.data.download_url);
+                  if (!raw.ok) return null;
+                  return Buffer.from(await raw.arrayBuffer());
+                }
+                if (resp.data.content) {
+                  return Buffer.from(resp.data.content, 'utf8');
+                }
+                return null;
+              } catch (e) {
+                if (e.status === 404 || e.status === 403) return null;
+                throw e;
+              }
+            }
+
+            for (const file of files) {
+              if (file.status === 'removed') {
+                const fullPath = safePath(repoDir, file.filename);
+                if (fullPath && fs.existsSync(fullPath)) fs.unlinkSync(fullPath);
+                continue;
+              }
+              if (file.status === 'renamed' && file.previous_filename) {
+                const oldPath = safePath(repoDir, file.previous_filename);
+                if (oldPath && fs.existsSync(oldPath)) fs.unlinkSync(oldPath);
+              }
+              const fullPath = safePath(repoDir, file.filename);
+              if (!fullPath) {
+                console.log(`Skipping ${file.filename}: path traversal detected`);
+                continue;
+              }
+              const content = await fetchContent(headOwner, headRepo, file.filename, headSha);
+              if (content === null) {
+                console.log(`Skipping ${file.filename}: could not fetch at head`);
+                continue;
+              }
+              fs.mkdirSync(path.dirname(fullPath), { recursive: true });
+              fs.writeFileSync(fullPath, content);
+            }
+            run('git add -A');
+            run('git commit --allow-empty -m "head"');
+
+            console.log('Review repo ready');
+            run('git log --oneline');
 
       - name: Install OpenCodeReview
         run: npm install -g @alibaba-group/open-code-review
@@ -66,15 +155,12 @@ jobs:
       - name: Run OpenCodeReview
         id: review
         run: |
-          BASE_REF="${{ github.event.pull_request.base.ref }}"
-          HEAD_SHA="${{ github.event.pull_request.head.sha }}"
-
-          echo "Reviewing PR: ${HEAD_SHA} against origin/${BASE_REF}"
+          echo "Reviewing PR in synthetic repo"
 
-          # Run OCR in range mode with JSON output
+          cd /tmp/review-repo
           ocr review \
-            --from "origin/${BASE_REF}" \
-            --to "${HEAD_SHA}" \
+            --from HEAD~1 \
+            --to HEAD \
             --format json \
             > /tmp/ocr-result.json 2>/tmp/ocr-stderr.log || true