diff --git a/tools/rebase-pr.js b/tools/rebase-pr.js
index eee42e4611..19c66ec03c 100644
--- a/tools/rebase-pr.js
+++ b/tools/rebase-pr.js
@@ -59,24 +59,73 @@ _main(...process.argv.slice(2)).catch(err => {
 
 // Helpers
 async function _main(repository, prNumber) {
-  console.log(`Determining target branch for PR ${prNumber} on ${repository}.`);
-  const targetBranch = await determineTargetBranch(repository, prNumber);
-  console.log(`Target branch is ${targetBranch}.`);
-  await exec(`git fetch origin ${targetBranch}`);
-  console.log(`Rebasing current branch on ${targetBranch}.`);
-  await exec(`git rebase origin/${targetBranch}`);
+  console.log(`Getting refs and SHAs for PR ${prNumber} on ${repository}.`);
+  const target = await determineTargetRefAndSha(repository, prNumber);
+  console.log(`Fetching target branch: ${target.baseRef}.`);
+  await exec(`git fetch origin ${target.baseRef}`);
+
+  // The sha of the latest commit on the target branch.
+  const {stdout: shaOfTargetBranchLatest} = await exec(`git rev-parse origin/${target.baseRef}`);
+  // The sha of the latest commit on the PR.
+  const {stdout: shaOfPRLatest} = await exec(`git rev-parse HEAD`);
+  // The first common SHA in the history of the target branch and the latest commit in the PR.
+  const {stdout: commonAncestorSha} =
+      await exec(`git merge-base origin/${target.baseRef} ${shaOfPRLatest}`);
+
+  // Log known refs and shas
+  console.log(`--------------------------------`);
+  console.log(`    Target Branch:                   ${target.baseRef}`);
+  console.log(`    Latest Commit for Target Branch: ${shaOfTargetBranchLatest.trim()}`);
+  console.log(`    Latest Commit for PR:            ${shaOfPRLatest.trim()}`);
+  console.log(`    First Common Ancestor SHA:       ${commonAncestorSha.trim()}`);
+  console.log(`--------------------------------`);
+  console.log();
+
+
+  // Get the count of commits between the latest commit from origin and the common ancestor SHA.
+  const {stdout: commitCount} =
+      await exec(`git rev-list --count origin/${target.baseRef}...${commonAncestorSha.trim()}`);
+  console.log(`Checking ${commitCount.trim()} commits for changes in the CircleCI config file.`);
+
+  // Check if the files changed between the latest commit from origin and the common ancestor SHA
+  // includes the CircleCI config.
+  const {stdout: circleCIConfigChanged} = await exec(
+      `git diff --name-only origin/${target.baseRef} ${commonAncestorSha.trim()} -- .circleci/config.yml`);
+
+  if (!!circleCIConfigChanged) {
+    throw Error(`
+        CircleCI config on ${target.baseRef} has been modified since commit ${commonAncestorSha.slice(0, 7)},
+        which this PR is based on.
+
+        Please rebase the PR on ${target.baseRef} after fetching from upstream.
+
+        Rebase instructions for PR Author, please run the following commands:
+
+          git fetch upstream ${target.baseRef};
+          git checkout ${target.headRef};
+          git rebase origin/${target.baseRef};
+          git push --force-with-lease;
+        `);
+  } else {
+    console.log('No change found in the CircleCI config file, continuing.');
+  }
+  console.log();
+
+  // Rebase the PR.
+  console.log(`Rebasing current branch on ${target.baseRef}.`);
+  await exec(`git rebase origin/${target.baseRef}`);
   console.log('Rebase successful.');
+
 }
 
-function determineTargetBranch(repository, prNumber) {
-  const pullsUrl = `https://api.github.com/repos/${repository}/pulls/${prNumber}`;
+async function requestDataFromGithub(url) {
   // GitHub requires a user agent: https://developer.github.com/v3/#user-agent-required
-  const options = {headers: {'User-Agent': repository}};
+  const options = {headers: {'User-Agent': 'angular'}};
 
   return new Promise((resolve, reject) => {
     https
         .get(
-            pullsUrl, options,
+            url, options,
             (res) => {
               const {statusCode} = res;
               const contentType = res.headers['content-type'];
@@ -84,7 +133,6 @@ function determineTargetBranch(repository, prNumber) {
 
               res.on('data', (chunk) => { rawData += chunk; });
               res.on('end', () => {
-
                 let error;
                 if (statusCode !== 200) {
                   error = new Error(
@@ -101,8 +149,7 @@ function determineTargetBranch(repository, prNumber) {
                 }
 
                 try {
-                  const parsedData = JSON.parse(rawData);
-                  resolve(parsedData['base']['ref']);
+                  resolve(JSON.parse(rawData));
                 } catch (e) {
                   reject(e);
                 }
@@ -111,3 +158,15 @@ function determineTargetBranch(repository, prNumber) {
         .on('error', (e) => { reject(e); });
   });
 }
+
+async function determineTargetRefAndSha(repository, prNumber) {
+  const pullsUrl = `https://api.github.com/repos/${repository}/pulls/${prNumber}`;
+
+  const result = await requestDataFromGithub(pullsUrl);
+  return {
+    baseRef: result.base.ref,
+    baseSha: result.base.sha,
+    headRef: result.head.ref,
+    headSha: result.head.sha,
+  };
+}