diff --git a/.circleci/config.yml b/.circleci/config.yml
index 1cb4baadc3..74dd57fb15 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -151,11 +151,33 @@ jobs:
       - setup_node_modules
       - run:
           name: Download artifacts for base revision
+          # TODO: The download-experimental-build.js script works by fetching
+          # artifacts from CI. CircleCI recently updated this endpoint to
+          # require an auth token. This is a problem for PR branches, where
+          # sizebot needs to run, because we don't want to leak the token to
+          # arbitrary code written by an outside contributor.
+          #
+          # This only affects PR branches. CI workflows that run on the main
+          # branch are allowed to access environment variables, because only those
+          # with push access can land code in main.
+          #
+          # As a temporary workaround, we'll fetch the assets from a mirror.
+          # Need to figure out a longer term solution for this.
+          #
+          # Original code
+          #
+          # command: |
+          #     git fetch origin main
+          #     cd ./scripts/release && yarn && cd ../../
+          #     scripts/release/download-experimental-build.js --commit=$(git merge-base HEAD origin/main) --allowBrokenCI
+          #     mv ./build ./base-build
+          #
+          # Workaround. Fetch the artifacts from react-builds.vercel.app. This
+          # is the same app that hosts the sizebot diff previews.
           command: |
-              git fetch origin main
-              cd ./scripts/release && yarn && cd ../../
-              scripts/release/download-experimental-build.js --commit=$(git merge-base HEAD origin/main) --allowBrokenCI
+            curl -L --retry 60 --retry-delay 10 --retry-max-time 600 https://react-builds.vercel.app/api/commits/$(git merge-base HEAD origin/main)/artifacts/build.tgz | tar -xz
               mv ./build ./base-build
+
       - run:
           # TODO: The `download-experimental-build` script copies the npm
           # packages into the `node_modules` directory. This is a historical