diff --git a/.github/workflows/build-one.yml b/.github/workflows/build-one.yml
index a33d7b2df..dd36bd380 100644
--- a/.github/workflows/build-one.yml
+++ b/.github/workflows/build-one.yml
@@ -6,7 +6,10 @@ on:
         description: 'Platform to build (e.g. hi3518ev200_lite_switcam-hs303-v2)'
         required: true
       commit:
-        description: 'Commit SHA to build (optional; defaults to current branch HEAD). Use this from `git bisect run` or for one-off historic rebuilds.'
+        description: 'Builder commit SHA to build (optional; defaults to current branch HEAD). Use this from `git bisect run` or for one-off historic rebuilds.'
+        required: false
+      firmware_ref:
+        description: 'OpenIPC/firmware ref (SHA/tag/branch) to clone — for cross-repo bisect of size/regression issues. Defaults to firmware HEAD.'
         required: false
 
 jobs:
@@ -60,6 +63,7 @@ jobs:
           BUILD_ID:       ${{ needs.resolve.outputs.tag_name }}
           BUILD_SHA:      ${{ needs.resolve.outputs.ref }}
           BUILD_PLATFORM: ${{ inputs.platform }}
+          OPENIPC_FW_REV: ${{ inputs.firmware_ref }}
         run: |
           export GIT_HASH=$(git rev-parse --short ${GITHUB_SHA})
           export GIT_BRANCH=${GITHUB_REF_NAME}
diff --git a/builder.sh b/builder.sh
index 1a5569935..aadf7c6c9 100755
--- a/builder.sh
+++ b/builder.sh
@@ -124,9 +124,18 @@ echo_c 33 "\nUpdating Builder"
 git pull
 
 rm -rf openipc
+# OPENIPC_FW_REV pins firmware to a specific ref (branch, tag, or SHA) for
+# cross-repo bisect of size/regression issues — set by build-one.yml's
+# firmware_ref input. When unset, clones HEAD of master as before.
 if [ ! -d "$FIRMWARE_DIR" ]; then
-    echo_c 33 "\nDownloading Firmware"
-    git clone --depth=1 https://github.com/OpenIPC/firmware.git "$FIRMWARE_DIR"
+    if [ -n "$OPENIPC_FW_REV" ]; then
+        echo_c 33 "\nDownloading Firmware @ ${OPENIPC_FW_REV}"
+        git clone https://github.com/OpenIPC/firmware.git "$FIRMWARE_DIR"
+        git -C "$FIRMWARE_DIR" checkout "$OPENIPC_FW_REV"
+    else
+        echo_c 33 "\nDownloading Firmware"
+        git clone --depth=1 https://github.com/OpenIPC/firmware.git "$FIRMWARE_DIR"
+    fi
     cd "$FIRMWARE_DIR"
 else
     echo_c 33 "\nUpdating Firmware"