diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3ea82211a..72850b95e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -33,8 +33,26 @@ jobs:
             cpu: amd64
           - os: windows
             cpu: amd64
-        branch: [~, upstream/version-1-6]
+        branch: [~, upstream/version-1-6, upstream/version-2-0]
+        exclude:
+          - target:
+              os: macos
+            branch: upstream/version-1-6
+          - target:
+              os: macos
+            branch: upstream/version-2-0
+          - target:
+              os: windows
+            branch: upstream/version-1-6
+          - target:
+              os: windows
+            branch: upstream/version-2-0
         include:
+          - branch: upstream/version-1-6
+            branch-short: version-1-6
+          - branch: upstream/version-2-0
+            branch-short: version-2-0
+            nimflags-extra: --mm:refc
           - target:
               os: linux
             builder: ubuntu-22.04
@@ -52,7 +70,7 @@ jobs:
       run:
         shell: ${{ matrix.shell }}
 
-    name: ${{ matrix.target.os }}-${{ matrix.target.cpu }}${{ matrix.branch != '' && ' (Nim ' || '' }}${{ matrix.branch }}${{ matrix.branch != '' && ')' || '' }}
+    name: ${{ matrix.target.os }}-${{ matrix.target.cpu }}${{ matrix.branch != '' && ' (Nim ' || '' }}${{ matrix.branch-short }}${{ matrix.branch != '' && ')' || '' }}
     runs-on: ${{ matrix.builder }}
     steps:
       - name: Checkout
@@ -112,9 +130,11 @@ jobs:
             else
               export NIMFLAGS="${NIMFLAGS} -d:limitStackUsage"
             fi
-            echo "NIMFLAGS=${NIMFLAGS}" >> $GITHUB_ENV
           fi
 
+          export NIMFLAGS="${NIMFLAGS} ${{ matrix.nimflags-extra }}"
+          echo "NIMFLAGS=${NIMFLAGS}" >> $GITHUB_ENV
+
           ncpu=""
           make_cmd="make"
           case '${{ runner.os }}' in
@@ -152,7 +172,7 @@ jobs:
 
       - name: Build binaries (with trace logging enabled)
         run: |
-          ${make_cmd} -j ${ncpu} V=1 NIM_COMMIT=${{ matrix.branch }} LOG_LEVEL=TRACE NIMFLAGS="-u:release --opt:none"
+          ${make_cmd} -j ${ncpu} V=1 NIM_COMMIT=${{ matrix.branch }} LOG_LEVEL=TRACE NIMFLAGS="-u:release --opt:none ${{ matrix.nimflags-extra }}"
           # The Windows image runs out of disk space, so make some room
           rm -rf build nimcache