diff --git a/.github/workflows/build_spack.yml b/.github/workflows/build_spack.yml
index 22b6f6e..16bcc73 100644
--- a/.github/workflows/build_spack.yml
+++ b/.github/workflows/build_spack.yml
@@ -5,7 +5,7 @@ on:
     tags:
       - '*'
     branches:
-      - '*'
+      - 'main'
   pull_request:
   workflow_dispatch:
   schedule:
diff --git a/spack.yaml b/spack.yaml
index 2519902..74c4c1b 100644
--- a/spack.yaml
+++ b/spack.yaml
@@ -2,14 +2,14 @@ spack:
   specs:
     - boost@1.86.0 +program_options +test +json
     - cmake@3.30.1
-    - edm4hep@0.10.5
+    - edm4hep@0.10.5 # 0.99.1 blocked due to clang14 in our CI
     - eigen@3.4.0
     - git-lfs@3.5.1
     - intel-tbb@2021.12.0
     - nlohmann-json@3.11.3
-    - podio@1.0.1
+    - podio@0.99
     - root@6.32.06 +aqua+opengl
-    - dd4hep@1.29 +xercesc
+    - dd4hep@1.29 +xercesc +edm4hep +hepmc3
     - geomodel@6.3.0 +geomodelg4
     - python@3.12
     - py-pybind11 @2.13.1
@@ -28,7 +28,7 @@ spack:
       variants: cxxstd=20 build_type=Release
 
   concretizer:
-    unify: when_possible
+    unify: true
 
   config:
     install_tree:
@@ -42,3 +42,17 @@ spack:
       access_pair:
         id_variable: GH_OCI_USER
         secret_variable: GH_OCI_TOKEN
+  compilers:
+  - compiler:
+      spec: apple-clang@=16.0.0
+      paths:
+        cc: /usr/bin/clang
+        cxx: /usr/bin/clang++
+        f77: null
+        fc: null
+      flags: {}
+      operating_system: sequoia
+      target: aarch64
+      modules: []
+      environment: {}
+      extra_rpaths: []