diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d64c025..4fd374d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -66,7 +66,7 @@ env:
   git_upstream: >-
     {
       "coatjava":          { "fork": "JeffersonLab/coatjava",          "ref": "development" },
-      "clas12Tags":        { "fork": "gemc/clas12Tags",                "ref": "5.4"         },
+      "clas12Tags":        { "fork": "gemc/clas12Tags",                "ref": ""            },
       "clas12-config":     { "fork": "JeffersonLab/clas12-config",     "ref": "main"        },
       "clas12-validation": { "fork": "JeffersonLab/clas12-validation", "ref": "main"        }
     }
@@ -124,8 +124,15 @@ jobs:
       ref_clas12config: ${{ steps.info.outputs.ref_clas12config }}
       fork_clas12validation: ${{ steps.info.outputs.fork_clas12validation }}
       ref_clas12validation: ${{ steps.info.outputs.ref_clas12validation }}
+      gemc_module_tag: ${{ steps.gemc_module_tag.outputs.gemc_module_tag }}
+      gemc_executable: ${{ steps.gemc_executable.outputs.gemc_executable }}
+      caller_repo: ${{ steps.gemc_executable.outputs.caller_repo }}
     steps:
-      - uses: actions/checkout@v4
+      - name: checkout clas12-validation
+        uses: actions/checkout@v4
+        with:
+          repository: JeffersonLab/clas12-validation
+          # ref: feature-branch # NOTE: if making a clas12-validation change that impacts this job, temporarily set the corresponding ref here
       - name: get dependency info
         id: info
         run: |
@@ -143,10 +150,10 @@ jobs:
           ref_clas12config=$(jq -r '."clas12-config".ref' deps.json)
           ref_clas12validation=$(jq -r '."clas12-validation".ref' deps.json)
           ### if any ref names are empty strings, use the highest tag instead
-          [ "$ref_coatjava" = "" ] && ref_coatjava=$(bin/get_highest_tag.rb $fork_coatjava)
-          [ "$ref_clas12tags" = "" ] && ref_clas12tags=$(bin/get_highest_tag.rb $fork_clas12tags)
-          [ "$ref_clas12config" = "" ] && ref_clas12config=$(bin/get_highest_tag.rb $fork_clas12config)
-          [ "$ref_clas12validation" = "" ] && ref_clas12validation=$(bin/get_highest_tag.rb $fork_clas12validation)
+          [ "$ref_coatjava" = "" ] && ref_coatjava=$(bin/get_highest_tag.rb JeffersonLab/coatjava) # NOTE: using the main forks here, since they have the semver tags
+          [ "$ref_clas12tags" = "" ] && ref_clas12tags=$(bin/get_highest_tag.rb gemc/clas12Tags)
+          [ "$ref_clas12config" = "" ] && ref_clas12config=$(bin/get_highest_tag.rb JeffersonLab/clas12-config)
+          [ "$ref_clas12validation" = "" ] && ref_clas12validation=$(bin/get_highest_tag.rb JeffersonLab/clas12-validation)
           ### set output vars
           echo fork_coatjava=$fork_coatjava >> $GITHUB_OUTPUT
           echo fork_clas12tags=$fork_clas12tags >> $GITHUB_OUTPUT
@@ -156,6 +163,23 @@ jobs:
           echo ref_clas12tags=$ref_clas12tags >> $GITHUB_OUTPUT
           echo ref_clas12config=$ref_clas12config >> $GITHUB_OUTPUT
           echo ref_clas12validation=$ref_clas12validation >> $GITHUB_OUTPUT
+      - name: set GEMC module tag
+        id: gemc_module_tag
+        run: |
+          ### if `ref_clas12tags` is a tag, use that; otherwise use the highest semver tag
+          ### FIXME probably won't work for triggers from backports to old `clas12Tags` tags
+          gemc_module_tag=${{ steps.info.outputs.ref_clas12tags }}
+          repo=gemc/clas12Tags
+          bin/is_a_tag.rb $gemc_module_tag $repo || gemc_module_tag=$(bin/get_highest_tag.rb $repo)
+          echo gemc_module_tag=$gemc_module_tag >> $GITHUB_OUTPUT
+      - name: set GEMC executable
+        id: gemc_executable
+        run: |
+          ### if the caller repo is `clas12Tags`, we want to locally build GEMC; if not, use the container version
+          caller_repo=$(echo "${{ github.repository }}" | sed 's;^.*/;;g')
+          [ "$caller_repo" = "clas12Tags" ] && gemc_executable=./clas12Tags/source/gemc || gemc_executable=gemc
+          echo gemc_executable=$gemc_executable >> $GITHUB_OUTPUT
+          echo caller_repo=$caller_repo >> $GITHUB_OUTPUT
       - name: dispatch summary
         run: |
           msg=$(echo '${{ github.event.pull_request.title || github.event.head_commit.message }}' | head -n1)
@@ -167,6 +191,10 @@ jobs:
           echo '| **`clas12-config` Fork and Ref:** | `${{ steps.info.outputs.fork_clas12config }}` | [`${{ steps.info.outputs.ref_clas12config }}`](https://github.com/${{ steps.info.outputs.fork_clas12config }}/tree/${{ steps.info.outputs.ref_clas12config }}) |' >> $GITHUB_STEP_SUMMARY
           echo '| **`clas12-validation` Fork and Ref:** | `${{ steps.info.outputs.fork_clas12validation }}` | [`${{ steps.info.outputs.ref_clas12validation }}`](https://github.com/${{ steps.info.outputs.fork_clas12validation }}/tree/${{ steps.info.outputs.ref_clas12validation }}) |' >> $GITHUB_STEP_SUMMARY
           echo '| | | |' >> $GITHUB_STEP_SUMMARY
+          echo '' >> $GITHUB_STEP_SUMMARY
+          echo '- Caller repository: `${{ steps.gemc_executable.outputs.caller_repo }}`' >> $GITHUB_STEP_SUMMARY
+          echo '- GEMC module: `gemc/${{ steps.gemc_module_tag.outputs.gemc_module_tag }}`' >> $GITHUB_STEP_SUMMARY
+          echo '- GEMC executable: `${{ steps.gemc_executable.outputs.gemc_executable }}`' >> $GITHUB_STEP_SUMMARY
 
   # build
   #############################################################################
@@ -206,6 +234,7 @@ jobs:
 
   build_gemc:
     name: Build GEMC
+    if: ${{ needs.dependency_info.outputs.caller_repo == 'clas12Tags' }} # only build GEMC if triggered by GEMC/clas12Tags
     needs:
       - dependency_info
     runs-on: ubuntu-latest
@@ -228,10 +257,10 @@ jobs:
           fetch-tags: true
           fetch-depth: 0
       - name: build
-        uses: docker://jeffersonlab/gemc:4.4.2-5.4-5.5-fedora36 ##### FIXME: need `latest` tag
+        uses: docker://jeffersonlab/gemc:dev-fedora36
         with:
-          entrypoint: bin/ci_gemc.sh ##### FIXME (in the script): switch to gemc/dev and use ubuntu + cvmfs action
-          args: build clas12Tags/source
+          entrypoint: bin/ci_gemc_build.sh
+          args: clas12Tags/source
       - name: tar
         run: tar cavf clas12Tags{.tar.zst,}
       - uses: actions/upload-artifact@v4
@@ -408,6 +437,7 @@ jobs:
   #############################################################################
   fulltest:
     name: Run
+    if: ${{ ! failure() && ! cancelled() }} # needed since `build_gemc` may be skipped
     needs:
       - event_generation
       - build_gemc
@@ -457,10 +487,10 @@ jobs:
           ls ${{ steps.files.outputs.gemcConfigFile }}
           ls ${{ steps.files.outputs.coatjavaConfigFile }}
       - name: simulation
-        uses: docker://jeffersonlab/gemc:4.4.2-5.4-5.5-fedora36 ##### FIXME: need `latest` tag
+        uses: docker://jeffersonlab/gemc:dev-fedora36
         with:
-          entrypoint: bin/ci_gemc.sh ##### FIXME (in the script): switch to gemc/dev and use ubuntu + cvmfs action
-          args: sim clas12Tags/source ${{ steps.files.outputs.gemcConfigFile }} ${{ steps.files.outputs.evgenFile }} ${{ steps.files.outputs.simFile }}
+          entrypoint: bin/ci_gemc_run.sh
+          args: ${{ needs.dependency_info.outputs.gemc_executable }} ${{ needs.dependency_info.outputs.gemc_module_tag }} ${{ steps.files.outputs.gemcConfigFile }} ${{ steps.files.outputs.evgenFile }} ${{ steps.files.outputs.simFile }}
       - name: check if output exists
         run: ls ${{ steps.files.outputs.simFile }}
       - name: reconstruction
@@ -497,6 +527,7 @@ jobs:
   #############################################################################
   final:
     name: Final
+    if: ${{ ! failure() && ! cancelled() }} # needed since `build_gemc` may be skipped
     needs:
       - fulltest
       - config_files
@@ -529,13 +560,13 @@ jobs:
 
   issue_bot:
     name: Issue bot
+    if: ${{ github.event_name == 'schedule' && ( cancelled() || failure() ) }}
     runs-on: ubuntu-latest
     needs:
       - final
     permissions:
       contents: read
       issues: write
-    if: ${{ github.event_name == 'schedule' && ( cancelled() || failure() ) }}
     steps:
       - uses: actions/checkout@v4
       - uses: JasonEtco/create-an-issue@v2
diff --git a/bin/ci_gemc.sh b/bin/ci_gemc.sh
deleted file mode 100755
index a8d0c42..0000000
--- a/bin/ci_gemc.sh
+++ /dev/null
@@ -1,79 +0,0 @@
-#!/bin/bash
-# CI `gemc` wrapper for CI workflow steps when `uses: [CONTAINER]` is used,
-# which does not seem to be compatible with multi-line commands
-
-set -e
-set -u
-
-### usage
-if [ $# -eq 0 ]; then
-  echo """
-  USAGE: $0 [command] [ARGS]...
-    commands:
-
-      build  compile GEMC
-             ARGS: [source_dir]
-
-      sim    run GEMC
-             ARGS: [source_dir] [config] [input] [output]
-
-  """ >&2
-  exit 2
-fi
-cmd=$1
-shift
-
-### parse arguments
-case $cmd in
-  build)
-    sourceDir=$1
-    ;;
-  sim)
-    sourceDir=$1
-    gemcConfigFile=$2
-    evgenFile=$3
-    simFile=$4
-    ;;
-  *)
-    echo "ERROR: unknown command '$cmd'" >&2
-    exit 1
-    ;;
-esac
-
-### source environment, and show available modules
-set +u
-source /app/localSetup.sh
-set -u
-echo "=============================="
-echo "MODULE AVAIL"
-echo "=============================="
-module avail --no-pager
-echo "=============================="
-
-### compile GEMC in $sourceDir
-if [ "$cmd" = "build" ]; then
-  pushd $sourceDir
-  scons -j4 OPT=1
-  popd
-fi
-
-### to get the dependencies correct, switch to this tag
-##### FIXME: switch to gemc/dev and use ubuntu + cvmfs action
-module switch gemc/5.4
-
-### run a simulation or print build info
-case $cmd in
-  sim)
-    ### run a simulation
-    $sourceDir/gemc \
-      $gemcConfigFile \
-      -INPUT_GEN_FILE="LUND, $evgenFile" \
-      -USE_GUI=0 \
-      -OUTPUT="hipo, $simFile"
-    ;;
-  build)
-    ### print build info
-    echo "### GEMC Build Information:"
-    $sourceDir/gemc -USE_GUI=0
-    ;;
-esac
diff --git a/bin/ci_gemc_build.sh b/bin/ci_gemc_build.sh
new file mode 100755
index 0000000..b7d4cd5
--- /dev/null
+++ b/bin/ci_gemc_build.sh
@@ -0,0 +1,24 @@
+#!/bin/bash
+# build GEMC wrapper
+
+set -e
+set -u
+
+sourceDir=$1
+
+### source environment
+set +u
+source /app/localSetup.sh
+set -u
+
+### show available modules
+echo "=============================="
+echo "MODULE AVAIL"
+echo "=============================="
+module avail --no-pager
+echo "=============================="
+
+### compile GEMC in $sourceDir
+pushd $sourceDir
+scons -j$(nproc) OPT=1
+popd
diff --git a/bin/ci_gemc_run.sh b/bin/ci_gemc_run.sh
new file mode 100755
index 0000000..9cc6cde
--- /dev/null
+++ b/bin/ci_gemc_run.sh
@@ -0,0 +1,35 @@
+#!/bin/bash
+# run GEMC wrapper
+
+set -e
+set -u
+
+gemcExe=$1
+gemcTag=$2
+gemcConfigFile=$3
+evgenFile=$4
+simFile=$5
+
+### source environment
+set +u
+source /app/localSetup.sh
+set -u
+
+### show available modules
+echo "=============================="
+echo "MODULE AVAIL"
+echo "=============================="
+module avail --no-pager
+echo "=============================="
+
+### switch to the gemc module
+### - if we do not have a custom build, this activates the appropriate gemc version
+### - if we do have a custom build, this just makes sure the dependencies are resolved correctly
+module switch gemc/$gemcTag
+
+### run a simulation
+$gemcExe \
+  $gemcConfigFile \
+  -INPUT_GEN_FILE="LUND, $evgenFile" \
+  -USE_GUI=0 \
+  -OUTPUT="hipo, $simFile"
diff --git a/bin/is_a_tag.rb b/bin/is_a_tag.rb
new file mode 100755
index 0000000..75e6aa9
--- /dev/null
+++ b/bin/is_a_tag.rb
@@ -0,0 +1,23 @@
+#!/usr/bin/ruby
+# check if a given tag is available in a repo; exit 0 if yes, 1 if no
+
+unless ARGV.length == 2
+  $stderr.puts "USAGE: #{$0} [tag] [repo owner]/[repo name]"
+  exit 2
+end
+check_tag, repo = ARGV
+
+api_args = [
+  '--silent',
+  '-L',
+  '-H "Accept: application/vnd.github+json"',
+  '-H "X-GitHub-Api-Version: 2022-11-28"',
+  "https://api.github.com/repos/#{repo}/tags",
+]
+tag_list = `curl #{api_args.join ' '} | jq -r '.[].name'`
+unless $?.success?
+  $stderr.puts "ERROR: failed to get list of tags from #{repo}"
+  exit 1
+end
+
+exit 1 unless tag_list.include? check_tag