diff --git a/.github/workflows/extra.yml b/.github/workflows/extra.yml
index 2168687f2..156514b50 100644
--- a/.github/workflows/extra.yml
+++ b/.github/workflows/extra.yml
@@ -54,6 +54,10 @@ jobs:
   coverage:
     if: "! github.event.pull_request.draft"
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [0, 1, 2, 3]
     services:
       redis:
         image: redis:7
@@ -78,12 +82,93 @@ jobs:
         run: pixi run -e py312e ./install/install_minq.sh
       - name: Install gpCAM
         run: pixi run -e py312e pip install gpcam==8.1.13
-      - name: Run canonical coverage suite
-        run: pixi run -e py312e ./libensemble/tests/run_tests.py -l -m -e --coverage --fail-fast --durations 25
-      - name: Upload coverage reports to Codecov
+      - name: Run coverage suite (shard 0 - unit + regression slice)
+        if: matrix.shard == 0
+        run: >-
+          pixi run -e py312e ./libensemble/tests/run_tests.py -l -m -e
+          --coverage --fail-fast --durations 25
+          --shard 0/4 --timings-json timings-coverage-0.json
+      - name: Run coverage suite (shards 1-3 - regression slice only)
+        if: matrix.shard != 0
+        run: >-
+          pixi run -e py312e ./libensemble/tests/run_tests.py -r -l -m -e
+          --coverage --fail-fast --durations 25
+          --shard ${{ matrix.shard }}/4 --timings-json timings-coverage-${{ matrix.shard }}.json
+      - name: Stage coverage files
+        if: always()
+        run: |
+          mkdir -p cov_staging
+          if [ -f libensemble/tests/.cov_merge_out ]; then
+            cp libensemble/tests/.cov_merge_out cov_staging/cov_merge_out_shard${{ matrix.shard }}.db
+          else
+            echo "::warning::no merged coverage db (.cov_merge_out) produced by shard ${{ matrix.shard }}"
+          fi
+          echo "Staged files:"; ls -lah cov_staging; du -sh cov_staging
+      - name: Upload coverage data
+        if: always()
+        uses: actions/upload-artifact@v7
+        with:
+          name: coverage-data-${{ matrix.shard }}
+          path: cov_staging/
+          include-hidden-files: true
+          if-no-files-found: warn
+      - name: Upload timings
+        if: always()
+        uses: actions/upload-artifact@v7
+        with:
+          name: coverage-timings-${{ matrix.shard }}
+          path: timings-coverage-${{ matrix.shard }}.json
+          if-no-files-found: ignore
+
+  coverage-combine:
+    if: always() && !cancelled() && needs.coverage.result != 'skipped'
+    needs: coverage
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          lfs: true
+      - uses: prefix-dev/setup-pixi@v0.11.0
+        with:
+          pixi-version: v0.69.0
+          cache: true
+          frozen: true
+          environments: py312e
+      - uses: actions/download-artifact@v7
+        with:
+          pattern: coverage-data-*
+          path: _coverage_data
+          merge-multiple: true
+      - name: Combine coverage reports
+        run: |
+          echo "Downloaded artifacts:"; find _coverage_data -type f | sort
+          DBS=$(find _coverage_data -type f -name "cov_merge_out_shard*.db" | sort)
+          if [ -z "$DBS" ]; then echo "::error::no per-shard coverage databases found"; exit 1; fi
+          DBS_ABS=$(for f in $DBS; do echo "$PWD/$f"; done)
+          cd libensemble/tests
+          pixi run -e py312e coverage combine $DBS_ABS
+          ls -lh .cov_merge_out
+          pixi run -e py312e coverage xml -o coverage.xml
+          pixi run -e py312e coverage report --skip-covered | tail -n 80
+          ls -lh coverage.xml
+      - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v7
+        with:
+          files: libensemble/tests/coverage.xml
+          fail_ci_if_error: true
         env:
           CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+      - name: Upload final timing artifacts
+        if: always()
+        uses: actions/download-artifact@v7
+        with:
+          pattern: coverage-timings-*
+          path: _timings
+          merge-multiple: true
+      - name: Merge and display timings
+        if: always()
+        run: |
+          find _timings -name "*.json" -exec cat {} \; 2>/dev/null | head -c 2000 || echo "no timings"
 
   external:
     runs-on: ubuntu-latest
diff --git a/libensemble/tests/unit_tests/test_manager_main.py b/libensemble/tests/unit_tests/test_manager_main.py
index 45b871938..f2425e672 100644
--- a/libensemble/tests/unit_tests/test_manager_main.py
+++ b/libensemble/tests/unit_tests/test_manager_main.py
@@ -67,6 +67,10 @@ def test_term_test_3():
 
 
 def test_send_work_order_persis_stop_empty_rows():
+    # Regression test: a PERSIS_STOP work order with zero H_rows (e.g. final_gen_send
+    # with nothing left to forward to a persistent gen) must still send the meta-message.
+    # Otherwise, a persistent worker blocked in PersistentSupport.recv() waiting on this
+    # message hangs forever, since nothing would ever be sent to it.
     hist, sim_specs, gen_specs, exit_criteria, al = setup.hist_setup2()
     final_libE_specs = dict(libE_specs, final_gen_send=True)
     fake_wcomm = FakeComm()