diff --git a/.github/scripts/prebuild-case-optimization.sh b/.github/scripts/prebuild-case-optimization.sh
index f580770330..7fb9af0041 100755
--- a/.github/scripts/prebuild-case-optimization.sh
+++ b/.github/scripts/prebuild-case-optimization.sh
@@ -22,12 +22,44 @@ case "$cluster" in
     *) echo "ERROR: Unknown cluster '$cluster'"; exit 1 ;;
 esac
 
+# Optional sharding (format "i/N", e.g. "1/2"), set by submit-slurm-job.sh's
+# [shard] argument via $job_shard: shard i builds every Nth case of the sorted
+# case list. Unset = build all cases in one job (default; other clusters).
+shard="${job_shard:-}"
+if [ -n "$shard" ]; then
+    # Validate full shape: must be exactly "digits/digits" — one slash with
+    # non-empty, purely numeric, non-leading-zero parts on both sides.
+    # Split first, then validate each part independently so that inputs like
+    # "1/" "/2" "//" "1/2/3" "a/b" "12" are all caught before any arithmetic.
+    shard_idx="${shard%%/*}"
+    shard_count="${shard##*/}"
+    # Reject if no slash (idx and count are equal and equal to the whole string)
+    case "$shard_idx" in
+        ''|*[!0-9]*|0*) echo "ERROR: bad shard '$shard' (expected i/N)"; exit 1 ;;
+    esac
+    case "$shard_count" in
+        ''|*[!0-9]*|0*) echo "ERROR: bad shard '$shard' (expected i/N)"; exit 1 ;;
+    esac
+    # Confirm the string is exactly "idx/count" — catches "12" (no slash) and
+    # "1/2/3" (extra slash, where idx=1 and count=2/3 would have failed above,
+    # but this is an extra safety net).
+    if [ "$shard" != "$shard_idx/$shard_count" ]; then
+        echo "ERROR: bad shard '$shard' (expected i/N)"; exit 1
+    fi
+    if [ "$shard_idx" -lt 1 ] || [ "$shard_idx" -gt "$shard_count" ]; then
+        echo "ERROR: bad shard '$shard' (expected i/N with 1 <= i <= N)"; exit 1
+    fi
+fi
+
 # Phoenix starts fresh (no prior dep build); other clusters pre-build deps via
 # build.sh first, so we must preserve them and only clean MFC target staging.
+# Sharded jobs share one workspace and run concurrently, so the workflow
+# cleans once before submitting them — cleaning here would wipe a sibling
+# shard's in-progress build.
 if [ "$cluster" = "phoenix" ]; then
     source .github/scripts/clean-build.sh
     clean_build
-else
+elif [ -z "$shard" ]; then
     find build/staging -maxdepth 1 -regex '.*/[0-9a-f]+' -type d -exec rm -rf {} + 2>/dev/null || true
     find build/install -maxdepth 1 -regex '.*/[0-9a-f]+' -type d -exec rm -rf {} + 2>/dev/null || true
 fi
@@ -40,7 +72,49 @@ case "$job_interface" in
     *)   echo "ERROR: prebuild requires gpu interface (acc or omp)"; exit 1 ;;
 esac
 
+# Case-optimized simulation builds land in per-case hash-named staging dirs,
+# but syscheck/pre_process/post_process hash identically across these cases.
+# Concurrent shards must not build those shared staging dirs simultaneously:
+# shard 1 builds them first and drops a done marker; other shards wait for it,
+# after which their builds no-op in the shared dirs.
+if [ -n "$shard" ] && [ "$shard_count" -gt 1 ]; then
+    shared_marker_done="build/.prebuild-shared-targets-done"
+    shared_marker_failed="build/.prebuild-shared-targets-failed"
+    set -- benchmarks/*/case.py
+    first_case="$1"
+    if [ "$shard_idx" -eq 1 ]; then
+        # Remove both markers at the start so reruns and manual invocations
+        # never observe stale state from a prior run.
+        rm -f "$shared_marker_done" "$shared_marker_failed"
+        echo "=== Shard 1/$shard_count: building shared targets ==="
+        # Write the failure marker if the build exits non-zero so other shards
+        # can detect the failure immediately instead of waiting 90 minutes.
+        trap 'touch "$shared_marker_failed"' ERR
+        ./mfc.sh build -i "$first_case" -t syscheck pre_process post_process --case-optimization $gpu_opts -j 8
+        trap - ERR
+        touch "$shared_marker_done"
+    else
+        echo "=== Shard $shard_idx/$shard_count: waiting for shard 1 to build shared targets ==="
+        waited=0
+        until [ -f "$shared_marker_done" ]; do
+            if [ -f "$shared_marker_failed" ]; then
+                echo "ERROR: shard 1 failed to build shared targets; see shard 1 log"; exit 1
+            fi
+            if [ "$waited" -ge 5400 ]; then
+                echo "ERROR: timed out waiting for $shared_marker_done"; exit 1
+            fi
+            sleep 30
+            waited=$((waited + 30))
+        done
+    fi
+fi
+
+idx=0
 for case in benchmarks/*/case.py; do
+    idx=$((idx + 1))
+    if [ -n "$shard" ] && [ $(((idx - 1) % shard_count)) -ne $((shard_idx - 1)) ]; then
+        continue
+    fi
     echo "=== Pre-building: $case ==="
     ./mfc.sh run "$case" --case-optimization $gpu_opts -j 8 --dry-run
 done
diff --git a/.github/scripts/run_parallel_benchmarks.sh b/.github/scripts/run_parallel_benchmarks.sh
index 15609d3c2b..7d5a410633 100755
--- a/.github/scripts/run_parallel_benchmarks.sh
+++ b/.github/scripts/run_parallel_benchmarks.sh
@@ -62,6 +62,12 @@ bash "${SCRIPT_DIR}/run_monitored_slurm_job.sh" "$pr_job_id" "pr/${job_slug}.out
 if [ "$pr_exit" -ne 0 ]; then
     echo "PR job exited with code: $pr_exit"
     tail -n 50 "pr/${job_slug}.out" 2>/dev/null || echo "  Could not read PR log"
+    # The PR benchmark run genuinely failed (cases crashed/hung/SIGTERM'd, not a
+    # monitor false-positive -- run_monitored_slurm_job.sh re-checks sacct). Fail
+    # the job instead of falling through to the YAML-exists check, which would let
+    # a broken PR pass green as long as a partial YAML was written. Scoped to PR
+    # only: a master/baseline infra flake stays a warning and does not red-cross.
+    exit 1
 else
     echo "PR job completed successfully"
 fi
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index c8e4cc61fa..2d0418564f 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -402,6 +402,13 @@ jobs:
             cluster_name: 'Oak Ridge | Frontier (AMD)'
             device: 'cpu'
             interface: 'none'
+            shard: '1/2'
+          - runner:       'frontier'
+            cluster:      'frontier_amd'
+            cluster_name: 'Oak Ridge | Frontier (AMD)'
+            device: 'cpu'
+            interface: 'none'
+            shard: '2/2'
     runs-on:
       group:  phoenix
       labels: ${{ matrix.runner }}
@@ -420,7 +427,7 @@ jobs:
 
       - name: Fetch Dependencies
         if:   matrix.cluster != 'phoenix'
-        timeout-minutes: 60
+        timeout-minutes: 120
         run:  bash .github/workflows/${{ matrix.cluster }}/build.sh ${{ matrix.device }} ${{ matrix.interface }}
 
       - name: Build
@@ -523,7 +530,22 @@ jobs:
 
       - name: Pre-Build (SLURM)
         if:   matrix.cluster == 'frontier_amd'
-        run:  bash .github/scripts/submit-slurm-job.sh .github/scripts/prebuild-case-optimization.sh gpu ${{ matrix.interface }} ${{ matrix.cluster }}
+        # AMD flang is slow enough that one serial pre-build job exceeds its
+        # walltime, so split the case list across two concurrent SLURM jobs.
+        # The shards share this workspace and skip their in-job staging clean,
+        # so clean once here on the login node before submitting.
+        run:  |
+          find build/staging -maxdepth 1 -regex '.*/[0-9a-f]+' -type d -exec rm -rf {} + 2>/dev/null || true
+          find build/install -maxdepth 1 -regex '.*/[0-9a-f]+' -type d -exec rm -rf {} + 2>/dev/null || true
+          rm -f build/.prebuild-shared-targets-done
+          bash .github/scripts/submit-slurm-job.sh .github/scripts/prebuild-case-optimization.sh gpu ${{ matrix.interface }} ${{ matrix.cluster }} 1/2 &
+          pid1=$!
+          bash .github/scripts/submit-slurm-job.sh .github/scripts/prebuild-case-optimization.sh gpu ${{ matrix.interface }} ${{ matrix.cluster }} 2/2 &
+          pid2=$!
+          rc=0
+          wait "$pid1" || rc=1
+          wait "$pid2" || rc=1
+          exit $rc
 
       - name: Build & Run Case-Optimization Tests
         if:   matrix.cluster != 'phoenix' && matrix.cluster != 'frontier_amd'
@@ -546,6 +568,8 @@ jobs:
         if:   always()
         run:  |
           for f in prebuild-case-optimization-${{ matrix.device }}-${{ matrix.interface }}.out \
+                   prebuild-case-optimization-${{ matrix.device }}-${{ matrix.interface }}-1-of-2.out \
+                   prebuild-case-optimization-${{ matrix.device }}-${{ matrix.interface }}-2-of-2.out \
                    run-case-optimization-${{ matrix.device }}-${{ matrix.interface }}.out; do
             [ -f "$f" ] && echo "=== $f ===" && cat "$f"
           done
@@ -556,5 +580,5 @@ jobs:
         with:
           name: case-opt-${{ strategy.job-index }}-${{ matrix.cluster }}-${{ matrix.interface }}
           path: |
-            prebuild-case-optimization-${{ matrix.device }}-${{ matrix.interface }}.out
+            prebuild-case-optimization-${{ matrix.device }}-${{ matrix.interface }}*.out
             run-case-optimization-${{ matrix.device }}-${{ matrix.interface }}.out
diff --git a/benchmarks/igr/case.py b/benchmarks/igr/case.py
index 423d7e1a41..d54e3bbdaf 100644
--- a/benchmarks/igr/case.py
+++ b/benchmarks/igr/case.py
@@ -102,10 +102,11 @@
             "patch_icpp(1)%length_x": 2 * math.pi * L,
             "patch_icpp(1)%length_y": 2 * math.pi * L,
             "patch_icpp(1)%length_z": 2 * math.pi * L,
-            "patch_icpp(1)%vel(1)": f"{V0}*sin(x/{L})*cos(y/{L})*sin(z/{L})",
-            "patch_icpp(1)%vel(2)": f"-{V0}*cos(x/{L})*sin(y/{L})*sin(z/{L})",
+            "patch_icpp(1)%vel(1)": 0.0,
+            "patch_icpp(1)%vel(2)": 0.0,
             "patch_icpp(1)%vel(3)": 0,
-            "patch_icpp(1)%pres": f"{P0} + ({rho0}*{V0}**2/16)*(cos(2*x/{L}) + cos(2*y/{L}))*(cos(2*z/{L}) + 2)",
+            "patch_icpp(1)%pres": 0.0,
+            "patch_icpp(1)%hcid": 380,
             "patch_icpp(1)%alpha_rho(1)": 1,
             "patch_icpp(1)%alpha(1)": 1,
             # Fluids Physical Parameters