diff --git a/.github/workflows/build_uw3_and_test.yaml b/.github/workflows/build_uw3_and_test.yaml
index e7286f99a..d049cedba 100644
--- a/.github/workflows/build_uw3_and_test.yaml
+++ b/.github/workflows/build_uw3_and_test.yaml
@@ -45,13 +45,16 @@ env:
 jobs:
   test:
     runs-on: ubuntu-latest
-    # The suite has outgrown 60 minutes. Measured 2026-08-15: the SERIAL phase
-    # alone is ~55 min (three batches at 14-15 min each), so the parallel phase
-    # - which runs last - was being cut off mid-run and reported as a failure
-    # that looked like a test failure. Any addition at all pushed a run over.
-    # This is a stop-gap that unblocks landing work; the durable fix is to split
-    # the batches across parallel jobs so wall-clock stops tracking total test
-    # time. See the CI-runtime issue for that.
+    # Raised past 60 minutes on a 2026-08-15 measurement (serial phase ~55 min).
+    # xdist has since cut that: the five successful runs before 2026-09-12 spent
+    # 34, 35, 43, 44 and 48 minutes in "Run tests", well inside this budget. The
+    # headroom is why the previously-deferred bands could simply be switched on
+    # (#721 follow-up) rather than traded against something else.
+    #
+    # Sharding the batches across parallel jobs remains the durable fix if this
+    # tightens again; it was referenced here as "the CI-runtime issue" but never
+    # actually filed, so the numbers above are recorded in the issue that this
+    # change opens.
     timeout-minutes: 120
 
     steps:
diff --git a/scripts/check_test_coverage.py b/scripts/check_test_coverage.py
index a384035f0..514f89f39 100644
--- a/scripts/check_test_coverage.py
+++ b/scripts/check_test_coverage.py
@@ -20,16 +20,11 @@
 
 # Deferred by maintainer decision, not by accident.
 DEFERRED = {
-    # No issue: this band was disabled in scripts/test.sh as "potentially
-    # problematic" without one being filed. Recorded as it stands rather than
-    # dressed up — #721 follow-up work re-enables it and removes this entry.
-    "test_06[0-9][0-9]_*": "regression suite disabled in test.sh, no issue filed",
-    # Narrow: test_1072 is pulled out of this band and run by name, so a broad
-    # test_107* would list a covered file as deferred.
-    "test_1070_*": "level_2/level_3 + tier_b/tier_c, awaiting triage (#504)",
-    "test_1071_*": "level_2/level_3 + tier_b/tier_c, awaiting triage (#504)",
-    "test_1073_*": "level_2/level_3 + tier_b/tier_c, awaiting triage (#504)",
-    "test_106*": "level_2/level_3 + tier_b/tier_c, awaiting triage (#504)",
+    # Empty, and that is the point. Every band that used to sit here — the
+    # test_06NN regression suite and test_106*/test_107* — is now batched in
+    # scripts/test.sh. Nothing is excluded from CI by its number any more; a
+    # test that should not gate says so with @pytest.mark.tier_c, which is a
+    # property of the test rather than of where it sits in the numbering.
 }
 
 
@@ -38,7 +33,10 @@ def globs_run_by(script):
     text = script.read_text().replace("\\\n", " ")
     patterns = set()
     for line in text.splitlines():
-        if "$PYTEST" in line and not line.lstrip().startswith("#"):
+        # Both spellings: the runner is a bash ARRAY, so call sites read
+        # "${PYTEST[@]}", but a plain $PYTEST is still worth matching in case
+        # one is left behind or reintroduced.
+        if re.search(r"\$\{?PYTEST", line) and not line.lstrip().startswith("#"):
             patterns.update(re.findall(r"tests/[A-Za-z0-9_\[\]*.\-]+", line))
     return patterns
 
diff --git a/scripts/test.sh b/scripts/test.sh
index 953d24b6a..3baceb943 100755
--- a/scripts/test.sh
+++ b/scripts/test.sh
@@ -78,14 +78,33 @@ export MKL_NUM_THREADS=1
 #
 # Unset (or 1) still runs everything in one process, which is what you want
 # when a test passes alone and fails in a full run.
+# An ARRAY, not a string. Every call site below expands it unquoted, so a string
+# would be word-split by bash: `-m 'not tier_c'` becomes the two words `'not` and
+# `tier_c'`, and pytest answers that by silently collecting NOTHING and exiting 0
+# — a green run that tested nothing. An array carries its elements intact through
+# "${PYTEST[@]}", and globs on the command line still expand normally.
+PYTEST=(pytest --config-file=tests/pytest.ini)
 if [ -n "$WORKERS" ] && [ "$WORKERS" -gt 1 ]; then
     echo "Serial batches: $WORKERS worker process(es)"
-    PYTEST="pytest --config-file=tests/pytest.ini --dist loadfile -n $WORKERS"
+    PYTEST+=(--dist loadfile -n "$WORKERS")
 else
     echo "Serial batches: in-process (set WORKERS=N to distribute)"
-    PYTEST="pytest --config-file=tests/pytest.ini"
 fi
 
+# Tier C never gates. A tier C failure demands an EXPLANATION, not a revert
+# (Charter S8, docs/developer/TESTING-RELIABILITY-SYSTEM.md): these are
+# characterisations that can fail because the code got BETTER, and comparisons
+# whose result moves with the fixture. They are excluded from the batches that
+# set $status and run in their own reporting pass at the end, which is read but
+# cannot fail the build.
+# Tier C never gates. A tier C failure demands an EXPLANATION, not a revert
+# (Charter S8, docs/developer/TESTING-RELIABILITY-SYSTEM.md): these are
+# characterisations that can fail because the code got BETTER, and comparisons
+# whose result moves with the fixture. They are excluded from the batches that
+# set $status and run in their own reporting pass at the end, which is read but
+# cannot fail the build.
+PYTEST+=(-m "not tier_c")
+
 # Run serial tests (unless --parallel-only specified)
 if [ $PARALLEL_ONLY -eq 0 ]; then
   echo "Running serial test suite..."
@@ -98,35 +117,37 @@ if [ $PARALLEL_ONLY -eq 0 ]; then
   python3 "$(dirname "$0")/check_test_coverage.py" || status=1
 
   # Run simple tests (0000-0299: basic functionality, imports, simple operations)
-  $PYTEST tests/test_00[0-4]*py || status=1
-  $PYTEST tests/test_0050*py || status=1
+  "${PYTEST[@]}" tests/test_00[0-4]*py || status=1
+  "${PYTEST[@]}" tests/test_0050*py || status=1
   # test_006[2-9] and test_007x matched NO batch glob and so never ran in CI:
   # the whole integration-point suite (0064-0067), swarm repopulation,
   # mid-time velocity and the VE stress history. They are not covered by the
   # disabled test_06*py line below either - that one is 0600-0699. The band is
   # taken whole (00[6-7]) rather than enumerated, so a test added next to its
   # siblings is not dark again. Verified passing (103 tests) before wiring in.
-  $PYTEST tests/test_005[1-9]*py tests/test_00[6-7]*py || status=1
-  $PYTEST tests/test_01*py || status=1
-  $PYTEST tests/test_02*py tests/test_03*py || status=1
+  "${PYTEST[@]}" tests/test_005[1-9]*py tests/test_00[6-7]*py || status=1
+  "${PYTEST[@]}" tests/test_01*py || status=1
+  "${PYTEST[@]}" tests/test_02*py tests/test_03*py || status=1
 
   # Intermediate tests (0500-0799: data structures, transformations, enhanced interfaces)
-  # NOTE: Temporarily disabling test_06*py regression tests (potentially problematic)
-  $PYTEST tests/test_05*py tests/test_07*py || status=1
-  # $PYTEST tests/test_06*py || status=1  # DISABLED - regression tests need validation
+  # test_06*py was disabled as "potentially problematic" and stayed that way. The
+  # whole band was measured 2026-09-12 and passes; a band is not a unit of trust,
+  # and holding one back for a suspicion nobody recorded meant its level_1 files
+  # never ran at all.
+  "${PYTEST[@]}" tests/test_05*py tests/test_06*py tests/test_07*py || status=1
 
   # Units system tests (0800-0899: unit-aware functions, arrays, and conversions)
-  $PYTEST tests/test_08*py || status=1
+  "${PYTEST[@]}" tests/test_08*py || status=1
 
   # Poisson solvers (including Darcy flow)
-  $PYTEST tests/test_100[0-9]*py tests/test_103*py || status=1
+  "${PYTEST[@]}" tests/test_100[0-9]*py tests/test_103*py || status=1
 
   # Solver / system tests (advanced solver problems)
   # test_101* / test_102* include the rotated free-slip suite (test_1018,
   # issue #504) and the MG / boundary-flux suites, which previously matched
   # no batch glob and never ran in CI.
-  $PYTEST tests/test_101*py tests/test_102*py || status=1
-  $PYTEST tests/test_105*py || status=1
+  "${PYTEST[@]}" tests/test_101*py tests/test_102*py || status=1
+  "${PYTEST[@]}" tests/test_105*py || status=1
 
   # The boundary-normal guard lives under tests/parallel/ but carries NO
   # mpi(min_size=2) mark, because the defect it guards is present in SERIAL in
@@ -134,26 +155,27 @@ if [ $PARALLEL_ONLY -eq 0 ]; then
   # spherical shell at np=1). Run it here so the serial job covers that path —
   # every other serial test of the default boundary normal is on a box, where
   # flat walls make the question vacuous. It also runs in the --p N batch below.
-  $PYTEST tests/parallel/test_1069_boundary_normal_parallel.py || status=1
-  # NOT yet batched (issue #504 audit): test_106*py and test_107*py contain
-  # level_2/level_3 + slow + tier_b/tier_c suites (e.g. test_1064) and need
-  # a triage/deselect decision before being wired into CI.
+  "${PYTEST[@]}" tests/parallel/test_1069_boundary_normal_parallel.py || status=1
+  # test_106*/test_107* were held back pending "a triage/deselect decision". That
+  # decision is made: the band is not the unit of exclusion. Of the 243 tests in
+  # the deferred bands only 26 are level_3 or tier_c; 55 are level_1 AND tier_a —
+  # hardened, fast, and exactly what CI exists to run. test_1064 is the slow one
+  # and it is excluded by WHAT IT IS (tier_c) below, not by its number.
+  "${PYTEST[@]}" tests/test_106*py tests/test_107*py || status=1
   #
-  # test_1072 is pulled forward out of that group, the same way test_1069 is
-  # above, because leaving it there defeats its purpose: it is the ONLY guard on
-  # the 3-D free-surface sign and relaxation rate, and #496 exists precisely
-  # because those regressions were invisible to CI. Landing the test into an
-  # unbatched file would have closed the issue without closing the gap.
-  # level_2/tier_b, ~55s serial; passes at np=1 and np=2.
-  $PYTEST tests/test_1072_free_surface_spherical.py || status=1
+  # test_1072 used to be pulled forward out of test_107* by name, because that
+  # band was not batched and it is the ONLY guard on the 3-D free-surface sign
+  # and relaxation rate (#496 exists because those regressions were invisible to
+  # CI). The band runs as a whole now, so the named line is gone rather than
+  # running it twice.
 
   # Diffusion / Advection tests
-  $PYTEST tests/test_1100*py || status=1
-  $PYTEST tests/test_1110*py tests/test_1120*py || status=1  # Annulus + vector SL
-  $PYTEST tests/test_1450*py || status=1
+  "${PYTEST[@]}" tests/test_1100*py || status=1
+  "${PYTEST[@]}" tests/test_1110*py tests/test_1120*py || status=1  # Annulus + vector SL
+  "${PYTEST[@]}" tests/test_1450*py || status=1
 
   # Named (un-numbered) test files - JIT, docstrings, projections
-  $PYTEST tests/test_docstring_utils.py tests/test_jit_cache.py \
+  "${PYTEST[@]}" tests/test_docstring_utils.py tests/test_jit_cache.py \
           tests/test_jit_deterministic_ordering.py \
           tests/test_multicomponent_projection.py \
           tests/test_snes_vector_asymmetric_jacobian.py \
@@ -249,6 +271,23 @@ else
   echo "⚠️  Skipping parallel tests (use --p N to enable)"
 fi
 
+# Tier C: run and report, never gate. Failures here are read by a human and
+# answered with an explanation or a re-characterisation, never with a revert.
+if [ $PARALLEL_ONLY -eq 0 ]; then
+  echo ""
+  echo "=========================================="
+  echo "Tier C characterisations (reported, not gating)"
+  echo "=========================================="
+  if pytest --config-file=tests/pytest.ini -m tier_c tests/; then
+    echo "Tier C: all characterisations still hold."
+  else
+    echo ""
+    echo "⚠️  A tier C characterisation changed. This does NOT fail the build."
+    echo "    Explain what moved and re-characterise the test — do not revert"
+    echo "    code to make it pass. See docs/developer/TESTING-RELIABILITY-SYSTEM.md."
+  fi
+fi
+
 #
 if [ $status -ne 0 ]; then
   echo ""