diff --git a/CLAUDE.md b/CLAUDE.md
index 8caffb4d8..0c7978aa5 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -204,10 +204,11 @@ pytest -m "level_1 and tier_a"     # quick
 pytest -m "tier_a or tier_b"       # full validation
 ```
 
-Tier A is hardened and reviewed — safe to build code around. Tier C is not mature
-enough to drive coding. Every bug fix ships the regression test that would have
-caught it, written first and shown to fail. Validate a new test's own correctness
-before changing library code to satisfy it.
+Tier A is hardened and reviewed — safe to build code around. Every bug fix ships
+the regression test that would have caught it, written first and shown to fail;
+such a proven test starts at tier B, so it gates. Tier C is only for a test not yet
+proven, so it cannot derail the code. Validate a new test's own correctness before
+changing library code to satisfy it.
 
 Every test file must be reachable from a glob in `scripts/test.sh`;
 `scripts/check_test_coverage.py` enforces it.
diff --git a/docs/developer/TESTING-RELIABILITY-SYSTEM.md b/docs/developer/TESTING-RELIABILITY-SYSTEM.md
index 4e8d072c8..a6d6f5ee8 100644
--- a/docs/developer/TESTING-RELIABILITY-SYSTEM.md
+++ b/docs/developer/TESTING-RELIABILITY-SYSTEM.md
@@ -291,11 +291,15 @@ pytest -m "not tier_c"
 
 ### When Writing a New Test
 
-1. **Start at Tier C**: All new tests begin as experimental
+1. **A proven test starts at Tier B**: a regression test written first and shown to
+   fail without the fix it guards, or a test asserting a hard baseline whose own
+   correctness has been checked, gates from the start. Tier C is for a test that is
+   not yet proven, so that it cannot derail the code it was written against (the C2
+   population above); it is otherwise close to obsolete (Louis, 2026-10-11).
 2. **Document Intent**: Clear docstring explaining what behavior is tested
 3. **Use xfail Appropriately**: If testing unimplemented feature, mark with xfail
 4. **Don't Break CI**: Tier C tests with xfail won't break automated testing
-5. **Promote Deliberately**: Don't rush promotion - let tests prove reliability
+5. **Promote Deliberately**: Tier B → A is the review step described above
 
 ### When a Test Fails
 
diff --git a/tests/test_0025_jit_compile_flags.py b/tests/test_0025_jit_compile_flags.py
index 5fe304997..006144840 100644
--- a/tests/test_0025_jit_compile_flags.py
+++ b/tests/test_0025_jit_compile_flags.py
@@ -14,7 +14,7 @@
 
 import underworld3 as uw
 
-pytestmark = [pytest.mark.level_1, pytest.mark.tier_c]
+pytestmark = [pytest.mark.level_1, pytest.mark.tier_b]
 
 
 def _compile_args(bundles):