diff --git a/toolchain/mfc/case_validator.py b/toolchain/mfc/case_validator.py
index 36fce10a0..476009fbd 100644
--- a/toolchain/mfc/case_validator.py
+++ b/toolchain/mfc/case_validator.py
@@ -19,9 +19,9 @@
 
 from . import eos
 from .common import MFCException
-from .params.definitions import CONSTRAINTS
+from .params.definitions import CONSTRAINTS, NAF, NB, NIB, NSM, NTS, NUM_PATCHES_MAX
 from .params.eos_families import EOS_FAMILIES
-from .params.namelist_parser import get_fortran_constants, get_fortran_real_constants
+from .params.namelist_parser import get_fortran_real_constants
 from .state import CFG
 
 # Above this the Ensemble-Averaged Bubble Model's O(alpha) expansion, which enters the closure as
@@ -790,7 +790,7 @@ def check_ibm(self):
             ib and num_ibs <= 0 and not has_particle_clouds,
             "num_ibs must be >= 1 when ib is enabled (or specify at least one particle_cloud with num_particles > 0)",
         )
-        num_ib_patches_max = get_fortran_constants().get("num_ib_patches_max_namelist", 50000)
+        num_ib_patches_max = NIB
         self.prohibit(
             ib and num_ibs > num_ib_patches_max,
             f"num_ibs must be <= {num_ib_patches_max} (num_ib_patches_max_namelist in m_constants.fpp)",
@@ -996,8 +996,8 @@ def check_ibm(self):
                             f"particle_cloud({i}) hemisphere shell {name}-extent must lie within {name}_domain",
                         )
 
-        num_ib_airfoils_max = get_fortran_constants().get("num_ib_airfoils_max", 5)
-        num_stl_models_max = get_fortran_constants().get("num_stl_models_max", 10)
+        num_ib_airfoils_max = NAF
+        num_stl_models_max = NSM
         num_stl_models = self.get("num_stl_models", 0)
         self.prohibit(
             num_stl_models < 0,
@@ -1436,7 +1436,7 @@ def check_synthetic_turbulence(self):
         num_turbulent_sources = self.get("num_turbulent_sources", 0) or 0
         self.prohibit(num_turbulent_sources <= 0, "num_turbulent_sources must be > 0 when synthetic_turbulence is enabled")
 
-        num_turb_sources_max = get_fortran_constants().get("num_turb_sources_max", 10)
+        num_turb_sources_max = NTS
         self.prohibit(
             num_turbulent_sources > num_turb_sources_max,
             f"num_turbulent_sources must be <= {num_turb_sources_max} (num_turb_sources_max in m_constants.fpp)",
@@ -2011,7 +2011,7 @@ def check_restart(self):
         self.prohibit(old_grid and t_step_old is None, "old_grid requires t_step_old to be set")
         self.prohibit(num_patches < 0, "num_patches must be non-negative")
         self.prohibit(num_patches == 0 and t_step_old is None, "num_patches must be positive for the non-restart case")
-        num_patches_max = get_fortran_constants().get("num_patches_max", 1000)
+        num_patches_max = NUM_PATCHES_MAX
         self.prohibit(
             num_patches > num_patches_max,
             f"num_patches must be <= {num_patches_max} (num_patches_max in m_constants.fpp)",
@@ -2407,7 +2407,7 @@ def check_patch_physics(self):
     def check_bc_patches(self):
         """Checks boundary condition patch geometry (pre-process)"""
         num_bc_patches = self.get("num_bc_patches", 0)
-        num_bc_patches_max = get_fortran_constants().get("num_bc_patches_max", 10)
+        num_bc_patches_max = NB
 
         if num_bc_patches <= 0:
             return
diff --git a/toolchain/mfc/params/definitions.py b/toolchain/mfc/params/definitions.py
index 159dd9aae..ba5613db2 100644
--- a/toolchain/mfc/params/definitions.py
+++ b/toolchain/mfc/params/definitions.py
@@ -35,6 +35,7 @@ def _fc(name: str, default: int) -> int:
 NAF = _fc("num_ib_airfoils_max", 5)  # ib_airfoil (Fortran array bound)
 NSM = _fc("num_stl_models_max", 10)  # stl_models (Fortran array bound)
 NPB = _fc("num_particle_clouds_max", 10)  # particle_cloud (Fortran array bound)
+NTS = _fc("num_turb_sources_max", 10)  # turb_pos, synth_L (Fortran array bound)
 # Enumeration limits for families not yet converted to IndexedFamily.
 # These are smaller than the Fortran array bounds to keep the registry compact.
 # The CONSTRAINTS dict below uses the Fortran constants for validation.
@@ -859,7 +860,6 @@ def _load():
     _r("num_turbulent_sources", INT, {"synthetic_turbulence"})
     _r("synth_U_inf", REAL, {"synthetic_turbulence"}, math=r"\f$U_\infty\f$")
     NSS = _fc("num_synth_shells_max", 50)
-    NTS = _fc("num_turb_sources_max", 10)
     for i in range(1, NSS + 1):
         _r(f"synth_n_waves_per_shell({i})", INT, {"synthetic_turbulence"})
         _r(f"synth_k_shell({i})", REAL, {"synthetic_turbulence"}, math=r"\f$k_s\f$")
diff --git a/toolchain/mfc/params_tests/test_definitions.py b/toolchain/mfc/params_tests/test_definitions.py
index 3c4cc913b..bb0c20b57 100644
--- a/toolchain/mfc/params_tests/test_definitions.py
+++ b/toolchain/mfc/params_tests/test_definitions.py
@@ -6,6 +6,8 @@
 
 import ast
 import inspect
+import pathlib
+import re
 import unittest
 
 from ..params import REGISTRY, definitions
@@ -194,3 +196,49 @@ def test_fallbacks_match_m_constants(self):
         for call in calls:
             name, default = (ast.literal_eval(arg) for arg in call.args)
             self.assertEqual(default, constants[name], f"_fc('{name}', {default}) fallback differs from m_constants.fpp")
+
+    def test_each_constant_is_resolved_once(self):
+        """No constant should be resolved by two _fc() calls.
+
+        A second _fc() for the same name is a second place to edit. _load()
+        had its own NTS = _fc("num_turb_sources_max", 10) shadowing the
+        module-level one, so importing NTS elsewhere would not have given a
+        single source of truth.
+        """
+        calls = [node for node in ast.walk(ast.parse(inspect.getsource(definitions))) if isinstance(node, ast.Call) and getattr(node.func, "id", None) == "_fc"]
+        names = [ast.literal_eval(call.args[0]) for call in calls]
+        duplicated = sorted({name for name in names if names.count(name) > 1})
+        self.assertEqual(duplicated, [], f"resolved by more than one _fc() call: {duplicated}")
+
+
+class TestFortranConstantGetFallbacks(unittest.TestCase):
+    """The same rule for get_fortran_constants().get(name, default) anywhere in the toolchain.
+
+    definitions.py resolves its constants through _fc(), which the test above
+    guards. Callers that re-resolve a constant with their own .get() default
+    are a second, unguarded copy of the same number: case_validator.py capped
+    num_ibs at 50000 while m_constants.fpp said 810000, and allowed 1000
+    num_patches while the Fortran array holds 10 -- too strict in one
+    direction and, worse, too permissive in the other. Those defaults are only
+    reached when src/ is absent, as in a Homebrew layout, so nothing at
+    development time notices they have drifted.
+
+    Prefer importing the already-resolved constant from definitions.py. This
+    test allows a .get() default only when it still matches the Fortran.
+    """
+
+    def test_get_fallbacks_match_m_constants(self):
+        constants = get_fortran_constants()
+        self.assertTrue(constants, "m_constants.fpp could not be parsed")
+
+        toolchain = pathlib.Path(__file__).resolve().parents[2]
+        pattern = re.compile(r"""get_fortran_constants\(\)\.get\(\s*["'](\w+)["']\s*,\s*(\d+)\s*\)""")
+
+        stale = []
+        for path in sorted(toolchain.rglob("*.py")):
+            for name, default in pattern.findall(path.read_text(encoding="utf-8", errors="ignore")):
+                if name in constants and int(default) != constants[name]:
+                    rel = path.relative_to(toolchain)
+                    stale.append(f"{rel}: .get('{name}', {default}) != {constants[name]} in m_constants.fpp")
+
+        self.assertEqual(stale, [], "stale Fortran-constant fallbacks:\n  " + "\n  ".join(stale))