diff --git a/toolchain/mfc/params/namelist_parser.py b/toolchain/mfc/params/namelist_parser.py
index 5ac3913499..8e4aa48c09 100644
--- a/toolchain/mfc/params/namelist_parser.py
+++ b/toolchain/mfc/params/namelist_parser.py
@@ -490,16 +490,29 @@ def get_fortran_constants() -> Dict[str, int]:
     """
     Get Fortran compile-time constants from m_constants.fpp.
 
-    Cached after first call. Returns empty dict if source unavailable.
+    Cached after first call. Falls back to built-in defaults when the Fortran
+    source is unavailable (e.g. Homebrew installs where src/ is not shipped).
     """
     global _FORTRAN_CONSTANTS_CACHE  # noqa: PLW0603
     if _FORTRAN_CONSTANTS_CACHE is None:
         root = get_mfc_root()
         path = root / "src" / "common" / "m_constants.fpp"
         _FORTRAN_CONSTANTS_CACHE = parse_fortran_constants(path)
+        if not _FORTRAN_CONSTANTS_CACHE:
+            _FORTRAN_CONSTANTS_CACHE = _FALLBACK_CONSTANTS.copy()
     return _FORTRAN_CONSTANTS_CACHE
 
 
+# Fallback values for when m_constants.fpp is not available at runtime.
+# Keep in sync with src/common/m_constants.fpp.
+_FALLBACK_CONSTANTS: Dict[str, int] = {
+    "num_fluids_max": 10,
+    "num_probes_max": 10,
+    "num_patches_max": 1000,
+    "num_bc_patches_max": 10,
+}
+
+
 def get_mfc_root() -> Path:
     """Get the MFC root directory from this file's location."""
     # This file is at toolchain/mfc/params/namelist_parser.py