diff --git a/src/V3Param.cpp b/src/V3Param.cpp
index 3e4e960d5..eac1fc1f5 100644
--- a/src/V3Param.cpp
+++ b/src/V3Param.cpp
@@ -1442,9 +1442,19 @@ class ParamProcessor final {
                 // Cast/CastSize default values are not yet folded by V3Width.
                 // Constify here so the comparison below sees a Const node.
                 // Other node kinds are handled in the branches above.
+                //
+                // Only fold when the cast is self-contained.  modvarp is the
+                // parameter on the shared module template and constifyParamsEdit
+                // is destructive: a default that reads another parameter
+                // (VarRef) or a type parameter (RefDType) must be evaluated per
+                // instance, and folding it here would evaluate it against the
+                // template's own defaults and bake that in for every later
+                // instance that relies on the default.
                 if (modvarp->valuep()
                     && (VN_IS(modvarp->valuep(), Cast) || VN_IS(modvarp->valuep(), CastSize))) {
-                    V3Const::constifyParamsEdit(modvarp->valuep());
+                    const bool dependent = modvarp->valuep()->exists(
+                        [](AstNode* np) { return VN_IS(np, VarRef) || VN_IS(np, RefDType); });
+                    if (!dependent) V3Const::constifyParamsEdit(modvarp->valuep());
                 }
                 UINFO(9, "cellPinCleanup: after constify " << pinp);
                 // String constants are parsed as logic arrays and converted to strings in V3Const.
diff --git a/test_regress/t/t_param_cast_default_struct.py b/test_regress/t/t_param_cast_default_struct.py
new file mode 100755
index 000000000..6fe7d000c
--- /dev/null
+++ b/test_regress/t/t_param_cast_default_struct.py
@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of either the GNU Lesser General Public License Version 3
+# or the Perl Artistic License Version 2.0.
+# SPDX-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile(verilator_flags2=["--binary"])
+
+test.execute()
+
+test.passes()
diff --git a/test_regress/t/t_param_cast_default_struct.v b/test_regress/t/t_param_cast_default_struct.v
new file mode 100644
index 000000000..263f6f6aa
--- /dev/null
+++ b/test_regress/t/t_param_cast_default_struct.v
@@ -0,0 +1,35 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
+// verilog_format: on
+
+// A parameter whose default is a cast of a member of a struct-typed
+// parameter.  
+
+typedef struct packed {int unsigned CAP;} cfg_t;
+
+module m #(
+  parameter cfg_t cfg = '{CAP: 0},
+  parameter int unsigned p_cap = int'(cfg.CAP)
+);
+endmodule
+
+module t;
+  localparam cfg_t c = '{CAP: 32};
+
+  m #(.cfg(c), .p_cap(64)) u_over ();  // explicit override, elaborated first
+  m #(.cfg(c)) u_dflt ();  // takes the default: int'(cfg.CAP) == 32
+
+  initial begin
+    `checkd(u_over.p_cap, 64);
+    `checkd(u_dflt.p_cap, 32);  // 0 before the fix
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule