diff --git a/src/V3Param.cpp b/src/V3Param.cpp
index 9787f13d1..d84a3c986 100644
--- a/src/V3Param.cpp
+++ b/src/V3Param.cpp
@@ -2293,20 +2293,21 @@ class ParamVisitor final : public VNVisitor {
                 nodep->v3error("Parameter without default value is never given value"
                                << " (IEEE 1800-2023 6.20.1): " << nodep->prettyNameQ());
             } else if (nodep->valuep()) {
-                // In visit(AstVar*) for localparams, check if expression contains VARXREF
-                // to another localparam (not parameter). Parameters are already const,
-                // but localparams may not be evaluated yet.
-                bool hasVarXRefToLparam = false;
+                // If the value expression contains a VarXRef to an interface
+                // localparam whose value is not yet constant, defer constification
+                // to avoid premature widthing with unresolved values (see
+                // t_lparam_dep_iface tests).  When the referenced localparam is
+                // already const, proceed normally so FUNCREFs with resolved iface
+                // param args get folded.
+                bool hasUnresolvedLparamXRef = false;
                 nodep->valuep()->foreach([&](const AstVarXRef* xrefp) {
-                    if (xrefp->varp() && xrefp->varp()->varType() == VVarType::LPARAM) {
-                        hasVarXRefToLparam = true;
+                    if (const AstVar* const varp = xrefp->varp()) {
+                        if (varp->varType() == VVarType::LPARAM && !VN_IS(varp->valuep(), Const)) {
+                            hasUnresolvedLparamXRef = true;
+                        }
                     }
                 });
-                if (hasVarXRefToLparam) {
-                    // Don't constify - let it be evaluated later
-                    return;
-                }
-
+                if (hasUnresolvedLparamXRef) return;
                 V3Const::constifyParamsEdit(nodep);
             }
         }
@@ -2328,6 +2329,13 @@ class ParamVisitor final : public VNVisitor {
             if (nodep->name() == candp->name()) {
                 if (AstVar* const varp = VN_CAST(candp, Var)) {
                     UINFO(9, "Found interface parameter: " << varp);
+                    // The interface may not have been visited yet (it is at a
+                    // deeper level in the work queue), so its localparams may
+                    // not be constified.  Eagerly constify here so that the
+                    // caller's hasUnresolvedLparamXRef check sees a Const.
+                    if (varp->isParam() && varp->valuep() && !VN_IS(varp->valuep(), Const)) {
+                        V3Const::constifyParamsEdit(varp);
+                    }
                     nodep->varp(varp);
                     return true;
                 } else if (const AstPin* const pinp = VN_CAST(candp, Pin)) {
diff --git a/test_regress/t/t_lparam_pkg_assign.py b/test_regress/t/t_lparam_pkg_assign.py
new file mode 100755
index 000000000..6fe7d000c
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign.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_lparam_pkg_assign.v b/test_regress/t/t_lparam_pkg_assign.v
new file mode 100644
index 000000000..d1190c23f
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign.v
@@ -0,0 +1,37 @@
+// DESCRIPTION: Verilator: Localparam with package function call using interface param
+//
+// 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
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+// verilog_format: on
+
+package pkg;
+  function automatic bit fn(int value);
+    return (value > 0) ? 1'b1 : 1'b0;
+  endfunction
+endpackage
+
+interface ifc();
+  localparam int PARAM = 1;
+endinterface
+
+module mod(ifc i);
+  localparam bit lpbit = pkg::fn(i.PARAM);
+endmodule
+
+module t;
+  ifc i();
+  mod m(.i);
+
+  initial begin
+    `checkd(m.lpbit, 1'b1);
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule
diff --git a/test_regress/t/t_lparam_pkg_assign2.py b/test_regress/t/t_lparam_pkg_assign2.py
new file mode 100755
index 000000000..6fe7d000c
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign2.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_lparam_pkg_assign2.v b/test_regress/t/t_lparam_pkg_assign2.v
new file mode 100644
index 000000000..4a7ee7dfb
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign2.v
@@ -0,0 +1,42 @@
+// DESCRIPTION: Verilator: Localparam with package function call using computed interface param
+//
+// 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
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+// verilog_format: on
+
+package pkg;
+  function automatic bit fn(int value);
+    return (value > 0) ? 1'b1 : 1'b0;
+  endfunction
+endpackage
+
+interface ifc #(parameter int WIDTH = 8);
+  localparam int DEPTH = $clog2(WIDTH);
+  localparam int COMPUTED = DEPTH * 2;
+endinterface
+
+module mod(ifc i);
+  // LPARAM references i.COMPUTED which depends on i.DEPTH which depends on WIDTH
+  localparam bit lpbit = pkg::fn(i.COMPUTED);
+  localparam int lpval = i.COMPUTED + 1;
+endmodule
+
+module t;
+  ifc #(.WIDTH(64)) i();
+  mod m(.i);
+
+  initial begin
+    // DEPTH = $clog2(64) = 6, COMPUTED = 6*2 = 12
+    `checkd(m.lpbit, 1'b1);  // fn(12) returns 1 since 12 > 0
+    `checkd(m.lpval, 13);  // lpval = 12 + 1 = 13
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule
diff --git a/test_regress/t/t_lparam_pkg_assign3.py b/test_regress/t/t_lparam_pkg_assign3.py
new file mode 100755
index 000000000..6fe7d000c
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign3.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_lparam_pkg_assign3.v b/test_regress/t/t_lparam_pkg_assign3.v
new file mode 100644
index 000000000..89640a45e
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign3.v
@@ -0,0 +1,45 @@
+// DESCRIPTION: Verilator: Localparam with function call using type-parameterized interface param
+//
+// 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
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+// verilog_format: on
+
+package pkg;
+  function automatic bit fn(int value);
+    return (value > 0) ? 1'b1 : 1'b0;
+  endfunction
+endpackage
+
+interface ifc #(parameter type some_type) ();
+  localparam int PARAM = 1;
+  localparam int TYPE_WIDTH = $bits(some_type);
+endinterface
+
+function automatic bit assert_func(bit value);
+  if (!value) $fatal(2, "DEAD");
+  return value;
+endfunction
+
+module mod(ifc i);
+  localparam bit lpbit = pkg::fn(i.PARAM);
+  localparam bit test = assert_func(i.TYPE_WIDTH == 32);
+endmodule
+
+module t;
+  ifc #(.some_type(int)) i();
+  mod m(.i);
+
+  initial begin
+    `checkd(m.lpbit, 1'b1);  // fn(1) returns 1 since 1 > 0
+    `checkd(m.test, 1'b1);  // $bits(int) == 32, so assert_func(1) returns 1
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule
diff --git a/test_regress/t/t_lparam_pkg_assign4.py b/test_regress/t/t_lparam_pkg_assign4.py
new file mode 100755
index 000000000..6fe7d000c
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign4.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_lparam_pkg_assign4.v b/test_regress/t/t_lparam_pkg_assign4.v
new file mode 100644
index 000000000..751991ee4
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign4.v
@@ -0,0 +1,49 @@
+// DESCRIPTION: Verilator: Interface array with multiple consumer modules using FUNCREF LPARAMs
+//
+// 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
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+// verilog_format: on
+
+package pkg;
+  function automatic int decode_width(int value);
+    return (value > 0) ? value * 2 : 0;
+  endfunction
+endpackage
+
+interface ifc #(parameter int WIDTH = 8);
+  localparam int DEPTH = $clog2(WIDTH);
+  localparam int DECODED = pkg::decode_width(DEPTH);
+endinterface
+
+module producer(ifc i);
+  localparam int BUF_SIZE = pkg::decode_width(i.DEPTH);
+  localparam int OUT_W = i.DECODED + 1;
+endmodule
+
+module consumer(ifc i);
+  localparam int HALF = i.DECODED / 2;
+  localparam int TAG_W = pkg::decode_width(i.DEPTH) + i.DECODED;
+endmodule
+
+module t;
+  ifc #(.WIDTH(64)) bus[2]();
+  producer p(.i(bus[0]));
+  consumer c(.i(bus[1]));
+
+  initial begin
+    // WIDTH=64, DEPTH=$clog2(64)=6, DECODED=decode_width(6)=12
+    `checkd(p.BUF_SIZE, 12);  // decode_width(6) = 12
+    `checkd(p.OUT_W, 13);  // 12 + 1 = 13
+    `checkd(c.HALF, 6);  // 12 / 2 = 6
+    `checkd(c.TAG_W, 24);  // decode_width(6) + 12 = 12 + 12 = 24
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule
diff --git a/test_regress/t/t_lparam_pkg_assign5.py b/test_regress/t/t_lparam_pkg_assign5.py
new file mode 100755
index 000000000..6fe7d000c
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign5.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_lparam_pkg_assign5.v b/test_regress/t/t_lparam_pkg_assign5.v
new file mode 100644
index 000000000..783e32245
--- /dev/null
+++ b/test_regress/t/t_lparam_pkg_assign5.v
@@ -0,0 +1,52 @@
+// DESCRIPTION: Verilator: Hierarchical interface pass-through with FUNCREF LPARAMs
+//
+// 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
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+// verilog_format: on
+
+package pkg;
+  function automatic int decode_width(int value);
+    return (value > 0) ? value * 2 : 0;
+  endfunction
+endpackage
+
+interface ifc #(parameter int WIDTH = 8);
+  localparam int DEPTH = $clog2(WIDTH);
+  localparam int DECODED = pkg::decode_width(DEPTH);
+endinterface
+
+// Leaf module: uses interface LPARAM in a FUNCREF-based localparam
+module leaf(ifc i);
+  localparam int BUF_SIZE = pkg::decode_width(i.DEPTH);
+  localparam int OUT_W = i.DECODED + 1;
+endmodule
+
+// Intermediate wrapper: passes interface through to leaf
+module wrapper(ifc i);
+  leaf u_leaf(.i);
+endmodule
+
+// Second level wrapper: adds another layer of hierarchy
+module subsystem(ifc bus);
+  wrapper u_wrap(.i(bus));
+endmodule
+
+module t;
+  ifc #(.WIDTH(64)) bus();
+  subsystem u_sub(.bus);
+
+  initial begin
+    // WIDTH=64, DEPTH=$clog2(64)=6, DECODED=decode_width(6)=12
+    `checkd(u_sub.u_wrap.u_leaf.BUF_SIZE, 12);  // decode_width(6) = 12
+    `checkd(u_sub.u_wrap.u_leaf.OUT_W, 13);  // 12 + 1 = 13
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule