diff --git a/core/prelude/operators/deref.carbon b/core/prelude/operators/deref.carbon
index 369304a4b469..5b99fa33e52d 100644
--- a/core/prelude/operators/deref.carbon
+++ b/core/prelude/operators/deref.carbon
@@ -7,5 +7,5 @@ package Core library "prelude/operators/deref";
 // TODO: Align with https://docs.carbon-lang.dev/docs/design/values.html#dereferencing-customization.
 interface CppUnsafeDeref {
   let Result:! type;
-  fn Op[ref self: Self]() -> ref Result;
+  fn Op[self: Self]() -> Result;
 }
diff --git a/toolchain/check/cpp/impl_lookup.cpp b/toolchain/check/cpp/impl_lookup.cpp
index a75fad0600fe..d5215db3de56 100644
--- a/toolchain/check/cpp/impl_lookup.cpp
+++ b/toolchain/check/cpp/impl_lookup.cpp
@@ -8,6 +8,8 @@
 #include "clang/Sema/Lookup.h"
 #include "clang/Sema/Sema.h"
 #include "toolchain/base/kind_switch.h"
+#include "toolchain/check/context.h"
+#include "toolchain/check/convert.h"
 #include "toolchain/check/core_identifier.h"
 #include "toolchain/check/cpp/import.h"
 #include "toolchain/check/cpp/location.h"
@@ -19,6 +21,7 @@
 #include "toolchain/check/impl_lookup.h"
 #include "toolchain/check/import_ref.h"
 #include "toolchain/check/inst.h"
+#include "toolchain/check/name_lookup.h"
 #include "toolchain/check/type.h"
 #include "toolchain/check/type_completion.h"
 #include "toolchain/sem_ir/builtin_function_kind.h"
@@ -187,8 +190,8 @@ static auto BuildCppUnsafeDerefWitness(
 
   // TODO: Parameterize the interface by the form of the operand and compute the
   // appropriate passing mode here.
-  SemIR::ClangDeclSignatureId signature_id =
-      MakeSignature(context, {}, SemIR::ClangDeclSignature::PassingMode::ByRef);
+  SemIR::ClangDeclSignatureId signature_id = MakeSignature(
+      context, {}, SemIR::ClangDeclSignature::PassingMode::ByValue);
 
   auto decl_info =
       DeclInfo{.decl = *candidates.begin(), .signature_id = signature_id};
@@ -197,17 +200,15 @@ static auto BuildCppUnsafeDerefWitness(
     return fn_id;
   }
 
-  auto result_type_id =
+  auto result_type_inst_id =
       context.functions()
           .Get(context.insts().GetAs<SemIR::FunctionDecl>(fn_id).function_id)
           .return_type_inst_id;
-  if (result_type_id == SemIR::ErrorInst::InstId) {
-    return SemIR::ErrorInst::InstId;
-  }
-
-  return BuildCustomWitness(context, loc_id, query_self_const_id,
-                            query_specific_interface_id,
-                            {result_type_id, fn_id});
+  return BuildCustomWitness(
+      context, loc_id, query_self_const_id, query_specific_interface_id,
+      {context.types().GetTypeInstId(context.types().GetUnqualifiedType(
+           context.types().GetTypeIdForTypeInstId(result_type_inst_id))),
+       fn_id});
 }
 
 static auto BuildDefaultWitness(
diff --git a/toolchain/check/testdata/interop/cpp/operators/deref.carbon b/toolchain/check/testdata/interop/cpp/operators/deref.carbon
index 6a2d917bdf74..974aad32221f 100644
--- a/toolchain/check/testdata/interop/cpp/operators/deref.carbon
+++ b/toolchain/check/testdata/interop/cpp/operators/deref.carbon
@@ -123,8 +123,8 @@ fn TestDerefFail(not_ptr: Cpp.NotAPtr) {
 // CHECK:STDOUT:   %Int.generic: %Int.type = struct_value () [concrete]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
 // CHECK:STDOUT:   %CppUnsafeDeref.type: type = facet_type <@CppUnsafeDeref> [concrete]
-// CHECK:STDOUT:   %Self: %CppUnsafeDeref.type = symbolic_binding Self, 0 [symbolic]
-// CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.type.393: type = fn_type @CppUnsafeDeref.WithSelf.Op, @CppUnsafeDeref.WithSelf(%Self) [symbolic]
+// CHECK:STDOUT:   %Self.a61: %CppUnsafeDeref.type = symbolic_binding Self, 0 [symbolic]
+// CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.type.393: type = fn_type @CppUnsafeDeref.WithSelf.Op, @CppUnsafeDeref.WithSelf(%Self.a61) [symbolic]
 // CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.0fa: %CppUnsafeDeref.WithSelf.Op.type.393 = struct_value () [symbolic]
 // CHECK:STDOUT:   %CppUnsafeDeref.assoc_type: type = assoc_entity_type @CppUnsafeDeref [concrete]
 // CHECK:STDOUT:   %assoc1: %CppUnsafeDeref.assoc_type = assoc_entity element1, imports.%Core.import_ref.a48 [concrete]
@@ -134,6 +134,7 @@ fn TestDerefFail(not_ptr: Cpp.NotAPtr) {
 // CHECK:STDOUT:   %operator_Star__carbon_thunk: %operator_Star__carbon_thunk.type = struct_value () [concrete]
 // CHECK:STDOUT:   %ConstDeref.Op.type: type = fn_type @ConstDeref.Op [concrete]
 // CHECK:STDOUT:   %ConstDeref.Op: %ConstDeref.Op.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Copy.type: type = facet_type <@Copy> [concrete]
 // CHECK:STDOUT:   %custom_witness.000: <witness> = custom_witness (%i32, %ConstDeref.Op), @CppUnsafeDeref [concrete]
 // CHECK:STDOUT:   %CppUnsafeDeref.facet.909: %CppUnsafeDeref.type = facet_value %ConstDeref, (%custom_witness.000) [concrete]
 // CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.type.6a5: type = fn_type @CppUnsafeDeref.WithSelf.Op, @CppUnsafeDeref.WithSelf(%CppUnsafeDeref.facet.909) [concrete]
@@ -144,6 +145,7 @@ fn TestDerefFail(not_ptr: Cpp.NotAPtr) {
 // CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
 // CHECK:STDOUT:     .Int = %Core.Int
 // CHECK:STDOUT:     .CppUnsafeDeref = %Core.CppUnsafeDeref
+// CHECK:STDOUT:     .Copy = %Core.Copy
 // CHECK:STDOUT:     import Core//prelude
 // CHECK:STDOUT:     import Core//prelude/...
 // CHECK:STDOUT:   }
@@ -161,6 +163,7 @@ fn TestDerefFail(not_ptr: Cpp.NotAPtr) {
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     <elided>
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.Copy: type = import_ref Core//prelude/copy, Copy, loaded [concrete = constants.%Copy.type]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @TestDerefPass(%int_ptr.param: ref %ConstDeref) -> ref %i32 {
@@ -169,13 +172,14 @@ fn TestDerefFail(not_ptr: Cpp.NotAPtr) {
 // CHECK:STDOUT:   %Core.ref: <namespace> = name_ref Core, imports.%Core [concrete = imports.%Core]
 // CHECK:STDOUT:   %CppUnsafeDeref.ref: type = name_ref CppUnsafeDeref, imports.%Core.CppUnsafeDeref [concrete = constants.%CppUnsafeDeref.type]
 // CHECK:STDOUT:   %Op.ref.loc13_38: %CppUnsafeDeref.assoc_type = name_ref Op, imports.%Core.import_ref.05c [concrete = constants.%assoc1]
-// CHECK:STDOUT:   %impl_witness_assoc_constant: type = impl_witness_assoc_constant @ConstDeref.cpp_operator.%i32.2 [concrete = constants.%i32]
+// CHECK:STDOUT:   %impl_witness_assoc_constant: type = impl_witness_assoc_constant constants.%i32 [concrete = constants.%i32]
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT:   %impl.elem1: %.b71 = impl_witness_access constants.%custom_witness.000, element1 [concrete = constants.%ConstDeref.Op]
 // CHECK:STDOUT:   %bound_method: <bound method> = bound_method %int_ptr.ref, %impl.elem1
+// CHECK:STDOUT:   %.loc13: %ConstDeref = acquire_value %int_ptr.ref
 // CHECK:STDOUT:   %Op.ref.loc13_43: %ConstDeref.cpp_operator.type = name_ref Op, imports.%ConstDeref.cpp_operator.decl [concrete = constants.%ConstDeref.cpp_operator]
-// CHECK:STDOUT:   %ConstDeref.cpp_operator.bound: <bound method> = bound_method %int_ptr.ref, %Op.ref.loc13_43
-// CHECK:STDOUT:   %operator_Star__carbon_thunk.call: ref %i32 = call imports.%operator_Star__carbon_thunk.decl(%int_ptr.ref)
+// CHECK:STDOUT:   %ConstDeref.cpp_operator.bound: <bound method> = bound_method %.loc13, %Op.ref.loc13_43
+// CHECK:STDOUT:   %operator_Star__carbon_thunk.call: ref %i32 = call imports.%operator_Star__carbon_thunk.decl(%.loc13)
 // CHECK:STDOUT:   return %operator_Star__carbon_thunk.call
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -220,10 +224,11 @@ fn TestDerefFail(not_ptr: Cpp.NotAPtr) {
 // CHECK:STDOUT:   %Core.ref: <namespace> = name_ref Core, imports.%Core [concrete = imports.%Core]
 // CHECK:STDOUT:   %CppUnsafeDeref.ref: type = name_ref CppUnsafeDeref, imports.%Core.CppUnsafeDeref [concrete = constants.%CppUnsafeDeref.type]
 // CHECK:STDOUT:   %Op.ref: %CppUnsafeDeref.assoc_type = name_ref Op, imports.%Core.import_ref.05c [concrete = constants.%assoc1]
-// CHECK:STDOUT:   %impl_witness_assoc_constant: type = impl_witness_assoc_constant @MutableDeref.cpp_operator.%i32.2 [concrete = constants.%i32]
+// CHECK:STDOUT:   %impl_witness_assoc_constant: type = impl_witness_assoc_constant constants.%i32 [concrete = constants.%i32]
 // CHECK:STDOUT:   %impl.elem1: %.a97 = impl_witness_access constants.%custom_witness.91c, element1 [concrete = constants.%MutableDeref.cpp_operator]
 // CHECK:STDOUT:   %bound_method: <bound method> = bound_method %int_ptr.ref, %impl.elem1
-// CHECK:STDOUT:   %MutableDeref.cpp_operator.call: ref %i32 = call %bound_method(%int_ptr.ref)
+// CHECK:STDOUT:   %.loc13: %MutableDeref = acquire_value %int_ptr.ref
+// CHECK:STDOUT:   %MutableDeref.cpp_operator.call: ref %i32 = call %bound_method(%.loc13)
 // CHECK:STDOUT:   return %MutableDeref.cpp_operator.call
 // CHECK:STDOUT: }
 // CHECK:STDOUT: