diff --git a/docs/design_rationale.md b/docs/design_rationale.md
index 6cc7841..bdb7918 100644
--- a/docs/design_rationale.md
+++ b/docs/design_rationale.md
@@ -218,12 +218,11 @@ zero changes to the library or to the box types.
 template <beman::monadics::box Box>
     requires beman::monadics::box<typename beman::monadics::get_box_traits<Box>::value_type>
 constexpr auto flatten(Box&& box) {
-    using Traits    = beman::monadics::get_box_traits<Box>;
-    using InnerBox  = typename Traits::value_type;
-    using ITraits   = beman::monadics::get_box_traits<InnerBox>;
+    using Traits   = beman::monadics::get_box_traits<Box>;
+    using InnerBox = typename Traits::value_type;
 
     if (!Traits::has_value(box))
-        return ITraits::make_error(Traits::error(std::forward<Box>(box)));
+        return beman::monadics::propagate_error<InnerBox>(std::forward<Box>(box));
     return Traits::value(std::forward<Box>(box));
 }
 ```
diff --git a/include/beman/monadics/CMakeLists.txt b/include/beman/monadics/CMakeLists.txt
index 50f3dde..9ece538 100644
--- a/include/beman/monadics/CMakeLists.txt
+++ b/include/beman/monadics/CMakeLists.txt
@@ -28,8 +28,8 @@ target_sources(
                 detail/meta_rebind.hpp
                 detail/or_else.hpp
                 detail/pipe_adaptor.hpp
-                detail/rebox_value.hpp
-                detail/rebox_error.hpp
+                detail/propagate_value.hpp
+                detail/propagate_error.hpp
                 detail/same_box.hpp
                 detail/same_unqualified_as.hpp
                 detail/transform.hpp
diff --git a/include/beman/monadics/detail/and_then.hpp b/include/beman/monadics/detail/and_then.hpp
index 95fe76b..0c3296f 100644
--- a/include/beman/monadics/detail/and_then.hpp
+++ b/include/beman/monadics/detail/and_then.hpp
@@ -6,7 +6,7 @@
 #include <beman/monadics/detail/get_box_traits.hpp>
 #include <beman/monadics/detail/invoke_with_value.hpp>
 #include <beman/monadics/detail/pipe_adaptor.hpp>
-#include <beman/monadics/detail/rebox_error.hpp>
+#include <beman/monadics/detail/propagate_error.hpp>
 #include <beman/monadics/detail/same_box.hpp>
 
 #include <utility>
@@ -37,7 +37,7 @@ class and_then_t {
 
         using NewBox = decltype(invoke_with_value(std::forward<Op>(op).callable(key), std::forward<Box>(box)));
 
-        return rebox_error<NewBox>(std::forward<Box>(box));
+        return propagate_error<NewBox>(std::forward<Box>(box));
     }
 };
 
diff --git a/include/beman/monadics/detail/invoke_with_error.hpp b/include/beman/monadics/detail/invoke_with_error.hpp
index 1e5a08e..e8436a8 100644
--- a/include/beman/monadics/detail/invoke_with_error.hpp
+++ b/include/beman/monadics/detail/invoke_with_error.hpp
@@ -11,13 +11,20 @@
 namespace beman::monadics::detail {
 
 template<typename Fn, typename Box, typename Traits = get_box_traits<Box>>
-[[nodiscard]] constexpr decltype(auto) invoke_with_error(Fn&& fn, Box&& box) noexcept
-    requires requires {
-        { Traits::error(std::forward<Box>(box)) };
-        { std::forward<Fn>(fn)(Traits::error(std::forward<Box>(box))) };
-    } || requires { requires std::invocable<Fn>; }
+concept invocable_with_error = requires {
+    requires has_error_channel<Box>;
+    { std::declval<Fn>()(Traits::error(std::declval<Box>())) };
+} || requires {
+    requires !has_error_channel<Box>;
+    requires std::invocable<Fn>;
+};
+
+template<typename Fn, box Box>
+[[nodiscard]] constexpr decltype(auto) invoke_with_error(Fn&& fn, Box&& box)
+    requires invocable_with_error<decltype(fn), decltype(box)>
 {
-    if constexpr (requires { Traits::error(std::forward<Box>(box)); }) {
+    using Traits = get_box_traits<Box>;
+    if constexpr (has_error_channel<Box>) {
         return std::forward<Fn>(fn)(Traits::error(std::forward<Box>(box)));
     } else {
         return std::forward<Fn>(fn)();
diff --git a/include/beman/monadics/detail/invoke_with_value.hpp b/include/beman/monadics/detail/invoke_with_value.hpp
index a773242..d0c3665 100644
--- a/include/beman/monadics/detail/invoke_with_value.hpp
+++ b/include/beman/monadics/detail/invoke_with_value.hpp
@@ -10,15 +10,19 @@
 
 namespace beman::monadics::detail {
 
-template<typename Fn, box Box, typename Traits = get_box_traits<Box>>
-[[nodiscard]] constexpr decltype(auto) invoke_with_value(Fn&& fn, Box&& box) noexcept
-    requires requires {
-        { Traits::value(std::forward<Box>(box)) } -> std::same_as<void>;
-        { fn() };
-    } || requires {
-        { fn(Traits::value(std::forward<Box>(box))) };
-    }
+template<typename Fn, typename Box, typename Traits = get_box_traits<Box>>
+concept invocable_with_value = requires {
+    requires std::is_void_v<typename Traits::value_type>;
+    requires std::invocable<Fn>;
+} || requires {
+    { std::declval<Fn>()(Traits::value(std::declval<Box>())) };
+};
+
+template<typename Fn, box Box>
+[[nodiscard]] constexpr decltype(auto) invoke_with_value(Fn&& fn, Box&& box)
+    requires invocable_with_value<decltype(fn), decltype(box)>
 {
+    using Traits = get_box_traits<Box>;
     if constexpr (std::is_void_v<typename Traits::value_type> && std::invocable<Fn>) {
         // should just invoke Traits::value(box);
         return std::forward<Fn>(fn)();
diff --git a/include/beman/monadics/detail/or_else.hpp b/include/beman/monadics/detail/or_else.hpp
index 035c3eb..4b55e6e 100644
--- a/include/beman/monadics/detail/or_else.hpp
+++ b/include/beman/monadics/detail/or_else.hpp
@@ -6,7 +6,7 @@
 #include <beman/monadics/detail/get_box_traits.hpp>
 #include <beman/monadics/detail/invoke_with_error.hpp>
 #include <beman/monadics/detail/pipe_adaptor.hpp>
-#include <beman/monadics/detail/rebox_value.hpp>
+#include <beman/monadics/detail/propagate_value.hpp>
 #include <beman/monadics/detail/same_box.hpp>
 
 #include <utility>
@@ -36,7 +36,7 @@ class or_else_t {
             return invoke_with_error(std::forward<Op>(op).callable(key), std::forward<Box>(box));
         }
 
-        return rebox_value<NewBox>(std::forward<Box>(box));
+        return propagate_value<NewBox>(std::forward<Box>(box));
     }
 };
 
diff --git a/include/beman/monadics/detail/propagate_error.hpp b/include/beman/monadics/detail/propagate_error.hpp
new file mode 100644
index 0000000..4cbb466
--- /dev/null
+++ b/include/beman/monadics/detail/propagate_error.hpp
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef BEMAN_MONADICS_DETAIL_PROPAGATE_ERROR_HPP
+#define BEMAN_MONADICS_DETAIL_PROPAGATE_ERROR_HPP
+
+#include <beman/monadics/detail/get_box_traits.hpp>
+#include <beman/monadics/detail/same_box.hpp>
+
+#include <utility>
+
+namespace beman::monadics::detail {
+
+template<typename NewBox, typename Box>
+concept propagatable_error = same_box<NewBox, Box> && (requires {
+    requires has_error_channel<Box>;
+    { get_box_traits<NewBox>::make_error(get_box_traits<Box>::error(std::declval<Box>())) };
+} || requires {
+    requires !has_error_channel<Box>;
+    { get_box_traits<NewBox>::make_error(get_box_traits<Box>::error()) };
+});
+
+template<typename NewBox, typename Box>
+    requires propagatable_error<NewBox, Box>
+[[nodiscard]] constexpr decltype(auto) propagate_error(Box&& box) {
+    using BoxTraits = get_box_traits<Box>;
+    using NewBoxTraits = get_box_traits<NewBox>;
+
+    if constexpr (has_error_channel<Box>) {
+        return NewBoxTraits::make_error(BoxTraits::error(std::forward<Box>(box)));
+    } else {
+        return NewBoxTraits::make_error(BoxTraits::error());
+    }
+}
+
+} // namespace beman::monadics::detail
+
+#endif // BEMAN_MONADICS_DETAIL_PROPAGATE_ERROR_HPP
diff --git a/include/beman/monadics/detail/rebox_value.hpp b/include/beman/monadics/detail/propagate_value.hpp
similarity index 51%
rename from include/beman/monadics/detail/rebox_value.hpp
rename to include/beman/monadics/detail/propagate_value.hpp
index 4bc80a1..4d82c06 100644
--- a/include/beman/monadics/detail/rebox_value.hpp
+++ b/include/beman/monadics/detail/propagate_value.hpp
@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#ifndef BEMAN_MONADICS_DETAIL_REBOX_VALUE_HPP
-#define BEMAN_MONADICS_DETAIL_REBOX_VALUE_HPP
+#ifndef BEMAN_MONADICS_DETAIL_PROPAGATE_VALUE_HPP
+#define BEMAN_MONADICS_DETAIL_PROPAGATE_VALUE_HPP
 
 #include <beman/monadics/detail/get_box_traits.hpp>
 #include <beman/monadics/detail/same_box.hpp>
@@ -12,8 +12,16 @@
 namespace beman::monadics::detail {
 
 template<typename NewBox, typename Box>
-    requires same_box<NewBox, Box>
-[[nodiscard]] constexpr decltype(auto) rebox_value(Box&& box) noexcept {
+concept propagatable_value = same_box<NewBox, Box> && (requires {
+    requires std::is_void_v<typename get_box_traits<NewBox>::value_type>;
+    { get_box_traits<NewBox>::make() };
+} || requires {
+    { get_box_traits<NewBox>::make(get_box_traits<Box>::value(std::declval<Box>())) };
+});
+
+template<typename NewBox, typename Box>
+    requires propagatable_value<NewBox, Box>
+[[nodiscard]] constexpr decltype(auto) propagate_value(Box&& box) {
     using BoxTraits = get_box_traits<Box>;
     using NewBoxTraits = get_box_traits<NewBox>;
 
@@ -26,4 +34,4 @@ template<typename NewBox, typename Box>
 
 } // namespace beman::monadics::detail
 
-#endif // BEMAN_MONADICS_DETAIL_REBOX_VALUE_HPP
+#endif // BEMAN_MONADICS_DETAIL_PROPAGATE_VALUE_HPP
diff --git a/include/beman/monadics/detail/rebox_error.hpp b/include/beman/monadics/detail/rebox_error.hpp
deleted file mode 100644
index bed5310..0000000
--- a/include/beman/monadics/detail/rebox_error.hpp
+++ /dev/null
@@ -1,28 +0,0 @@
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-#ifndef BEMAN_MONADICS_DETAIL_REBOX_ERROR_HPP
-#define BEMAN_MONADICS_DETAIL_REBOX_ERROR_HPP
-
-#include <beman/monadics/detail/get_box_traits.hpp>
-#include <beman/monadics/detail/same_box.hpp>
-
-#include <utility>
-
-namespace beman::monadics::detail {
-
-template<typename NewBox, typename Box>
-    requires same_box<NewBox, Box>
-[[nodiscard]] constexpr decltype(auto) rebox_error(Box&& box) noexcept {
-    using BoxTraits = get_box_traits<Box>;
-    using NewBoxTraits = get_box_traits<NewBox>;
-
-    if constexpr (requires { BoxTraits::error(std::forward<Box>(box)); }) {
-        return NewBoxTraits::make_error(BoxTraits::error(std::forward<Box>(box)));
-    } else {
-        return NewBoxTraits::make_error(BoxTraits::error());
-    }
-}
-
-} // namespace beman::monadics::detail
-
-#endif // BEMAN_MONADICS_DETAIL_REBOX_ERROR_HPP
diff --git a/include/beman/monadics/detail/transform.hpp b/include/beman/monadics/detail/transform.hpp
index 18a5f64..cb932e5 100644
--- a/include/beman/monadics/detail/transform.hpp
+++ b/include/beman/monadics/detail/transform.hpp
@@ -6,7 +6,7 @@
 #include <beman/monadics/detail/get_box_traits.hpp>
 #include <beman/monadics/detail/invoke_with_value.hpp>
 #include <beman/monadics/detail/pipe_adaptor.hpp>
-#include <beman/monadics/detail/rebox_error.hpp>
+#include <beman/monadics/detail/propagate_error.hpp>
 #include <beman/monadics/detail/same_box.hpp>
 
 #include <type_traits>
@@ -42,7 +42,7 @@ class transform_t {
             }
         }
 
-        return rebox_error<NewBox>(std::forward<Box>(box));
+        return propagate_error<NewBox>(std::forward<Box>(box));
     }
 };
 
diff --git a/include/beman/monadics/detail/transform_error.hpp b/include/beman/monadics/detail/transform_error.hpp
index b8a4c8c..f1ad663 100644
--- a/include/beman/monadics/detail/transform_error.hpp
+++ b/include/beman/monadics/detail/transform_error.hpp
@@ -36,7 +36,7 @@ class transform_error_t {
                                                               std::forward<Box>(box)));
         }
 
-        return rebox_value<NewBox>(std::forward<Box>(box));
+        return propagate_value<NewBox>(std::forward<Box>(box));
 
         // gcc11/12 internal crash with pipe_adaptor_t
         // return std::forward<Box>(box) | or_else([f = std::forward<Op>(op).callable(key)](auto&& e) {
diff --git a/include/beman/monadics/monadics.hpp b/include/beman/monadics/monadics.hpp
index 1d8c2b7..e8e5972 100644
--- a/include/beman/monadics/monadics.hpp
+++ b/include/beman/monadics/monadics.hpp
@@ -6,8 +6,12 @@
 #include <beman/monadics/detail/access_key.hpp>
 #include <beman/monadics/detail/and_then.hpp>
 #include <beman/monadics/detail/get_box_traits.hpp>
+#include <beman/monadics/detail/invoke_with_error.hpp>
+#include <beman/monadics/detail/invoke_with_value.hpp>
 #include <beman/monadics/detail/or_else.hpp>
 #include <beman/monadics/detail/pipe_adaptor.hpp>
+#include <beman/monadics/detail/propagate_error.hpp>
+#include <beman/monadics/detail/propagate_value.hpp>
 #include <beman/monadics/detail/transform.hpp>
 #include <beman/monadics/detail/transform_error.hpp>
 
@@ -20,9 +24,13 @@ using detail::box;
 using detail::box_traits;
 using detail::get_box_traits;
 using detail::has_error_channel;
+using detail::invoke_with_error;
+using detail::invoke_with_value;
 using detail::or_else;
 using detail::or_elseable;
 using detail::pipe_adaptor;
+using detail::propagate_error;
+using detail::propagate_value;
 using detail::transform;
 using detail::transform_error;
 using detail::transform_errorable;
diff --git a/tests/beman/monadics/detail/CMakeLists.txt b/tests/beman/monadics/detail/CMakeLists.txt
index 00c45b0..96e8aff 100644
--- a/tests/beman/monadics/detail/CMakeLists.txt
+++ b/tests/beman/monadics/detail/CMakeLists.txt
@@ -23,8 +23,8 @@ set(tests
     meta_rebind_error
     meta_rebind
     pipe_adaptor
-    rebox_error
-    rebox_value
+    propagate_error
+    propagate_value
     same_box
 )
 
diff --git a/tests/beman/monadics/detail/has_box_traits.test.cpp b/tests/beman/monadics/detail/has_box_traits.test.cpp
new file mode 100644
index 0000000..b330d55
--- /dev/null
+++ b/tests/beman/monadics/detail/has_box_traits.test.cpp
@@ -0,0 +1,353 @@
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <catch2/catch_template_test_macros.hpp>
+#include <catch2/catch_test_macros.hpp>
+
+#include <beman/monadics/detail/has_box_traits.hpp>
+
+namespace beman::monadics::detail::tests {
+
+namespace {
+
+template<typename T>
+struct box_traits {};
+
+// ── has_valid_has_value_fn fixtures ──────────────────────────────────────────
+
+struct HasValueCorrect {};
+template<>
+struct box_traits<HasValueCorrect> {
+    using value_type = int;
+    static bool has_value(const HasValueCorrect&) noexcept { return true; }
+};
+
+struct HasValueBadReturn {};
+struct NoBool {};
+template<>
+struct box_traits<HasValueBadReturn> {
+    using value_type = int;
+    static NoBool has_value(const HasValueBadReturn&) noexcept { return {}; }
+};
+
+struct HasValueNotNoexcept {};
+template<>
+struct box_traits<HasValueNotNoexcept> {
+    using value_type = int;
+    static bool has_value(const HasValueNotNoexcept&) { return true; } // missing noexcept
+};
+
+// ── has_valid_value_fn fixtures ───────────────────────────────────────────────
+
+struct ValueCorrect {
+    int val{};
+};
+template<>
+struct box_traits<ValueCorrect> {
+    using value_type = int;
+    static int value(ValueCorrect&&) { return 0; }
+};
+
+struct ValueWrongReturn {
+    int val{};
+};
+template<>
+struct box_traits<ValueWrongReturn> {
+    using value_type = int;
+    static double value(ValueWrongReturn&&) { return 3.14; }
+};
+
+// ── has_valid_error_fn fixtures ───────────────────────────────────────────────
+
+struct ErrorNullaryCorrect {};
+template<>
+struct box_traits<ErrorNullaryCorrect> {
+    using error_type = int;
+    static int error() { return 0; }
+};
+
+struct ErrorUnaryCorrect {
+    int err{};
+};
+template<>
+struct box_traits<ErrorUnaryCorrect> {
+    using error_type = int;
+    static int error(ErrorUnaryCorrect&& b) { return b.err; }
+};
+
+struct ErrorWrongReturn {};
+template<>
+struct box_traits<ErrorWrongReturn> {
+    using error_type = int;
+    static double error(ErrorWrongReturn&&) { return 3.14; }
+};
+
+// ── has_valid_make_fn fixtures ────────────────────────────────────────────────
+
+struct MakeCorrect {
+    int val{};
+};
+template<>
+struct box_traits<MakeCorrect> {
+    using value_type = int;
+    static MakeCorrect make(int v) { return {v}; }
+};
+
+struct OtherBox {
+    int val{};
+};
+
+struct MakeWrongReturn {
+    int val{};
+};
+template<>
+struct box_traits<MakeWrongReturn> {
+    using value_type = int;
+    static OtherBox make(int v) { return {v}; }
+};
+
+// ── has_valid_make_error_fn fixtures ─────────────────────────────────────────
+
+struct MakeErrorCorrect {
+    int err{};
+};
+template<>
+struct box_traits<MakeErrorCorrect> {
+    using error_type = int;
+    static MakeErrorCorrect make_error(int e) { return {e}; }
+};
+
+struct MakeErrorWrongReturn {
+    int err{};
+};
+template<>
+struct box_traits<MakeErrorWrongReturn> {
+    using error_type = int;
+    static OtherBox make_error(int e) { return {e}; }
+};
+
+// ── rebind / rebind_error fixtures ────────────────────────────────────────────
+
+template<typename T, typename E>
+struct ExpectedLike {
+    using value_type = T;
+    using error_type = E;
+
+    template<typename U>
+    using rebind = ExpectedLike<U, E>;
+
+    template<typename F>
+    using rebind_error = ExpectedLike<T, F>;
+};
+
+template<typename T>
+struct OptionalLike {
+    using value_type = T;
+};
+
+template<typename T, typename E>
+struct box_traits<ExpectedLike<T, E>> {
+    using value_type = T;
+    using error_type = E;
+
+    template<typename U>
+    using rebind = ExpectedLike<U, E>;
+
+    template<typename F>
+    using rebind_error = ExpectedLike<T, F>;
+
+    static bool has_value(const ExpectedLike<T, E>&) noexcept { return true; }
+    static T value(ExpectedLike<T, E>&&) { return {}; }
+    static E error(ExpectedLike<T, E>&&) { return {}; }
+    static ExpectedLike<T, E> make(T) { return {}; }
+    static ExpectedLike<T, E> make_error(E) { return {}; }
+};
+
+// rebind goes to the wrong template family
+template<typename T, typename E>
+struct BadRebindTraits {
+    using value_type = T;
+    using error_type = E;
+
+    template<typename U>
+    using rebind = OptionalLike<U>; // wrong template family
+
+    template<typename F>
+    using rebind_error = ExpectedLike<T, F>;
+};
+
+// rebind preserves the right shape but hard-codes the wrong error type
+template<typename T, typename E>
+struct WrongErrorRebindTraits {
+    using value_type = T;
+    using error_type = E;
+
+    template<typename U>
+    using rebind = ExpectedLike<U, int>; // error always becomes int
+
+    template<typename F>
+    using rebind_error = ExpectedLike<T, F>;
+};
+
+// rebind_error goes to the wrong template family
+template<typename T, typename E>
+struct BadRebindErrorTraits {
+    using value_type = T;
+    using error_type = E;
+
+    template<typename U>
+    using rebind = ExpectedLike<U, E>;
+
+    template<typename F>
+    using rebind_error = OptionalLike<F>; // wrong template family
+};
+
+// rebind_error preserves the right shape but hard-codes the wrong value type
+template<typename T, typename E>
+struct WrongValueRebindErrorTraits {
+    using value_type = T;
+    using error_type = E;
+
+    template<typename U>
+    using rebind = ExpectedLike<U, E>;
+
+    template<typename F>
+    using rebind_error = ExpectedLike<double, F>; // value always becomes double
+};
+
+} // namespace
+
+// ── has_valid_has_value_fn ────────────────────────────────────────────────────
+
+TEMPLATE_TEST_CASE_SIG("has_valid_has_value_fn",
+                       "",
+                       ((typename Box, bool Valid), Box, Valid),
+                       (int, true),                // no has_value -> passes
+                       (HasValueCorrect, true),    // correct signature
+                       (HasValueBadReturn, false), // non-bool-convertible return
+                       (HasValueNotNoexcept, false)) {
+    using Traits = box_traits<Box>;
+    if constexpr (Valid) {
+        STATIC_REQUIRE(has_valid_has_value_fn<Box, Traits>);
+    } else {
+        STATIC_REQUIRE_FALSE(has_valid_has_value_fn<Box, Traits>);
+    }
+}
+
+// ── has_valid_value_fn ────────────────────────────────────────────────────────
+
+TEMPLATE_TEST_CASE_SIG("has_valid_value_fn",
+                       "",
+                       ((typename Box, bool Valid), Box, Valid),
+                       (int, true),               // no value -> passes
+                       (ValueCorrect, true),      // correct return type
+                       (ValueWrongReturn, false)) // returns double instead of int
+{
+    using Traits = box_traits<Box>;
+    if constexpr (Valid) {
+        STATIC_REQUIRE(has_valid_value_fn<Box, Traits>);
+    } else {
+        STATIC_REQUIRE_FALSE(has_valid_value_fn<Box, Traits>);
+    }
+}
+
+// ── has_valid_error_fn ────────────────────────────────────────────────────────
+
+TEMPLATE_TEST_CASE_SIG("has_valid_error_fn",
+                       "",
+                       ((typename Box, bool Valid), Box, Valid),
+                       (int, true),                 // no error -> passes
+                       (ErrorNullaryCorrect, true), // nullary, correct return
+                       (ErrorUnaryCorrect, true),   // unary, correct return
+                       (ErrorWrongReturn, false))   // unary, wrong return type
+{
+    using Traits = box_traits<Box>;
+    if constexpr (Valid) {
+        STATIC_REQUIRE(has_valid_error_fn<Box, Traits>);
+    } else {
+        STATIC_REQUIRE_FALSE(has_valid_error_fn<Box, Traits>);
+    }
+}
+
+// ── has_valid_make_fn ─────────────────────────────────────────────────────────
+
+TEMPLATE_TEST_CASE_SIG("has_valid_make_fn",
+                       "",
+                       ((typename Box, bool Valid), Box, Valid),
+                       (int, true),              // no make -> passes
+                       (MakeCorrect, true),      // returns Box
+                       (MakeWrongReturn, false)) // returns OtherBox
+{
+    using Traits = box_traits<Box>;
+    if constexpr (Valid) {
+        STATIC_REQUIRE(has_valid_make_fn<Box, Traits>);
+    } else {
+        STATIC_REQUIRE_FALSE(has_valid_make_fn<Box, Traits>);
+    }
+}
+
+// ── has_valid_make_error_fn ───────────────────────────────────────────────────
+
+TEMPLATE_TEST_CASE_SIG("has_valid_make_error_fn",
+                       "",
+                       ((typename Box, bool Valid), Box, Valid),
+                       (int, true),                   // no make_error -> passes
+                       (MakeErrorCorrect, true),      // returns Box
+                       (MakeErrorWrongReturn, false)) // returns OtherBox
+{
+    using Traits = box_traits<Box>;
+    if constexpr (Valid) {
+        STATIC_REQUIRE(has_valid_make_error_fn<Box, Traits>);
+    } else {
+        STATIC_REQUIRE_FALSE(has_valid_make_error_fn<Box, Traits>);
+    }
+}
+
+// ── has_valid_rebind ──────────────────────────────────────────────────────────
+
+using ExpectedIntDouble = ExpectedLike<int, double>;
+
+TEST_CASE("has_valid_rebind - correct traits") {
+    STATIC_REQUIRE(has_valid_rebind<ExpectedIntDouble, box_traits<ExpectedIntDouble>>);
+}
+
+TEST_CASE("has_valid_rebind - rebind returns wrong template family") {
+    STATIC_REQUIRE_FALSE(has_valid_rebind<ExpectedIntDouble, BadRebindTraits<int, double>>);
+}
+
+TEST_CASE("has_valid_rebind - rebind corrupts error type") {
+    STATIC_REQUIRE_FALSE(has_valid_rebind<ExpectedIntDouble, WrongErrorRebindTraits<int, double>>);
+}
+
+// ── has_valid_rebind_error ────────────────────────────────────────────────────
+
+TEST_CASE("has_valid_rebind_error - correct traits") {
+    STATIC_REQUIRE(has_valid_rebind_error<ExpectedIntDouble, box_traits<ExpectedIntDouble>>);
+}
+
+TEST_CASE("has_valid_rebind_error - rebind_error returns wrong template family") {
+    STATIC_REQUIRE_FALSE(has_valid_rebind_error<ExpectedIntDouble, BadRebindErrorTraits<int, double>>);
+}
+
+TEST_CASE("has_valid_rebind_error - rebind_error corrupts value type") {
+    STATIC_REQUIRE_FALSE(has_valid_rebind_error<ExpectedIntDouble, WrongValueRebindErrorTraits<int, double>>);
+}
+
+// ── has_box_traits ────────────────────────────────────────────────────────────
+
+TEST_CASE("has_box_traits - fully correct traits") {
+    STATIC_REQUIRE(has_box_traits<ExpectedIntDouble, box_traits<ExpectedIntDouble>>);
+}
+
+TEST_CASE("has_box_traits - empty traits passes") {
+    STATIC_REQUIRE(has_box_traits<int, box_traits<int>>);
+}
+
+TEST_CASE("has_box_traits - bad rebind fails") {
+    STATIC_REQUIRE_FALSE(has_box_traits<ExpectedIntDouble, BadRebindTraits<int, double>>);
+}
+
+TEST_CASE("has_box_traits - bad rebind_error fails") {
+    STATIC_REQUIRE_FALSE(has_box_traits<ExpectedIntDouble, BadRebindErrorTraits<int, double>>);
+}
+
+} // namespace beman::monadics::detail::tests
diff --git a/tests/beman/monadics/detail/rebox_error.test.cpp b/tests/beman/monadics/detail/propagate_error.test.cpp
similarity index 88%
rename from tests/beman/monadics/detail/rebox_error.test.cpp
rename to tests/beman/monadics/detail/propagate_error.test.cpp
index 2f556d6..641157d 100644
--- a/tests/beman/monadics/detail/rebox_error.test.cpp
+++ b/tests/beman/monadics/detail/propagate_error.test.cpp
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#include <beman/monadics/detail/rebox_error.hpp>
+#include <beman/monadics/detail/propagate_error.hpp>
 
 #include <catch2/catch_template_test_macros.hpp>
 
@@ -45,7 +45,7 @@ namespace beman::monadics::detail::tests {
 TEST_CASE("with-error-channel") {
     constexpr auto result = []() {
         Box<int, float> src{1.0f};
-        return rebox_error<Box<int, double>>(std::move(src));
+        return propagate_error<Box<int, double>>(std::move(src));
     }();
 
     STATIC_REQUIRE(std::get<1>(result) == 1.0);
@@ -54,7 +54,7 @@ TEST_CASE("with-error-channel") {
 TEST_CASE("without-error-channel") {
     constexpr auto result = []() {
         std::optional<int> src{std::nullopt};
-        return rebox_error<std::optional<double>>(std::move(src));
+        return propagate_error<std::optional<double>>(std::move(src));
     }();
 
     STATIC_REQUIRE_FALSE(result.has_value());
diff --git a/tests/beman/monadics/detail/rebox_value.test.cpp b/tests/beman/monadics/detail/propagate_value.test.cpp
similarity index 89%
rename from tests/beman/monadics/detail/rebox_value.test.cpp
rename to tests/beman/monadics/detail/propagate_value.test.cpp
index 67955dc..7830fcf 100644
--- a/tests/beman/monadics/detail/rebox_value.test.cpp
+++ b/tests/beman/monadics/detail/propagate_value.test.cpp
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#include <beman/monadics/detail/rebox_value.hpp>
+#include <beman/monadics/detail/propagate_value.hpp>
 
 #include <catch2/catch_test_macros.hpp>
 
@@ -51,7 +51,7 @@ namespace beman::monadics::detail::tests {
 TEST_CASE("with-error-channel") {
     constexpr auto result = []() {
         Box<int, double> src{1};
-        return rebox_value<Box<double, double>>(std::move(src));
+        return propagate_value<Box<double, double>>(std::move(src));
     }();
 
     STATIC_REQUIRE(std::get<0>(result) == 1.0);
@@ -60,7 +60,7 @@ TEST_CASE("with-error-channel") {
 TEST_CASE("without-error-channel") {
     constexpr auto result = []() {
         std::optional<int> src{10};
-        return rebox_value<std::optional<double>>(std::move(src));
+        return propagate_value<std::optional<double>>(std::move(src));
     }();
 
     STATIC_REQUIRE(result.value() == 10.0);