diff --git a/include/mp/proxy-types.h b/include/mp/proxy-types.h
index f81cfa03..f120268e 100644
--- a/include/mp/proxy-types.h
+++ b/include/mp/proxy-types.h
@@ -662,6 +662,19 @@ decltype(auto) CustomReadField(TypeList<LocalType> param,
     return read_dest.update([&](auto& value) { ReadOne<0>(param, invoke_context, input, value); });
 }
 
+//! Overload CustomReadField to serialize objects that have CustomReadMessage
+//! overloads. Defining a CustomReadMessage overload is simpler than defining a
+//! CustomReadField overload because it only requires defining a normal
+//! function, not a template function, but less flexible.
+template <typename LocalType, typename Reader, typename ReadDest>
+decltype(auto) CustomReadField(TypeList<LocalType>, Priority<2>, InvokeContext& invoke_context, Reader&& reader,
+                               ReadDest&& read_dest,
+                               decltype(CustomReadMessage(invoke_context, reader.get(),
+                                                          std::declval<LocalType&>()))* enable = nullptr)
+{
+    return read_dest.update([&](auto& value) { if (reader.has()) CustomReadMessage(invoke_context, reader.get(), value); });
+}
+
 template <typename... LocalTypes, typename... Args>
 decltype(auto) ReadField(TypeList<LocalTypes...>, Args&&... args)
 {
@@ -719,6 +732,17 @@ bool CustomHasValue(InvokeContext& invoke_context, Values&&... value)
     return true;
 }
 
+//! Overload CustomBuildField to serialize objects that have CustomBuildMessage
+//! overloads. Defining a CustomBuildMessage overload is simpler than defining a
+//! CustomBuildField overload because it only requires defining a normal
+//! function, not a template function, but less flexible.
+template <typename LocalType, typename Value, typename Output>
+void CustomBuildField(TypeList<LocalType>, Priority<2>, InvokeContext& invoke_context, Value&& value, Output&& output,
+                      decltype(CustomBuildMessage(invoke_context, value, std::move(output.get())))* enable = nullptr)
+{
+    CustomBuildMessage(invoke_context, value, std::move(output.init()));
+}
+
 template <typename... LocalTypes, typename Context, typename... Values, typename Output>
 void BuildField(TypeList<LocalTypes...>, Context& context, Output&& output, Values&&... values)
 {
@@ -1389,10 +1413,61 @@ struct ServerExcept : Parent
     }
 };
 
+//! Helper for CustomPassField below. Call Accessor::get method if it has one,
+//! otherwise return capnp::Void.
+template <typename Accessor, typename Message>
+decltype(auto) MaybeGet(Message&& message, decltype(Accessor::get(message))* enable = nullptr)
+{
+    return Accessor::get(message);
+}
+
+template <typename Accessor>
+::capnp::Void MaybeGet(...)
+{
+    return {};
+}
+
+//! Helper for CustomPassField below. Call Accessor::init method if it has one,
+//! otherwise do nothing.
+template <typename Accessor, typename Message>
+decltype(auto) MaybeInit(Message&& message, decltype(Accessor::get(message))* enable = nullptr)
+{
+    return Accessor::init(message);
+}
+
+template <typename Accessor>
+::capnp::Void MaybeInit(...)
+{
+    return {};
+}
+
+//! Overload CustomPassField to serialize objects that have CustomPassMessage
+//! overloads. Defining a CustomPassMessage overload is simpler than defining a
+//! CustomPassField overload because it only requires defining a normal
+//! function, not a template function, but less flexible.
+template <typename Accessor, typename... LocalTypes, typename ServerContext, typename Fn, typename... Args>
+auto CustomPassField(TypeList<LocalTypes...>, ServerContext& server_context, Fn&& fn, Args&&... args)
+    -> decltype(CustomPassMessage(server_context, MaybeGet<Accessor>(server_context.call_context.getParams()),
+                                  MaybeGet<Accessor>(server_context.call_context.getResults()), nullptr))
+{
+    CustomPassMessage(server_context, MaybeGet<Accessor>(server_context.call_context.getParams()),
+                      MaybeInit<Accessor>(server_context.call_context.getResults()),
+                      [&](LocalTypes... param) { fn.invoke(server_context, std::forward<Args>(args)..., param...); });
+}
+
 template <class Accessor>
 void CustomPassField();
 
 //! PassField override calling CustomPassField function, if it exists.
+//! Defining a CustomPassField or CustomPassMessage overload is useful for
+//! input/output parameters. If an overload is not defined these parameters will
+//! just be deserialized on the server side with ReadField into a temporary
+//! variable, then the server method will be called passing the temporary
+//! variable as a parameter, then the temporary variable will be serialized and
+//! sent back to the client with BuildField. But if a PassField or PassMessage
+//! overload is defined, the overload is called with a callback to invoke and
+//! pass parameters to the server side function, and run arbitrary code before
+//! and after invoking the function.
 template <typename Accessor, typename... Args>
 auto PassField(Priority<2>, Args&&... args) -> decltype(CustomPassField<Accessor>(std::forward<Args>(args)...))
 {
diff --git a/test/mp/test/foo-types.h b/test/mp/test/foo-types.h
index 41961eb7..347be20b 100644
--- a/test/mp/test/foo-types.h
+++ b/test/mp/test/foo-types.h
@@ -28,6 +28,45 @@ decltype(auto) CustomReadField(TypeList<FooCustom>, Priority<1>, InvokeContext&
 }
 
 } // namespace test
+
+inline void CustomBuildMessage(InvokeContext& invoke_context,
+                        const test::FooMessage& src,
+                        test::messages::FooMessage::Builder&& builder)
+{
+    builder.setMessage(src.message + " build");
+}
+
+inline void CustomReadMessage(InvokeContext& invoke_context,
+                       const test::messages::FooMessage::Reader& reader,
+                       test::FooMessage& dest)
+{
+    dest.message = std::string{reader.getMessage()} + " read";
+}
+
+inline void CustomBuildMessage(InvokeContext& invoke_context,
+                        const test::FooMutable& src,
+                        test::messages::FooMutable::Builder&& builder)
+{
+    builder.setMessage(src.message + " build");
+}
+
+inline void CustomReadMessage(InvokeContext& invoke_context,
+                       const test::messages::FooMutable::Reader& reader,
+                       test::FooMutable& dest)
+{
+    dest.message = std::string{reader.getMessage()} + " read";
+}
+
+inline void CustomPassMessage(InvokeContext& invoke_context,
+                       const test::messages::FooMutable::Reader& reader,
+                       test::messages::FooMutable::Builder builder,
+                       std::function<void(test::FooMutable&)>&& fn)
+{
+    test::FooMutable mut;
+    mut.message = std::string{reader.getMessage()} + " pass";
+    fn(mut);
+    builder.setMessage(mut.message + " return");
+}
 } // namespace mp
 
 #endif // MP_TEST_FOO_TYPES_H
diff --git a/test/mp/test/foo.capnp b/test/mp/test/foo.capnp
index 9080ee3c..caa87ea8 100644
--- a/test/mp/test/foo.capnp
+++ b/test/mp/test/foo.capnp
@@ -25,6 +25,8 @@ interface FooInterface $Proxy.wrap("mp::test::FooImplementation") {
     callbackExtended @10 (context :Proxy.Context, callback :ExtendedCallback, arg: Int32) -> (result :Int32);
     passCustom @11 (arg :FooCustom) -> (result :FooCustom);
     passEmpty @12 (arg :FooEmpty) -> (result :FooEmpty);
+    passMessage @13 (arg :FooMessage) -> (result :FooMessage);
+    passMutable @14 (arg :FooMutable) -> (arg :FooMutable);
 }
 
 interface FooCallback $Proxy.wrap("mp::test::FooCallback") {
@@ -50,6 +52,14 @@ struct FooCustom $Proxy.wrap("mp::test::FooCustom") {
 struct FooEmpty $Proxy.wrap("mp::test::FooEmpty") {
 }
 
+struct FooMessage {
+    message @0 :Text;
+}
+
+struct FooMutable {
+    message @0 :Text;
+}
+
 struct Pair(T1, T2) {
     first @0 :T1;
     second @1 :T2;
diff --git a/test/mp/test/foo.h b/test/mp/test/foo.h
index 43f4205b..a98fa19e 100644
--- a/test/mp/test/foo.h
+++ b/test/mp/test/foo.h
@@ -31,6 +31,16 @@ struct FooEmpty
 {
 };
 
+struct FooMessage
+{
+    std::string message;
+};
+
+struct FooMutable
+{
+    std::string message;
+};
+
 class FooCallback
 {
 public:
@@ -60,6 +70,8 @@ class FooImplementation
     int callbackExtended(ExtendedCallback& callback, int arg) { return callback.callExtended(arg); }
     FooCustom passCustom(FooCustom foo) { return foo; }
     FooEmpty passEmpty(FooEmpty foo) { return foo; }
+    FooMessage passMessage(FooMessage foo) { foo.message += " call"; return foo; }
+    void passMutable(FooMutable& foo) { foo.message += " call"; }
     std::shared_ptr<FooCallback> m_callback;
 };
 
diff --git a/test/mp/test/test.cpp b/test/mp/test/test.cpp
index ed845cd6..f58b0c69 100644
--- a/test/mp/test/test.cpp
+++ b/test/mp/test/test.cpp
@@ -108,6 +108,16 @@ KJ_TEST("Call FooInterface methods")
 
     foo->passEmpty(FooEmpty{});
 
+    FooMessage message1;
+    message1.message = "init";
+    FooMessage message2{foo->passMessage(message1)};
+    KJ_EXPECT(message2.message == "init build read call build read");
+
+    FooMutable mut;
+    mut.message = "init";
+    foo->passMutable(mut);
+    KJ_EXPECT(mut.message == "init build pass call return read");
+
     disconnect_client();
     thread.join();