diff --git a/.gitignore b/.gitignore
index b0e5a8f4..28191207 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 examples/tmp*.pdf
 examples/tmp*.svg
 examples/tmp*.gif
+**/__pycache__/
\ No newline at end of file
diff --git a/.gitmodules b/.gitmodules
index 40e419c2..9af83fe7 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -56,5 +56,5 @@
 	shallow = true
 [submodule "nanobind_json"]
 	path = gitmodules/nanobind_json
-	url = https://github.com/Griger5/nanobind_json
+	url = https://github.com/Griger5/nanobind_json.git
 	shallow = true
diff --git a/gitmodules/nanobind_json b/gitmodules/nanobind_json
index 6e9e15fe..cb8a8565 160000
--- a/gitmodules/nanobind_json
+++ b/gitmodules/nanobind_json
@@ -1 +1 @@
-Subproject commit 6e9e15fee7fee798ec3259cb1fa1829c3991b567
+Subproject commit cb8a8565cc3bc7510b448cea6e6ec9cff98a599b
diff --git a/src/aero_data.hpp b/src/aero_data.hpp
index 1bf1d279..b57a22f2 100644
--- a/src/aero_data.hpp
+++ b/src/aero_data.hpp
@@ -48,7 +48,7 @@ struct AeroData {
         f_aero_data_from_camp(this->ptr.f_arg(), camp_core.ptr.f_arg());
     }
 
-    AeroData(const nlohmann::json &json) :
+    AeroData(const nlohmann::ordered_json &json) :
         ptr(f_aero_data_ctor, f_aero_data_dtor)
     {
         if (!InputJSONResource::unique_keys(json))
diff --git a/src/aero_dist.hpp b/src/aero_dist.hpp
index fe867561..a1f6375f 100644
--- a/src/aero_dist.hpp
+++ b/src/aero_dist.hpp
@@ -44,7 +44,7 @@ struct AeroDist {
 
     AeroDist(
         std::shared_ptr<AeroData> aero_data,
-        const nlohmann::json &json
+        const nlohmann::ordered_json &json
     ):
         ptr(f_aero_dist_ctor, f_aero_dist_dtor),
         aero_data(aero_data)
diff --git a/src/aero_mode.hpp b/src/aero_mode.hpp
index 51549eca..46f98cbc 100644
--- a/src/aero_mode.hpp
+++ b/src/aero_mode.hpp
@@ -138,7 +138,7 @@ struct AeroMode {
         ptr(f_aero_mode_ctor, f_aero_mode_dtor)
     {}
 
-    AeroMode(AeroData &aero_data, const nlohmann::json &json) :
+    AeroMode(AeroData &aero_data, const nlohmann::ordered_json &json) :
         ptr(f_aero_mode_ctor, f_aero_mode_dtor)
     {
         if (json.size() != 1 || !json.is_object() || !json.begin().value().is_object())
@@ -149,7 +149,7 @@ struct AeroMode {
         guard.check_parameters();
     }
 
-    static void check_mode_json(const nlohmann::json &mode) {
+    static void check_mode_json(const nlohmann::ordered_json &mode) {
         for (auto key : std::set<std::string>({"mass_frac", "mode_type"})) // TODO #320: more...
             if (mode.find(key) == mode.end())
                 throw std::runtime_error("mode parameters dict must include key '" + key + "'");
diff --git a/src/env_state.hpp b/src/env_state.hpp
index 990742d5..4fa1bd8c 100644
--- a/src/env_state.hpp
+++ b/src/env_state.hpp
@@ -29,7 +29,7 @@ extern "C" void f_env_state_air_dens(const void *ptr, double *air_density) noexc
 struct EnvState {
     PMCResource ptr;
 
-    EnvState(const nlohmann::json &json) :
+    EnvState(const nlohmann::ordered_json &json) :
         ptr(f_env_state_ctor, f_env_state_dtor)
     {
         JSONResourceGuard<InputJSONResource> guard(json);
diff --git a/src/gas_data.hpp b/src/gas_data.hpp
index cecadaca..58af81a2 100644
--- a/src/gas_data.hpp
+++ b/src/gas_data.hpp
@@ -11,7 +11,7 @@
 #include "gas_data_parameters.hpp"
 #include "camp_core.hpp"
 #include "nanobind/nanobind.h"
-#include "nanobind_json/nanobind_json.hpp"
+#include "nanobind_json/nanobind_json.h"
 
 extern "C" void f_gas_data_ctor(void *ptr) noexcept;
 extern "C" void f_gas_data_dtor(void *ptr) noexcept;
@@ -26,7 +26,7 @@ extern "C" void f_gas_data_spec_name_by_index(const void *ptr, const int *i_spec
 
 struct GasData {
     PMCResource ptr;
-    const nlohmann::json json;
+    const nlohmann::ordered_json json;
 
     GasData(const CampCore &CampCore) :
         ptr(f_gas_data_ctor, f_gas_data_dtor)
@@ -38,11 +38,11 @@ struct GasData {
         ptr(f_gas_data_ctor, f_gas_data_dtor),
         json(tpl)
     {
-        auto json_array = nlohmann::json::array();
+        auto json_array = nlohmann::ordered_json::array();
         for (const auto item : tpl)
-            json_array.push_back(nlohmann::json::object({{
+            json_array.push_back(nlohmann::ordered_json::object({{
                 nanobind::cast<std::string>(item),
-                nlohmann::json::array()
+                nlohmann::ordered_json::array()
             }}));
 
         JSONResourceGuard<InputJSONResource> guard(json_array);
diff --git a/src/gas_state.hpp b/src/gas_state.hpp
index 23dcc390..6b1ad688 100644
--- a/src/gas_state.hpp
+++ b/src/gas_state.hpp
@@ -115,7 +115,7 @@ struct GasState {
         return data;
     }
 
-    static void set_mix_rats(const GasState &self, const nlohmann::json &json) {
+    static void set_mix_rats(const GasState &self, const nlohmann::ordered_json &json) {
         if (json.size() == 0)
             throw std::runtime_error("Non-empty sequence of mixing ratios expected");
 
diff --git a/src/input_guard.hpp b/src/input_guard.hpp
index 0bb238c9..0b638bd0 100644
--- a/src/input_guard.hpp
+++ b/src/input_guard.hpp
@@ -6,7 +6,7 @@
 
 struct InputGuard {
   public:
-    InputGuard(const nlohmann::json &j) {
+    InputGuard(const nlohmann::ordered_json &j) {
         process_json(j);
 
         this->dict_key_present = false;
@@ -71,8 +71,8 @@ struct InputGuard {
 
     bool dict_key_present;
 
-    void process_json(const nlohmann::json &j) {
-        nlohmann::json flat = j.flatten();
+    void process_json(const nlohmann::ordered_json &j) {
+        nlohmann::ordered_json flat = j.flatten();
 
         // JSON Pointer, as in a string syntax for identifying a specific value in JSON
         std::vector<std::string> json_pointers;
diff --git a/src/json_resource.hpp b/src/json_resource.hpp
index dd558e4c..76b99409 100644
--- a/src/json_resource.hpp
+++ b/src/json_resource.hpp
@@ -19,8 +19,8 @@
 struct JSONResource {
   private:
     std::set<std::string> vars;
-    const nlohmann::json *json;
-    std::stack<const nlohmann::json*> json_parent;
+    const nlohmann::ordered_json *json;
+    std::stack<const nlohmann::ordered_json*> json_parent;
 
     std::unique_ptr<InputGuard> input_guard_ptr;
 
@@ -34,7 +34,7 @@ struct JSONResource {
 
     JSONResource() {}
 
-    JSONResource(const nlohmann::json &json) {
+    JSONResource(const nlohmann::ordered_json &json) {
         this->set_current_json_ptr(&json);
         for (auto &entry : this->json->items()) {
             this->vars.insert(entry.key());
@@ -43,7 +43,7 @@ struct JSONResource {
         input_guard_ptr = std::make_unique<InputGuard>(json);
     };
 
-    void set_current_json_ptr(const nlohmann::json *ptr) {
+    void set_current_json_ptr(const nlohmann::ordered_json *ptr) {
         this->json = ptr;
     }
 
@@ -86,8 +86,8 @@ struct JSONResource {
 
     void zoom_in(const bpstd::string_view &sub) noexcept {
         auto it = this->json->is_array() 
-            ? this->json->at(this->json->size()-1).find(sub)
-            : this->json->find(sub);
+            ? this->json->at(this->json->size()-1).find(sub.to_string())
+            : this->json->find(sub.to_string());
 
         assert(*it != NULL);
         this->json_parent.push(this->json);
@@ -157,7 +157,7 @@ struct JSONResource {
         const bpstd::string_view name,
         T *var
     ) {
-        *var = this->find(name)->get<T>();
+        *var = this->find(name.to_string())->get<T>();
     }
 
     void read_str(
@@ -165,7 +165,7 @@ struct JSONResource {
         char* var_data,
         int* var_size
     ) noexcept {
-        auto it = this->find(name);
+        auto it = this->find(name.to_string());
         if (it == this->json->end())
         {
             assert(false);
@@ -240,7 +240,7 @@ struct InputJSONResource: JSONResource {
 
   public:
     InputJSONResource(
-        const nlohmann::json &json,
+        const nlohmann::ordered_json &json,
         const std::string key_cond = "",
         const std::string key_name = "",
         const std::size_t max_zoom_level = 3
@@ -290,7 +290,7 @@ struct InputJSONResource: JSONResource {
         return 1;
     }
 
-    static bool unique_keys(const nlohmann::json &json) {
+    static bool unique_keys(const nlohmann::ordered_json &json) {
         std::set<std::string> keys;
         for (auto i=0u; i<json.size(); ++i) {
             for (auto &entry : json.at(i).items()) {
@@ -304,9 +304,9 @@ struct InputJSONResource: JSONResource {
 };
 
 struct OutputJSONResource: JSONResource {
-    std::unique_ptr<nlohmann::json> guard;
+    std::unique_ptr<nlohmann::ordered_json> guard;
 
-    OutputJSONResource() : guard(std::make_unique<nlohmann::json>()) {
+    OutputJSONResource() : guard(std::make_unique<nlohmann::ordered_json>()) {
         this->set_current_json_ptr(this->guard.get());
     }
 
@@ -331,12 +331,12 @@ struct JSONResourceGuard {
         json_resource_ptr() = std::make_unique<T>();
     }
 
-    JSONResourceGuard(const nlohmann::json & json) {
+    JSONResourceGuard(const nlohmann::ordered_json & json) {
         json_resource_ptr() = std::make_unique<T>(json);
     }
 
     JSONResourceGuard(
-        const nlohmann::json & json,
+        const nlohmann::ordered_json & json,
         const std::string key_cond,
         const std::string key_name
     ) {
@@ -344,7 +344,7 @@ struct JSONResourceGuard {
     }
 
     JSONResourceGuard(
-        const nlohmann::json & json,
+        const nlohmann::ordered_json & json,
         const std::string key_cond,
         const std::string key_name,
         const int max_zoom_level
diff --git a/src/pypartmc.cpp b/src/pypartmc.cpp
index c758a2a0..9f532729 100644
--- a/src/pypartmc.cpp
+++ b/src/pypartmc.cpp
@@ -14,7 +14,7 @@
 #include "nanobind/ndarray.h"
 #undef snprintf // required to fix an issue with std::snprintf in nlohmann::json
 #include "nlohmann/json.hpp"
-#include "nanobind_json/nanobind_json.hpp"
+#include "nanobind_json/nanobind_json.h"
 #include "sundials/sundials_config.h"
 #include "camp/version.h"
 #include "tl/optional.hpp"
@@ -174,7 +174,7 @@ NB_MODULE(_PyPartMC, m) {
         )pbdoc"
     )
         .def(nb::init<const CampCore&>())
-        .def(nb::init<const nlohmann::json&>())
+        .def(nb::init<const nlohmann::ordered_json&>())
         .def("spec_by_name", AeroData::spec_by_name,
              "Return the number of the species in AeroData with the given name.")
         .def("__len__", AeroData::__len__, "Return number of aerosol species.")
@@ -404,7 +404,7 @@ NB_MODULE(_PyPartMC, m) {
             scenario_t.
         )pbdoc"
     )
-        .def(nb::init<const nlohmann::json&>())
+        .def(nb::init<const nlohmann::ordered_json&>())
         .def("set_temperature", EnvState::set_temperature,
             "Set the temperature of the environment state.")
         .def_prop_ro("temp", EnvState::temp,
@@ -463,7 +463,7 @@ NB_MODULE(_PyPartMC, m) {
             nb::init<
                 const GasData&,
                 const AeroData&,
-                const nlohmann::json&
+                const nlohmann::ordered_json&
             >(),
             "instantiates and initializes from a JSON object"
         )
@@ -525,7 +525,7 @@ NB_MODULE(_PyPartMC, m) {
         "RunPartOpt",
         "Options controlling the execution of run_part()."
     )
-        .def(nb::init<const nlohmann::json&>())
+        .def(nb::init<const nlohmann::ordered_json&>())
         .def_prop_ro("t_max", RunPartOpt::t_max, "Total simulation time.")
         .def_prop_ro("del_t", RunPartOpt::del_t, "Time step.")
     ;
@@ -534,7 +534,7 @@ NB_MODULE(_PyPartMC, m) {
         "RunSectOpt",
         "Options controlling the execution of run_sect()."
     )
-        .def(nb::init<const nlohmann::json&, EnvState&>())
+        .def(nb::init<const nlohmann::ordered_json&, EnvState&>())
         .def_prop_ro("t_max", RunSectOpt::t_max, "Total simulation time.")
         .def_prop_ro("del_t", RunSectOpt::del_t, "Time step.")
     ;
@@ -543,7 +543,7 @@ NB_MODULE(_PyPartMC, m) {
         "RunExactOpt",
         "Options controlling the execution of run_exact()."
     )
-        .def(nb::init<const nlohmann::json&, EnvState&>())
+        .def(nb::init<const nlohmann::ordered_json&, EnvState&>())
         .def_prop_ro("t_max", RunExactOpt::t_max, "Total simulation time.")
     ;
 
@@ -556,9 +556,8 @@ NB_MODULE(_PyPartMC, m) {
         .def_prop_ro("widths", BinGrid::widths, "Return bin widths.")
     ;
 
-    nb::class_<AeroMode>(m,"AeroMode",
-        "An aerosol size distribution mode.")
-        .def(nb::init<AeroData&, const nlohmann::json&>())
+    nb::class_<AeroMode>(m,"AeroMode")
+        .def(nb::init<AeroData&, const nlohmann::ordered_json&>())
         .def_prop_rw("num_conc", &AeroMode::get_num_conc, &AeroMode::set_num_conc,
              "Provide access (read or write) to the total number concentration of a mode.")
         .def("num_dist", &AeroMode::num_dist,
@@ -584,7 +583,7 @@ NB_MODULE(_PyPartMC, m) {
     ;
 
     nb::class_<AeroDist>(m,"AeroDist")
-        .def(nb::init<std::shared_ptr<AeroData>, const nlohmann::json&>())
+        .def(nb::init<std::shared_ptr<AeroData>, const nlohmann::ordered_json&>())
         .def_prop_ro("n_mode", &AeroDist::get_n_mode,
             "Number of aerosol modes in the distribution.")
         .def_prop_ro("num_conc", &AeroDist::get_total_num_conc,
diff --git a/src/run_exact_opt.hpp b/src/run_exact_opt.hpp
index fbf22a76..fdede19c 100644
--- a/src/run_exact_opt.hpp
+++ b/src/run_exact_opt.hpp
@@ -18,10 +18,10 @@ extern "C" void f_run_exact_opt_t_max(const void *ptr, double *t_max) noexcept;
 struct RunExactOpt {
     PMCResource ptr;
 
-    RunExactOpt(const nlohmann::json &json, EnvState &env_state) :
+    RunExactOpt(const nlohmann::ordered_json &json, EnvState &env_state) :
         ptr(f_run_exact_opt_ctor, f_run_exact_opt_dtor)
     {
-        nlohmann::json json_copy(json);
+        nlohmann::ordered_json json_copy(json);
 
         for (auto key : std::set<std::string>({
             "t_output"
diff --git a/src/run_part_opt.hpp b/src/run_part_opt.hpp
index 423f1f55..279d72da 100644
--- a/src/run_part_opt.hpp
+++ b/src/run_part_opt.hpp
@@ -19,10 +19,10 @@ struct RunPartOpt {
     PMCResource ptr;
     bool allow_halving, allow_doubling;
 
-    RunPartOpt(const nlohmann::json &json) :
+    RunPartOpt(const nlohmann::ordered_json &json) :
         ptr(f_run_part_opt_ctor, f_run_part_opt_dtor)
     {
-        nlohmann::json json_copy(json);
+        nlohmann::ordered_json json_copy(json);
 
         if (json_copy.find("do_parallel") != json_copy.end() && json_copy["do_parallel"])
             throw std::runtime_error("setting do_parallel=true not supported in PyPartMC");
diff --git a/src/run_sect_opt.hpp b/src/run_sect_opt.hpp
index cab6a187..175a80a4 100644
--- a/src/run_sect_opt.hpp
+++ b/src/run_sect_opt.hpp
@@ -19,10 +19,10 @@ extern "C" void f_run_sect_opt_del_t(const void *ptr, double *del_t) noexcept;
 struct RunSectOpt {
     PMCResource ptr;
 
-    RunSectOpt(const nlohmann::json &json, EnvState &env_state) :
+    RunSectOpt(const nlohmann::ordered_json &json, EnvState &env_state) :
         ptr(f_run_sect_opt_ctor, f_run_sect_opt_dtor)
     {
-        nlohmann::json json_copy(json);
+        nlohmann::ordered_json json_copy(json);
 
         for (auto key : std::set<std::string>({
             "t_output", "t_progress"
diff --git a/src/scenario.hpp b/src/scenario.hpp
index 8ba25b69..893f661e 100644
--- a/src/scenario.hpp
+++ b/src/scenario.hpp
@@ -85,12 +85,12 @@ extern "C" void f_scenario_aero_background_time(
 
 struct Scenario {
     PMCResource ptr;
-    const nlohmann::json json;
+    const nlohmann::ordered_json json;
 
     Scenario(
         const GasData &gas_data,
         const AeroData &aero_data,
-        const nlohmann::json &json
+        const nlohmann::ordered_json &json
     ) :
         ptr(f_scenario_ctor, f_scenario_dtor),
         json(json)
diff --git a/tests/test_aero_dist.py b/tests/test_aero_dist.py
index c6af191c..3cb9a09e 100644
--- a/tests/test_aero_dist.py
+++ b/tests/test_aero_dist.py
@@ -100,10 +100,7 @@ def test_ctor():
         "order",
         (
             range,
-            pytest.param(
-                lambda n_modes: reversed(range(n_modes)),
-                marks=(pytest.mark.xfail(strict=True),),
-            ),  # TODO #213
+            lambda n_modes: reversed(range(n_modes)),
         ),
     )
     def test_ctor_multimode(n_modes, order):
@@ -124,11 +121,14 @@ def test_ctor_multimode(n_modes, order):
 
         # assert
         assert sut.n_mode == n_modes
-        assert sut.num_conc == np.sum(num_concs)
+        np.testing.assert_approx_equal(
+            actual=sut.num_conc, desired=np.sum(num_concs), significant=10
+        )
         for i in range(sut.n_mode):
-            assert sut.mode(i).type == modes[i]["mode_type"]
-            assert sut.mode(i).num_conc == modes[i]["num_conc"]
-            assert sut.mode(i).name == f"mode_{tuple(order(n_modes))[i]}"
+            j = tuple(order(n_modes))[i]
+            assert sut.mode(i).type == modes[j]["mode_type"]
+            assert sut.mode(i).num_conc == modes[j]["num_conc"]
+            assert sut.mode(i).name == f"mode_{j}"
 
     @staticmethod
     def test_ctor_modes_in_order(n_modes=4):
diff --git a/tests/test_aero_mode.py b/tests/test_aero_mode.py
index 29c5dee9..a2f649a6 100644
--- a/tests/test_aero_mode.py
+++ b/tests/test_aero_mode.py
@@ -323,11 +323,11 @@ def test_fixed_segfault_case_on_circular_reference():
         )
 
         # act
-        with pytest.raises(TypeError) as exc_info:
+        with pytest.raises(RuntimeError) as exc_info:
             ppmc.AeroMode(aero_data, fishy_ctor_arg)
 
         # assert
-        assert "incompatible function arguments" in str(exc_info.value)
+        assert "Circular reference detected" in str(exc_info.value)
 
     @staticmethod
     @pytest.mark.skipif(platform.machine() == "arm64", reason="TODO #348")
diff --git a/tests/test_scenario.py b/tests/test_scenario.py
index 98701044..ad8b75b1 100644
--- a/tests/test_scenario.py
+++ b/tests/test_scenario.py
@@ -170,12 +170,7 @@ def test_ctor_fails_with_no_values_in_time_array():
     @staticmethod
     @pytest.mark.parametrize(
         "mode_names",
-        (
-            ("A", "B"),
-            pytest.param(
-                ("B", "A"), marks=(pytest.mark.xfail(strict=True),)
-            ),  # TODO #213
-        ),
+        (("A", "B"), pytest.param(("B", "A"))),
     )
     def test_multi_mode(mode_names):
         # arrange
@@ -207,12 +202,7 @@ def test_multi_mode(mode_names):
     @pytest.mark.parametrize("key", ("aero_emissions", "aero_background"))
     @pytest.mark.parametrize(
         "mode_names",
-        (
-            ("A", "B"),
-            pytest.param(
-                ("B", "A"), marks=(pytest.mark.xfail(strict=True),)
-            ),  # TODO #213
-        ),
+        (("A", "B"), pytest.param(("B", "A"))),
     )
     def test_time_varying_aero_multimode(key, mode_names):
         # arrange