diff --git a/src/IO/JSON/JSONIOHandlerImpl.cpp b/src/IO/JSON/JSONIOHandlerImpl.cpp
index e5db1ef2c6..8ffb33a7f9 100644
--- a/src/IO/JSON/JSONIOHandlerImpl.cpp
+++ b/src/IO/JSON/JSONIOHandlerImpl.cpp
@@ -386,6 +386,20 @@ JSONIOHandlerImpl::JSONIOHandlerImpl(
     , m_fileFormat{format}
     , m_originalExtension{std::move(originalExtension)}
 {
+    // set the defaults
+    switch (m_fileFormat)
+    {
+    case FileFormat::Json:
+        // @todo take the switch to openPMD 2.0 as a chance to switch to
+        // short attribute mode as a default here
+        m_attributeMode = AttributeMode::Long;
+        m_mode = IOMode::Dataset;
+        break;
+    case FileFormat::Toml:
+        m_attributeMode = AttributeMode::Short;
+        m_mode = IOMode::Template;
+        break;
+    }
     std::tie(
         m_mode, m_IOModeSpecificationVia, m_printedSkippedWriteWarningAlready) =
         retrieveDatasetMode(config);
diff --git a/test/SerialIOTest.cpp b/test/SerialIOTest.cpp
index 93a233da75..e0286418f9 100644
--- a/test/SerialIOTest.cpp
+++ b/test/SerialIOTest.cpp
@@ -1241,7 +1241,12 @@ inline void dtype_test(
                   "../samples/dtype_test." + backend,
                   Access::CREATE,
                   activateTemplateMode.value())
-            : Series("../samples/dtype_test." + backend, Access::CREATE);
+            :
+            // test TOML long attribute mode by default
+            Series(
+                "../samples/dtype_test." + backend,
+                Access::CREATE,
+                R"({"toml":{"attribute":{"mode":"long"}}})");
         bool adios1 = s.backend() == "ADIOS1" || s.backend() == "MPI_ADIOS1";
 
         char c = 'c';
@@ -1836,7 +1841,8 @@ inline void fileBased_write_test(const std::string &backend)
     {
         Series o = Series(
             "../samples/subdir/serial_fileBased_write%03T." + backend,
-            Access::CREATE);
+            Access::CREATE,
+            R"({"toml":{"dataset":{"mode":"dataset"}}})");
 
         ParticleSpecies &e_1 = o.iterations[1].particles["e"];
 
@@ -7397,7 +7403,10 @@ void groupbased_read_write(std::string const &ext)
     std::string filename = "../samples/groupbased_read_write." + ext;
 
     {
-        Series write(filename, Access::CREATE);
+        Series write(
+            filename,
+            Access::CREATE,
+            R"({"toml":{"dataset":{"mode":"dataset"}}})");
         auto E_x = write.iterations[0].meshes["E"]["x"];
         auto E_y = write.iterations[0].meshes["E"]["y"];
         E_x.resetDataset(ds);
@@ -7410,7 +7419,10 @@ void groupbased_read_write(std::string const &ext)
     }
 
     {
-        Series write(filename, Access::READ_WRITE);
+        Series write(
+            filename,
+            Access::READ_WRITE,
+            R"({"toml":{"dataset":{"mode":"dataset"}}})");
         // create a new iteration
         auto E_x = write.iterations[1].meshes["E"]["x"];
         E_x.resetDataset(ds);
@@ -7450,7 +7462,10 @@ void groupbased_read_write(std::string const &ext)
 
     // check that truncation works correctly
     {
-        Series write(filename, Access::CREATE);
+        Series write(
+            filename,
+            Access::CREATE,
+            R"({"toml":{"dataset":{"mode":"dataset"}}})");
         // create a new iteration
         auto E_x = write.iterations[2].meshes["E"]["x"];
         E_x.resetDataset(ds);