diff --git a/src/kg/include/kg/io/Engine.inl b/src/kg/include/kg/io/Engine.inl
index 00092d894..1a6808fa5 100644
--- a/src/kg/include/kg/io/Engine.inl
+++ b/src/kg/include/kg/io/Engine.inl
@@ -15,15 +15,9 @@ inline Engine::Engine(File&& file, MPI_Comm comm) : file_{std::move(file)}
   MPI_Comm_size(comm, &mpi_size_);
 }
 
-inline void Engine::beginStep(StepMode mode)
-{
-  file_.beginStep(mode);
-}
+inline void Engine::beginStep(StepMode mode) { file_.beginStep(mode); }
 
-inline void Engine::endStep()
-{
-  file_.endStep();
-}
+inline void Engine::endStep() { file_.endStep(); }
 
 // ----------------------------------------------------------------------
 // put
@@ -78,18 +72,12 @@ inline void Engine::get(const std::string& pfx, T& datum, Args&&... args)
 // ----------------------------------------------------------------------
 // performPuts
 
-inline void Engine::performPuts()
-{
-  file_.performPuts();
-}
+inline void Engine::performPuts() { file_.performPuts(); }
 
 // ----------------------------------------------------------------------
 // performGets
 
-inline void Engine::performGets()
-{
-  file_.performGets();
-}
+inline void Engine::performGets() { file_.performGets(); }
 
 // ----------------------------------------------------------------------
 // internal
@@ -105,7 +93,7 @@ inline void Engine::putVariable(const T* data, const Mode launch,
 template <typename T>
 inline void Engine::putAttribute(const T& datum)
 {
-  file_.putAttribute(prefix(), &datum, 1);
+  file_.putAttribute(prefix(), datum);
 }
 
 template <typename T>
@@ -150,20 +138,11 @@ inline Dims Engine::variableShape()
 // ----------------------------------------------------------------------
 // close
 
-inline void Engine::close()
-{
-  file_.close();
-}
+inline void Engine::close() { file_.close(); }
 
-inline int Engine::mpiRank() const
-{
-  return mpi_rank_;
-}
+inline int Engine::mpiRank() const { return mpi_rank_; }
 
-inline int Engine::mpiSize() const
-{
-  return mpi_size_;
-}
+inline int Engine::mpiSize() const { return mpi_size_; }
 
 } // namespace io
 } // namespace kg
diff --git a/src/kg/include/kg/io/File.h b/src/kg/include/kg/io/File.h
index 20d1661e9..b1d81b746 100644
--- a/src/kg/include/kg/io/File.h
+++ b/src/kg/include/kg/io/File.h
@@ -41,6 +41,9 @@ class File
   template <typename T>
   void getAttribute(const std::string& name, T* data);
 
+  template <typename T>
+  void putAttribute(const std::string& name, const T& datum);
+
   template <typename T>
   void putAttribute(const std::string& name, const T* data, size_t size);
 
diff --git a/src/kg/include/kg/io/File.inl b/src/kg/include/kg/io/File.inl
index e15b5e7e9..873208e8e 100644
--- a/src/kg/include/kg/io/File.inl
+++ b/src/kg/include/kg/io/File.inl
@@ -6,25 +6,13 @@ namespace io
 
 inline File::File(FileBase* impl) : impl_{impl} {}
 
-inline File::~File()
-{
-  close();
-}
+inline File::~File() { close(); }
 
-inline void File::close()
-{
-  impl_.reset();
-}
+inline void File::close() { impl_.reset(); }
 
-inline void File::beginStep(StepMode mode)
-{
-  impl_->beginStep(mode);
-}
+inline void File::beginStep(StepMode mode) { impl_->beginStep(mode); }
 
-inline void File::endStep()
-{
-  impl_->endStep();
-}
+inline void File::endStep() { impl_->endStep(); }
 
 inline void File::performPuts()
 {
@@ -73,6 +61,14 @@ inline void File::getAttribute(const std::string& name, T* data)
   impl_->getAttribute(name, dataVar);
 }
 
+template <typename T>
+inline void File::putAttribute(const std::string& name, const T& datum)
+{
+  assert(impl_);
+  FileBase::Type datumVar = datum;
+  impl_->putAttribute(name, datumVar);
+}
+
 template <typename T>
 inline void File::putAttribute(const std::string& name, const T* data,
                                size_t size)
diff --git a/src/kg/include/kg/io/FileAdios2.h b/src/kg/include/kg/io/FileAdios2.h
index 22da36303..81c59ac51 100644
--- a/src/kg/include/kg/io/FileAdios2.h
+++ b/src/kg/include/kg/io/FileAdios2.h
@@ -35,6 +35,7 @@ class FileAdios2 : public FileBase
   Dims shapeVariable(const std::string& name) const override;
 
   void getAttribute(const std::string& name, TypePointer data) override;
+  void putAttribute(const std::string& name, const Type& datum) override;
   void putAttribute(const std::string& name, TypeConstPointer data,
                     size_t size) override;
   size_t sizeAttribute(const std::string& name) const override;
@@ -44,6 +45,7 @@ class FileAdios2 : public FileBase
   struct GetVariable;
   struct GetAttribute;
   struct PutAttribute;
+  struct PutAttributeScalar;
 
   template <typename T>
   void putVariable(const std::string& name, const T* data, Mode launch,
@@ -57,6 +59,9 @@ class FileAdios2 : public FileBase
   template <typename T>
   void getAttribute(const std::string& name, T* data);
 
+  template <typename T>
+  void putAttribute(const std::string& name, const T& datum);
+
   template <typename T>
   void putAttribute(const std::string& name, const T* data, size_t size);
 
diff --git a/src/kg/include/kg/io/FileAdios2.inl b/src/kg/include/kg/io/FileAdios2.inl
index dda3c0e48..1926d9678 100644
--- a/src/kg/include/kg/io/FileAdios2.inl
+++ b/src/kg/include/kg/io/FileAdios2.inl
@@ -21,6 +21,7 @@ inline FileAdios2::FileAdios2(adios2::ADIOS& ad, const std::string& name,
     io_name_ = io_name;
   }
   io_ = ad.DeclareIO(io_name_);
+  // io_.SetEngine("BP4");
   adios2::Mode adios2_mode;
   if (mode == Mode::Read) {
     adios2_mode = adios2::Mode::Read;
@@ -179,6 +180,21 @@ inline Dims FileAdios2::shapeVariable(const std::string& name) const
   std::abort();
 }
 
+template <typename T>
+inline void FileAdios2::putAttribute(const std::string& name, const T& datum)
+{
+  // if (mpiRank() != 0) { // FIXME, should we do this?
+  //   return;
+  // }
+  auto attr = io_.InquireAttribute<T>(name);
+  if (attr) {
+    mprintf("attr '%s' already exists -- ignoring it!\n", name.c_str());
+  } else {
+    // FIXME? we're never using "single value", just an array of size 1
+    io_.DefineAttribute<T>(name, datum);
+  }
+}
+
 template <typename T>
 inline void FileAdios2::putAttribute(const std::string& name, const T* data,
                                      size_t size)
@@ -224,6 +240,27 @@ inline void FileAdios2::getAttribute(const std::string& name, TypePointer data)
   mpark::visit(GetAttribute{*this, name}, data);
 }
 
+struct FileAdios2::PutAttributeScalar
+{
+  PutAttributeScalar(FileAdios2& self, const std::string& name)
+    : self{self}, name{name}
+  {}
+
+  template <typename T>
+  void operator()(const T& datum)
+  {
+    self.putAttribute(name, datum);
+  }
+
+  FileAdios2& self;
+  const std::string& name;
+};
+
+inline void FileAdios2::putAttribute(const std::string& name, const Type& datum)
+{
+  mpark::visit(PutAttributeScalar{*this, name}, datum);
+}
+
 struct FileAdios2::PutAttribute
 {
   PutAttribute(FileAdios2& self, const std::string& name, size_t size)
diff --git a/src/kg/include/kg/io/FileBase.h b/src/kg/include/kg/io/FileBase.h
index a9388cd77..a9d4d95ed 100644
--- a/src/kg/include/kg/io/FileBase.h
+++ b/src/kg/include/kg/io/FileBase.h
@@ -14,6 +14,8 @@ namespace io
 class FileBase
 {
 public:
+  using Type = mpark::variant<int, unsigned int, unsigned long,
+                              unsigned long long, float, double, std::string>;
   using TypePointer =
     mpark::variant<int*, unsigned int*, unsigned long*, unsigned long long*,
                    float*, double*, std::string*>;
@@ -41,6 +43,7 @@ class FileBase
   virtual Dims shapeVariable(const std::string& name) const = 0;
 
   virtual void getAttribute(const std::string& name, TypePointer data) = 0;
+  virtual void putAttribute(const std::string& name, const Type& datum) = 0;
   virtual void putAttribute(const std::string& name, TypeConstPointer data,
                             size_t size) = 0;
   virtual size_t sizeAttribute(const std::string& name) const = 0;
diff --git a/src/kg/testing/io/TestIOAdios2.cxx b/src/kg/testing/io/TestIOAdios2.cxx
index 927e06e7d..8ef6c2b33 100644
--- a/src/kg/testing/io/TestIOAdios2.cxx
+++ b/src/kg/testing/io/TestIOAdios2.cxx
@@ -8,7 +8,7 @@ TEST(IOAdios2, CtorDtor) { auto io = kg::io::IOAdios2{}; }
 TEST(IOAdios2, OpenWrite)
 {
   auto io = kg::io::IOAdios2{};
-  auto file = io.openFile("test1", kg::io::Mode::Write);
+  auto file = io.openFile("test1.bp", kg::io::Mode::Write);
 }
 
 TEST(IOAdios2, OpenReadMissingFile)
@@ -76,6 +76,8 @@ TEST(IOAdios2, FilePutGetAttribute)
     file.beginStep(kg::io::StepMode::Append);
     auto dbl = std::vector<double>{1., 2., 3., 4., 5.};
     file.putAttribute("attr_dbl", dbl.data(), dbl.size());
+    double val = 99.;
+    file.putAttribute("attr_dbl_scalar", val);
     file.endStep();
   }
   {
@@ -84,7 +86,10 @@ TEST(IOAdios2, FilePutGetAttribute)
     auto size = file.sizeAttribute("attr_dbl");
     auto dbl = std::vector<double>(size);
     file.getAttribute("attr_dbl", dbl.data());
+    double val;
+    file.getAttribute("attr_dbl_scalar", &val);
     EXPECT_EQ(dbl, (std::vector<double>{1., 2., 3., 4., 5.}));
+    EXPECT_EQ(val, 99.);
     file.endStep();
   }
 }