diff --git a/test/fd/fmt.cpp b/test/fd/fmt.cpp
index 726b5e5..fe1fed4 100644
--- a/test/fd/fmt.cpp
+++ b/test/fd/fmt.cpp
@@ -1,7 +1,10 @@
 #include <gtest/gtest.h>
 
+#include <filesystem>
+#include <memory>
 #include <stdplus/fd/fmt.hpp>
 #include <stdplus/fd/managed.hpp>
+#include <stdplus/gtest/tmp.hpp>
 #include <stdplus/util/cexec.hpp>
 #include <sys/mman.h>
 
@@ -31,5 +34,42 @@
     EXPECT_EQ(4106, fd.lseek(0, Whence::Cur));
 }
 
+class FormatToFileTest : public gtest::TestWithTmp
+{
+  protected:
+    std::string tmpname;
+    std::unique_ptr<FormatToFile> file;
+
+    FormatToFileTest() :
+        tmpname(fmt::format("{}/tmp.XXXXXX", CaseTmpDir())),
+        file(std::make_unique<FormatToFile>(tmpname))
+    {
+        tmpname = file->getTmpname();
+        EXPECT_TRUE(std::filesystem::exists(tmpname));
+    }
+
+    ~FormatToFileTest() noexcept(true)
+    {
+        file.reset();
+        EXPECT_FALSE(std::filesystem::exists(tmpname));
+    }
+};
+
+TEST_F(FormatToFileTest, NoCommit)
+{
+    file->append("hi\n");
+    EXPECT_EQ(0, std::filesystem::file_size(tmpname));
+}
+
+TEST_F(FormatToFileTest, Basic)
+{
+    file->append("hi\n");
+    EXPECT_EQ(0, std::filesystem::file_size(tmpname));
+    auto filename = fmt::format("{}/out", CaseTmpDir());
+    file->commit(filename);
+    EXPECT_FALSE(std::filesystem::exists(tmpname));
+    EXPECT_EQ(3, std::filesystem::file_size(filename));
+}
+
 } // namespace fd
 } // namespace stdplus