diff --git a/test/fd/fmt.cpp b/test/fd/fmt.cpp
new file mode 100644
index 0000000..726b5e5
--- /dev/null
+++ b/test/fd/fmt.cpp
@@ -0,0 +1,35 @@
+#include <gtest/gtest.h>
+
+#include <stdplus/fd/fmt.hpp>
+#include <stdplus/fd/managed.hpp>
+#include <stdplus/util/cexec.hpp>
+#include <sys/mman.h>
+
+namespace stdplus
+{
+namespace fd
+{
+
+TEST(FormatBuffer, Basic)
+{
+    auto fd = ManagedFd(CHECK_ERRNO(memfd_create("test", 0), "memfd_create"));
+    {
+        FormatBuffer buf(fd, 4096);
+        buf.append("hi\n");
+        EXPECT_EQ(0, fd.lseek(0, Whence::Cur));
+        buf.flush();
+
+        EXPECT_EQ(3, fd.lseek(0, Whence::Cur));
+        buf.append("{}", std::string(2050, 'a'));
+        EXPECT_EQ(3, fd.lseek(0, Whence::Cur));
+        buf.append("{}", std::string(2050, 'a'));
+        EXPECT_EQ(4103, fd.lseek(0, Whence::Cur));
+
+        buf.append("hi\n");
+        EXPECT_EQ(4103, fd.lseek(0, Whence::Cur));
+    }
+    EXPECT_EQ(4106, fd.lseek(0, Whence::Cur));
+}
+
+} // namespace fd
+} // namespace stdplus