diff --git a/CHANGELOG.md b/CHANGELOG.md
index 39378647..05752ea1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 * Use static_string for WebSocket handshakes
 * Simplify get_lowest_layer test
 * Add test_allocator to extras/test
+* More flat_streambuf tests
 
 --------------------------------------------------------------------------------
 
diff --git a/extras/beast/test/test_allocator.hpp b/extras/beast/test/test_allocator.hpp
index 851b0712..13bd7019 100644
--- a/extras/beast/test/test_allocator.hpp
+++ b/extras/beast/test/test_allocator.hpp
@@ -23,45 +23,50 @@ struct test_allocator_info
 };
 
 template<class T,
-    bool Assign, bool Move, bool Swap, bool Select>
+    bool Equal, bool Assign, bool Move, bool Swap, bool Select>
 class test_allocator;
 
 template<class T,
-    bool Assign, bool Move, bool Swap, bool Select>
+    bool Equal, bool Assign, bool Move, bool Swap, bool Select>
 struct test_allocator_base
 {
 };
 
 template<class T,
-    bool Assign, bool Move, bool Swap>
-struct test_allocator_base<T, Assign, Move, Swap, true>
+    bool Equal, bool Assign, bool Move, bool Swap>
+struct test_allocator_base<
+    T, Equal, Assign, Move, Swap, true>
 {
     static
-    test_allocator<T, Assign, Move, Swap, true>
-    select_on_container_copy_construction(
-        test_allocator<T, Assign, Move, Swap, true> const& a)
+    test_allocator<T, Equal, Assign, Move, Swap, true>
+    select_on_container_copy_construction(test_allocator<
+        T, Equal, Assign, Move, Swap, true> const& a)
     {
-        return test_allocator<T, Assign, Move, Swap, true>{};
+        return test_allocator<T,
+            Equal, Assign, Move, Swap, true>{};
     }
 };
 
 template<class T,
-    bool Assign, bool Move, bool Swap, bool Select>
+    bool Equal, bool Assign, bool Move, bool Swap, bool Select>
 class test_allocator : public test_allocator_base<
-        T, Assign, Move, Swap, Select>
+        T, Equal, Assign, Move, Swap, Select>
 {
     std::size_t id_;
     std::shared_ptr<test_allocator_info> info_;
 
-    template<class, bool, bool, bool, bool>
+    template<class, bool, bool, bool, bool, bool>
     friend class test_allocator;
 
 public:
     using value_type = T;
+
     using propagate_on_container_copy_assignment =
         std::integral_constant<bool, Assign>;
+
     using propagate_on_container_move_assignment =
         std::integral_constant<bool, Move>;
+
     using propagate_on_container_swap =
         std::integral_constant<bool, Swap>;
 
@@ -69,7 +74,7 @@ public:
     struct rebind
     {
         using other = test_allocator<
-            U, Assign, Move, Swap, Select>;
+            U, Equal, Assign, Move, Swap, Select>;
     };
 
     test_allocator()
@@ -91,8 +96,8 @@ public:
     }
 
     template<class U>
-    test_allocator(test_allocator<
-            U, Assign, Move, Swap, Select> const& u) noexcept
+    test_allocator(test_allocator<U,
+            Equal, Assign, Move, Swap, Select> const& u) noexcept
         : id_(u.id_)
         , info_(u.info_)
     {
@@ -119,6 +124,18 @@ public:
         ::operator delete(p);
     }
 
+    bool
+    operator==(test_allocator const& other) const
+    {
+        return id_ == other.id_ || Equal;
+    }
+
+    bool
+    operator!=(test_allocator const& other) const
+    {
+        return ! this->operator==(other);
+    }
+
     std::size_t
     id() const
     {
diff --git a/test/core/flat_streambuf.cpp b/test/core/flat_streambuf.cpp
index c6c034aa..77d79000 100644
--- a/test/core/flat_streambuf.cpp
+++ b/test/core/flat_streambuf.cpp
@@ -10,7 +10,9 @@
 
 #include "buffer_test.hpp"
 #include <beast/core/to_string.hpp>
+#include <beast/test/test_allocator.hpp>
 #include <beast/unit_test/suite.hpp>
+#include <algorithm>
 
 namespace beast {
 
@@ -29,6 +31,121 @@ public:
         return to_string(sb1.data()) == to_string(sb2.data());
     }
 
+    template<
+        bool Equal, bool Assign, bool Move, bool Swap, bool Select>
+    void
+    testCtor()
+    {
+        using allocator = test::test_allocator<char,
+            Equal, Assign, Move, Swap, Select>;
+        {
+            using boost::asio::buffer_size;
+            basic_flat_streambuf<allocator> b1{10};
+            BEAST_EXPECT(b1.size() == 0);
+            BEAST_EXPECT(b1.capacity() == 0);
+            BEAST_EXPECT(b1.max_size() == 10);
+            b1.prepare(1);
+            b1.commit(1);
+            basic_flat_streambuf<allocator> b2{std::move(b1)};
+            BEAST_EXPECT(b1.capacity() == 0);
+            BEAST_EXPECT(b1.max_size() == 10);
+            BEAST_EXPECT(b2.size() == 1);
+            BEAST_EXPECT(b2.max_size() == 10);
+            BEAST_EXPECT(buffer_size(b1.data()) == 0);
+            BEAST_EXPECT(buffer_size(b1.prepare(1)) == 1);
+        }
+        {
+            basic_flat_streambuf<allocator> b1{10};
+            basic_flat_streambuf<allocator> b2{std::move(b1), allocator{}};
+        }
+        {
+            basic_flat_streambuf<allocator> b1{10};
+            basic_flat_streambuf<allocator> b2{b1};
+        }
+        {
+            basic_flat_streambuf<allocator> b1{10};
+            basic_flat_streambuf<allocator> b2{b1, allocator{}};
+        }
+        {
+            flat_streambuf b1{10};
+            b1.prepare(1);
+            b1.commit(1);
+            basic_flat_streambuf<allocator> b2{b1};
+            BEAST_EXPECT(b2.size() == 1);
+        }
+        {
+            basic_flat_streambuf<allocator> b1{10};
+        }
+        {
+            basic_flat_streambuf<allocator> b1{allocator{}, 10};
+        }
+    }
+
+    void
+    testCtors()
+    {
+        testCtor<false, false, false, false, false>();
+        testCtor<false, false, false, false,  true>();
+        testCtor<false, false, false,  true, false>();
+        testCtor<false, false, false,  true,  true>();
+        testCtor<false, false,  true, false, false>();
+        testCtor<false, false,  true, false,  true>();
+        testCtor<false, false,  true,  true, false>();
+        testCtor<false, false,  true,  true,  true>();
+        testCtor<false,  true, false, false, false>();
+        testCtor<false,  true, false, false,  true>();
+        testCtor<false,  true, false,  true, false>();
+        testCtor<false,  true, false,  true,  true>();
+        testCtor<false,  true,  true, false, false>();
+        testCtor<false,  true,  true, false,  true>();
+        testCtor<false,  true,  true,  true, false>();
+        testCtor<false,  true,  true,  true,  true>();
+        testCtor< true, false, false, false, false>();
+        testCtor< true, false, false, false,  true>();
+        testCtor< true, false, false,  true, false>();
+        testCtor< true, false, false,  true,  true>();
+        testCtor< true, false,  true, false, false>();
+        testCtor< true, false,  true, false,  true>();
+        testCtor< true, false,  true,  true, false>();
+        testCtor< true, false,  true,  true,  true>();
+        testCtor< true,  true, false, false, false>();
+        testCtor< true,  true, false, false,  true>();
+        testCtor< true,  true, false,  true, false>();
+        testCtor< true,  true, false,  true,  true>();
+        testCtor< true,  true,  true, false, false>();
+        testCtor< true,  true,  true, false,  true>();
+        testCtor< true,  true,  true,  true, false>();
+        testCtor< true,  true,  true,  true,  true>();
+    }
+
+    void
+    testOperations()
+    {
+        //
+        // reserve
+        //
+
+        {
+            flat_streambuf b{10};
+            b.prepare(1);
+            b.commit(1);
+            b.reserve(2);
+            BEAST_EXPECT(b.size() == 1);
+        }
+        {
+            flat_streambuf b{10};
+            try
+            {
+                b.reserve(11);
+                fail("", __FILE__, __LINE__);
+            }
+            catch(std::length_error const&)
+            {
+                pass();
+            }
+        }
+    }
+
     void
     testSpecialMembers()
     {
@@ -159,6 +276,9 @@ public:
     void
     run() override
     {
+        testCtors();
+        testOperations();
+
         testSpecialMembers();
         testStream();
         testPrepare();
diff --git a/test/core/streambuf.cpp b/test/core/streambuf.cpp
index 6d110cbc..2d3440de 100644
--- a/test/core/streambuf.cpp
+++ b/test/core/streambuf.cpp
@@ -109,14 +109,14 @@ public:
         // VFALCO This needs work
         {
             using alloc_type =
-                test_allocator<char, false, false, false, false>;
+                test_allocator<char, false, false, false, false, false>;
             using sb_type = basic_streambuf<alloc_type>;
             sb_type sb;
             BEAST_EXPECT(sb.get_allocator().id() == 1);
         }
         {
             using alloc_type =
-                test_allocator<char, false, false, false, false>;
+                test_allocator<char, false, false, false, false, false>;
             using sb_type = basic_streambuf<alloc_type>;
             sb_type sb;
             BEAST_EXPECT(sb.get_allocator().id() == 2);