diff --git a/include/fast_io_dsal/impl/deque.h b/include/fast_io_dsal/impl/deque.h
index f92ffae9..06191ed6 100644
--- a/include/fast_io_dsal/impl/deque.h
+++ b/include/fast_io_dsal/impl/deque.h
@@ -958,7 +958,18 @@ class deque FAST_IO_TRIVIALLY_RELOCATABLE_IF_ELIGIBLE
 	{
 		this->copy_construct_impl(other.controller);
 	}
-	inline constexpr deque &operator=(deque const &) = delete;
+	inline constexpr deque &operator=(deque const &other) noexcept(::std::is_nothrow_copy_constructible_v<value_type>)
+	{
+		if (__builtin_addressof(other) == this)
+		{
+			return *this;
+		}
+		deque temp(other);
+		destroy_deque_controller(this->controller);
+		this->controller = temp.controller;
+		temp.controller = {{}, {}, {}};
+		return *this;
+	}
 
 	inline constexpr deque(deque &&other) noexcept : controller(other.controller)
 	{
diff --git a/tests/0026.container/0003.deque/constructors/copy_assignment.cc b/tests/0026.container/0003.deque/constructors/copy_assignment.cc
new file mode 100644
index 00000000..7df96c3b
--- /dev/null
+++ b/tests/0026.container/0003.deque/constructors/copy_assignment.cc
@@ -0,0 +1,141 @@
+#include <fast_io.h>
+#include <fast_io_dsal/deque.h>
+#include <fast_io_dsal/string.h>
+
+template <typename T>
+inline void test_copy_assignment()
+{
+	::fast_io::io::perr("=== copy assignment test ===\n");
+
+	// Build source deque
+	::fast_io::deque<T> src;
+	for (::std::size_t i{}; i != 4096u; ++i)
+	{
+		if constexpr (::std::same_as<T, ::std::size_t>)
+		{
+			src.push_back(i);
+		}
+		else
+		{
+			src.push_back(::fast_io::string(::fast_io::concat_fast_io(i)));
+		}
+	}
+
+	// Build destination deque with garbage data
+	::fast_io::deque<T> dst;
+	for (::std::size_t i{}; i != 1234u; ++i)
+	{
+		if constexpr (::std::same_as<T, ::std::size_t>)
+		{
+			dst.push_back(999999u);
+		}
+		else
+		{
+			dst.emplace_back("garbage");
+		}
+	}
+
+	// Perform copy assignment
+	dst = src;
+
+	// Check size
+	if (dst.size() != src.size())
+	{
+		::fast_io::io::panic("ERROR: dst.size() != src.size() after copy assignment\n");
+	}
+
+	// Check element equality
+	::std::size_t idx{};
+	for (T const &e : dst)
+	{
+		if constexpr (::std::same_as<T, ::std::size_t>)
+		{
+			if (e != idx)
+			{
+				::fast_io::io::panicln("ERROR: dst element mismatch: ", e);
+			}
+		}
+		else
+		{
+			::fast_io::string expected(::fast_io::concat_fast_io(idx));
+			if (e != expected)
+			{
+				::fast_io::io::panicln("ERROR: dst string mismatch: ", e);
+			}
+		}
+		++idx;
+	}
+
+	// Modify src to ensure deep copy
+	if constexpr (::std::same_as<T, ::std::size_t>)
+	{
+		for (::std::size_t i{}; i != src.size(); ++i)
+		{
+			src[i] = src[i] + 1u;
+		}
+	}
+	else
+	{
+		for (::std::size_t i{}; i != src.size(); ++i)
+		{
+			src[i].append("_changed");
+		}
+	}
+
+	// dst must remain unchanged
+	idx = 0u;
+	for (T const &e : dst)
+	{
+		if constexpr (::std::same_as<T, ::std::size_t>)
+		{
+			if (e != idx)
+			{
+				::fast_io::io::panicln("ERROR: dst changed after modifying src: ", e);
+			}
+		}
+		else
+		{
+			::fast_io::string expected(::fast_io::concat_fast_io(idx));
+			if (e != expected)
+			{
+				::fast_io::io::panicln("ERROR: dst string changed after modifying src: ", e);
+			}
+		}
+		++idx;
+	}
+
+	// Self-assignment test
+	dst = dst;
+
+	// Verify unchanged after self-assignment
+	idx = 0u;
+	for (T const &e : dst)
+	{
+		if constexpr (::std::same_as<T, ::std::size_t>)
+		{
+			if (e != idx)
+			{
+				::fast_io::io::panicln("ERROR: dst changed after self-assignment: ", e);
+			}
+		}
+		else
+		{
+			::fast_io::string expected(::fast_io::concat_fast_io(idx));
+			if (e != expected)
+			{
+				::fast_io::io::panicln("ERROR: dst string changed after self-assignment: ", e);
+			}
+		}
+		++idx;
+	}
+
+	::fast_io::io::print("copy assignment test finished\n");
+}
+
+int main()
+{
+	test_copy_assignment<::std::size_t>();
+	test_copy_assignment<::fast_io::string>();
+
+	::fast_io::io::print("All copy assignment tests finished\n");
+}