diff --git a/include/asio/detail/config.hpp b/include/asio/detail/config.hpp
index 1172b870cb..cb960b02df 100644
--- a/include/asio/detail/config.hpp
+++ b/include/asio/detail/config.hpp
@@ -1658,4 +1658,38 @@
 # define ASIO_VERSIONED_NAME(name) asio_ ## name
 #endif // defined(ASIO_VERSION_NAMESPACE)
 
+// Named C++20 modules require CPOs (customization point objects) to have
+// external linkage.  The anonymous-namespace singleton idiom used by asio's
+// prefer/require/query/require_concept CPOs creates TU-local entities that
+// GCC 15+ (and future Clang in strict mode) reject when those headers are
+// #included inside a named module's global module fragment.
+//
+// Define ASIO_CPO_INLINE_CONSTEXPR to replace the anonymous-namespace
+// reference pattern with an inline constexpr variable.  This is safe for all
+// compilers: the impl types are stateless function objects with constexpr
+// constructors, so `inline constexpr impl cpo{}` has identical ODR semantics.
+// This flag is compiler-neutral; enable it for any C++20 named-module build.
+#if !defined(ASIO_CPO_INLINE_CONSTEXPR) \
+  && !defined(ASIO_DISABLE_CPO_INLINE_CONSTEXPR)
+// Automatically activate when the user opts in via a build-system flag.
+// In GCC named-module builds, pass -DASIO_GCC_NAMED_MODULES to trigger this.
+# if defined(ASIO_GCC_NAMED_MODULES)
+#  define ASIO_CPO_INLINE_CONSTEXPR 1
+# endif // defined(ASIO_GCC_NAMED_MODULES)
+#endif // !defined(ASIO_CPO_INLINE_CONSTEXPR)
+
+// Namespace-scope `const` variables (non-inline) have internal linkage in
+// C++.  When included in a named module's global module fragment they become
+// TU-local entities; GCC 15+ rejects module functions that reference them.
+// ASIO_DETAIL_CONSTEXPR_VAR expands to `inline constexpr` under
+// ASIO_CPO_INLINE_CONSTEXPR, giving these variables external linkage and
+// making them safe to reference from named modules.
+// NOTE: this block must come AFTER the ASIO_CPO_INLINE_CONSTEXPR detection
+// above so that the #if test is evaluated with the correct macro state.
+#if defined(ASIO_CPO_INLINE_CONSTEXPR)
+# define ASIO_DETAIL_CONSTEXPR_VAR inline constexpr
+#else
+# define ASIO_DETAIL_CONSTEXPR_VAR const
+#endif // defined(ASIO_CPO_INLINE_CONSTEXPR)
+
 #endif // ASIO_DETAIL_CONFIG_HPP
diff --git a/include/asio/detail/socket_types.hpp b/include/asio/detail/socket_types.hpp
index 97585fad11..55c59ec580 100644
--- a/include/asio/detail/socket_types.hpp
+++ b/include/asio/detail/socket_types.hpp
@@ -99,8 +99,8 @@ ASIO_INLINE_NAMESPACE_BEGIN
 namespace detail {
 
 #if defined(ASIO_WINDOWS_RUNTIME)
-const int max_addr_v4_str_len = 256;
-const int max_addr_v6_str_len = 256;
+ASIO_DETAIL_CONSTEXPR_VAR int max_addr_v4_str_len = 256;
+ASIO_DETAIL_CONSTEXPR_VAR int max_addr_v6_str_len = 256;
 typedef unsigned __int32 u_long_type;
 typedef unsigned __int16 u_short_type;
 struct in4_addr_type { u_long_type s_addr; };
@@ -185,10 +185,10 @@ typedef int signed_size_type;
 # define ASIO_OS_DEF_SA_NOCLDWAIT 0x4
 #elif defined(ASIO_WINDOWS) || defined(ASIO_CYGWIN_W32_SOCKETS)
 typedef SOCKET socket_type;
-const SOCKET invalid_socket = INVALID_SOCKET;
-const int socket_error_retval = SOCKET_ERROR;
-const int max_addr_v4_str_len = 256;
-const int max_addr_v6_str_len = 256;
+ASIO_DETAIL_CONSTEXPR_VAR SOCKET invalid_socket = INVALID_SOCKET;
+ASIO_DETAIL_CONSTEXPR_VAR int socket_error_retval = SOCKET_ERROR;
+ASIO_DETAIL_CONSTEXPR_VAR int max_addr_v4_str_len = 256;
+ASIO_DETAIL_CONSTEXPR_VAR int max_addr_v6_str_len = 256;
 typedef sockaddr socket_addr_type;
 typedef in_addr in4_addr_type;
 typedef ip_mreq in4_mreq_type;
@@ -291,22 +291,22 @@ struct sockaddr_un_type { u_short sun_family; char sun_path[108]; };
 #  define ASIO_OS_DEF_AI_ADDRCONFIG 0
 # endif
 # if defined (_WIN32_WINNT)
-const int max_iov_len = 64;
+ASIO_DETAIL_CONSTEXPR_VAR int max_iov_len = 64;
 # else
-const int max_iov_len = 16;
+ASIO_DETAIL_CONSTEXPR_VAR int max_iov_len = 16;
 # endif
 # define ASIO_OS_DEF_SA_RESTART 0x1
 # define ASIO_OS_DEF_SA_NOCLDSTOP 0x2
 # define ASIO_OS_DEF_SA_NOCLDWAIT 0x4
 #else
 typedef int socket_type;
-const int invalid_socket = -1;
-const int socket_error_retval = -1;
-const int max_addr_v4_str_len = INET_ADDRSTRLEN;
+ASIO_DETAIL_CONSTEXPR_VAR int invalid_socket = -1;
+ASIO_DETAIL_CONSTEXPR_VAR int socket_error_retval = -1;
+ASIO_DETAIL_CONSTEXPR_VAR int max_addr_v4_str_len = INET_ADDRSTRLEN;
 #if defined(INET6_ADDRSTRLEN)
-const int max_addr_v6_str_len = INET6_ADDRSTRLEN + 1 + IF_NAMESIZE;
+ASIO_DETAIL_CONSTEXPR_VAR int max_addr_v6_str_len = INET6_ADDRSTRLEN + 1 + IF_NAMESIZE;
 #else // defined(INET6_ADDRSTRLEN)
-const int max_addr_v6_str_len = 256;
+ASIO_DETAIL_CONSTEXPR_VAR int max_addr_v6_str_len = 256;
 #endif // defined(INET6_ADDRSTRLEN)
 typedef sockaddr socket_addr_type;
 typedef in_addr in4_addr_type;
@@ -412,10 +412,10 @@ typedef int signed_size_type;
 #  define ASIO_OS_DEF_AI_ADDRCONFIG 0
 # endif
 # if defined(IOV_MAX)
-const int max_iov_len = IOV_MAX;
+ASIO_DETAIL_CONSTEXPR_VAR int max_iov_len = IOV_MAX;
 # else
 // POSIX platforms are not required to define IOV_MAX.
-const int max_iov_len = 16;
+ASIO_DETAIL_CONSTEXPR_VAR int max_iov_len = 16;
 # endif
 # define ASIO_OS_DEF_SA_RESTART SA_RESTART
 # define ASIO_OS_DEF_SA_NOCLDSTOP SA_NOCLDSTOP
@@ -425,9 +425,9 @@ const int max_iov_len = 16;
 #  define ASIO_OS_DEF_SA_NOCLDWAIT 0
 # endif // defined(SA_NOCLDWAIT)
 #endif
-const int custom_socket_option_level = 0xA5100000;
-const int enable_connection_aborted_option = 1;
-const int always_fail_option = 2;
+ASIO_DETAIL_CONSTEXPR_VAR int custom_socket_option_level = 0xA5100000;
+ASIO_DETAIL_CONSTEXPR_VAR int enable_connection_aborted_option = 1;
+ASIO_DETAIL_CONSTEXPR_VAR int always_fail_option = 2;
 
 } // namespace detail
 ASIO_INLINE_NAMESPACE_END
diff --git a/include/asio/prefer.hpp b/include/asio/prefer.hpp
index 48890a14c8..af42d52f32 100644
--- a/include/asio/prefer.hpp
+++ b/include/asio/prefer.hpp
@@ -520,12 +520,16 @@ const T static_instance<T>::instance = {};
 } // namespace ASIO_VERSIONED_NAME(prefer_fn)
 namespace asio {
 ASIO_INLINE_NAMESPACE_BEGIN
+#if defined(ASIO_CPO_INLINE_CONSTEXPR)
+inline constexpr ASIO_VERSIONED_NAME(prefer_fn)::impl prefer{};
+#else // defined(ASIO_CPO_INLINE_CONSTEXPR)
 namespace {
 
 static constexpr const ASIO_VERSIONED_NAME(prefer_fn)::impl&
   prefer = ASIO_VERSIONED_NAME(prefer_fn)::static_instance<>::instance;
 
 } // namespace
+#endif // defined(ASIO_CPO_INLINE_CONSTEXPR)
 
 typedef ASIO_VERSIONED_NAME(prefer_fn)::impl prefer_t;
 
diff --git a/include/asio/query.hpp b/include/asio/query.hpp
index ebb1791db4..051e191433 100644
--- a/include/asio/query.hpp
+++ b/include/asio/query.hpp
@@ -257,12 +257,16 @@ const T static_instance<T>::instance = {};
 } // namespace ASIO_VERSIONED_NAME(query_fn)
 namespace asio {
 ASIO_INLINE_NAMESPACE_BEGIN
+#if defined(ASIO_CPO_INLINE_CONSTEXPR)
+inline constexpr ASIO_VERSIONED_NAME(query_fn)::impl query{};
+#else // defined(ASIO_CPO_INLINE_CONSTEXPR)
 namespace {
 
 static constexpr const ASIO_VERSIONED_NAME(query_fn)::impl&
   query = ASIO_VERSIONED_NAME(query_fn)::static_instance<>::instance;
 
 } // namespace
+#endif // defined(ASIO_CPO_INLINE_CONSTEXPR)
 
 typedef ASIO_VERSIONED_NAME(query_fn)::impl query_t;
 
diff --git a/include/asio/require.hpp b/include/asio/require.hpp
index 3a1029b2a3..78884883ee 100644
--- a/include/asio/require.hpp
+++ b/include/asio/require.hpp
@@ -375,12 +375,16 @@ const T static_instance<T>::instance = {};
 } // namespace ASIO_VERSIONED_NAME(require_fn)
 namespace asio {
 ASIO_INLINE_NAMESPACE_BEGIN
+#if defined(ASIO_CPO_INLINE_CONSTEXPR)
+inline constexpr ASIO_VERSIONED_NAME(require_fn)::impl require{};
+#else // defined(ASIO_CPO_INLINE_CONSTEXPR)
 namespace {
 
 static constexpr const ASIO_VERSIONED_NAME(require_fn)::impl&
   require = ASIO_VERSIONED_NAME(require_fn)::static_instance<>::instance;
 
 } // namespace
+#endif // defined(ASIO_CPO_INLINE_CONSTEXPR)
 
 typedef ASIO_VERSIONED_NAME(require_fn)::impl require_t;
 
diff --git a/include/asio/require_concept.hpp b/include/asio/require_concept.hpp
index d1ba31654e..3befd3b286 100644
--- a/include/asio/require_concept.hpp
+++ b/include/asio/require_concept.hpp
@@ -285,6 +285,9 @@ const T static_instance<T>::instance = {};
 } // namespace ASIO_VERSIONED_NAME(require_concept_fn)
 namespace asio {
 ASIO_INLINE_NAMESPACE_BEGIN
+#if defined(ASIO_CPO_INLINE_CONSTEXPR)
+inline constexpr ASIO_VERSIONED_NAME(require_concept_fn)::impl require_concept{};
+#else // defined(ASIO_CPO_INLINE_CONSTEXPR)
 namespace {
 
 static constexpr const ASIO_VERSIONED_NAME(require_concept_fn)::impl&
@@ -292,6 +295,7 @@ static constexpr const ASIO_VERSIONED_NAME(require_concept_fn)::impl&
     require_concept_fn)::static_instance<>::instance;
 
 } // namespace
+#endif // defined(ASIO_CPO_INLINE_CONSTEXPR)
 
 typedef ASIO_VERSIONED_NAME(require_concept_fn)::impl require_concept_t;