diff --git a/crates/ironrdp-pdu/src/gcc/cluster_data.rs b/crates/ironrdp-pdu/src/gcc/cluster_data.rs
index 9e2f499b6d..6f5c6ef4b9 100644
--- a/crates/ironrdp-pdu/src/gcc/cluster_data.rs
+++ b/crates/ironrdp-pdu/src/gcc/cluster_data.rs
@@ -52,8 +52,11 @@ impl<'de> Decode<'de> for ClientClusterData {
         let flags_with_version = src.read_u32();
         let redirected_session_id = src.read_u32();
 
-        let flags = RedirectionFlags::from_bits(flags_with_version & !REDIRECTION_VERSION_MASK)
-            .ok_or_else(|| invalid_field_err!("flags", "invalid redirection flags", in: src))?;
+        // [MS-RDPBCGR] 3.3.5.3.3 asks the server to validate only the Client
+        // Network Data bounds among the settings blocks, never this bit set;
+        // retain unknown bits (crate-wide policy since #1144) rather than
+        // refusing the client at GCC.
+        let flags = RedirectionFlags::from_bits_retain(flags_with_version & !REDIRECTION_VERSION_MASK);
         let redirection_version = RedirectionVersion::from_u32((flags_with_version & REDIRECTION_VERSION_MASK) >> 2)
             .ok_or_else(|| invalid_field_err!("redirVersion", "invalid redirection version", in: src))?;
 
diff --git a/crates/ironrdp-pdu/src/gcc/monitor_data.rs b/crates/ironrdp-pdu/src/gcc/monitor_data.rs
index cbb3243a49..d60f6cbdca 100644
--- a/crates/ironrdp-pdu/src/gcc/monitor_data.rs
+++ b/crates/ironrdp-pdu/src/gcc/monitor_data.rs
@@ -116,8 +116,11 @@ impl<'de> Decode<'de> for Monitor {
         let top = src.read_i32();
         let right = src.read_i32();
         let bottom = src.read_i32();
-        let flags = MonitorFlags::from_bits(src.read_u32())
-            .ok_or_else(|| invalid_field_err!("flags", "invalid monitor flags", in: src))?;
+        // [MS-RDPBCGR] 2.2.1.3.6.1 defines only TS_MONITOR_PRIMARY here, and
+        // 3.3.5.3.3 never asks the server to validate the bit set; retain
+        // unknown bits (crate-wide policy since #1144) rather than refusing a
+        // multimon client at GCC.
+        let flags = MonitorFlags::from_bits_retain(src.read_u32());
 
         Ok(Self {
             left,
diff --git a/crates/ironrdp-pdu/src/gcc/multi_transport_channel_data.rs b/crates/ironrdp-pdu/src/gcc/multi_transport_channel_data.rs
index 0949c0a04c..c5d5806bb1 100644
--- a/crates/ironrdp-pdu/src/gcc/multi_transport_channel_data.rs
+++ b/crates/ironrdp-pdu/src/gcc/multi_transport_channel_data.rs
@@ -1,7 +1,5 @@
 use bitflags::bitflags;
-use ironrdp_core::{
-    Decode, DecodeResult, Encode, EncodeResult, ReadCursor, WriteCursor, ensure_fixed_part_size, invalid_field_err,
-};
+use ironrdp_core::{Decode, DecodeResult, Encode, EncodeResult, ReadCursor, WriteCursor, ensure_fixed_part_size};
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 #[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
@@ -37,8 +35,12 @@ impl<'de> Decode<'de> for MultiTransportChannelData {
     fn decode(src: &mut ReadCursor<'de>) -> DecodeResult<Self> {
         ensure_fixed_part_size!(in: src);
 
-        let flags = MultiTransportFlags::from_bits(src.read_u32())
-            .ok_or_else(|| invalid_field_err!("flags", "invalid multitransport flags", in: src))?;
+        // [MS-RDPBCGR] 2.2.1.3.8's transportFlags list has grown before
+        // (UDP_PREFERRED and SOFTSYNC_TCP_TO_UDP are later additions), and
+        // 3.3.5.3.3 never asks the receiver to validate it; retain unknown
+        // bits (crate-wide policy since #1144) rather than failing the GCC
+        // exchange.
+        let flags = MultiTransportFlags::from_bits_retain(src.read_u32());
 
         Ok(Self { flags })
     }
diff --git a/crates/ironrdp-testsuite-core/tests/pdu/gcc.rs b/crates/ironrdp-testsuite-core/tests/pdu/gcc.rs
index e34672339c..acde4242cb 100644
--- a/crates/ironrdp-testsuite-core/tests/pdu/gcc.rs
+++ b/crates/ironrdp-testsuite-core/tests/pdu/gcc.rs
@@ -248,6 +248,23 @@ fn from_buffer_correctly_parses_client_cluster_data() {
     assert_eq!(*CLUSTER_DATA, decode(buffer).unwrap());
 }
 
+#[test]
+fn cluster_data_with_undefined_flag_bits_decodes_and_retains_them() {
+    // [MS-RDPBCGR] 3.3.5.3.3 never asks the server to validate this bit set;
+    // an unknown flag must not fail the GCC exchange. The bit is retained so
+    // re-encoding preserves the wire value.
+    const UNDEFINED_BIT: u32 = 0x0000_0080; // undefined in 2.2.1.3.5
+
+    let mut buffer = CLUSTER_DATA_BUFFER.to_vec();
+    let flags = u32::from_le_bytes(buffer[0..4].try_into().unwrap()) | UNDEFINED_BIT;
+    buffer[0..4].copy_from_slice(&flags.to_le_bytes());
+
+    let data: ClientClusterData = decode(buffer.as_slice()).expect("undefined redirection bits are not fatal");
+
+    assert_ne!(data.flags.bits() & UNDEFINED_BIT, 0);
+    assert_eq!(encode_vec(&data).unwrap(), buffer);
+}
+
 #[test]
 fn to_buffer_correctly_serializes_client_cluster_data() {
     let data = CLUSTER_DATA.clone();
@@ -555,6 +572,25 @@ fn from_buffer_correctly_parses_client_monitor_data_with_monitors() {
     );
 }
 
+#[test]
+fn monitor_with_undefined_flag_bits_decodes_and_retains_them() {
+    // [MS-RDPBCGR] 2.2.1.3.6.1 defines only TS_MONITOR_PRIMARY; a vendor or
+    // future bit must not refuse a multimon client at GCC. The bit is
+    // retained so re-encoding preserves the wire value.
+    const UNDEFINED_BIT: u32 = 0x0000_0002; // undefined in 2.2.1.3.6.1
+
+    let mut buffer = ironrdp_testsuite_core::monitor_data::MONITOR_DATA_WITH_MONITORS_BUFFER.to_vec();
+    // First monitor starts after the 8-byte header; its flags are the fifth
+    // dword of the 20-byte TS_MONITOR_DEF.
+    let flags = u32::from_le_bytes(buffer[24..28].try_into().unwrap()) | UNDEFINED_BIT;
+    buffer[24..28].copy_from_slice(&flags.to_le_bytes());
+
+    let data: ClientMonitorData = decode(buffer.as_slice()).expect("undefined monitor bits are not fatal");
+
+    assert_ne!(data.monitors[0].flags.bits() & UNDEFINED_BIT, 0);
+    assert_eq!(encode_vec(&data).unwrap(), buffer);
+}
+
 #[test]
 fn to_buffer_correctly_serializes_client_monitor_data_without_monitors() {
     let data = ironrdp_testsuite_core::monitor_data::MONITOR_DATA_WITHOUT_MONITORS.clone();
@@ -666,6 +702,24 @@ fn from_buffer_correctly_parses_server_multi_transport_channel_data() {
     );
 }
 
+#[test]
+fn multi_transport_with_undefined_flag_bits_decodes_and_retains_them() {
+    // [MS-RDPBCGR] 2.2.1.3.8's transportFlags list has grown before; an
+    // unknown bit must not fail the GCC exchange. The bit is retained so
+    // re-encoding preserves the wire value.
+    const UNDEFINED_BIT: u32 = 0x0000_0002; // undefined in 2.2.1.3.8
+
+    let mut buffer =
+        ironrdp_testsuite_core::multi_transport_channel_data::SERVER_GCC_MULTI_TRANSPORT_CHANNEL_BLOCK_BUFFER.to_vec();
+    let flags = u32::from_le_bytes(buffer[0..4].try_into().unwrap()) | UNDEFINED_BIT;
+    buffer[0..4].copy_from_slice(&flags.to_le_bytes());
+
+    let data: MultiTransportChannelData = decode(buffer.as_slice()).expect("undefined transport bits are not fatal");
+
+    assert_ne!(data.flags.bits() & UNDEFINED_BIT, 0);
+    assert_eq!(encode_vec(&data).unwrap(), buffer);
+}
+
 #[test]
 fn to_buffer_correctly_serializes_server_multi_transport_channel_data() {
     let data = ironrdp_testsuite_core::multi_transport_channel_data::SERVER_GCC_MULTI_TRANSPORT_CHANNEL_BLOCK.clone();