diff --git a/crates/ironrdp-pdu/src/input/fast_path.rs b/crates/ironrdp-pdu/src/input/fast_path.rs
index 7835d73113..a22ec8414c 100644
--- a/crates/ironrdp-pdu/src/input/fast_path.rs
+++ b/crates/ironrdp-pdu/src/input/fast_path.rs
@@ -198,8 +198,13 @@ impl<'de> Decode<'de> for FastPathInputEvent {
             FastpathInputEventType::ScanCode => {
                 ensure_size!(in: src, size: 1);
                 let code = src.read_u8();
-                let flags = KeyboardFlags::from_bits(flags)
-                    .ok_or_else(|| invalid_field_err!("flags", "input keyboard flags unsupported", in: src))?;
+                // Retain unknown eventFlags bits (crate-wide policy since
+                // #1144): the slow-path decoder for the same keystroke
+                // already does (scan_code.rs), and a rejection here is
+                // session-fatal mid-use. [MS-RDPBCGR] 3.3.5.8.2's SHOULD-drop
+                // covers unknown event TYPES, which stays enforced above,
+                // not unknown flag bits.
+                let flags = KeyboardFlags::from_bits_retain(flags);
                 FastPathInputEvent::KeyboardEvent(flags, code)
             }
             FastpathInputEventType::Mouse => {
@@ -215,15 +220,16 @@ impl<'de> Decode<'de> for FastPathInputEvent {
                 FastPathInputEvent::MouseEventRel(mouse_event)
             }
             FastpathInputEventType::Sync => {
-                let flags = SynchronizeFlags::from_bits(flags)
-                    .ok_or_else(|| invalid_field_err!("flags", "input synchronize flags unsupported", in: src))?;
+                // Same rationale as ScanCode above; the slow-path sync
+                // decoder (sync.rs) already retains unknown toggle bits.
+                let flags = SynchronizeFlags::from_bits_retain(flags);
                 FastPathInputEvent::SyncEvent(flags)
             }
             FastpathInputEventType::Unicode => {
                 ensure_size!(in: src, size: 2);
                 let code = src.read_u16();
-                let flags = KeyboardFlags::from_bits(flags)
-                    .ok_or_else(|| invalid_field_err!("flags", "input keyboard flags unsupported", in: src))?;
+                // Same rationale as ScanCode above.
+                let flags = KeyboardFlags::from_bits_retain(flags);
                 FastPathInputEvent::UnicodeKeyboardEvent(flags, code)
             }
             FastpathInputEventType::QoeTimestamp => {
diff --git a/crates/ironrdp-testsuite-core/tests/input/fastpath_packets.rs b/crates/ironrdp-testsuite-core/tests/input/fastpath_packets.rs
index 92064d2171..a0219f6996 100644
--- a/crates/ironrdp-testsuite-core/tests/input/fastpath_packets.rs
+++ b/crates/ironrdp-testsuite-core/tests/input/fastpath_packets.rs
@@ -361,3 +361,52 @@ fn wheel_rotations_out_of_range_is_clamped(#[case] rotation_units: i16, #[case]
         })
     );
 }
+
+#[test]
+fn fastpath_keyboard_event_with_undefined_flag_bits_decodes_and_retains_them() {
+    // The 5-bit fast-path eventFlags field defines RELEASE, EXTENDED and
+    // EXTENDED1 for keyboard events ([MS-RDPBCGR] 2.2.8.1.2.2.1); an unknown
+    // bit must not kill a live session, and the slow-path decoder for the
+    // same keystroke already tolerates it. The bit is retained so
+    // re-encoding preserves the wire value.
+    const UNDEFINED_BIT: u8 = 0x10;
+
+    // eventCode = FASTPATH_INPUT_EVENT_SCANCODE (0) in bits 5..8,
+    // eventFlags = RELEASE | UNDEFINED_BIT in bits 0..5, then the scancode.
+    let buffer = [0x01 | UNDEFINED_BIT, 0x1d];
+
+    let event: FastPathInputEvent = ironrdp_core::decode(&buffer).expect("undefined flag bits are not fatal");
+
+    let FastPathInputEvent::KeyboardEvent(flags, code) = event else {
+        panic!("expected a keyboard event");
+    };
+    assert_eq!(code, 0x1d);
+    assert!(flags.contains(KeyboardFlags::RELEASE));
+    assert_ne!(flags.bits() & UNDEFINED_BIT, 0);
+    assert_eq!(
+        ironrdp_core::encode_vec(&FastPathInputEvent::KeyboardEvent(flags, code)).unwrap(),
+        buffer
+    );
+}
+
+#[test]
+fn fastpath_sync_event_with_undefined_flag_bits_decodes_and_retains_them() {
+    // [MS-RDPBCGR] 2.2.8.1.2.2.5 defines the four lock flags; bit 0x10 of
+    // the 5-bit eventFlags field is undefined and must not be fatal.
+    const UNDEFINED_BIT: u8 = 0x10;
+
+    // eventCode = FASTPATH_INPUT_EVENT_SYNC (3) in bits 5..8.
+    let buffer = [(3 << 5) | 0x01 | UNDEFINED_BIT];
+
+    let event: FastPathInputEvent = ironrdp_core::decode(&buffer).expect("undefined flag bits are not fatal");
+
+    let FastPathInputEvent::SyncEvent(flags) = event else {
+        panic!("expected a sync event");
+    };
+    assert!(flags.contains(SynchronizeFlags::SCROLL_LOCK));
+    assert_ne!(flags.bits() & UNDEFINED_BIT, 0);
+    assert_eq!(
+        ironrdp_core::encode_vec(&FastPathInputEvent::SyncEvent(flags)).unwrap(),
+        buffer
+    );
+}