diff --git a/README.md b/README.md
index fc7dc82..e363c9f 100644
--- a/README.md
+++ b/README.md
@@ -47,6 +47,7 @@ Platform-neutral remote client relay session:
 - relay QUIC connection with pinned relay certificate trust
 - client hello with room, virtual MAC, and datagram budget
 - welcome/reject handling with assigned peer id and effective TAP MTU
+- QUIC DATAGRAM support and negotiated datagram budget diagnostics
 - Ethernet frame send/receive helpers over QUIC DATAGRAM
 - client tunnel statistics for frame/datagram rx/tx and drops
 
diff --git a/crates/lanparty-client-core/src/lib.rs b/crates/lanparty-client-core/src/lib.rs
index 2d0c073..2d13e51 100644
--- a/crates/lanparty-client-core/src/lib.rs
+++ b/crates/lanparty-client-core/src/lib.rs
@@ -22,7 +22,7 @@ use lanparty_ctrl::{
     CONTROL_LENGTH_PREFIX_LEN, ControlMessage, EndpointHello, MAX_CONTROL_MESSAGE_LEN, RELAY_ALPN,
     RoomCode, ServerWelcome, decode_control_frame, encode_control_message,
 };
-use lanparty_obs::TunnelStats;
+use lanparty_obs::{QuicDiagnostics, TunnelStats};
 use lanparty_proto::{EthernetFrame, FrameType, MacAddr, decode_datagram, encode_datagram};
 use quinn::{ClientConfig, Endpoint, crypto::rustls::QuicClientConfig};
 use rustls::pki_types::CertificateDer;
@@ -214,6 +214,7 @@ pub struct ClientSession {
     connection: quinn::Connection,
     config: ClientSessionConfig,
     welcome: ServerWelcome,
+    quic_max_datagram_size: u16,
     stats: Arc<ClientTunnelStats>,
 }
 
@@ -246,6 +247,16 @@ impl ClientSession {
         &self.welcome
     }
 
+    #[must_use]
+    pub const fn quic_max_datagram_size(&self) -> u16 {
+        self.quic_max_datagram_size
+    }
+
+    #[must_use]
+    pub const fn quic_diagnostics(&self) -> QuicDiagnostics {
+        QuicDiagnostics::new(true, Some(self.quic_max_datagram_size))
+    }
+
     #[must_use]
     pub fn relay_io(&self) -> ClientRelayIo {
         ClientRelayIo::new(
@@ -415,9 +426,8 @@ pub async fn connect_client(config: ClientSessionConfig) -> Result<ClientSession
     let peer_datagram_size = connection
         .max_datagram_size()
         .context("relay did not negotiate QUIC DATAGRAM support")?;
-    let hello_datagram_size = usize::from(config.max_datagram_size())
-        .min(peer_datagram_size)
-        .min(usize::from(u16::MAX)) as u16;
+    let hello_datagram_size =
+        negotiated_quic_datagram_size(config.max_datagram_size(), peer_datagram_size);
     let hello = EndpointHello::client(
         config.room().clone(),
         config.virtual_mac(),
@@ -432,6 +442,7 @@ pub async fn connect_client(config: ClientSessionConfig) -> Result<ClientSession
             connection,
             config,
             welcome,
+            quic_max_datagram_size: hello_datagram_size,
             stats: Arc::default(),
         }),
         ControlMessage::Reject(reject) => bail!(
@@ -443,6 +454,11 @@ pub async fn connect_client(config: ClientSessionConfig) -> Result<ClientSession
     }
 }
 
+#[must_use]
+fn negotiated_quic_datagram_size(configured: u16, peer: usize) -> u16 {
+    usize::from(configured).min(peer).min(usize::from(u16::MAX)) as u16
+}
+
 fn relay_client_config(relay_ca_cert_der: &[u8]) -> Result<ClientConfig> {
     let mut roots = rustls::RootCertStore::empty();
     roots
@@ -550,6 +566,16 @@ mod tests {
         assert!(identity.virtual_mac().is_valid_client_identity());
     }
 
+    #[test]
+    fn clamps_negotiated_quic_datagram_size() {
+        assert_eq!(negotiated_quic_datagram_size(1400, 1350), 1350);
+        assert_eq!(negotiated_quic_datagram_size(1300, 1400), 1300);
+        assert_eq!(
+            negotiated_quic_datagram_size(u16::MAX, usize::MAX),
+            u16::MAX
+        );
+    }
+
     #[test]
     fn identity_store_creates_and_reuses_mac() {
         let path = unique_temp_identity_path();
@@ -648,6 +674,12 @@ mod tests {
         );
         assert_eq!(client.welcome().room_id(), 7);
         assert_eq!(client.welcome().peer_id(), 2);
+        assert!(client.quic_max_datagram_size() <= 1400);
+        assert!(client.quic_diagnostics().datagram_supported());
+        assert_eq!(
+            client.quic_diagnostics().max_datagram_size(),
+            Some(client.quic_max_datagram_size())
+        );
         let relay_io = client.relay_io();
         assert_eq!(relay_io.welcome().peer_id(), 2);