diff --git a/mm-server/src/server.rs b/mm-server/src/server.rs
index b9bb9a6..866c577 100644
--- a/mm-server/src/server.rs
+++ b/mm-server/src/server.rs
@@ -17,6 +17,7 @@ use std::net::SocketAddr;
 use std::sync::Arc;
 use tracing::debug_span;
 use tracing::trace;
+use tracing::trace_span;
 use tracing::warn;
 use tracing::{debug, error};
 
@@ -66,7 +67,8 @@ pub struct ClientConnection {
     conn: quiche::Connection,
     timer: mio_timerfd::TimerFd,
     timeout_token: mio::Token,
-    partial: HashMap<u64, BytesMut>,
+    partial_reads: HashMap<u64, BytesMut>,
+    partial_writes: HashMap<u64, Bytes>,
     in_flight: HashMap<u64, StreamWorker>,
     dgram_recv: Receiver<protocol::MessageType>,
     dgram_send: WakingSender<protocol::MessageType>,
@@ -264,6 +266,9 @@ impl Server {
 
             // Demux packets from in-flight requests and datagrams from attachments.
             for client in self.clients.values_mut() {
+                let conn_span = trace_span!("conn_write", conn_id = ?client.conn_id);
+                let _guard = conn_span.enter();
+
                 if client.conn.is_draining() {
                     continue;
                 }
@@ -302,7 +307,14 @@ impl Server {
                         continue;
                     }
 
+                    if !client.flush_partial_write(sid)? {
+                        continue;
+                    }
+
                     loop {
+                        let span = trace_span!("stream_write", sid);
+                        let _guard = span.enter();
+
                         match client
                             .in_flight
                             .get(&sid)
@@ -311,7 +323,10 @@ impl Server {
                             .try_recv()
                         {
                             Ok(msg) => {
-                                client.write_message(sid, msg, false, &mut self.scratch)?;
+                                if !client.write_message(sid, msg, false, &mut self.scratch)? {
+                                    // No more write capacity at the moment.
+                                    break;
+                                }
                             }
                             Err(TryRecvError::Disconnected) => {
                                 client.conn.stream_send(sid, &[], true)?;
@@ -439,7 +454,8 @@ impl Server {
                     timer,
                     timeout_token,
                     in_flight: streams,
-                    partial: HashMap::new(),
+                    partial_reads: HashMap::new(),
+                    partial_writes: HashMap::new(),
                     dgram_recv,
                     dgram_send,
                 };
@@ -546,7 +562,7 @@ impl Server {
 
         // Clean up partial data for closed streams.
         client
-            .partial
+            .partial_reads
             .retain(|sid, _| !client.conn.stream_finished(*sid));
 
         Ok(())
@@ -591,7 +607,7 @@ impl ClientConnection {
     ) -> anyhow::Result<(Vec<protocol::MessageType>, bool)> {
         // Start with partial data from the previous call to read_messages.
         scratch.truncate(0);
-        if let Some(partial) = self.partial.remove(&sid) {
+        if let Some(partial) = self.partial_reads.remove(&sid) {
             scratch.unsplit(partial);
         }
 
@@ -629,7 +645,8 @@ impl ClientConnection {
                         sid,
                         n
                     );
-                    self.partial.insert(sid, buf);
+
+                    self.partial_reads.insert(sid, buf);
                     break;
                 }
                 Err(e) => return Err(e.into()),
@@ -649,26 +666,73 @@ impl ClientConnection {
         Ok((messages, stream_fin))
     }
 
-    /// Send a message on a stream.
+    /// Send a message on a stream. Returns Ok(false) if the stream is full.
     fn write_message(
         &mut self,
         sid: u64,
         msg: protocol::MessageType,
         fin: bool,
         scratch: &mut BytesMut,
-    ) -> anyhow::Result<()> {
+    ) -> anyhow::Result<bool> {
         scratch.resize(protocol::MAX_MESSAGE_SIZE, 0);
-        let len = protocol::encode_message(&msg, scratch).unwrap();
+        let len = protocol::encode_message(&msg, scratch)?;
 
-        trace!(
-            conn_id = ?self.conn_id,
-            stream_id = sid,
-            len,
-            "sending {}", msg
-        );
+        trace!(len, "sending {}", msg);
 
-        self.conn.stream_send(sid, &scratch[..len], fin)?;
-        Ok(())
+        match self.conn.stream_send(sid, &scratch[..len], fin) {
+            Ok(n) if n != len => {
+                // Partial write.
+                assert!(n < len);
+                trace!(n, "partial write");
+
+                let partial = scratch.split_to(len).split_off(n).freeze();
+                let old = self.partial_writes.insert(sid, partial);
+                assert_eq!(None, old);
+
+                Ok(false)
+            }
+            Err(quiche::Error::Done) => {
+                trace!("stream blocked");
+
+                let data = scratch.split_to(len).freeze();
+                let old = self.partial_writes.insert(sid, data);
+                assert_eq!(None, old);
+
+                Ok(false)
+            }
+            v => {
+                assert_eq!(len, v?);
+                Ok(true)
+            }
+        }
+    }
+
+    /// Flushes previous partial writes.
+    fn flush_partial_write(&mut self, sid: u64) -> anyhow::Result<bool> {
+        use std::collections::hash_map::Entry;
+
+        match self.partial_writes.entry(sid) {
+            Entry::Vacant(_) => Ok(true),
+            Entry::Occupied(mut entry) => {
+                let partial = entry.get().clone();
+                trace!(len = partial.len(), "flushing previous partial");
+
+                match self.conn.stream_send(sid, &partial, false) {
+                    Ok(n) if n != entry.get().len() => {
+                        // Partial write.
+                        entry.get_mut().advance(n);
+                        trace!(len = entry.get().len(), "remaining partial");
+                        Ok(false)
+                    }
+                    Ok(_) => {
+                        entry.remove();
+                        Ok(true)
+                    }
+                    Err(quiche::Error::Done) => Ok(false),
+                    Err(e) => Err(anyhow!(e)),
+                }
+            }
+        }
     }
 
     /// Send a message as a datagram.