diff --git a/Cargo.lock b/Cargo.lock
index 9e6ead0..f87259c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1778,7 +1778,7 @@ dependencies = [
 
 [[package]]
 name = "transbeam"
-version = "0.3.0"
+version = "0.4.0"
 dependencies = [
  "actix",
  "actix-files",
diff --git a/Cargo.toml b/Cargo.toml
index 58d0229..ee3496f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "transbeam"
-version = "0.3.0"
+version = "0.4.0"
 authors = ["xenofem <xenofem@xeno.science>"]
 edition = "2021"
 license = "MIT"
diff --git a/cli/transbeam-cli b/cli/transbeam-cli
index 7299398..331a725 100755
--- a/cli/transbeam-cli
+++ b/cli/transbeam-cli
@@ -51,10 +51,6 @@ async def send(paths, host, password, lifetime, collection_name=None):
         loader = file_loader([(paths[i], fileMetadata[i]["size"]) for i in range(len(paths))])
         for data in loader:
             await ws.send(data)
-            resp = await ws.recv()
-            if resp != "ack":
-                tqdm.write("unexpected response: {}".format(resp))
-                exit(1)
 
 parser = argparse.ArgumentParser(description="Upload files to transbeam")
 parser.add_argument("-l", "--lifetime", type=int, default=7, help="Lifetime in days for files (default 7)")
diff --git a/src/upload.rs b/src/upload.rs
index 94f80b8..4f59490 100644
--- a/src/upload.rs
+++ b/src/upload.rs
@@ -211,10 +211,6 @@ impl StreamHandler<Result<ws::Message, ws::ProtocolError>> for Uploader {
     }
 }
 
-fn ack(ctx: &mut Context) {
-    ctx.text("ack");
-}
-
 impl Uploader {
     fn notify_error_and_cleanup(&mut self, e: Error, ctx: &mut Context) {
         error!("{}", e);
@@ -350,13 +346,10 @@ impl Uploader {
                     }),
                 );
             }
-            ws::Message::Binary(data) | ws::Message::Continuation(Item::Last(data)) => {
-                let result = self.handle_data(data)?;
-                ack(ctx);
-                return Ok(result);
-            }
-            ws::Message::Continuation(Item::FirstBinary(data))
-            | ws::Message::Continuation(Item::Continue(data)) => {
+            ws::Message::Binary(data)
+            | ws::Message::Continuation(Item::FirstBinary(data))
+            | ws::Message::Continuation(Item::Continue(data))
+            | ws::Message::Continuation(Item::Last(data)) => {
                 return self.handle_data(data);
             }
             ws::Message::Close(reason) => {
diff --git a/static/js/upload.js b/static/js/upload.js
index debf733..9ec6e79 100644
--- a/static/js/upload.js
+++ b/static/js/upload.js
@@ -2,6 +2,8 @@ const FILE_CHUNK_SIZE = 16384;
 const MAX_FILES = 256;
 const SAMPLE_WINDOW = 100;
 const STALL_THRESHOLD = 1000;
+const MAX_WS_BUFFER = 1048576;
+const WS_BUFFER_DELAY = 10;
 
 let files = [];
 
@@ -204,47 +206,42 @@ function sendManifest() {
 }
 
 function handleMessage(msg) {
-    if (bytesSent === 0) {
-        let reply;
-        try {
-            reply = JSON.parse(msg.data);
-        } catch (error) {
-            socket.close();
-            displayError('Received an invalid response from the server');
-            console.error(error);
-            return;
-        }
-        if (reply.type === 'ready') {
-            downloadCode.textContent = reply.code;
-            updateProgress();
-            document.body.className = 'uploading';
-            sendData();
-            return;
-        }
+    if (bytesSent > 0) {
+        console.warn('Received unexpected message from server during upload', msg.data);
+        return;
+    }
 
-        // we're going to display a more useful error message
-        socket.removeEventListener('close', handleClose);
+    let reply;
+    try {
+        reply = JSON.parse(msg.data);
+    } catch (error) {
         socket.close();
-        if (reply.type === 'too_big') {
-            maxSize = reply.max_size;
-            updateFiles();
-        } else if (reply.type === 'too_long') {
-            updateMaxLifetime(reply.max_lifetime);
-            displayError(`The maximum retention time for uploads is ${reply.max_lifetime} days`);
-        } else if (reply.type === 'incorrect_password') {
-            messageBox.textContent = ('Incorrect password');
-            document.body.className = 'error landing';
-        } else if (reply.type === 'error') {
-            displayError(reply.details);
-        }
-    } else {
-        if (msg.data === 'ack') {
-            sendData();
-        } else {
-            console.error('Received unexpected message from server instead of ack', msg.data);
-            displayError();
-            socket.close();
-        }
+        displayError('Received an invalid response from the server');
+        console.error(error);
+        return;
+    }
+    if (reply.type === 'ready') {
+        downloadCode.textContent = reply.code;
+        updateProgress();
+        document.body.className = 'uploading';
+        sendData();
+        return;
+    }
+
+    // we're going to display a more useful error message
+    socket.removeEventListener('close', handleClose);
+    socket.close();
+    if (reply.type === 'too_big') {
+        maxSize = reply.max_size;
+        updateFiles();
+    } else if (reply.type === 'too_long') {
+        updateMaxLifetime(reply.max_lifetime);
+        displayError(`The maximum retention time for uploads is ${reply.max_lifetime} days`);
+    } else if (reply.type === 'incorrect_password') {
+        messageBox.textContent = ('Incorrect password');
+        document.body.className = 'error landing';
+    } else if (reply.type === 'error') {
+        displayError(reply.details);
     }
 }
 
@@ -262,32 +259,43 @@ function updateMaxLifetime(lifetime) {
 }
 
 function sendData() {
-    if (fileIndex >= files.length) {
-        finishSending();
+    if (socket.readyState !== 1) {
         return;
     }
-    const currentFile = files[fileIndex];
-    if (byteIndex < currentFile.size) {
-        const endpoint = Math.min(byteIndex+FILE_CHUNK_SIZE, currentFile.size);
-        const data = currentFile.slice(byteIndex, endpoint);
-        socket.send(data);
-        byteIndex = endpoint;
-        bytesSent += data.size;
 
-        // It's ok if the monotonically increasing fields like
-        // percentage are updating super quickly, but it's awkward for
-        // rate and ETA
-        const now = Date.now() / 1000;
-        if (timestamps.length === 0 || now - timestamps.at(-1)[0] > 1) {
-            timestamps.push([now, bytesSent]);
-            if (timestamps.length > SAMPLE_WINDOW) { timestamps.shift(); }
+    while (true) {
+        if (fileIndex >= files.length) {
+            finishSending();
+            return;
         }
 
-        updateProgress();
-    } else {
-        fileIndex += 1;
-        byteIndex = 0;
-        sendData();
+        if (socket.bufferedAmount >= MAX_WS_BUFFER) {
+            setTimeout(sendData, WS_BUFFER_DELAY);
+            return;
+        }
+
+        const currentFile = files[fileIndex];
+        if (byteIndex < currentFile.size) {
+            const endpoint = Math.min(byteIndex+FILE_CHUNK_SIZE, currentFile.size);
+            const data = currentFile.slice(byteIndex, endpoint);
+            socket.send(data);
+            byteIndex = endpoint;
+            bytesSent += data.size;
+
+            // It's ok if the monotonically increasing fields like
+            // percentage are updating super quickly, but it's awkward for
+            // rate and ETA
+            const now = Date.now() / 1000;
+            if (timestamps.length === 0 || now - timestamps.at(-1)[0] > 1) {
+                timestamps.push([now, bytesSent]);
+                if (timestamps.length > SAMPLE_WINDOW) { timestamps.shift(); }
+            }
+
+            updateProgress();
+        } else {
+            fileIndex += 1;
+            byteIndex = 0;
+        }
     }
 }