diff --git a/src-tauri/src/sync2/live.rs b/src-tauri/src/sync2/live.rs
index 7098b264..58f9a32e 100644
--- a/src-tauri/src/sync2/live.rs
+++ b/src-tauri/src/sync2/live.rs
@@ -20,6 +20,7 @@ use tokio_tungstenite::tungstenite::{Bytes, Message};
 use super::actions::{local, stopped};
 use super::engine::Result;
 use super::live_hub::{announce, receive, set_server_online, subscribe};
+use super::live_seal::MAX_LIVE_MESSAGE;
 use super::sealed::random_bytes;
 use super::server::{self, ServerStore, StoreError};
 use super::service::sync_after;
@@ -27,8 +28,6 @@ use super::state::{StoreKind, load_group};
 use crate::sync::error::SyncError;
 
 const PROTOCOL: &str = "flow-sync.v1";
-/// The most a live message or a notice may hold (spec/api.md, Limits).
-const MAX_MESSAGE: usize = 64 * 1024;
 const PING_EVERY: Duration = Duration::from_secs(15);
 const RECONNECT_AFTER: Duration = Duration::from_secs(20);
 /// Several devices may store at once: one sync shortly after covers them all.
@@ -77,8 +76,8 @@ impl ServerStore {
         }
         let upgraded = response.upgrade().await.map_err(StoreError::Network)?;
         let config = WebSocketConfig::default()
-            .max_message_size(Some(MAX_MESSAGE))
-            .max_frame_size(Some(MAX_MESSAGE));
+            .max_message_size(Some(MAX_LIVE_MESSAGE))
+            .max_frame_size(Some(MAX_LIVE_MESSAGE));
         Ok(WebSocketStream::from_raw_socket(upgraded, Role::Client, Some(config)).await)
     }
 }
diff --git a/src-tauri/src/sync2/live_seal.rs b/src-tauri/src/sync2/live_seal.rs
index dd6d163f..d4cea817 100644
--- a/src-tauri/src/sync2/live_seal.rs
+++ b/src-tauri/src/sync2/live_seal.rs
@@ -16,6 +16,9 @@ use super::roster::Role;
 use super::store::Store;
 use crate::sync::canonical::to_canonical_json;
 
+/// The most a live message or a notice may hold, on any transport (spec/store.md, Untrusted input).
+pub const MAX_LIVE_MESSAGE: usize = 64 * 1024;
+
 /// What sealing and opening live messages needs, taken from a session when it opened.
 #[derive(Clone)]
 pub struct LiveKeys {
@@ -83,6 +86,9 @@ impl LiveKeys {
     /// Checks and opens a message from another current device. The caller still checks that its
     /// counter is above the last one accepted from that device.
     pub fn open(&self, bytes: &[u8]) -> Result<Received> {
+        if bytes.len() > MAX_LIVE_MESSAGE {
+            return Err(Error::Length.into());
+        }
         let object = parse_object(bytes, &self.group)?;
         let from = hex::encode(object.header.sender);
         let key = self.others.get(&from).ok_or(Error::Sender)?;
@@ -113,6 +119,14 @@ mod tests {
     use super::*;
     use crate::sync2::fuzz::{json, member, mutations, seq};
     use crate::sync2::live_message::{Answer, DeviceKind};
+    use crate::sync2::sealed::random_bytes;
+
+    fn random_bytes_of(len: usize) -> Vec<u8> {
+        std::iter::repeat_with(random_bytes::<32>)
+            .flatten()
+            .take(len)
+            .collect()
+    }
 
     /// Two devices of one group, each with the other's signing key.
     static PAIR: LazyLock<(LiveKeys, LiveKeys)> = LazyLock::new(|| {
@@ -152,6 +166,28 @@ mod tests {
         ]
     }
 
+    #[test]
+    fn a_live_message_over_the_limit_is_refused_on_any_transport() {
+        let (a, b) = &*PAIR;
+        let with_reason = |bytes: usize| {
+            a.seal(
+                1,
+                &LiveMessage::Result(Answer {
+                    to: b.me(),
+                    reference: 1,
+                    ok: false,
+                    reason: Some(hex::encode(random_bytes_of(bytes))),
+                }),
+            )
+            .unwrap()
+        };
+        let small = with_reason(1024);
+        assert!(b.open(&small).is_ok());
+        let large = with_reason(128 * 1024);
+        assert!(large.len() > MAX_LIVE_MESSAGE);
+        assert!(b.open(&large).is_err());
+    }
+
     proptest! {
         #[test]
         fn live_messages_round_trip(seq in seq(), message in message()) {