perf(protocol): send usage changes without full session status

crates/protocol/src/event.rs

@@ -68,6 +68,10 @@ pub enum ServerEvent {
     },
     SessionEvent(StoredEvent),
     SessionStatusReplaced(Box<SessionStatus>),
+    SessionUsageChanged {
+        usage: Option<TokenUsage>,
+        context_window: Option<u64>,
+    },
     SessionPlanReplaced(SessionPlan),
     ProvidersReplaced(ProvidersStatus),
     GitStatusReplaced(GitStatusStatus),

crates/protocol/src/lib.rs

@@ -150,6 +150,8 @@ pub use wire::{
 // host then sends the text as an ordinary turn. Index subscriptions may carry
 // known_index; matching content receives IndexUnchanged instead of a snapshot.
 // ReadFilePreview rejects files larger than 512 KiB before transferring bytes.
+// SessionUsageChanged carries usage and meter capacity when the rest of the
+// session status is unchanged; subscription replies remain full statuses.
 pub const PROTOCOL_VERSION: u32 = 12;
 
 #[cfg(test)]

crates/runtime/src/app/snapshots.rs

@@ -109,12 +109,25 @@ impl DomainDiff {
             .collect();
         changed_ids.sort_unstable();
         for id in changed_ids {
-            let status = session_statuses[id].clone();
+            let status = &session_statuses[id];
+            let usage_only = self.session_statuses.get_mut(id).is_some_and(|previous| {
+                previous.usage = status.usage;
+                previous.context_window = status.context_window;
+                previous == status
+            });
+            let event = if usage_only {
+                ServerEvent::SessionUsageChanged {
+                    usage: status.usage,
+                    context_window: status.context_window,
+                }
+            } else {
+                ServerEvent::SessionStatusReplaced(Box::new(status.clone()))
+            };
             emit_replacement(
                 Topic::SessionStatus {
                     session_id: id.to_string(),
                 },
-                ServerEvent::SessionStatusReplaced(Box::new(status)),
+                event,
                 cx,
             );
         }

crates/ui/src/store/mod.rs

@@ -1342,6 +1342,30 @@ impl WorkspaceStore {
                     self.sync_active_conversation_ui();
                 }
             }
+            (
+                Topic::SessionStatus { session_id },
+                ServerEvent::SessionUsageChanged {
+                    usage,
+                    context_window,
+                },
+            ) if self.baseline_topics.contains(&envelope.topic) => {
+                if let Some(status) = self
+                    .threads
+                    .get_mut(session_id)
+                    .and_then(|thread| thread.status.as_mut())
+                {
+                    status.usage = *usage;
+                    status.context_window = *context_window;
+                }
+                if let Some(status) = self
+                    .session_status_replica
+                    .as_mut()
+                    .filter(|status| status.session_id == *session_id)
+                {
+                    status.usage = *usage;
+                    status.context_window = *context_window;
+                }
+            }
             (Topic::SessionPlan { session_id }, ServerEvent::SessionPlanReplaced(plan))
                 if plan.session_id == *session_id =>
             {