agent2: Token count (#36496)

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/agent_ui/src/acp/thread_view.rs

@@ -816,7 +816,7 @@ impl AcpThreadView {
                 self.thread_retry_status.take();
                 self.thread_state = ThreadState::ServerExited { status: *status };
             }
-            AcpThreadEvent::TitleUpdated => {}
+            AcpThreadEvent::TitleUpdated | AcpThreadEvent::TokenUsageUpdated => {}
         }
         cx.notify();
     }
@@ -2794,6 +2794,7 @@ impl AcpThreadView {
                     .child(
                         h_flex()
                             .gap_1()
+                            .children(self.render_token_usage(cx))
                             .children(self.profile_selector.clone())
                             .children(self.model_selector.clone())
                             .child(self.render_send_button(cx)),
@@ -2816,6 +2817,44 @@ impl AcpThreadView {
             .thread(acp_thread.session_id(), cx)
     }
 
+    fn render_token_usage(&self, cx: &mut Context<Self>) -> Option<Div> {
+        let thread = self.thread()?.read(cx);
+        let usage = thread.token_usage()?;
+        let is_generating = thread.status() != ThreadStatus::Idle;
+
+        let used = crate::text_thread_editor::humanize_token_count(usage.used_tokens);
+        let max = crate::text_thread_editor::humanize_token_count(usage.max_tokens);
+
+        Some(
+            h_flex()
+                .flex_shrink_0()
+                .gap_0p5()
+                .mr_1()
+                .child(
+                    Label::new(used)
+                        .size(LabelSize::Small)
+                        .color(Color::Muted)
+                        .map(|label| {
+                            if is_generating {
+                                label
+                                    .with_animation(
+                                        "used-tokens-label",
+                                        Animation::new(Duration::from_secs(2))
+                                            .repeat()
+                                            .with_easing(pulsating_between(0.6, 1.)),
+                                        |label, delta| label.alpha(delta),
+                                    )
+                                    .into_any()
+                            } else {
+                                label.into_any_element()
+                            }
+                        }),
+                )
+                .child(Label::new("/").size(LabelSize::Small).color(Color::Muted))
+                .child(Label::new(max).size(LabelSize::Small).color(Color::Muted)),
+        )
+    }
+
     fn toggle_burn_mode(
         &mut self,
         _: &ToggleBurnMode,

crates/agent_ui/src/agent_diff.rs

@@ -1526,6 +1526,7 @@ impl AgentDiff {
                 self.update_reviewing_editors(workspace, window, cx);
             }
             AcpThreadEvent::TitleUpdated
+            | AcpThreadEvent::TokenUsageUpdated
             | AcpThreadEvent::EntriesRemoved(_)
             | AcpThreadEvent::ToolAuthorizationRequired
             | AcpThreadEvent::Retry(_) => {}