fix(tui): repaint the header context window on a /model switch

After `/model`, the header kept the previous model's window ("15K /
1.0M" on a 262K model) until the next turn or a restart. The pager
caches the window inside its context snapshot (streaming `totalTokens`
updates refresh it from the catalog) and nothing refreshed the cache
when the session's model changed. Comparing cached numbers against
catalog windows cannot tell a catalog-derived total from a shell
effective limit (KIGI_DEBUG_CONTEXT_WINDOW, header upgrades), so the
shell, which installs the effective window, now reports it:

- kigi-shell: `handle_set_session_model` returns the installed window;
  `model_switch::apply` stamps it as `totalContextTokens` in the
  `session/set_model` response meta and carries it on the
  `ModelChanged` broadcast (`context_window`, serde-default: old pagers
  ignore it, old leaders omit it). The response meta previously
  serialized the actor's inner `Result` and reported success on an
  actor failure; that error now propagates.
- kigi-tui: `Effect::SwitchModel` reads the meta; `SwitchModelComplete`
  carries `Ok(Option<u64>)`; the completion handler and the follower
  `ModelChanged` arm call `AgentView::apply_context_window`, which
  re-derives the snapshot (pct, free tokens) and reports whether it
  changed so a follower redraws on a window-only change.
- Tests: the bare `/model <name>` dispatch path asserts the snapshot and
  the drawn header; follower with/without the field; same-model new
  window redraw; actor returns the window; serde round trip.

Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4

crates/codegen/kigi-tui/src/app/acp_handler/session_notification.rs

@@ -812,6 +812,7 @@ pub(super) fn handle_session_notification(notif: &acp::ExtNotification, app: &mu
         XaiSessionUpdate::ModelChanged {
             model_id,
             reasoning_effort,
+            context_window,
         } => {
             if agent.session.model_switch_pending {
                 tracing::debug!(
@@ -847,6 +848,8 @@ pub(super) fn handle_session_notification(notif: &acp::ExtNotification, app: &mu
                 .session
                 .models
                 .set_current(new_model_id.clone(), effort);
+            let window_changed =
+                context_window.is_some_and(|total| agent.apply_context_window(total));
             agent.session.user_model_preference = Some(new_model_id.clone());
             let resolved_effort = agent.session.models.reasoning_effort;
             let actually_changed =
@@ -858,7 +861,7 @@ pub(super) fn handle_session_notification(notif: &acp::ExtNotification, app: &mu
                     "ModelChanged broadcast applied (remote switch)"
                 );
             }
-            actually_changed
+            actually_changed || window_changed
         }
         XaiSessionUpdate::MemoryFiles { files } => {
             let entries = crate::views::memory_modal::build_entries(files);

crates/codegen/kigi-tui/src/app/acp_handler/tests/mod.rs

@@ -1701,12 +1701,14 @@ pub(super) fn model_changed_ext(
     session_id: &str,
     model_id: &str,
     reasoning_effort: Option<&str>,
+    context_window: Option<u64>,
 ) -> acp::ExtNotification {
     let payload = SessionNotification {
         session_id: acp::SessionId::new(session_id),
         update: XaiSessionUpdate::ModelChanged {
             model_id: model_id.to_string(),
             reasoning_effort: reasoning_effort.map(String::from),
+            context_window,
         },
         meta: None,
     };
@@ -1723,6 +1725,7 @@ pub(super) fn model_changed_ext_with_event(
         update: XaiSessionUpdate::ModelChanged {
             model_id: model_id.to_string(),
             reasoning_effort: None,
+            context_window: None,
         },
         meta: Some(serde_json::json!({ "eventId" : event_id })),
     };

crates/codegen/kigi-tui/src/app/acp_handler/tests/models.rs

@@ -110,6 +110,70 @@
         );
     }
 
+    fn seed_windowed_models(agent: &mut AgentView) {
+        for (slug, window) in [("wide", 1_000_000u64), ("narrow", 262_144)] {
+            let mut info = make_model_info(slug);
+            info.meta = serde_json::json!({ "totalContextTokens": window })
+                .as_object()
+                .cloned();
+            agent
+                .session
+                .models
+                .available
+                .insert(acp::ModelId::new(std::sync::Arc::from(slug)), info);
+        }
+        agent
+            .session
+            .models
+            .set_current(acp::ModelId::new(std::sync::Arc::from("wide")), None);
+        agent.apply_context_used(15_000, 1_000_000);
+    }
+
+    /// A follower mirrors a remote switch and installs the effective window
+    /// the broadcast carries, like the invoking client does.
+    #[test]
+    fn model_changed_broadcast_installs_the_reported_context_window() {
+        let mut app = make_app_with_agent("sess-1");
+        seed_windowed_models(app.agents.get_mut(&AgentId(0)).unwrap());
+
+        let notif = model_changed_ext("sess-1", "narrow", None, Some(262_144));
+        assert!(handle_ext_notification(&notif, &mut app));
+
+        let snap = app.agents[&AgentId(0)].context_state.as_ref().unwrap();
+        assert_eq!((snap.used, snap.total, snap.usage_pct), (15_000, 262_144, 6));
+    }
+
+    /// Same model, same effort, new effective window (the catalog was
+    /// reloaded and re-applied): the follower must still request a redraw.
+    #[test]
+    fn model_changed_same_model_new_window_requests_redraw() {
+        let mut app = make_app_with_agent("sess-1");
+        seed_windowed_models(app.agents.get_mut(&AgentId(0)).unwrap());
+
+        let notif = model_changed_ext("sess-1", "wide", None, Some(2_000_000));
+        assert!(handle_ext_notification(&notif, &mut app));
+
+        let snap = app.agents[&AgentId(0)].context_state.as_ref().unwrap();
+        assert_eq!((snap.used, snap.total), (15_000, 2_000_000));
+
+        let again = model_changed_ext("sess-1", "wide", None, Some(2_000_000));
+        assert!(!handle_ext_notification(&again, &mut app));
+    }
+
+    /// A broadcast from a leader that predates the field carries no window;
+    /// the snapshot is left for the next streaming update to refresh.
+    #[test]
+    fn model_changed_broadcast_without_window_keeps_the_snapshot() {
+        let mut app = make_app_with_agent("sess-1");
+        seed_windowed_models(app.agents.get_mut(&AgentId(0)).unwrap());
+
+        let notif = model_changed_ext("sess-1", "narrow", None, None);
+        assert!(handle_ext_notification(&notif, &mut app));
+
+        let snap = app.agents[&AgentId(0)].context_state.as_ref().unwrap();
+        assert_eq!((snap.used, snap.total), (15_000, 1_000_000));
+    }
+
     #[test]
     fn models_update_without_active_agent_uses_shell_default() {
         let (tx, _rx) = tokio::sync::mpsc::unbounded_channel();
@@ -232,7 +296,7 @@
         let scrollback_before = agent.scrollback.len();
         assert!(!agent.session.model_switch_pending);
 
-        let notif = model_changed_ext("sess-1", "kigi-4", None);
+        let notif = model_changed_ext("sess-1", "kigi-4", None, None);
         let changed = handle_ext_notification(&notif, &mut app);
         assert!(
             changed,
@@ -278,7 +342,7 @@
             Some(acp::ModelId::new(std::sync::Arc::from("heavy")));
         assert!(!agent.session.model_switch_pending);
 
-        let notif = model_changed_ext("sess-1", "auto", None);
+        let notif = model_changed_ext("sess-1", "auto", None, None);
         let changed = handle_ext_notification(&notif, &mut app);
         assert!(
             changed,
@@ -328,7 +392,7 @@
         agent.session.model_switch_pending = true;
         let scrollback_before = agent.scrollback.len();
 
-        let notif = model_changed_ext("sess-1", "kigi-4", None);
+        let notif = model_changed_ext("sess-1", "kigi-4", None, None);
         let changed = handle_ext_notification(&notif, &mut app);
         assert!(
             !changed,
@@ -369,7 +433,7 @@
         let agent = app.agents.get_mut(&AgentId(0)).unwrap();
         seed_models(agent, "kigi-3", &["kigi-3", "kigi-4"]);
 
-        let notif = model_changed_ext("sess-1", "kigi-99-unknown", None);
+        let notif = model_changed_ext("sess-1", "kigi-99-unknown", None, None);
         let changed = handle_ext_notification(&notif, &mut app);
         assert!(
             !changed,
@@ -400,7 +464,7 @@
         let agent = app.agents.get_mut(&AgentId(0)).unwrap();
         seed_models(agent, "kigi-3", &["kigi-3", "kigi-4"]);
 
-        let notif = model_changed_ext("sess-1", "kigi-4", Some("high"));
+        let notif = model_changed_ext("sess-1", "kigi-4", Some("high"), None);
         assert!(handle_ext_notification(&notif, &mut app));
 
         let agent = app.agents.get(&AgentId(0)).unwrap();
@@ -422,7 +486,7 @@
         let agent = app.agents.get_mut(&AgentId(0)).unwrap();
         seed_models(agent, "kigi-3", &["kigi-3", "kigi-4"]);
 
-        let notif = model_changed_ext("sess-OTHER", "kigi-4", None);
+        let notif = model_changed_ext("sess-OTHER", "kigi-4", None, None);
         let changed = handle_ext_notification(&notif, &mut app);
         assert!(!changed);
 

crates/codegen/kigi-tui/src/app/actions.rs

@@ -2112,7 +2112,10 @@ pub enum TaskResult {
         agent_id: AgentId,
         model_id: acp::ModelId,
         effort: Option<ReasoningEffort>,
-        result: Result<(), SwitchModelError>,
+        /// Ok carries the effective context window the shell reported
+        /// (`totalContextTokens` in the response meta; `None` from a shell
+        /// that predates it).
+        result: Result<Option<u64>, SwitchModelError>,
         /// Forwarded from `Effect::SwitchModel.prev_model_id` for
         /// rollback on `IncompatibleAgent`.
         prev_model_id: Option<acp::ModelId>,

crates/codegen/kigi-tui/src/app/agent_view/session.rs

@@ -733,6 +733,20 @@ impl AgentView {
             }
         }
     }
+    /// The shell reported the session's effective context window (model
+    /// switch). The snapshot caches the previous model's window, so the
+    /// header would show it until the next streaming update. Returns
+    /// whether the snapshot changed, so the caller can request a redraw.
+    pub fn apply_context_window(&mut self, total: u64) -> bool {
+        let Some((used, cached)) = self.context_state.as_ref().map(|c| (c.used, c.total)) else {
+            return false;
+        };
+        if cached == total {
+            return false;
+        }
+        self.apply_context_used(used, total);
+        true
+    }
     /// Record a key event to the input flight recorder.
     ///
     /// Zero heap allocations — stores raw `Copy` types in the ring buffer.
@@ -1262,6 +1276,36 @@ mod resolve_turn_activity_tests {
     }
 }
 #[cfg(test)]
+mod apply_context_window_tests {
+    use super::super::test_agent_view;
+    fn view() -> super::AgentView {
+        test_agent_view(Some("s1"), std::path::PathBuf::from("/tmp"))
+    }
+    #[test]
+    fn reported_window_replaces_the_cached_total() {
+        let mut agent = view();
+        agent.apply_context_used(15_000, 1_000_000);
+        assert!(agent.apply_context_window(262_144));
+        let snap = agent.context_state.as_ref().unwrap();
+        assert_eq!(
+            (snap.used, snap.total, snap.usage_pct, snap.free_tokens),
+            (15_000, 262_144, 6, 262_144 - 15_000)
+        );
+    }
+    #[test]
+    fn same_window_reports_no_change() {
+        let mut agent = view();
+        agent.apply_context_used(15_000, 1_000_000);
+        assert!(!agent.apply_context_window(1_000_000));
+    }
+    #[test]
+    fn no_snapshot_stays_none() {
+        let mut agent = view();
+        assert!(!agent.apply_context_window(262_144));
+        assert!(agent.context_state.is_none());
+    }
+}
+#[cfg(test)]
 mod status_window_tests {
     use super::super::test_agent_view;
     #[test]

crates/codegen/kigi-tui/src/app/dispatch/session/lifecycle.rs

@@ -978,13 +978,13 @@ pub(in crate::app::dispatch) fn handle_switch_model_complete(
     agent_id: AgentId,
     model_id: acp::ModelId,
     effort: Option<ReasoningEffort>,
-    result: Result<(), SwitchModelError>,
+    result: Result<Option<u64>, SwitchModelError>,
     prev_model_id: Option<acp::ModelId>,
 ) -> Vec<Effect> {
     if let Some(agent) = app.agents.get_mut(&agent_id) {
         agent.session.model_switch_pending = false;
         let mut effects = match result {
-            Ok(()) => {
+            Ok(context_window) => {
                 agent.session.user_model_preference = Some(model_id.clone());
                 let display_name = agent
                     .session
@@ -996,6 +996,9 @@ pub(in crate::app::dispatch) fn handle_switch_model_complete(
                 let prev_model = agent.session.models.current.clone();
                 let prev_effort = agent.session.models.reasoning_effort;
                 agent.session.models.set_current(model_id.clone(), effort);
+                if let Some(total) = context_window {
+                    agent.apply_context_window(total);
+                }
                 let resolved_effort = agent.session.models.reasoning_effort;
                 let unchanged =
                     prev_model.as_ref() == Some(&model_id) && prev_effort == resolved_effort;

crates/codegen/kigi-tui/src/app/dispatch/tests/prompt.rs

@@ -1789,7 +1789,7 @@ fn switch_model_holds_prompt_until_complete() {
             agent_id: id,
             model_id,
             effort: None,
-            result: Ok(()),
+            result: Ok(None),
             prev_model_id: None,
         }),
         &mut app,

crates/codegen/kigi-tui/src/app/dispatch/tests/settings.rs

@@ -282,7 +282,7 @@ fn model_switch_pending_resets_correctly_across_success_and_failure() {
             agent_id: id,
             model_id: model_a,
             effort: None,
-            result: Ok(()),
+            result: Ok(None),
             prev_model_id: None,
         }),
         &mut app,

crates/codegen/kigi-tui/src/app/dispatch/tests/task_result.rs

@@ -535,7 +535,7 @@ fn switch_model_complete_success_updates_model_and_pushes_message() {
             agent_id: id,
             model_id: model_id.clone(),
             effort: None,
-            result: Ok(()),
+            result: Ok(None),
             prev_model_id: None,
         }),
         &mut app,
@@ -554,6 +554,96 @@ fn switch_model_complete_success_updates_model_and_pushes_message() {
     ));
 }
 
+/// The bare `/model <name>` path: `SetDefaultModel` moves `models.current`
+/// before the ACP round trip; the completion then installs the effective
+/// window the shell reported, so the header repaints at once.
+#[test]
+fn switch_model_complete_installs_the_reported_context_window() {
+    let mut app = test_app_with_agent();
+    let id = AgentId(0);
+    let model = |slug: &str, window: u64| {
+        let mid = acp::ModelId::new(std::sync::Arc::from(slug));
+        let info = acp::ModelInfo::new(mid.clone(), slug.to_string()).meta(
+            serde_json::json!({ "totalContextTokens": window })
+                .as_object()
+                .cloned(),
+        );
+        (mid, info)
+    };
+    let (wide, wide_info) = model("wide", 1_000_000);
+    let (narrow, narrow_info) = model("narrow", 262_144);
+    {
+        let agent = app.agents.get_mut(&id).unwrap();
+        agent
+            .session
+            .models
+            .available
+            .insert(wide.clone(), wide_info);
+        agent
+            .session
+            .models
+            .available
+            .insert(narrow.clone(), narrow_info);
+        agent.session.models.set_current(wide.clone(), None);
+        agent.apply_context_used(15_000, 1_000_000);
+    }
+
+    let effects = dispatch(Action::SetDefaultModel(narrow.clone()), &mut app);
+    assert!(
+        effects
+            .iter()
+            .any(|e| matches!(e, Effect::SwitchModel { .. }))
+    );
+    assert!(app.agents[&id].session.model_switch_pending);
+    assert_eq!(
+        app.agents[&id].context_state.as_ref().unwrap().total,
+        1_000_000
+    );
+
+    dispatch(
+        Action::TaskComplete(TaskResult::SwitchModelComplete {
+            agent_id: id,
+            model_id: narrow,
+            effort: None,
+            result: Ok(Some(262_144)),
+            prev_model_id: Some(wide),
+        }),
+        &mut app,
+    );
+
+    let agent = app.agents.get_mut(&id).unwrap();
+    let snap = agent.context_state.as_ref().unwrap();
+    assert_eq!(
+        (snap.used, snap.total, snap.usage_pct),
+        (15_000, 262_144, 6)
+    );
+    let area = ratatui::layout::Rect::new(0, 0, 100, 40);
+    let mut buf = ratatui::buffer::Buffer::empty(area);
+    agent.draw(
+        area,
+        &mut buf,
+        &crate::actions::ActionRegistry::defaults(),
+        &mut crate::scrollback::render::ScratchBuffer::new(),
+        None,
+        false,
+        0,
+        None,
+        &crate::app::bundle::BundleState::default(),
+        false,
+        &mut Vec::new(),
+    );
+    let screen: String = (0..area.height)
+        .map(|y| {
+            (0..area.width)
+                .filter_map(|x| buf.cell((x, y)).map(|c| c.symbol().to_string()))
+                .collect::<String>()
+        })
+        .collect::<Vec<_>>()
+        .join("\n");
+    assert!(screen.contains("15K / 262K"), "header: {screen:?}");
+    assert!(!screen.contains("1.0M"), "stale window: {screen:?}");
+}
+
 #[test]
 fn switch_model_complete_skips_message_and_persist_when_unchanged() {
     let mut app = test_app_with_agent();
@@ -575,7 +665,7 @@ fn switch_model_complete_skips_message_and_persist_when_unchanged() {
             agent_id: id,
             model_id: model_id.clone(),
             effort: None,
-            result: Ok(()),
+            result: Ok(None),
             prev_model_id: None,
         }),
         &mut app,
@@ -630,7 +720,7 @@ fn switch_model_complete_persists_resolved_effort_from_catalog_meta() {
             model_id: model_id.clone(),
             // user typed `/model Blackbox 4.7` with no effort
             effort: None,
-            result: Ok(()),
+            result: Ok(None),
             prev_model_id: None,
         }),
         &mut app,
@@ -696,7 +786,7 @@ fn switch_to_non_reasoning_model_clears_persisted_effort() {
             agent_id: id,
             model_id: model_id.clone(),
             effort: None,
-            result: Ok(()),
+            result: Ok(None),
             prev_model_id: None,
         }),
         &mut app,
@@ -930,7 +1020,7 @@ fn same_agent_type_switch_no_modal() {
             agent_id: id,
             model_id: model_b.clone(),
             effort: None,
-            result: Ok(()),
+            result: Ok(None),
             prev_model_id: None,
         }),
         &mut app,
@@ -969,7 +1059,7 @@ fn switch_model_pending_lifecycle() {
             agent_id: id,
             model_id,
             effort: None,
-            result: Ok(()),
+            result: Ok(None),
             prev_model_id: None,
         }),
         &mut app,

crates/codegen/kigi-tui/src/app/effects/mod.rs

@@ -1587,7 +1587,12 @@ pub(crate) fn execute(
                         .meta(meta);
                     let result = acp_send(req, &tx)
                         .await
-                        .map(|_| ())
+                        .map(|resp| {
+                            resp.meta
+                                .as_ref()
+                                .and_then(|m| m.get("totalContextTokens"))
+                                .and_then(serde_json::Value::as_u64)
+                        })
                         .map_err(|e| {
                             use kigi_shell::agent::config::ModelSwitchIncompatibleAgentError;
                             if let Some(typed) = ModelSwitchIncompatibleAgentError::from_acp_error(