fix(tui): persist selected model mode

CHANGELOG.md

@@ -105,6 +105,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   stays preloaded in Agent mode, so creating a file no longer stops at the
   deferred-tool schema hydration message before the normal approval/execution
   path (#1825, #1841).
+- **Saved sessions keep the selected model mode.** Changing from `auto` to a
+  concrete model now updates existing session metadata, and resumed sessions
+  recompute the `auto` flag from the saved model instead of falling back to the
+  startup default.
 
 ### Changed
 

crates/tui/CHANGELOG.md

@@ -105,6 +105,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   stays preloaded in Agent mode, so creating a file no longer stops at the
   deferred-tool schema hydration message before the normal approval/execution
   path (#1825, #1841).
+- **Saved sessions keep the selected model mode.** Changing from `auto` to a
+  concrete model now updates existing session metadata, and resumed sessions
+  recompute the `auto` flag from the saved model instead of falling back to the
+  startup default.
 
 ### Changed
 

crates/tui/src/commands/config.rs

@@ -370,9 +370,7 @@ pub fn set_config_value(app: &mut App, key: &str, value: &str, persist: bool) ->
         "model" => {
             // Support "/model auto" — auto-select model based on request complexity
             if value.trim().eq_ignore_ascii_case("auto") {
-                app.auto_model = true;
-                app.model = "auto".to_string();
-                app.last_effective_model = None;
+                app.set_model_selection("auto".to_string());
                 app.reasoning_effort = ReasoningEffort::Auto;
                 app.last_effective_reasoning_effort = None;
                 app.update_model_compaction_budget();
@@ -384,15 +382,13 @@ pub fn set_config_value(app: &mut App, key: &str, value: &str, persist: bool) ->
                 );
             }
             // Clear auto mode when a specific model is set
-            app.auto_model = false;
-            app.last_effective_model = None;
             let Some(model) = normalize_model_name_for_provider(app.api_provider, value) else {
                 return CommandResult::error(format!(
                     "Invalid model '{value}'. Expected a DeepSeek model ID. Common models: {}",
                     COMMON_DEEPSEEK_MODELS.join(", ")
                 ));
             };
-            app.model = model.clone();
+            app.set_model_selection(model.clone());
             app.update_model_compaction_budget();
             app.session.last_prompt_tokens = None;
             app.session.last_completion_tokens = None;
@@ -550,9 +546,7 @@ pub fn set_config_value(app: &mut App, key: &str, value: &str, persist: bool) ->
         }
         "default_model" => {
             if let Some(ref model) = settings.default_model {
-                app.auto_model = model.trim().eq_ignore_ascii_case("auto");
-                app.model.clone_from(model);
-                app.last_effective_model = None;
+                app.set_model_selection(model.clone());
                 if app.auto_model {
                     app.reasoning_effort = ReasoningEffort::Auto;
                     app.last_effective_reasoning_effort = None;

crates/tui/src/tui/app.rs

@@ -4150,6 +4150,25 @@ impl App {
             compaction_threshold_for_model_and_effort(&model, self.reasoning_effort.api_value());
     }
 
+    pub fn set_model_selection(&mut self, model: String) {
+        let auto_model = model.trim().eq_ignore_ascii_case("auto");
+        self.model = if auto_model {
+            "auto".to_string()
+        } else {
+            model
+        };
+        self.auto_model = auto_model;
+        self.last_effective_model = None;
+    }
+
+    pub fn model_selection_for_persistence(&self) -> String {
+        if self.auto_model || self.model.trim().eq_ignore_ascii_case("auto") {
+            "auto".to_string()
+        } else {
+            self.model.clone()
+        }
+    }
+
     pub fn effective_model_for_budget(&self) -> &str {
         if self.auto_model {
             return self

crates/tui/src/tui/ui.rs

@@ -1497,8 +1497,7 @@ async fn run_event_loop(
                         if app.auto_model {
                             app.last_effective_model = Some(model);
                         } else {
-                            app.model = model;
-                            app.last_effective_model = None;
+                            app.set_model_selection(model);
                         }
                         app.update_model_compaction_budget();
                         app.workspace = workspace;
@@ -3513,6 +3512,7 @@ async fn run_cache_warmup(app: &App, config: &Config) -> Result<Usage> {
 // `format_*` chip/message builders moved to `tui/format_helpers.rs`.
 
 fn build_session_snapshot(app: &App, manager: &SessionManager) -> SavedSession {
+    let model = app.model_selection_for_persistence();
     if let Some(ref existing_id) = app.current_session_id
         && let Ok(existing) = manager.load_session(existing_id)
     {
@@ -3522,6 +3522,7 @@ fn build_session_snapshot(app: &App, manager: &SessionManager) -> SavedSession {
             u64::from(app.session.total_tokens),
             app.system_prompt.as_ref(),
         );
+        updated.metadata.model = model;
         updated.metadata.mode = Some(app.mode.as_setting().to_string());
         app.sync_cost_to_metadata(&mut updated.metadata);
         updated.context_references = app.session_context_references.clone();
@@ -3532,7 +3533,7 @@ fn build_session_snapshot(app: &App, manager: &SessionManager) -> SavedSession {
             create_saved_session_with_id_and_mode(
                 existing_id.clone(),
                 &app.api_messages,
-                &app.model,
+                &model,
                 &app.workspace,
                 u64::from(app.session.total_tokens),
                 app.system_prompt.as_ref(),
@@ -3541,7 +3542,7 @@ fn build_session_snapshot(app: &App, manager: &SessionManager) -> SavedSession {
         } else {
             create_saved_session_with_mode(
                 &app.api_messages,
-                &app.model,
+                &model,
                 &app.workspace,
                 u64::from(app.session.total_tokens),
                 app.system_prompt.as_ref(),
@@ -4111,16 +4112,16 @@ async fn apply_model_picker_choice(
     }
 
     if model_changed {
-        app.auto_model = model_is_auto;
-        app.last_effective_model = None;
-        app.model = model.clone();
-        app.update_model_compaction_budget();
+        app.set_model_selection(model.clone());
         app.clear_model_scoped_telemetry();
     }
     if effort_changed {
         app.reasoning_effort = effort;
         app.last_effective_reasoning_effort = None;
     }
+    if model_changed || effort_changed {
+        app.update_model_compaction_budget();
+    }
 
     // Best-effort persist; surface a status warning if the settings file
     // can't be written rather than aborting the in-memory change.
@@ -4236,7 +4237,7 @@ async fn switch_provider(
     let new_model = config.default_model();
     let cache_scope_changed = previous_provider != target || previous_model != new_model;
     app.api_provider = target;
-    app.model = new_model.clone();
+    app.set_model_selection(new_model.clone());
     app.update_model_compaction_budget();
     if cache_scope_changed {
         app.clear_model_scoped_telemetry();
@@ -4672,7 +4673,7 @@ async fn apply_command_result(
                         *config = new_config.clone();
                         app.api_provider = config.api_provider();
                         let new_model = config.default_model();
-                        app.model = new_model.clone();
+                        app.set_model_selection(new_model.clone());
                         app.update_model_compaction_budget();
                         app.session.last_prompt_tokens = None;
                         app.session.last_completion_tokens = None;
@@ -6255,7 +6256,7 @@ fn apply_loaded_session(app: &mut App, config: &Config, session: &SavedSession)
     app.sync_context_references_from_session(&session.context_references, &message_to_cell);
     app.mark_history_updated();
     app.viewport.transcript_selection.clear();
-    app.model.clone_from(&session.metadata.model);
+    app.set_model_selection(session.metadata.model.clone());
     app.update_model_compaction_budget();
     apply_workspace_runtime_state(app, config, session.metadata.workspace.clone());
     app.session.total_tokens = u32::try_from(session.metadata.total_tokens).unwrap_or(u32::MAX);

crates/tui/src/tui/ui/tests.rs

@@ -3912,6 +3912,64 @@ fn first_snapshot_preserves_current_session_id_for_artifact_ownership() {
     assert_eq!(snapshot.metadata.id, "session-123");
 }
 
+#[test]
+fn existing_session_snapshot_updates_model_selection() {
+    let tmp = tempfile::tempdir().expect("tempdir");
+    let manager =
+        crate::session_manager::SessionManager::new(tmp.path().join("sessions")).expect("manager");
+    let mut existing = saved_session_with_messages(vec![text_message("user", "hello")]);
+    existing.metadata.model = "auto".to_string();
+    manager
+        .save_session(&existing)
+        .expect("save existing session");
+
+    let mut app = create_test_app();
+    app.current_session_id = Some(existing.metadata.id.clone());
+    app.api_messages.push(text_message("user", "hello"));
+    app.set_model_selection("deepseek-v4-flash".to_string());
+
+    let snapshot = build_session_snapshot(&app, &manager);
+
+    assert_eq!(snapshot.metadata.id, existing.metadata.id);
+    assert_eq!(snapshot.metadata.model, "deepseek-v4-flash");
+}
+
+#[test]
+fn apply_loaded_session_restores_concrete_model_mode() {
+    let mut app = create_test_app();
+    app.set_model_selection("auto".to_string());
+    let mut session = saved_session_with_messages(vec![
+        text_message("user", "hello"),
+        text_message("assistant", "hi"),
+    ]);
+    session.metadata.model = "deepseek-v4-flash".to_string();
+
+    let recovered = apply_loaded_session(&mut app, &Config::default(), &session);
+
+    assert!(!recovered);
+    assert!(!app.auto_model);
+    assert_eq!(app.model, "deepseek-v4-flash");
+    assert_eq!(app.model_selection_for_persistence(), "deepseek-v4-flash");
+}
+
+#[test]
+fn apply_loaded_session_restores_auto_model_mode() {
+    let mut app = create_test_app();
+    app.set_model_selection("deepseek-v4-pro".to_string());
+    let mut session = saved_session_with_messages(vec![
+        text_message("user", "hello"),
+        text_message("assistant", "hi"),
+    ]);
+    session.metadata.model = "auto".to_string();
+
+    let recovered = apply_loaded_session(&mut app, &Config::default(), &session);
+
+    assert!(!recovered);
+    assert!(app.auto_model);
+    assert_eq!(app.model, "auto");
+    assert_eq!(app.model_selection_for_persistence(), "auto");
+}
+
 #[test]
 fn apply_loaded_session_restores_artifact_registry() {
     let mut app = create_test_app();