fix(tui): refresh prompt on model switch

crates/tui/src/core/engine.rs

@@ -789,10 +789,12 @@ impl Engine {
                         .send(Event::status(format!("Mode changed to: {mode:?}")))
                         .await;
                 }
-                Op::SetModel { model } => {
+                Op::SetModel { model, mode } => {
                     self.session.auto_model = model.trim().eq_ignore_ascii_case("auto");
                     self.session.model = model;
                     self.config.model.clone_from(&self.session.model);
+                    self.refresh_system_prompt(mode);
+                    self.emit_session_updated().await;
                     let _ = self
                         .tx_event
                         .send(Event::status(format!(

crates/tui/src/core/engine/tests.rs

@@ -1238,6 +1238,61 @@ async fn session_update_preserves_reasoning_tool_only_turn() {
     assert_eq!(messages, vec![assistant]);
 }
 
+#[tokio::test]
+async fn set_model_reloads_instruction_sources_and_updates_session_prompt() {
+    let tmp = tempdir().expect("tempdir");
+    let instructions = tmp.path().join("instructions.md");
+    fs::write(&instructions, "FLASH_INSTRUCTIONS_MARKER").expect("write instructions");
+    let config = EngineConfig {
+        workspace: tmp.path().to_path_buf(),
+        model: "deepseek-v4-flash".to_string(),
+        instructions: vec![instructions.clone().into()],
+        ..Default::default()
+    };
+    let (engine, handle) = Engine::new(config, &Config::default());
+    fs::write(&instructions, "PRO_INSTRUCTIONS_MARKER").expect("rewrite instructions");
+
+    let run = tokio::spawn(engine.run());
+    handle
+        .send(Op::SetModel {
+            model: "deepseek-v4-pro".to_string(),
+            mode: AppMode::Agent,
+        })
+        .await
+        .expect("send set model");
+
+    let (model, prompt) = {
+        let mut rx = handle.rx_event.write().await;
+        loop {
+            let event = tokio::time::timeout(std::time::Duration::from_secs(1), rx.recv())
+                .await
+                .expect("session update after model switch")
+                .expect("event");
+            if let Event::SessionUpdated {
+                model,
+                system_prompt,
+                ..
+            } = event
+            {
+                let prompt = match system_prompt.expect("system prompt") {
+                    SystemPrompt::Text(text) => text,
+                    SystemPrompt::Blocks(blocks) => blocks
+                        .into_iter()
+                        .map(|block| block.text)
+                        .collect::<Vec<_>>()
+                        .join("\n"),
+                };
+                break (model, prompt);
+            }
+        }
+    };
+    run.abort();
+
+    assert_eq!(model, "deepseek-v4-pro");
+    assert!(prompt.contains("PRO_INSTRUCTIONS_MARKER"));
+    assert!(!prompt.contains("FLASH_INSTRUCTIONS_MARKER"));
+}
+
 #[test]
 fn detects_context_length_errors_from_provider_payloads() {
     let msg = r#"SSE stream request failed: HTTP 400 Bad Request: {"error":{"message":"This model's maximum context length is 131072 tokens. However, you requested 153056 tokens (148960 in the messages, 4096 in the completion).","type":"invalid_request_error"}}"#;

crates/tui/src/core/ops.rs

@@ -63,9 +63,9 @@ pub enum Op {
     #[allow(dead_code)]
     ChangeMode { mode: AppMode },
 
-    /// Update the model being used
+    /// Update the model being used and refresh the prompt for the current mode.
     #[allow(dead_code)]
-    SetModel { model: String },
+    SetModel { model: String, mode: AppMode },
 
     /// Update auto-compaction settings
     SetCompaction { config: CompactionConfig },

crates/tui/src/tui/ui.rs

@@ -3437,6 +3437,7 @@ async fn run_event_loop(
                         let _ = engine_handle
                             .send(Op::SetModel {
                                 model: app.model.clone(),
+                                mode: app.mode,
                             })
                             .await;
                     }
@@ -4748,10 +4749,12 @@ async fn dispatch_user_message(
 async fn apply_model_and_compaction_update(
     engine_handle: &EngineHandle,
     compaction: crate::compaction::CompactionConfig,
+    mode: AppMode,
 ) {
     let _ = engine_handle
         .send(Op::SetModel {
             model: compaction.model.clone(),
+            mode,
         })
         .await;
     let _ = engine_handle
@@ -4779,6 +4782,7 @@ async fn drain_web_config_events(
                             apply_model_and_compaction_update(
                                 engine_handle,
                                 app.compaction_config(),
+                                app.mode,
                             )
                             .await;
                         }
@@ -4803,6 +4807,7 @@ async fn drain_web_config_events(
                             apply_model_and_compaction_update(
                                 engine_handle,
                                 app.compaction_config(),
+                                app.mode,
                             )
                             .await;
                         }
@@ -4888,7 +4893,7 @@ async fn apply_model_picker_choice(
     }
 
     if model_changed {
-        apply_model_and_compaction_update(engine_handle, app.compaction_config()).await;
+        apply_model_and_compaction_update(engine_handle, app.compaction_config(), app.mode).await;
     }
 
     let model_summary = if model_is_auto {
@@ -5237,7 +5242,7 @@ async fn apply_command_result(
                 }
             }
             AppAction::UpdateCompaction(compaction) => {
-                apply_model_and_compaction_update(engine_handle, compaction).await;
+                apply_model_and_compaction_update(engine_handle, compaction, app.mode).await;
             }
             AppAction::OpenConfigEditor(mode) => match mode {
                 ConfigUiMode::Native => {
@@ -5267,6 +5272,7 @@ async fn apply_command_result(
                                 apply_model_and_compaction_update(
                                     engine_handle,
                                     app.compaction_config(),
+                                    app.mode,
                                 )
                                 .await;
                             }
@@ -6743,7 +6749,8 @@ async fn handle_view_events(
                 if let Some(action) = result.action {
                     match action {
                         AppAction::UpdateCompaction(compaction) => {
-                            apply_model_and_compaction_update(engine_handle, compaction).await;
+                            apply_model_and_compaction_update(engine_handle, compaction, app.mode)
+                                .await;
                         }
                         AppAction::OpenConfigView => {}
                         _ => {}

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

@@ -2021,11 +2021,12 @@ async fn model_change_update_syncs_engine_model_before_compaction() {
     let compaction = app.compaction_config();
     let mut engine = crate::core::engine::mock_engine_handle();
 
-    apply_model_and_compaction_update(&engine.handle, compaction).await;
+    apply_model_and_compaction_update(&engine.handle, compaction, app.mode).await;
 
     match engine.rx_op.recv().await.expect("set model op") {
-        crate::core::ops::Op::SetModel { model } => {
+        crate::core::ops::Op::SetModel { model, mode } => {
             assert_eq!(model, "deepseek-v4-flash");
+            assert_eq!(mode, app.mode);
         }
         other => panic!("expected SetModel, got {other:?}"),
     }