fix(tui): keep fixed-model auto thinking local

Closes #973

Squashed from #976 after local verification and green CI.

crates/tui/src/tools/subagent/mod.rs

@@ -3390,7 +3390,7 @@ pub(crate) async fn resolve_subagent_assignment_route(
     let explicit_model = configured_model.is_some();
     let mut route = fallback_subagent_assignment_route(runtime, configured_model, prompt);
 
-    if (runtime.auto_model || runtime.reasoning_effort_auto)
+    if should_use_subagent_flash_router(runtime)
         && let Ok(Some(recommendation)) = subagent_flash_router(runtime, prompt).await
     {
         if runtime.auto_model && !explicit_model {
@@ -3407,6 +3407,10 @@ pub(crate) async fn resolve_subagent_assignment_route(
     route
 }
 
+fn should_use_subagent_flash_router(runtime: &SubAgentRuntime) -> bool {
+    runtime.auto_model
+}
+
 fn fallback_subagent_assignment_route(
     runtime: &SubAgentRuntime,
     configured_model: Option<String>,

crates/tui/src/tools/subagent/tests.rs

@@ -442,6 +442,26 @@ fn subagent_auto_reasoning_resolves_to_distinct_v4_tiers() {
     );
 }
 
+#[test]
+fn fixed_model_subagent_auto_reasoning_skips_flash_router() {
+    let runtime = stub_runtime().with_reasoning_effort(Some("high".to_string()), true);
+
+    assert!(
+        !should_use_subagent_flash_router(&runtime),
+        "fixed-model auto thinking should resolve locally without a hidden router request"
+    );
+}
+
+#[test]
+fn auto_model_subagent_assignments_still_use_flash_router() {
+    let runtime = stub_runtime().with_auto_model(true);
+
+    assert!(
+        should_use_subagent_flash_router(&runtime),
+        "auto-model sub-agent assignments still need router guidance"
+    );
+}
+
 #[test]
 fn subagent_router_prompt_frames_assignment_as_auto_routing() {
     let runtime = stub_runtime()

crates/tui/src/tui/ui.rs

@@ -3628,7 +3628,7 @@ async fn dispatch_user_message(
         persistence_actor::persist(PersistRequest::Checkpoint(session));
     }
 
-    let auto_selection = if app.auto_model || app.reasoning_effort == ReasoningEffort::Auto {
+    let auto_selection = if should_resolve_auto_model_selection(app) {
         Some(resolve_auto_model_selection(app, config, &message, &content).await)
     } else {
         None
@@ -3698,6 +3698,10 @@ async fn dispatch_user_message(
     Ok(())
 }
 
+fn should_resolve_auto_model_selection(app: &App) -> bool {
+    app.auto_model
+}
+
 async fn resolve_auto_model_selection(
     app: &App,
     config: &Config,

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

@@ -787,6 +787,31 @@ async fn dispatch_user_message_failed_send_clears_loading_state() {
     assert!(app.last_send_at.is_none());
 }
 
+#[test]
+fn fixed_model_auto_thinking_skips_auto_model_router() {
+    let mut app = create_test_app();
+    app.auto_model = false;
+    app.model = "deepseek-v4-pro".to_string();
+    app.reasoning_effort = ReasoningEffort::Auto;
+
+    assert!(
+        !should_resolve_auto_model_selection(&app),
+        "fixed-model auto thinking must stay local instead of starting a hidden router request"
+    );
+}
+
+#[test]
+fn auto_model_still_uses_auto_model_router() {
+    let mut app = create_test_app();
+    app.auto_model = true;
+    app.reasoning_effort = ReasoningEffort::Auto;
+
+    assert!(
+        should_resolve_auto_model_selection(&app),
+        "auto model still needs the router to choose the concrete model"
+    );
+}
+
 fn init_git_repo() -> TempDir {
     let dir = tempfile::tempdir().expect("tempdir");