handle Features::Personality gracefully

codex-rs/core/tests/suite/personality.rs

@@ -114,7 +114,7 @@ async fn base_instructions_override_disables_personality_template() {
     config.base_instructions = Some("override instructions".to_string());
 
     let model_info =
-        codex_core::test_support::construct_model_info_offline("gpt-5.3-codex", &config);
+        codex_core::test_support::construct_model_info_offline("exp-codex-personality", &config);
 
     assert_eq!(model_info.model_messages, None);
     assert_eq!(model_info.get_model_instructions(config.personality), "");
@@ -443,7 +443,7 @@ async fn user_turn_personality_same_value_does_not_add_update_message() -> anyho
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn instructions_are_empty_if_feature_disabled() -> anyhow::Result<()> {
+async fn instructions_use_default_if_feature_disabled() -> anyhow::Result<()> {
     let codex_home = TempDir::new().expect("create temp dir");
     let mut config = load_default_config_for_test(&codex_home).await;
     config
@@ -453,8 +453,13 @@ async fn instructions_are_empty_if_feature_disabled() -> anyhow::Result<()> {
     config.personality = Some(Personality::Friendly);
 
     let model_info =
-        codex_core::test_support::construct_model_info_offline("gpt-5.3-codex", &config);
-    assert_eq!(model_info.get_model_instructions(config.personality), "");
+        codex_core::test_support::construct_model_info_offline("exp-codex-personality", &config);
+    let default_instructions = model_info.get_model_instructions(/*personality*/ None);
+    assert!(!default_instructions.is_empty());
+    assert_eq!(
+        model_info.get_model_instructions(config.personality),
+        default_instructions
+    );
 
     Ok(())
 }
@@ -512,6 +517,11 @@ async fn user_turn_personality_skips_if_feature_disabled() -> anyhow::Result<()>
 
     let requests = resp_mock.requests();
     assert_eq!(requests.len(), 2, "expected two requests");
+    assert!(!requests[0].instructions_text().is_empty());
+    assert_eq!(
+        requests[0].instructions_text(),
+        requests[1].instructions_text()
+    );
     let request = requests
         .last()
         .expect("expected personality update request");

codex-rs/models-manager/src/model_info.rs

@@ -52,8 +52,19 @@ pub fn with_config_overrides(mut model: ModelInfo, config: &ModelsManagerConfig)
         };
     }
 
-    if config.base_instructions.is_some() || !config.personality_enabled {
+    if config.base_instructions.is_some() {
         model.model_messages = None;
+    } else if !config.personality_enabled
+        && let Some(model_messages) = model.model_messages.as_mut()
+    {
+        let personality_default = model_messages
+            .get_personality_message(/*personality*/ None)
+            .unwrap_or_default();
+        if let Some(instructions_template) = model_messages.instructions_template.as_mut() {
+            *instructions_template =
+                instructions_template.replace(PERSONALITY_PLACEHOLDER, &personality_default);
+        }
+        model_messages.instructions_variables = None;
     }
 
     model

codex-rs/models-manager/src/model_info_tests.rs

@@ -90,8 +90,16 @@ fn base_instructions_override_disables_model_messages() {
 }
 
 #[test]
-fn disabled_personality_clears_model_messages() {
-    let model = model_info_from_slug("gpt-5.2-codex");
+fn disabled_personality_uses_default_model_messages() {
+    let mut model = model_info_from_slug("gpt-5.2-codex");
+    model
+        .model_messages
+        .as_mut()
+        .expect("model messages")
+        .instructions_variables
+        .as_mut()
+        .expect("instruction variables")
+        .personality_default = Some("default personality".to_string());
     let config = ModelsManagerConfig {
         personality_enabled: false,
         ..Default::default()
@@ -99,7 +107,12 @@ fn disabled_personality_clears_model_messages() {
 
     let updated = with_config_overrides(model.clone(), &config);
     let mut expected = model;
-    expected.model_messages = None;
+    expected.model_messages = Some(ModelMessages {
+        instructions_template: Some(format!(
+            "{DEFAULT_PERSONALITY_HEADER}\n\ndefault personality\n\n{BASE_INSTRUCTIONS}"
+        )),
+        instructions_variables: None,
+    });
 
     assert_eq!(updated, expected);
 }