diff --git a/codex-rs/tui/src/app.rs b/codex-rs/tui/src/app.rs
index b7f2b9df6..554df264e 100644
--- a/codex-rs/tui/src/app.rs
+++ b/codex-rs/tui/src/app.rs
@@ -51,6 +51,7 @@ use crate::legacy_core::config::ConfigBuilder;
 use crate::legacy_core::config::ConfigOverrides;
 use crate::legacy_core::config::PermissionProfileSnapshot;
 use crate::legacy_core::config::edit::ConfigEditsBuilder;
+use crate::managed_new_thread_defaults::apply_managed_new_thread_defaults;
 use crate::model_catalog::ModelCatalog;
 use crate::model_migration::ModelMigrationOutcome;
 use crate::model_migration::migration_copy_for_models;
@@ -794,7 +795,18 @@ impl App {
             None => app_server.bootstrap(&config).await?,
         };
         let bootstrap_ms = bootstrap.duration.as_millis();
-        let mut model = bootstrap.default_model;
+        if matches!(
+            &session_selection,
+            SessionSelection::StartFresh | SessionSelection::Exit
+        ) {
+            apply_managed_new_thread_defaults(
+                &mut config,
+                app_server.managed_new_thread_defaults(),
+                &cli_kv_overrides,
+                &harness_overrides,
+            );
+        }
+        let mut model = config.model.clone().unwrap_or(bootstrap.default_model);
         let available_models = bootstrap.available_models;
         let remote_connection = crate::status::remote_connection::remote_connection_status_value(
             &app_server_target,
diff --git a/codex-rs/tui/src/app/session_lifecycle.rs b/codex-rs/tui/src/app/session_lifecycle.rs
index a961658c8..b4bbbd231 100644
--- a/codex-rs/tui/src/app/session_lifecycle.rs
+++ b/codex-rs/tui/src/app/session_lifecycle.rs
@@ -539,7 +539,13 @@ impl App {
         self.refresh_in_memory_config_from_disk_best_effort("starting a new thread")
             .await;
         let model = self.chat_widget.current_model().to_string();
-        let config = self.fresh_session_config();
+        let mut config = self.fresh_session_config();
+        apply_managed_new_thread_defaults(
+            &mut config,
+            app_server.managed_new_thread_defaults(),
+            &self.cli_kv_overrides,
+            &self.harness_overrides,
+        );
         let summary = session_summary(
             self.chat_widget.token_usage(),
             self.chat_widget.thread_id(),
diff --git a/codex-rs/tui/src/app_server_session.rs b/codex-rs/tui/src/app_server_session.rs
index 0405bb75a..aeca17091 100644
--- a/codex-rs/tui/src/app_server_session.rs
+++ b/codex-rs/tui/src/app_server_session.rs
@@ -24,6 +24,7 @@ use codex_app_server_protocol::AskForApproval;
 use codex_app_server_protocol::AuthMode;
 use codex_app_server_protocol::ClientRequest;
 use codex_app_server_protocol::ConfigBatchWriteParams;
+use codex_app_server_protocol::ConfigRequirementsReadResponse;
 use codex_app_server_protocol::ConfigWriteResponse;
 use codex_app_server_protocol::ExternalAgentConfigDetectParams;
 use codex_app_server_protocol::ExternalAgentConfigDetectResponse;
@@ -39,6 +40,7 @@ use codex_app_server_protocol::MemoryResetResponse;
 use codex_app_server_protocol::Model as ApiModel;
 use codex_app_server_protocol::ModelListParams;
 use codex_app_server_protocol::ModelListResponse;
+use codex_app_server_protocol::NewThreadModelDefaults;
 use codex_app_server_protocol::RateLimitSnapshot;
 use codex_app_server_protocol::RequestId;
 use codex_app_server_protocol::ReviewDelivery;
@@ -178,6 +180,7 @@ pub(crate) struct AppServerSession {
     thread_settings_update_supported: bool,
     default_model: Option<String>,
     available_models: Vec<ModelPreset>,
+    managed_new_thread_defaults: Option<NewThreadModelDefaults>,
     external_agent_config_import_completion_pending: AtomicBool,
 }
 
@@ -222,6 +225,7 @@ impl AppServerSession {
             thread_settings_update_supported: true,
             default_model: None,
             available_models: Vec::new(),
+            managed_new_thread_defaults: None,
             external_agent_config_import_completion_pending: AtomicBool::new(false),
         }
     }
@@ -260,6 +264,21 @@ impl AppServerSession {
     pub(crate) async fn bootstrap(&mut self, config: &Config) -> Result<AppServerBootstrap> {
         let started_at = Instant::now();
         let account = self.read_account().await?;
+        let requirements_request_id = self.next_request_id();
+        let requirements: ConfigRequirementsReadResponse = self
+            .client
+            .request_typed(ClientRequest::ConfigRequirementsRead {
+                request_id: requirements_request_id,
+                params: None,
+            })
+            .await
+            .map_err(|err| {
+                bootstrap_request_error("configRequirements/read failed during TUI bootstrap", err)
+            })?;
+        self.managed_new_thread_defaults = requirements
+            .requirements
+            .and_then(|requirements| requirements.models)
+            .and_then(|models| models.new_thread);
         let model_request_id = self.next_request_id();
         let models: ModelListResponse = self
             .client
@@ -350,6 +369,10 @@ impl AppServerSession {
         })
     }
 
+    pub(crate) fn managed_new_thread_defaults(&self) -> Option<&NewThreadModelDefaults> {
+        self.managed_new_thread_defaults.as_ref()
+    }
+
     /// Fetches the current account info without refreshing the auth token.
     ///
     /// Used by both `bootstrap` (to populate the initial UI) and `get_login_status`
diff --git a/codex-rs/tui/src/lib.rs b/codex-rs/tui/src/lib.rs
index 897f1ec83..96f442a88 100644
--- a/codex-rs/tui/src/lib.rs
+++ b/codex-rs/tui/src/lib.rs
@@ -137,6 +137,7 @@ mod line_truncation;
 pub(crate) mod live_wrap;
 pub use live_wrap::RowBuilder;
 mod local_chatgpt_auth;
+mod managed_new_thread_defaults;
 mod markdown;
 mod markdown_render;
 mod markdown_stream;
diff --git a/codex-rs/tui/src/managed_new_thread_defaults.rs b/codex-rs/tui/src/managed_new_thread_defaults.rs
new file mode 100644
index 000000000..ad9ad30b8
--- /dev/null
+++ b/codex-rs/tui/src/managed_new_thread_defaults.rs
@@ -0,0 +1,51 @@
+use crate::legacy_core::config::Config;
+use crate::legacy_core::config::ConfigOverrides;
+use codex_app_server_protocol::NewThreadModelDefaults;
+use codex_protocol::config_types::ServiceTier;
+use toml::Value as TomlValue;
+
+pub(crate) fn apply_managed_new_thread_defaults(
+    config: &mut Config,
+    defaults: Option<&NewThreadModelDefaults>,
+    cli_kv_overrides: &[(String, TomlValue)],
+    harness_overrides: &ConfigOverrides,
+) {
+    let Some(defaults) = defaults else {
+        return;
+    };
+    // Managed values are defaults rather than enforcement. Preserve explicit launch choices from
+    // dedicated flags such as `-m` (`harness_overrides`) and generic `-c key=value` settings
+    // (`cli_kv_overrides`), then fill only the fields that were not selected for this invocation.
+    // Model and reasoning effort are a compatibility-sensitive pair, so an explicit override of
+    // either opts out of both managed values. For example, `codex -m gpt-5.4` keeps that model and
+    // its existing/default effort, while `-c model_reasoning_effort=low` does not switch to the
+    // managed model. Service tier remains independent and is resolved against the selected model
+    // before the thread starts.
+    let has_cli_override = |key: &str| cli_kv_overrides.iter().any(|(path, _value)| path == key);
+    let has_explicit_model_settings = harness_overrides.model.is_some()
+        || has_cli_override("model")
+        || has_cli_override("model_reasoning_effort");
+
+    if !has_explicit_model_settings && let Some(model) = defaults.model.as_ref() {
+        config.model = Some(model.clone());
+    }
+    if !has_explicit_model_settings
+        && let Some(reasoning_effort) = defaults.model_reasoning_effort.as_ref()
+    {
+        config.model_reasoning_effort = Some(reasoning_effort.clone());
+    }
+    if harness_overrides.service_tier.is_none()
+        && !has_cli_override("service_tier")
+        && let Some(service_tier) = defaults.service_tier.as_ref()
+    {
+        config.service_tier = Some(
+            ServiceTier::from_request_value(service_tier)
+                .map(|tier| tier.request_value().to_string())
+                .unwrap_or_else(|| service_tier.clone()),
+        );
+    }
+}
+
+#[cfg(test)]
+#[path = "managed_new_thread_defaults_tests.rs"]
+mod tests;
diff --git a/codex-rs/tui/src/managed_new_thread_defaults_tests.rs b/codex-rs/tui/src/managed_new_thread_defaults_tests.rs
new file mode 100644
index 000000000..d73d6459d
--- /dev/null
+++ b/codex-rs/tui/src/managed_new_thread_defaults_tests.rs
@@ -0,0 +1,110 @@
+use super::*;
+use crate::legacy_core::config::ConfigBuilder;
+use codex_protocol::openai_models::ReasoningEffort;
+use pretty_assertions::assert_eq;
+
+async fn test_config() -> Config {
+    let codex_home = tempfile::tempdir().expect("tempdir").keep();
+    ConfigBuilder::default()
+        .codex_home(codex_home)
+        .build()
+        .await
+        .expect("config")
+}
+
+fn defaults() -> NewThreadModelDefaults {
+    NewThreadModelDefaults {
+        model: Some("managed-model".to_string()),
+        model_reasoning_effort: Some(ReasoningEffort::High),
+        service_tier: Some("fast".to_string()),
+    }
+}
+
+#[tokio::test]
+async fn applies_managed_defaults_to_a_new_thread_config() {
+    let mut actual = test_config().await;
+    actual.model = Some("configured-model".to_string());
+    actual.model_reasoning_effort = Some(ReasoningEffort::Low);
+    actual.service_tier = Some("flex".to_string());
+    let mut expected = actual.clone();
+    expected.model = Some("managed-model".to_string());
+    expected.model_reasoning_effort = Some(ReasoningEffort::High);
+    expected.service_tier = Some(ServiceTier::Fast.request_value().to_string());
+
+    apply_managed_new_thread_defaults(
+        &mut actual,
+        Some(&defaults()),
+        &[],
+        &ConfigOverrides::default(),
+    );
+
+    assert_eq!(actual, expected);
+}
+
+#[tokio::test]
+async fn explicit_model_skips_managed_model_and_reasoning_effort() {
+    let mut actual = test_config().await;
+    actual.model = Some("explicit-model".to_string());
+    actual.model_reasoning_effort = None;
+    actual.service_tier = Some("flex".to_string());
+    let mut expected = actual.clone();
+    expected.service_tier = Some(ServiceTier::Fast.request_value().to_string());
+    let harness_overrides = ConfigOverrides {
+        model: Some("explicit-model".to_string()),
+        ..ConfigOverrides::default()
+    };
+
+    apply_managed_new_thread_defaults(&mut actual, Some(&defaults()), &[], &harness_overrides);
+
+    assert_eq!(actual, expected);
+}
+
+#[tokio::test]
+async fn explicit_reasoning_effort_skips_managed_model_and_reasoning_effort() {
+    let mut actual = test_config().await;
+    actual.model = Some("configured-model".to_string());
+    actual.model_reasoning_effort = Some(ReasoningEffort::Low);
+    actual.service_tier = Some("flex".to_string());
+    let mut expected = actual.clone();
+    expected.service_tier = Some(ServiceTier::Fast.request_value().to_string());
+    let cli_kv_overrides = vec![(
+        "model_reasoning_effort".to_string(),
+        TomlValue::String("low".to_string()),
+    )];
+
+    apply_managed_new_thread_defaults(
+        &mut actual,
+        Some(&defaults()),
+        &cli_kv_overrides,
+        &ConfigOverrides::default(),
+    );
+
+    assert_eq!(actual, expected);
+}
+
+#[tokio::test]
+async fn explicit_launch_overrides_take_precedence() {
+    let mut actual = test_config().await;
+    actual.model = Some("explicit-model".to_string());
+    actual.model_reasoning_effort = Some(ReasoningEffort::Low);
+    actual.service_tier = Some("flex".to_string());
+    let expected = actual.clone();
+    let cli_kv_overrides = vec![(
+        "model_reasoning_effort".to_string(),
+        TomlValue::String("low".to_string()),
+    )];
+    let harness_overrides = ConfigOverrides {
+        model: Some("explicit-model".to_string()),
+        service_tier: Some(Some("flex".to_string())),
+        ..ConfigOverrides::default()
+    };
+
+    apply_managed_new_thread_defaults(
+        &mut actual,
+        Some(&defaults()),
+        &cli_kv_overrides,
+        &harness_overrides,
+    );
+
+    assert_eq!(actual, expected);
+}