codex: normalize legacy service tier overrides

codex-rs/core/src/config/config_tests.rs

@@ -6509,6 +6509,28 @@ async fn explicit_null_service_tier_override_sets_fast_default_opt_out() -> std:
     Ok(())
 }
 
+#[tokio::test]
+async fn legacy_fast_service_tier_override_maps_to_priority_id() -> std::io::Result<()> {
+    let fixture = create_test_fixture()?;
+
+    let config = Config::load_from_base_config_with_overrides(
+        fixture.cfg.clone(),
+        ConfigOverrides {
+            cwd: Some(fixture.cwd_path()),
+            service_tier: Some(Some(ServiceTier::from(SERVICE_TIER_FAST_LEGACY))),
+            ..Default::default()
+        },
+        fixture.codex_home(),
+    )
+    .await?;
+
+    assert_eq!(
+        config.service_tier,
+        Some(ServiceTier::from(SERVICE_TIER_PRIORITY))
+    );
+    Ok(())
+}
+
 #[tokio::test]
 async fn service_tier_id_takes_precedence_over_deprecated_service_tier() -> std::io::Result<()> {
     let fixture = create_test_fixture()?;

codex-rs/core/src/config/mod.rs

@@ -2667,7 +2667,7 @@ impl Config {
         let deprecated_config_service_tier =
             cfg.service_tier.clone().map(normalize_deprecated_service_tier);
         let service_tier = match service_tier_override {
-            Some(Some(service_tier)) => Some(service_tier),
+            Some(Some(service_tier)) => Some(normalize_deprecated_service_tier(service_tier)),
             Some(None) => {
                 // Preserve explicit standard/clear intent after the nested override
                 // collapses into `Config.service_tier = None`.