diff --git a/codex-rs/app-server-protocol/src/protocol/v2.rs b/codex-rs/app-server-protocol/src/protocol/v2.rs
index ea019d3b61..d652bb5372 100644
--- a/codex-rs/app-server-protocol/src/protocol/v2.rs
+++ b/codex-rs/app-server-protocol/src/protocol/v2.rs
@@ -30,6 +30,10 @@ use codex_protocol::config_types::ServiceTier as CoreServiceTier;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::config_types::WebSearchMode;
 use codex_protocol::config_types::WebSearchToolConfig;
+use codex_protocol::config_types::flex_service_tier;
+use codex_protocol::config_types::is_flex_service_tier;
+use codex_protocol::config_types::is_priority_service_tier;
+use codex_protocol::config_types::priority_service_tier;
 use codex_protocol::items::AgentMessageContent as CoreAgentMessageContent;
 use codex_protocol::items::TurnItem as CoreTurnItem;
 use codex_protocol::mcp::CallToolResult as CoreMcpCallToolResult;
@@ -129,36 +133,26 @@ pub enum ServiceTier {
     Flex,
 }
 
-impl ServiceTier {
-    pub fn fast() -> Self {
-        Self::Fast
+pub fn service_tier_to_core(service_tier: ServiceTier) -> CoreServiceTier {
+    match service_tier {
+        ServiceTier::Fast => priority_service_tier(),
+        ServiceTier::Flex => flex_service_tier(),
     }
+}
 
-    pub fn flex() -> Self {
-        Self::Flex
-    }
-
-    pub fn to_core(self) -> CoreServiceTier {
-        match self {
-            Self::Fast => CoreServiceTier::priority(),
-            Self::Flex => CoreServiceTier::flex(),
-        }
-    }
-
-    pub fn from_core(service_tier: &CoreServiceTier) -> Option<Self> {
-        if service_tier.is_priority() {
-            Some(Self::Fast)
-        } else if service_tier.is_flex() {
-            Some(Self::Flex)
-        } else {
-            None
-        }
+pub fn service_tier_from_core(service_tier: &CoreServiceTier) -> Option<ServiceTier> {
+    if is_priority_service_tier(service_tier) {
+        Some(ServiceTier::Fast)
+    } else if is_flex_service_tier(service_tier) {
+        Some(ServiceTier::Flex)
+    } else {
+        None
     }
 }
 
 impl From<ServiceTier> for CoreServiceTier {
     fn from(value: ServiceTier) -> Self {
-        value.to_core()
+        service_tier_to_core(value)
     }
 }
 
diff --git a/codex-rs/app-server/src/codex_message_processor.rs b/codex-rs/app-server/src/codex_message_processor.rs
index 72dbf019d2..5bc7c30e7c 100644
--- a/codex-rs/app-server/src/codex_message_processor.rs
+++ b/codex-rs/app-server/src/codex_message_processor.rs
@@ -241,6 +241,8 @@ use codex_app_server_protocol::WindowsSandboxSetupMode;
 use codex_app_server_protocol::WindowsSandboxSetupStartParams;
 use codex_app_server_protocol::WindowsSandboxSetupStartResponse;
 use codex_app_server_protocol::build_turns_from_rollout_items;
+use codex_app_server_protocol::service_tier_from_core;
+use codex_app_server_protocol::service_tier_to_core;
 use codex_arg0::Arg0DispatchPaths;
 use codex_backend_client::AddCreditsNudgeCreditType as BackendAddCreditsNudgeCreditType;
 use codex_backend_client::Client as BackendClient;
@@ -2920,7 +2922,10 @@ impl CodexMessageProcessor {
                 thread: thread.clone(),
                 model: config_snapshot.model,
                 model_provider: config_snapshot.model_provider_id,
-                service_tier: config_snapshot.service_tier.as_ref().and_then(ServiceTier::from_core),
+                service_tier: config_snapshot
+                    .service_tier
+                    .as_ref()
+                    .and_then(service_tier_from_core),
                 cwd: config_snapshot.cwd,
                 instruction_sources,
                 approval_policy: config_snapshot.approval_policy.into(),
@@ -3023,7 +3028,7 @@ impl CodexMessageProcessor {
     fn resolve_service_tier_override(
         service_tier: Option<Option<ServiceTier>>,
     ) -> Option<Option<codex_protocol::config_types::ServiceTier>> {
-        service_tier.map(|service_tier| service_tier.map(ServiceTier::to_core))
+        service_tier.map(|service_tier| service_tier.map(service_tier_to_core))
     }
 
     async fn thread_archive(&self, request_id: ConnectionRequestId, params: ThreadArchiveParams) {
@@ -4509,7 +4514,7 @@ impl CodexMessageProcessor {
                     service_tier: session_configured
                         .service_tier
                         .as_ref()
-                        .and_then(ServiceTier::from_core),
+                        .and_then(service_tier_from_core),
                     cwd: session_configured.cwd,
                     instruction_sources,
                     approval_policy: session_configured.approval_policy.into(),
@@ -5147,7 +5152,7 @@ impl CodexMessageProcessor {
                 service_tier: session_configured
                     .service_tier
                     .as_ref()
-                    .and_then(ServiceTier::from_core),
+                    .and_then(service_tier_from_core),
                 cwd: session_configured.cwd,
                 instruction_sources,
                 approval_policy: session_configured.approval_policy.into(),
@@ -8505,7 +8510,7 @@ async fn handle_pending_thread_resume_request(
         thread,
         model,
         model_provider: model_provider_id,
-        service_tier: service_tier.as_ref().and_then(ServiceTier::from_core),
+        service_tier: service_tier.as_ref().and_then(service_tier_from_core),
         cwd,
         instruction_sources,
         approval_policy: approval_policy.into(),
@@ -8675,13 +8680,13 @@ fn collect_resume_override_mismatches(
         ));
     }
     if let Some(requested_service_tier) = request.service_tier.as_ref()
-        && requested_service_tier.map(ServiceTier::to_core).as_ref()
+        && requested_service_tier.map(service_tier_to_core).as_ref()
             != config_snapshot.service_tier.as_ref()
     {
         let active_service_tier = config_snapshot
             .service_tier
             .as_ref()
-            .and_then(ServiceTier::from_core);
+            .and_then(service_tier_from_core);
         mismatch_details.push(format!(
             "service_tier requested={requested_service_tier:?} active={active_service_tier:?}"
         ));
@@ -10065,7 +10070,7 @@ mod tests {
     use codex_model_provider_info::ModelProviderInfo;
     use codex_model_provider_info::WireApi;
     use codex_protocol::ThreadId;
-    use codex_protocol::config_types::ServiceTier as CoreServiceTier;
+    use codex_protocol::config_types::flex_service_tier;
     use codex_protocol::openai_models::ReasoningEffort;
     use codex_protocol::permissions::FileSystemAccessMode;
     use codex_protocol::permissions::FileSystemPath;
@@ -10546,7 +10551,7 @@ mod tests {
             path: None,
             model: None,
             model_provider: None,
-            service_tier: Some(Some(ServiceTier::fast())),
+            service_tier: Some(Some(ServiceTier::Fast)),
             cwd: None,
             approval_policy: None,
             approvals_reviewer: None,
@@ -10562,7 +10567,7 @@ mod tests {
         let config_snapshot = ThreadConfigSnapshot {
             model: "gpt-5".to_string(),
             model_provider_id: "openai".to_string(),
-            service_tier: Some(CoreServiceTier::flex()),
+            service_tier: Some(flex_service_tier()),
             approval_policy: codex_protocol::protocol::AskForApproval::OnRequest,
             approvals_reviewer: codex_protocol::config_types::ApprovalsReviewer::User,
             permission_profile: codex_protocol::models::PermissionProfile::Disabled,
diff --git a/codex-rs/app-server/src/models.rs b/codex-rs/app-server/src/models.rs
index 74c4f2d066..13f8ece9ce 100644
--- a/codex-rs/app-server/src/models.rs
+++ b/codex-rs/app-server/src/models.rs
@@ -5,6 +5,7 @@ use codex_app_server_protocol::ModelUpgradeInfo;
 use codex_app_server_protocol::ReasoningEffortOption;
 use codex_core::ThreadManager;
 use codex_models_manager::manager::RefreshStrategy;
+use codex_protocol::config_types::is_priority_service_tier;
 use codex_protocol::openai_models::ModelPreset;
 use codex_protocol::openai_models::ModelServiceTier as CoreModelServiceTier;
 use codex_protocol::openai_models::ReasoningEffortPreset;
@@ -52,7 +53,7 @@ fn model_from_preset(preset: ModelPreset) -> Model {
 fn legacy_additional_speed_tiers(service_tiers: &[CoreModelServiceTier]) -> Vec<String> {
     service_tiers
         .iter()
-        .filter_map(|tier| tier.id.is_priority().then_some("fast".to_string()))
+        .filter_map(|tier| is_priority_service_tier(&tier.id).then_some("fast".to_string()))
         .collect()
 }
 
diff --git a/codex-rs/app-server/tests/suite/v2/model_list.rs b/codex-rs/app-server/tests/suite/v2/model_list.rs
index ffb360e9c6..d18c73ed48 100644
--- a/codex-rs/app-server/tests/suite/v2/model_list.rs
+++ b/codex-rs/app-server/tests/suite/v2/model_list.rs
@@ -12,6 +12,7 @@ use codex_app_server_protocol::ModelListResponse;
 use codex_app_server_protocol::ModelUpgradeInfo;
 use codex_app_server_protocol::ReasoningEffortOption;
 use codex_app_server_protocol::RequestId;
+use codex_protocol::config_types::is_priority_service_tier;
 use codex_protocol::openai_models::ModelPreset;
 use codex_protocol::openai_models::ModelServiceTier as CoreModelServiceTier;
 use pretty_assertions::assert_eq;
@@ -59,7 +60,7 @@ fn model_from_preset(preset: &ModelPreset) -> Model {
 fn legacy_additional_speed_tiers(service_tiers: &[CoreModelServiceTier]) -> Vec<String> {
     service_tiers
         .iter()
-        .filter_map(|tier| tier.id.is_priority().then_some("fast".to_string()))
+        .filter_map(|tier| is_priority_service_tier(&tier.id).then_some("fast".to_string()))
         .collect()
 }
 
diff --git a/codex-rs/app-server/tests/suite/v2/thread_start.rs b/codex-rs/app-server/tests/suite/v2/thread_start.rs
index cca4d085a6..8f88943109 100644
--- a/codex-rs/app-server/tests/suite/v2/thread_start.rs
+++ b/codex-rs/app-server/tests/suite/v2/thread_start.rs
@@ -340,7 +340,7 @@ async fn thread_start_accepts_flex_service_tier() -> Result<()> {
 
     let req_id = mcp
         .send_thread_start_request(ThreadStartParams {
-            service_tier: Some(Some(ServiceTier::flex())),
+            service_tier: Some(Some(ServiceTier::Flex)),
             ..Default::default()
         })
         .await?;
@@ -352,7 +352,7 @@ async fn thread_start_accepts_flex_service_tier() -> Result<()> {
     .await??;
     let ThreadStartResponse { service_tier, .. } = to_response::<ThreadStartResponse>(resp)?;
 
-    assert_eq!(service_tier, Some(ServiceTier::flex()));
+    assert_eq!(service_tier, Some(ServiceTier::Flex));
     Ok(())
 }
 
diff --git a/codex-rs/core/src/client_common_tests.rs b/codex-rs/core/src/client_common_tests.rs
index 732ae63e4d..e9e5dd7cfa 100644
--- a/codex-rs/core/src/client_common_tests.rs
+++ b/codex-rs/core/src/client_common_tests.rs
@@ -3,6 +3,7 @@ use codex_api::ResponsesApiRequest;
 use codex_api::TextControls;
 use codex_api::create_text_param_for_request;
 use codex_protocol::config_types::ServiceTier;
+use codex_protocol::config_types::flex_service_tier;
 use codex_protocol::models::FunctionCallOutputPayload;
 use pretty_assertions::assert_eq;
 
@@ -155,7 +156,7 @@ fn serializes_flex_service_tier_when_set() {
         stream: true,
         include: vec![],
         prompt_cache_key: None,
-        service_tier: Some(ServiceTier::flex().to_string()),
+        service_tier: Some(flex_service_tier().to_string()),
         text: None,
         client_metadata: None,
     };
diff --git a/codex-rs/core/src/config/mod.rs b/codex-rs/core/src/config/mod.rs
index e6a5bf09a9..eeb5bebf1d 100644
--- a/codex-rs/core/src/config/mod.rs
+++ b/codex-rs/core/src/config/mod.rs
@@ -89,6 +89,7 @@ use codex_protocol::config_types::Verbosity;
 use codex_protocol::config_types::WebSearchConfig;
 use codex_protocol::config_types::WebSearchMode;
 use codex_protocol::config_types::WindowsSandboxLevel;
+use codex_protocol::config_types::is_priority_service_tier;
 use codex_protocol::models::ActivePermissionProfile;
 use codex_protocol::models::ActivePermissionProfileModification;
 use codex_protocol::models::PermissionProfile;
@@ -2659,7 +2660,10 @@ impl Config {
             None => config_profile.service_tier.or(cfg.service_tier),
         };
         let service_tier = match service_tier {
-            Some(service_tier) if service_tier.is_priority() && !features.enabled(Feature::FastMode) => {
+            Some(service_tier)
+                if is_priority_service_tier(&service_tier)
+                    && !features.enabled(Feature::FastMode) =>
+            {
                 None
             }
             other => other,
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index 2393eab5b3..37b136a617 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -314,6 +314,7 @@ use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::WindowsSandboxLevel;
+use codex_protocol::config_types::priority_service_tier;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseInputItem;
 use codex_protocol::models::ResponseItem;
@@ -788,7 +789,7 @@ fn get_service_tier(
 
     account_plan_type
         .is_some_and(is_enterprise_default_service_tier_plan)
-        .then(ServiceTier::priority)
+        .then(priority_service_tier)
 }
 
 fn is_enterprise_default_service_tier_plan(plan_type: AccountPlanType) -> bool {
diff --git a/codex-rs/core/src/session/tests.rs b/codex-rs/core/src/session/tests.rs
index 4b7fa855ce..9376127f52 100644
--- a/codex-rs/core/src/session/tests.rs
+++ b/codex-rs/core/src/session/tests.rs
@@ -34,6 +34,7 @@ use codex_protocol::ThreadId;
 use codex_protocol::account::PlanType as AccountPlanType;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::TrustLevel;
+use codex_protocol::config_types::priority_service_tier;
 use codex_protocol::exec_output::ExecToolCallOutput;
 use codex_protocol::models::FileSystemPermissions;
 use codex_protocol::models::FunctionCallOutputBody;
@@ -2801,7 +2802,7 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
             Some(AccountPlanType::Enterprise),
             /*fast_mode_enabled*/ true,
         ),
-        Some(ServiceTier::priority())
+        Some(priority_service_tier())
     );
     assert_eq!(
         get_service_tier(
@@ -2810,7 +2811,7 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
             Some(AccountPlanType::EnterpriseCbpUsageBased),
             /*fast_mode_enabled*/ true,
         ),
-        Some(ServiceTier::priority())
+        Some(priority_service_tier())
     );
     assert_eq!(
         get_service_tier(
@@ -2819,7 +2820,7 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
             Some(AccountPlanType::Business),
             /*fast_mode_enabled*/ true,
         ),
-        Some(ServiceTier::priority())
+        Some(priority_service_tier())
     );
     assert_eq!(
         get_service_tier(
@@ -2828,7 +2829,7 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
             Some(AccountPlanType::Team),
             /*fast_mode_enabled*/ true,
         ),
-        Some(ServiceTier::priority())
+        Some(priority_service_tier())
     );
     assert_eq!(
         get_service_tier(
@@ -2837,7 +2838,7 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
             Some(AccountPlanType::SelfServeBusinessUsageBased),
             /*fast_mode_enabled*/ true,
         ),
-        Some(ServiceTier::priority())
+        Some(priority_service_tier())
     );
 }
 
diff --git a/codex-rs/core/tests/suite/agent_websocket.rs b/codex-rs/core/tests/suite/agent_websocket.rs
index f765c3c137..56dae67672 100644
--- a/codex-rs/core/tests/suite/agent_websocket.rs
+++ b/codex-rs/core/tests/suite/agent_websocket.rs
@@ -1,6 +1,7 @@
 use anyhow::Result;
 use codex_features::Feature;
 use codex_protocol::config_types::ServiceTier;
+use codex_protocol::config_types::priority_service_tier;
 use core_test_support::responses::WebSocketConnectionConfig;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
@@ -269,7 +270,7 @@ async fn websocket_v2_first_turn_uses_updated_fast_tier_after_startup_prewarm()
     assert_eq!(warmup["generate"].as_bool(), Some(false));
     assert_eq!(warmup.get("service_tier"), None);
 
-    test.submit_turn_with_service_tier("hello", Some(ServiceTier::priority()))
+    test.submit_turn_with_service_tier("hello", Some(priority_service_tier()))
         .await?;
 
     assert_eq!(server.handshakes().len(), 1);
@@ -313,7 +314,7 @@ async fn websocket_v2_first_turn_drops_fast_tier_after_startup_prewarm() -> Resu
             .features
             .enable(Feature::ResponsesWebsocketsV2)
             .expect("test config should allow feature update");
-        config.service_tier = Some(ServiceTier::priority());
+        config.service_tier = Some(priority_service_tier());
     });
     let test = builder.build_with_websocket_server(&server).await?;
 
@@ -385,7 +386,7 @@ async fn websocket_v2_next_turn_uses_updated_service_tier() -> Result<()> {
     assert_eq!(warmup["generate"].as_bool(), Some(false));
     assert_eq!(warmup.get("service_tier"), None);
 
-    test.submit_turn_with_service_tier("first", Some(ServiceTier::priority()))
+    test.submit_turn_with_service_tier("first", Some(priority_service_tier()))
         .await?;
     test.submit_turn_with_service_tier("second", /*service_tier*/ None)
         .await?;
diff --git a/codex-rs/core/tests/suite/model_switching.rs b/codex-rs/core/tests/suite/model_switching.rs
index aa8d6d8ceb..173a87ae91 100644
--- a/codex-rs/core/tests/suite/model_switching.rs
+++ b/codex-rs/core/tests/suite/model_switching.rs
@@ -5,6 +5,8 @@ use codex_login::CodexAuth;
 use codex_models_manager::manager::RefreshStrategy;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::ServiceTier;
+use codex_protocol::config_types::flex_service_tier;
+use codex_protocol::config_types::priority_service_tier;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::openai_models::ConfigShellToolType;
 use codex_protocol::openai_models::InputModality;
@@ -297,7 +299,7 @@ async fn service_tier_change_is_applied_on_next_http_turn() -> Result<()> {
 
     let test = test_codex().build(&server).await?;
 
-    test.submit_turn_with_service_tier("fast turn", Some(ServiceTier::priority()))
+    test.submit_turn_with_service_tier("fast turn", Some(priority_service_tier()))
         .await?;
     test.submit_turn_with_service_tier("standard turn", /*service_tier*/ None)
         .await?;
@@ -323,7 +325,7 @@ async fn flex_service_tier_is_applied_to_http_turn() -> Result<()> {
 
     let test = test_codex().build(&server).await?;
 
-    test.submit_turn_with_service_tier("flex turn", Some(ServiceTier::flex()))
+    test.submit_turn_with_service_tier("flex turn", Some(flex_service_tier()))
         .await?;
 
     let request = resp_mock.single_request();
diff --git a/codex-rs/memories/write/src/startup_tests.rs b/codex-rs/memories/write/src/startup_tests.rs
index 1b1053fa44..98befeee37 100644
--- a/codex-rs/memories/write/src/startup_tests.rs
+++ b/codex-rs/memories/write/src/startup_tests.rs
@@ -4,6 +4,7 @@ use codex_git_utils::diff_since_latest_init;
 use codex_git_utils::reset_git_repository;
 use codex_protocol::ThreadId;
 use codex_protocol::config_types::ServiceTier;
+use codex_protocol::config_types::priority_service_tier;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
@@ -253,14 +254,14 @@ async fn memories_startup_phase1_uses_live_thread_service_tier() -> anyhow::Resu
             model: None,
             effort: None,
             summary: None,
-            service_tier: Some(Some(ServiceTier::priority())),
+            service_tier: Some(Some(priority_service_tier())),
             collaboration_mode: None,
             personality: None,
         })
         .await?;
 
-    let config_snapshot = wait_for_service_tier(&test, Some(ServiceTier::priority())).await?;
-    assert_eq!(config_snapshot.service_tier, Some(ServiceTier::priority()));
+    let config_snapshot = wait_for_service_tier(&test, Some(priority_service_tier())).await?;
+    assert_eq!(config_snapshot.service_tier, Some(priority_service_tier()));
 
     let context = crate::runtime::MemoryStartupContext::new(
         Arc::clone(&test.thread_manager),
@@ -277,7 +278,7 @@ async fn memories_startup_phase1_uses_live_thread_service_tier() -> anyhow::Resu
             ReasoningEffort::Low,
         )
         .await;
-    assert_eq!(request_context.service_tier, Some(ServiceTier::priority()));
+    assert_eq!(request_context.service_tier, Some(priority_service_tier()));
 
     shutdown_test_codex(&test).await?;
     Ok(())
diff --git a/codex-rs/model-provider/src/amazon_bedrock/catalog.rs b/codex-rs/model-provider/src/amazon_bedrock/catalog.rs
index 47d2c75619..0686bcc076 100644
--- a/codex-rs/model-provider/src/amazon_bedrock/catalog.rs
+++ b/codex-rs/model-provider/src/amazon_bedrock/catalog.rs
@@ -1,7 +1,7 @@
 use codex_models_manager::model_info::BASE_INSTRUCTIONS;
 use codex_protocol::config_types::ReasoningSummary;
-use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Verbosity;
+use codex_protocol::config_types::priority_service_tier;
 use codex_protocol::openai_models::ApplyPatchToolType;
 use codex_protocol::openai_models::ConfigShellToolType;
 use codex_protocol::openai_models::InputModality;
@@ -49,7 +49,7 @@ fn gpt_5_4_cmb_bedrock_model(priority: i32) -> ModelInfo {
         supported_in_api: true,
         priority,
         service_tiers: vec![ModelServiceTier {
-            id: ServiceTier::priority(),
+            id: priority_service_tier(),
             name: "Fast".to_string(),
             description: "Fast tier".to_string(),
         }],
diff --git a/codex-rs/protocol/src/config_types.rs b/codex-rs/protocol/src/config_types.rs
index b4433aeae2..eb54ab8a25 100644
--- a/codex-rs/protocol/src/config_types.rs
+++ b/codex-rs/protocol/src/config_types.rs
@@ -355,47 +355,41 @@ pub const SERVICE_TIER_FLEX: &str = "flex";
 #[ts(type = "string")]
 pub struct ServiceTier(String);
 
-impl ServiceTier {
-    pub fn new(id: impl Into<String>) -> Self {
-        Self(id.into())
-    }
+pub fn priority_service_tier() -> ServiceTier {
+    ServiceTier::from(SERVICE_TIER_PRIORITY)
+}
 
-    pub fn as_str(&self) -> &str {
-        self.0.as_str()
-    }
+pub fn flex_service_tier() -> ServiceTier {
+    ServiceTier::from(SERVICE_TIER_FLEX)
+}
 
-    pub fn priority() -> Self {
-        Self::new(SERVICE_TIER_PRIORITY)
-    }
+pub fn service_tier_id(service_tier: &ServiceTier) -> &str {
+    service_tier.as_ref()
+}
 
-    pub fn flex() -> Self {
-        Self::new(SERVICE_TIER_FLEX)
-    }
+pub fn is_priority_service_tier(service_tier: &ServiceTier) -> bool {
+    service_tier_id(service_tier) == SERVICE_TIER_PRIORITY
+}
 
-    pub fn is_priority(&self) -> bool {
-        self.as_str() == SERVICE_TIER_PRIORITY
-    }
-
-    pub fn is_flex(&self) -> bool {
-        self.as_str() == SERVICE_TIER_FLEX
-    }
+pub fn is_flex_service_tier(service_tier: &ServiceTier) -> bool {
+    service_tier_id(service_tier) == SERVICE_TIER_FLEX
 }
 
 impl AsRef<str> for ServiceTier {
     fn as_ref(&self) -> &str {
-        self.as_str()
+        self.0.as_str()
     }
 }
 
 impl From<&str> for ServiceTier {
     fn from(value: &str) -> Self {
-        Self::new(value)
+        Self(value.to_string())
     }
 }
 
 impl From<String> for ServiceTier {
     fn from(value: String) -> Self {
-        Self::new(value)
+        Self(value)
     }
 }
 
@@ -407,7 +401,7 @@ impl From<ServiceTier> for String {
 
 impl std::fmt::Display for ServiceTier {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.write_str(self.as_str())
+        f.write_str(service_tier_id(self))
     }
 }
 
diff --git a/codex-rs/protocol/src/openai_models.rs b/codex-rs/protocol/src/openai_models.rs
index 6f2daccea9..5afbee4098 100644
--- a/codex-rs/protocol/src/openai_models.rs
+++ b/codex-rs/protocol/src/openai_models.rs
@@ -19,6 +19,7 @@ use crate::config_types::Personality;
 use crate::config_types::ReasoningSummary;
 use crate::config_types::ServiceTier;
 use crate::config_types::Verbosity;
+use crate::config_types::priority_service_tier;
 
 const PERSONALITY_PLACEHOLDER: &str = "{{ personality }}";
 
@@ -469,19 +470,24 @@ impl From<ModelInfo> for ModelPreset {
     }
 }
 
+pub fn model_supports_service_tier(model: &ModelPreset, service_tier: &ServiceTier) -> bool {
+    model
+        .service_tiers
+        .iter()
+        .any(|tier| tier.id == *service_tier)
+}
+
+pub fn model_service_tier<'a>(
+    model: &'a ModelPreset,
+    service_tier: &ServiceTier,
+) -> Option<&'a ModelServiceTier> {
+    model
+        .service_tiers
+        .iter()
+        .find(|tier| tier.id == *service_tier)
+}
+
 impl ModelPreset {
-    pub fn supports_service_tier(&self, service_tier: &ServiceTier) -> bool {
-        self.service_tiers
-            .iter()
-            .any(|tier| tier.id == *service_tier)
-    }
-
-    pub fn service_tier(&self, service_tier: &ServiceTier) -> Option<&ModelServiceTier> {
-        self.service_tiers
-            .iter()
-            .find(|tier| tier.id == *service_tier)
-    }
-
     /// Filter models based on authentication mode.
     ///
     /// In ChatGPT mode, all models are visible. Otherwise, only API-supported models are shown.
@@ -831,7 +837,7 @@ mod tests {
                 message: "Try Spark.".to_string(),
             }),
             service_tiers: vec![ModelServiceTier {
-                id: ServiceTier::priority(),
+                id: priority_service_tier(),
                 name: "Fast".to_string(),
                 description: "Fast tier".to_string(),
             }],
@@ -844,6 +850,9 @@ mod tests {
                 message: "Try Spark.".to_string(),
             })
         );
-        assert!(preset.supports_service_tier(&ServiceTier::priority()));
+        assert!(model_supports_service_tier(
+            &preset,
+            &priority_service_tier()
+        ));
     }
 }