diff --git a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
index 5e24845729..d9daa3b6b1 100644
--- a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
+++ b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
@@ -7157,6 +7157,12 @@
               "null"
             ]
           },
+          "service_tier_id": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
           "tools": {
             "anyOf": [
               {
@@ -12829,6 +12835,12 @@
               "null"
             ]
           },
+          "service_tier_id": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
           "tools": {
             "anyOf": [
               {
diff --git a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
index 6153a54eb9..7b6412de92 100644
--- a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
+++ b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
@@ -3613,6 +3613,12 @@
             "null"
           ]
         },
+        "service_tier_id": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "tools": {
           "anyOf": [
             {
@@ -9440,6 +9446,12 @@
             "null"
           ]
         },
+        "service_tier_id": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "tools": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ConfigReadResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ConfigReadResponse.json
index 87a826e5af..f5fabb42c6 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ConfigReadResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ConfigReadResponse.json
@@ -357,6 +357,12 @@
             "null"
           ]
         },
+        "service_tier_id": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "tools": {
           "anyOf": [
             {
@@ -659,6 +665,12 @@
             "null"
           ]
         },
+        "service_tier_id": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "tools": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/typescript/v2/Config.ts b/codex-rs/app-server-protocol/schema/typescript/v2/Config.ts
index cc7e340ea3..31eae0af4a 100644
--- a/codex-rs/app-server-protocol/schema/typescript/v2/Config.ts
+++ b/codex-rs/app-server-protocol/schema/typescript/v2/Config.ts
@@ -19,4 +19,4 @@ export type Config = {model: string | null, review_model: string | null, model_c
  * [UNSTABLE] Optional default for where approval requests are routed for
  * review.
  */
-approvals_reviewer: ApprovalsReviewer | null, sandbox_mode: SandboxMode | null, sandbox_workspace_write: SandboxWorkspaceWrite | null, forced_chatgpt_workspace_id: string | null, forced_login_method: ForcedLoginMethod | null, web_search: WebSearchMode | null, tools: ToolsV2 | null, profile: string | null, profiles: { [key in string]?: ProfileV2 }, instructions: string | null, developer_instructions: string | null, compact_prompt: string | null, model_reasoning_effort: ReasoningEffort | null, model_reasoning_summary: ReasoningSummary | null, model_verbosity: Verbosity | null, service_tier: string | null, analytics: AnalyticsConfig | null} & ({ [key in string]?: number | string | boolean | Array<JsonValue> | { [key in string]?: JsonValue } | null });
+approvals_reviewer: ApprovalsReviewer | null, sandbox_mode: SandboxMode | null, sandbox_workspace_write: SandboxWorkspaceWrite | null, forced_chatgpt_workspace_id: string | null, forced_login_method: ForcedLoginMethod | null, web_search: WebSearchMode | null, tools: ToolsV2 | null, profile: string | null, profiles: { [key in string]?: ProfileV2 }, instructions: string | null, developer_instructions: string | null, compact_prompt: string | null, model_reasoning_effort: ReasoningEffort | null, model_reasoning_summary: ReasoningSummary | null, model_verbosity: Verbosity | null, service_tier: string | null, service_tier_id: string | null, analytics: AnalyticsConfig | null} & ({ [key in string]?: number | string | boolean | Array<JsonValue> | { [key in string]?: JsonValue } | null });
diff --git a/codex-rs/app-server-protocol/schema/typescript/v2/ProfileV2.ts b/codex-rs/app-server-protocol/schema/typescript/v2/ProfileV2.ts
index d05038701c..572a5dae3e 100644
--- a/codex-rs/app-server-protocol/schema/typescript/v2/ProfileV2.ts
+++ b/codex-rs/app-server-protocol/schema/typescript/v2/ProfileV2.ts
@@ -15,4 +15,4 @@ export type ProfileV2 = {model: string | null, model_provider: string | null, ap
  * are routed for review. If omitted, the enclosing config default is
  * used.
  */
-approvals_reviewer: ApprovalsReviewer | null, service_tier: string | null, model_reasoning_effort: ReasoningEffort | null, model_reasoning_summary: ReasoningSummary | null, model_verbosity: Verbosity | null, web_search: WebSearchMode | null, tools: ToolsV2 | null, chatgpt_base_url: string | null} & ({ [key in string]?: number | string | boolean | Array<JsonValue> | { [key in string]?: JsonValue } | null });
+approvals_reviewer: ApprovalsReviewer | null, service_tier: string | null, service_tier_id: string | null, model_reasoning_effort: ReasoningEffort | null, model_reasoning_summary: ReasoningSummary | null, model_verbosity: Verbosity | null, web_search: WebSearchMode | null, tools: ToolsV2 | null, chatgpt_base_url: string | null} & ({ [key in string]?: number | string | boolean | Array<JsonValue> | { [key in string]?: JsonValue } | null });
diff --git a/codex-rs/app-server-protocol/src/protocol/v2/config.rs b/codex-rs/app-server-protocol/src/protocol/v2/config.rs
index 8bc50bb1f2..10d3897fc3 100644
--- a/codex-rs/app-server-protocol/src/protocol/v2/config.rs
+++ b/codex-rs/app-server-protocol/src/protocol/v2/config.rs
@@ -137,6 +137,7 @@ pub struct ProfileV2 {
     #[experimental("config/read.approvalsReviewer")]
     pub approvals_reviewer: Option<ApprovalsReviewer>,
     pub service_tier: Option<String>,
+    pub service_tier_id: Option<String>,
     pub model_reasoning_effort: Option<ReasoningEffort>,
     pub model_reasoning_summary: Option<ReasoningSummary>,
     pub model_verbosity: Option<Verbosity>,
@@ -248,6 +249,7 @@ pub struct Config {
     pub model_reasoning_summary: Option<ReasoningSummary>,
     pub model_verbosity: Option<Verbosity>,
     pub service_tier: Option<String>,
+    pub service_tier_id: Option<String>,
     pub analytics: Option<AnalyticsConfig>,
     #[experimental("config/read.apps")]
     #[serde(default)]
diff --git a/codex-rs/app-server/tests/suite/v2/turn_start.rs b/codex-rs/app-server/tests/suite/v2/turn_start.rs
index e5c5c5adbb..1b7bb8376b 100644
--- a/codex-rs/app-server/tests/suite/v2/turn_start.rs
+++ b/codex-rs/app-server/tests/suite/v2/turn_start.rs
@@ -60,6 +60,7 @@ use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::ModeKind;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Settings;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::user_input::MAX_USER_INPUT_TEXT_CHARS;
@@ -423,6 +424,82 @@ async fn turn_start_sends_service_tier_id_to_model_request() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn turn_start_configured_service_tier_id_takes_precedence() -> Result<()> {
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp-1"),
+        responses::ev_assistant_message("msg-1", "Done"),
+        responses::ev_completed("resp-1"),
+    ]);
+    let response_mock = responses::mount_sse_once(&server, body).await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(
+        codex_home.path(),
+        &server.uri(),
+        "never",
+        &BTreeMap::default(),
+    )?;
+    let config_path = codex_home.path().join("config.toml");
+    let config_toml = std::fs::read_to_string(&config_path)?;
+    let service_tier_id = "experimental-tier-id".to_string();
+    let (config_root, config_rest) = config_toml
+        .split_once("\n[features]\n")
+        .expect("test config should contain features section");
+    std::fs::write(
+        &config_path,
+        format!(
+            "{config_root}\nservice_tier = \"{}\"\nservice_tier_id = \"{service_tier_id}\"\n\n[features]\n{config_rest}",
+            ServiceTier::Fast
+        ),
+    )?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let thread_req = mcp
+        .send_thread_start_request(ThreadStartParams {
+            model: Some("mock-model".to_string()),
+            ..Default::default()
+        })
+        .await?;
+    let thread_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(thread_req)),
+    )
+    .await??;
+    let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(thread_resp)?;
+
+    let turn_req = mcp
+        .send_turn_start_request(TurnStartParams {
+            thread_id: thread.id,
+            input: vec![V2UserInput::Text {
+                text: "Hello".to_string(),
+                text_elements: Vec::new(),
+            }],
+            ..Default::default()
+        })
+        .await?;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(turn_req)),
+    )
+    .await??;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("turn/completed"),
+    )
+    .await??;
+
+    assert_eq!(
+        response_mock.single_request().body_json()["service_tier"],
+        json!(service_tier_id)
+    );
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn thread_start_omits_empty_instruction_overrides_from_model_request() -> Result<()> {
     let server = responses::start_mock_server().await;
diff --git a/codex-rs/config/src/config_toml.rs b/codex-rs/config/src/config_toml.rs
index d2eeaf927f..d19386f3e8 100644
--- a/codex-rs/config/src/config_toml.rs
+++ b/codex-rs/config/src/config_toml.rs
@@ -310,6 +310,10 @@ pub struct ConfigToml {
     /// Optional explicit service tier preference for new turns (`fast` or `flex`).
     pub service_tier: Option<ServiceTier>,
 
+    /// Optional explicit service tier id preference for new turns. Takes precedence over
+    /// `service_tier` when both are set.
+    pub service_tier_id: Option<String>,
+
     /// Base URL for requests to ChatGPT (as opposed to the OpenAI API).
     pub chatgpt_base_url: Option<String>,
 
diff --git a/codex-rs/config/src/profile_toml.rs b/codex-rs/config/src/profile_toml.rs
index fab78a128c..d1870dc514 100644
--- a/codex-rs/config/src/profile_toml.rs
+++ b/codex-rs/config/src/profile_toml.rs
@@ -26,6 +26,9 @@ pub struct ConfigProfile {
     pub model: Option<String>,
     /// Optional explicit service tier preference for new turns (`fast` or `flex`).
     pub service_tier: Option<ServiceTier>,
+    /// Optional explicit service tier id preference for new turns. Takes precedence over
+    /// `service_tier` when both are set.
+    pub service_tier_id: Option<String>,
     /// The key in the `model_providers` map identifying the
     /// [`ModelProviderInfo`] to use.
     pub model_provider: Option<String>,
diff --git a/codex-rs/core/config.schema.json b/codex-rs/core/config.schema.json
index 6eab1d4c55..b07da2c46f 100644
--- a/codex-rs/core/config.schema.json
+++ b/codex-rs/core/config.schema.json
@@ -677,6 +677,10 @@
           ],
           "description": "Optional explicit service tier preference for new turns (`fast` or `flex`)."
         },
+        "service_tier_id": {
+          "description": "Optional explicit service tier id preference for new turns. Takes precedence over `service_tier` when both are set.",
+          "type": "string"
+        },
         "tools": {
           "$ref": "#/definitions/ToolsToml"
         },
@@ -4514,6 +4518,10 @@
       ],
       "description": "Optional explicit service tier preference for new turns (`fast` or `flex`)."
     },
+    "service_tier_id": {
+      "description": "Optional explicit service tier id preference for new turns. Takes precedence over `service_tier` when both are set.",
+      "type": "string"
+    },
     "shell_environment_policy": {
       "allOf": [
         {
diff --git a/codex-rs/core/src/compact.rs b/codex-rs/core/src/compact.rs
index 9d1c82eb65..08af8ec92d 100644
--- a/codex-rs/core/src/compact.rs
+++ b/codex-rs/core/src/compact.rs
@@ -543,7 +543,7 @@ async fn drain_to_completed(
             &turn_context.session_telemetry,
             turn_context.reasoning_effort,
             turn_context.reasoning_summary,
-            turn_context.config.service_tier.clone(),
+            turn_context.config.service_tier_id.clone(),
             turn_metadata_header,
             // Rollout tracing currently models remote compaction only; local compaction streams
             // are left untraced until the reducer has a first-class local compaction lifecycle.
diff --git a/codex-rs/core/src/compact_remote.rs b/codex-rs/core/src/compact_remote.rs
index eb9e454677..24344e975a 100644
--- a/codex-rs/core/src/compact_remote.rs
+++ b/codex-rs/core/src/compact_remote.rs
@@ -197,7 +197,7 @@ async fn run_remote_compact_task_inner_impl(
             CompactConversationRequestSettings {
                 effort: turn_context.reasoning_effort,
                 summary: turn_context.reasoning_summary,
-                service_tier: turn_context.config.service_tier.clone(),
+                service_tier: turn_context.config.service_tier_id.clone(),
             },
             &turn_context.session_telemetry,
             &compaction_trace,
diff --git a/codex-rs/core/src/compact_remote_v2.rs b/codex-rs/core/src/compact_remote_v2.rs
index 7705616d37..e2476ab7cf 100644
--- a/codex-rs/core/src/compact_remote_v2.rs
+++ b/codex-rs/core/src/compact_remote_v2.rs
@@ -252,7 +252,7 @@ async fn run_remote_compaction_request_v2(
             &turn_context.session_telemetry,
             turn_context.reasoning_effort,
             turn_context.reasoning_summary,
-            turn_context.config.service_tier.clone(),
+            turn_context.config.service_tier_id.clone(),
             turn_metadata_header,
             &InferenceTraceContext::disabled(),
         )
diff --git a/codex-rs/core/src/config/config_tests.rs b/codex-rs/core/src/config/config_tests.rs
index bc9af28a67..74780d1cdd 100644
--- a/codex-rs/core/src/config/config_tests.rs
+++ b/codex-rs/core/src/config/config_tests.rs
@@ -5238,6 +5238,34 @@ async fn set_model_updates_profile() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn clearing_service_tier_id_also_clears_legacy_service_tier() -> anyhow::Result<()> {
+    let codex_home = TempDir::new()?;
+    let config_path = codex_home.path().join(CONFIG_TOML_FILE);
+
+    tokio::fs::write(
+        &config_path,
+        r#"
+service_tier = "fast"
+service_tier_id = "priority"
+"#,
+    )
+    .await?;
+
+    ConfigEditsBuilder::new(codex_home.path())
+        .set_service_tier(/*service_tier*/ None)
+        .set_service_tier_id(/*service_tier_id*/ None)
+        .apply()
+        .await?;
+
+    let serialized = tokio::fs::read_to_string(config_path).await?;
+    let parsed: ConfigToml = toml::from_str(&serialized)?;
+
+    assert_eq!((parsed.service_tier, parsed.service_tier_id), (None, None));
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn set_model_updates_existing_profile() -> anyhow::Result<()> {
     let codex_home = TempDir::new()?;
@@ -6946,6 +6974,7 @@ async fn test_precedence_fixture_with_o3_profile() -> std::io::Result<()> {
             model_context_window: None,
             model_auto_compact_token_limit: None,
             service_tier: None,
+            service_tier_id: None,
             model_provider_id: "openai".to_string(),
             model_provider: fixture.openai_provider.clone(),
             permissions: Permissions {
@@ -7126,14 +7155,14 @@ async fn explicit_null_service_tier_override_sets_fast_default_opt_out() -> std:
         fixture.cfg.clone(),
         ConfigOverrides {
             cwd: Some(fixture.cwd_path()),
-            service_tier: Some(None),
+            service_tier_id: Some(None),
             ..Default::default()
         },
         fixture.codex_home(),
     )
     .await?;
 
-    assert_eq!(config.service_tier, None);
+    assert_eq!(config.service_tier_id, None);
     assert_eq!(config.notices.fast_default_opt_out, Some(true));
     Ok(())
 }
@@ -7146,7 +7175,7 @@ async fn legacy_fast_service_tier_override_uses_priority_request_value() -> std:
         fixture.cfg.clone(),
         ConfigOverrides {
             cwd: Some(fixture.cwd_path()),
-            service_tier: Some(Some("fast".to_string())),
+            service_tier: Some(Some(ServiceTier::Fast)),
             ..Default::default()
         },
         fixture.codex_home(),
@@ -7154,8 +7183,40 @@ async fn legacy_fast_service_tier_override_uses_priority_request_value() -> std:
     .await?;
 
     assert_eq!(
-        config.service_tier,
-        Some(ServiceTier::Fast.request_value().to_string())
+        config,
+        Config {
+            service_tier: Some(ServiceTier::Fast),
+            service_tier_id: Some(ServiceTier::Fast.request_value().to_string()),
+            ..config.clone()
+        }
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn service_tier_id_takes_precedence_over_service_tier() -> std::io::Result<()> {
+    let fixture = create_test_fixture()?;
+    let mut cfg = fixture.cfg.clone();
+    cfg.service_tier = Some(ServiceTier::Fast);
+    cfg.service_tier_id = Some("flex".to_string());
+
+    let config = Config::load_from_base_config_with_overrides(
+        cfg,
+        ConfigOverrides {
+            cwd: Some(fixture.cwd_path()),
+            ..Default::default()
+        },
+        fixture.codex_home(),
+    )
+    .await?;
+
+    assert_eq!(
+        config,
+        Config {
+            service_tier: Some(ServiceTier::Fast),
+            service_tier_id: Some("flex".to_string()),
+            ..config.clone()
+        }
     );
     Ok(())
 }
@@ -7179,7 +7240,7 @@ async fn fast_default_opt_out_notice_config_is_respected() -> std::io::Result<()
     )
     .await?;
 
-    assert_eq!(config.service_tier, None);
+    assert_eq!(config.service_tier_id, None);
     assert_eq!(config.notices.fast_default_opt_out, Some(true));
     Ok(())
 }
@@ -7205,6 +7266,7 @@ async fn test_precedence_fixture_with_gpt3_profile() -> std::io::Result<()> {
         model_context_window: None,
         model_auto_compact_token_limit: None,
         service_tier: None,
+        service_tier_id: None,
         model_provider_id: "openai-custom".to_string(),
         model_provider: fixture.openai_custom_provider.clone(),
         permissions: Permissions {
@@ -7363,6 +7425,7 @@ async fn test_precedence_fixture_with_zdr_profile() -> std::io::Result<()> {
         model_context_window: None,
         model_auto_compact_token_limit: None,
         service_tier: None,
+        service_tier_id: None,
         model_provider_id: "openai".to_string(),
         model_provider: fixture.openai_provider.clone(),
         permissions: Permissions {
@@ -7506,6 +7569,7 @@ async fn test_precedence_fixture_with_gpt5_profile() -> std::io::Result<()> {
         model_context_window: None,
         model_auto_compact_token_limit: None,
         service_tier: None,
+        service_tier_id: None,
         model_provider_id: "openai".to_string(),
         model_provider: fixture.openai_provider.clone(),
         permissions: Permissions {
diff --git a/codex-rs/core/src/config/edit.rs b/codex-rs/core/src/config/edit.rs
index 1362103364..ccaa8f13dc 100644
--- a/codex-rs/core/src/config/edit.rs
+++ b/codex-rs/core/src/config/edit.rs
@@ -32,8 +32,10 @@ pub enum ConfigEdit {
         model: Option<String>,
         effort: Option<ReasoningEffort>,
     },
-    /// Update the service tier preference for future turns.
+    /// Update the legacy service tier preference for future turns.
     SetServiceTier { service_tier: Option<ServiceTier> },
+    /// Update the service tier id preference for future turns.
+    SetServiceTierId { service_tier_id: Option<String> },
     /// Update the active (or default) model personality.
     SetModelPersonality { personality: Option<Personality> },
     /// Toggle the acknowledgement flag under `[notice]`.
@@ -538,6 +540,12 @@ impl ConfigDocument {
                 &["service_tier"],
                 service_tier.map(|service_tier| value(service_tier.to_string())),
             )),
+            ConfigEdit::SetServiceTierId { service_tier_id } => Ok(self.write_profile_value(
+                &["service_tier_id"],
+                service_tier_id
+                    .as_ref()
+                    .map(|service_tier_id| value(service_tier_id.clone())),
+            )),
             ConfigEdit::SetModelPersonality { personality } => Ok(self.write_profile_value(
                 &["personality"],
                 personality.map(|personality| value(personality.to_string())),
@@ -1114,6 +1122,12 @@ impl ConfigEditsBuilder {
         self
     }
 
+    pub fn set_service_tier_id(mut self, service_tier_id: Option<String>) -> Self {
+        self.edits
+            .push(ConfigEdit::SetServiceTierId { service_tier_id });
+        self
+    }
+
     pub fn set_service_tier(mut self, service_tier: Option<ServiceTier>) -> Self {
         self.edits.push(ConfigEdit::SetServiceTier { service_tier });
         self
diff --git a/codex-rs/core/src/config/mod.rs b/codex-rs/core/src/config/mod.rs
index b2dfbcb2b8..5136c05ee4 100644
--- a/codex-rs/core/src/config/mod.rs
+++ b/codex-rs/core/src/config/mod.rs
@@ -402,8 +402,12 @@ pub struct Config {
     /// Optional override of model selection.
     pub model: Option<String>,
 
-    /// Effective service tier request id preference for new turns.
-    pub service_tier: Option<String>,
+    /// Effective legacy service tier preference for new turns.
+    pub service_tier: Option<ServiceTier>,
+
+    /// Effective service tier id preference for new turns. Takes precedence over
+    /// `service_tier` when both are set.
+    pub service_tier_id: Option<String>,
 
     /// Model used specifically for review sessions.
     pub review_model: Option<String>,
@@ -1870,7 +1874,8 @@ pub struct ConfigOverrides {
     pub permission_profile: Option<PermissionProfile>,
     pub default_permissions: Option<String>,
     pub model_provider: Option<String>,
-    pub service_tier: Option<Option<String>>,
+    pub service_tier: Option<Option<ServiceTier>>,
+    pub service_tier_id: Option<Option<String>>,
     pub config_profile: Option<String>,
     pub codex_self_exe: Option<PathBuf>,
     pub codex_linux_sandbox_exe: Option<PathBuf>,
@@ -2139,6 +2144,7 @@ impl Config {
             default_permissions: default_permissions_override,
             model_provider,
             service_tier: service_tier_override,
+            service_tier_id: service_tier_id_override,
             config_profile: config_profile_key,
             codex_self_exe,
             codex_linux_sandbox_exe,
@@ -2733,25 +2739,39 @@ impl Config {
         let service_tier = match service_tier_override {
             Some(Some(service_tier)) => Some(service_tier),
             Some(None) => {
-                // Preserve explicit standard/clear intent after the nested override
-                // collapses into `Config.service_tier = None`.
                 notices.fast_default_opt_out = Some(true);
                 None
             }
-            None => config_profile
-                .service_tier
-                .or(cfg.service_tier)
-                .map(|service_tier| service_tier.request_value().to_string()),
+            None => config_profile.service_tier.or(cfg.service_tier),
         };
-        let service_tier = service_tier.and_then(|service_tier| {
-            match ServiceTier::from_request_value(&service_tier) {
-                Some(ServiceTier::Fast) => features
-                    .enabled(Feature::FastMode)
-                    .then(|| ServiceTier::Fast.request_value().to_string()),
-                Some(ServiceTier::Flex) => Some(ServiceTier::Flex.request_value().to_string()),
-                None => Some(service_tier),
+        let service_tier = match service_tier {
+            Some(ServiceTier::Fast) if features.enabled(Feature::FastMode) => {
+                Some(ServiceTier::Fast)
             }
-        });
+            Some(ServiceTier::Fast) => None,
+            Some(ServiceTier::Flex) => Some(ServiceTier::Flex),
+            None => None,
+        };
+        let service_tier_id = match service_tier_id_override {
+            Some(Some(service_tier_id)) => Some(service_tier_id),
+            Some(None) => {
+                // Preserve explicit standard/clear intent after the nested override
+                // collapses into `Config.service_tier_id = None`.
+                notices.fast_default_opt_out = Some(true);
+                None
+            }
+            None => config_profile.service_tier_id.or(cfg.service_tier_id).or_else(|| {
+                service_tier
+                    .as_ref()
+                    .map(|service_tier| service_tier.request_value().to_string())
+            }),
+        };
+        let service_tier_id =
+            match service_tier_id.as_deref() {
+                Some("priority") if features.enabled(Feature::FastMode) => service_tier_id,
+                Some("priority") => None,
+                Some(_) | None => service_tier_id,
+            };
 
         let compact_prompt = compact_prompt.or(cfg.compact_prompt).and_then(|value| {
             let trimmed = value.trim();
@@ -2981,6 +3001,7 @@ impl Config {
         let config = Self {
             model,
             service_tier,
+            service_tier_id,
             review_model,
             model_context_window: cfg.model_context_window,
             model_auto_compact_token_limit: cfg.model_auto_compact_token_limit,
diff --git a/codex-rs/core/src/session/config_lock.rs b/codex-rs/core/src/session/config_lock.rs
index 10f2242647..2d854d7ae2 100644
--- a/codex-rs/core/src/session/config_lock.rs
+++ b/codex-rs/core/src/session/config_lock.rs
@@ -113,6 +113,7 @@ fn save_session_resolved_fields(sc: &SessionConfiguration, lock_config: &mut Con
         .service_tier
         .as_deref()
         .and_then(codex_protocol::config_types::ServiceTier::from_request_value);
+    lock_config.service_tier_id = sc.service_tier.clone();
     lock_config.instructions = Some(sc.base_instructions.clone());
     lock_config.developer_instructions = sc.developer_instructions.clone();
     lock_config.compact_prompt = sc.compact_prompt.clone();
@@ -291,7 +292,7 @@ mod tests {
         assert_eq!(lock.model, None);
         assert_eq!(lock.model_reasoning_effort, None);
         assert_eq!(lock.model_reasoning_summary, None);
-        assert_eq!(lock.service_tier, None);
+        assert_eq!(lock.service_tier_id, None);
         assert_eq!(lock.instructions, None);
         assert_eq!(lock.developer_instructions, None);
         assert_eq!(lock.compact_prompt, None);
@@ -300,6 +301,24 @@ mod tests {
         assert_eq!(lock.approvals_reviewer, None);
     }
 
+    #[tokio::test]
+    async fn lock_clears_legacy_service_tier_when_session_tier_is_cleared() {
+        let mut sc = crate::session::tests::make_session_configuration_for_tests().await;
+        let mut config = (*sc.original_config_do_not_use).clone();
+        config.service_tier = Some(codex_protocol::config_types::ServiceTier::Fast);
+        config.service_tier_id = Some("priority".to_string());
+        sc.original_config_do_not_use = Arc::new(config);
+        sc.service_tier = None;
+
+        let lockfile = sc.to_config_lockfile_toml().expect("lock should serialize");
+        let lock = &lockfile.config;
+
+        assert_eq!(
+            (lock.service_tier, lock.service_tier_id.clone()),
+            (None, None)
+        );
+    }
+
     #[tokio::test]
     async fn lock_validation_reports_config_diff() {
         let sc = crate::session::tests::make_session_configuration_for_tests().await;
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index 6d4b275421..b3c83f538f 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -595,8 +595,8 @@ impl Codex {
         let account_plan_type = auth_manager
             .auth_cached()
             .and_then(|auth| auth.account_plan_type());
-        let service_tier = get_service_tier(
-            config.service_tier.clone(),
+        let service_tier = get_service_tier_id(
+            config.service_tier_id.clone(),
             config.notices.fast_default_opt_out.unwrap_or(false),
             account_plan_type,
             config.features.enabled(Feature::FastMode),
@@ -793,14 +793,14 @@ impl Codex {
     }
 }
 
-fn get_service_tier(
-    configured_service_tier: Option<String>,
+fn get_service_tier_id(
+    configured_service_tier_id: Option<String>,
     fast_default_opt_out: bool,
     account_plan_type: Option<AccountPlanType>,
     fast_mode_enabled: bool,
 ) -> Option<String> {
-    if configured_service_tier.is_some() || fast_default_opt_out || !fast_mode_enabled {
-        return configured_service_tier;
+    if configured_service_tier_id.is_some() || fast_default_opt_out || !fast_mode_enabled {
+        return configured_service_tier_id;
     }
 
     account_plan_type
diff --git a/codex-rs/core/src/session/tests.rs b/codex-rs/core/src/session/tests.rs
index b63b16cbf4..a9f247d270 100644
--- a/codex-rs/core/src/session/tests.rs
+++ b/codex-rs/core/src/session/tests.rs
@@ -3011,10 +3011,10 @@ fn session_telemetry(
 }
 
 #[test]
-fn get_service_tier_defaults_enterprise_accounts_to_fast() {
+fn get_service_tier_id_defaults_enterprise_accounts_to_fast() {
     assert_eq!(
-        get_service_tier(
-            /*configured_service_tier*/ None,
+        get_service_tier_id(
+            /*configured_service_tier_id*/ None,
             /*fast_default_opt_out*/ false,
             Some(AccountPlanType::Enterprise),
             /*fast_mode_enabled*/ true,
@@ -3022,8 +3022,8 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
         Some(ServiceTier::Fast.request_value().to_string())
     );
     assert_eq!(
-        get_service_tier(
-            /*configured_service_tier*/ None,
+        get_service_tier_id(
+            /*configured_service_tier_id*/ None,
             /*fast_default_opt_out*/ false,
             Some(AccountPlanType::EnterpriseCbpUsageBased),
             /*fast_mode_enabled*/ true,
@@ -3031,8 +3031,8 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
         Some(ServiceTier::Fast.request_value().to_string())
     );
     assert_eq!(
-        get_service_tier(
-            /*configured_service_tier*/ None,
+        get_service_tier_id(
+            /*configured_service_tier_id*/ None,
             /*fast_default_opt_out*/ false,
             Some(AccountPlanType::Business),
             /*fast_mode_enabled*/ true,
@@ -3040,8 +3040,8 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
         Some(ServiceTier::Fast.request_value().to_string())
     );
     assert_eq!(
-        get_service_tier(
-            /*configured_service_tier*/ None,
+        get_service_tier_id(
+            /*configured_service_tier_id*/ None,
             /*fast_default_opt_out*/ false,
             Some(AccountPlanType::Team),
             /*fast_mode_enabled*/ true,
@@ -3049,8 +3049,8 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
         Some(ServiceTier::Fast.request_value().to_string())
     );
     assert_eq!(
-        get_service_tier(
-            /*configured_service_tier*/ None,
+        get_service_tier_id(
+            /*configured_service_tier_id*/ None,
             /*fast_default_opt_out*/ false,
             Some(AccountPlanType::SelfServeBusinessUsageBased),
             /*fast_mode_enabled*/ true,
@@ -3060,10 +3060,10 @@ fn get_service_tier_defaults_enterprise_accounts_to_fast() {
 }
 
 #[test]
-fn get_service_tier_respects_fast_default_opt_out() {
+fn get_service_tier_id_respects_fast_default_opt_out() {
     assert_eq!(
-        get_service_tier(
-            /*configured_service_tier*/ None,
+        get_service_tier_id(
+            /*configured_service_tier_id*/ None,
             /*fast_default_opt_out*/ true,
             Some(AccountPlanType::Enterprise),
             /*fast_mode_enabled*/ true,
@@ -3073,10 +3073,10 @@ fn get_service_tier_respects_fast_default_opt_out() {
 }
 
 #[test]
-fn get_service_tier_does_not_default_non_enterprise_or_disabled_fast_mode() {
+fn get_service_tier_id_does_not_default_non_enterprise_or_disabled_fast_mode() {
     assert_eq!(
-        get_service_tier(
-            /*configured_service_tier*/ None,
+        get_service_tier_id(
+            /*configured_service_tier_id*/ None,
             /*fast_default_opt_out*/ false,
             Some(AccountPlanType::Pro),
             /*fast_mode_enabled*/ true,
@@ -3084,8 +3084,8 @@ fn get_service_tier_does_not_default_non_enterprise_or_disabled_fast_mode() {
         None
     );
     assert_eq!(
-        get_service_tier(
-            /*configured_service_tier*/ None,
+        get_service_tier_id(
+            /*configured_service_tier_id*/ None,
             /*fast_default_opt_out*/ false,
             Some(AccountPlanType::Enterprise),
             /*fast_mode_enabled*/ false,
diff --git a/codex-rs/core/src/session/turn.rs b/codex-rs/core/src/session/turn.rs
index 1723904cff..0314eb84c0 100644
--- a/codex-rs/core/src/session/turn.rs
+++ b/codex-rs/core/src/session/turn.rs
@@ -702,7 +702,7 @@ async fn track_turn_resolved_config_analytics(
             reasoning_summary: Some(turn_context.reasoning_summary),
             service_tier: turn_context
                 .config
-                .service_tier
+                .service_tier_id
                 .as_deref()
                 .and_then(ServiceTier::from_request_value),
             approval_policy: turn_context.approval_policy.value(),
@@ -1856,7 +1856,7 @@ async fn try_run_sampling_request(
             &turn_context.session_telemetry,
             turn_context.reasoning_effort,
             turn_context.reasoning_summary,
-            turn_context.config.service_tier.clone(),
+            turn_context.config.service_tier_id.clone(),
             turn_metadata_header,
             &inference_trace,
         )
diff --git a/codex-rs/core/src/session/turn_context.rs b/codex-rs/core/src/session/turn_context.rs
index d4fe30063f..62789ec51c 100644
--- a/codex-rs/core/src/session/turn_context.rs
+++ b/codex-rs/core/src/session/turn_context.rs
@@ -414,7 +414,11 @@ impl Session {
         per_turn_config.model_reasoning_effort =
             session_configuration.collaboration_mode.reasoning_effort();
         per_turn_config.model_reasoning_summary = session_configuration.model_reasoning_summary;
-        per_turn_config.service_tier = session_configuration.service_tier.clone();
+        per_turn_config.service_tier = session_configuration
+            .service_tier
+            .as_deref()
+            .and_then(codex_protocol::config_types::ServiceTier::from_request_value);
+        per_turn_config.service_tier_id = session_configuration.service_tier.clone();
         per_turn_config.personality = session_configuration.personality;
         per_turn_config.approvals_reviewer = session_configuration.approvals_reviewer;
         per_turn_config.permissions.permission_profile =
diff --git a/codex-rs/core/src/session_startup_prewarm.rs b/codex-rs/core/src/session_startup_prewarm.rs
index 93d14ff7d6..a90481d52d 100644
--- a/codex-rs/core/src/session_startup_prewarm.rs
+++ b/codex-rs/core/src/session_startup_prewarm.rs
@@ -232,7 +232,7 @@ async fn schedule_startup_prewarm_inner(
             &startup_turn_context.session_telemetry,
             startup_turn_context.reasoning_effort,
             startup_turn_context.reasoning_summary,
-            startup_turn_context.config.service_tier.clone(),
+            startup_turn_context.config.service_tier_id.clone(),
             startup_turn_metadata_header.as_deref(),
         )
         .await?;
diff --git a/codex-rs/core/tests/suite/agent_websocket.rs b/codex-rs/core/tests/suite/agent_websocket.rs
index 6e985eebe0..4a5f32982f 100644
--- a/codex-rs/core/tests/suite/agent_websocket.rs
+++ b/codex-rs/core/tests/suite/agent_websocket.rs
@@ -313,7 +313,7 @@ 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::Fast.request_value().to_string());
+        config.service_tier_id = Some(ServiceTier::Fast.request_value().to_string());
     });
     let test = builder.build_with_websocket_server(&server).await?;
 
diff --git a/codex-rs/core/tests/suite/compact_remote.rs b/codex-rs/core/tests/suite/compact_remote.rs
index 3da841ba79..7452c5829d 100644
--- a/codex-rs/core/tests/suite/compact_remote.rs
+++ b/codex-rs/core/tests/suite/compact_remote.rs
@@ -445,7 +445,7 @@ async fn assert_remote_manual_compact_request_parity(
     let mut builder = test_codex().with_auth(auth);
     if let Some(service_tier) = configured_service_tier {
         builder = builder.with_config(move |config| {
-            config.service_tier = Some(service_tier.request_value().to_string());
+            config.service_tier_id = Some(service_tier.request_value().to_string());
         });
     }
     let harness = TestCodexHarness::with_builder(builder).await?;
diff --git a/codex-rs/exec/src/lib.rs b/codex-rs/exec/src/lib.rs
index b035a19517..3a81eb3570 100644
--- a/codex-rs/exec/src/lib.rs
+++ b/codex-rs/exec/src/lib.rs
@@ -411,6 +411,7 @@ pub async fn run_main(cli: Cli, arg0_paths: Arg0DispatchPaths) -> anyhow::Result
         cwd: resolved_cwd,
         model_provider: model_provider.clone(),
         service_tier: None,
+        service_tier_id: None,
         codex_self_exe: arg0_paths.codex_self_exe.clone(),
         codex_linux_sandbox_exe: arg0_paths.codex_linux_sandbox_exe.clone(),
         main_execve_wrapper_exe: arg0_paths.main_execve_wrapper_exe.clone(),
diff --git a/codex-rs/memories/write/src/startup_tests.rs b/codex-rs/memories/write/src/startup_tests.rs
index 4fcb1d409b..db8cdec063 100644
--- a/codex-rs/memories/write/src/startup_tests.rs
+++ b/codex-rs/memories/write/src/startup_tests.rs
@@ -240,7 +240,7 @@ async fn memories_startup_phase1_uses_live_thread_service_tier() -> anyhow::Resu
     let server = start_mock_server().await;
     let home = Arc::new(TempDir::new()?);
     let test = build_test_codex(&server, home).await?;
-    assert_eq!(test.config.service_tier, None);
+    assert_eq!(test.config.service_tier_id, None);
 
     test.codex
         .submit(Op::OverrideTurnContext {
diff --git a/codex-rs/thread-manager-sample/src/main.rs b/codex-rs/thread-manager-sample/src/main.rs
index 6817f677e6..0dd4850b10 100644
--- a/codex-rs/thread-manager-sample/src/main.rs
+++ b/codex-rs/thread-manager-sample/src/main.rs
@@ -161,6 +161,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         startup_warnings: Vec::new(),
         model,
         service_tier: None,
+        service_tier_id: None,
         review_model: None,
         model_context_window: None,
         model_auto_compact_token_limit: None,
diff --git a/codex-rs/tui/src/app/event_dispatch.rs b/codex-rs/tui/src/app/event_dispatch.rs
index 670c6f7de3..5499d065e2 100644
--- a/codex-rs/tui/src/app/event_dispatch.rs
+++ b/codex-rs/tui/src/app/event_dispatch.rs
@@ -1274,11 +1274,13 @@ impl App {
             AppEvent::PersistServiceTierSelection { service_tier } => {
                 self.refresh_status_line();
                 let profile = self.active_profile.as_deref();
-                self.config.service_tier =
+                self.config.service_tier = service_tier;
+                self.config.service_tier_id =
                     service_tier.map(|service_tier| service_tier.request_value().to_string());
                 let mut edits = ConfigEditsBuilder::new(&self.config.codex_home)
                     .with_profile(profile)
-                    .set_service_tier(service_tier);
+                    .set_service_tier(service_tier)
+                    .set_service_tier_id(self.config.service_tier_id.clone());
                 if service_tier.is_none() {
                     self.config.notices.fast_default_opt_out = Some(true);
                     edits = edits.set_fast_default_opt_out(/*opted_out*/ true);
diff --git a/codex-rs/tui/src/app/session_lifecycle.rs b/codex-rs/tui/src/app/session_lifecycle.rs
index b114d08be9..6c933d94a9 100644
--- a/codex-rs/tui/src/app/session_lifecycle.rs
+++ b/codex-rs/tui/src/app/session_lifecycle.rs
@@ -617,10 +617,12 @@ impl App {
 
     pub(super) fn fresh_session_config(&self) -> Config {
         let mut config = self.config.clone();
-        config.service_tier = self
-            .chat_widget
-            .configured_service_tier()
-            .map(|service_tier| service_tier.request_value().to_string());
+        config.service_tier = self.chat_widget.configured_service_tier();
+        config.service_tier_id = self.config.service_tier_id.clone().or_else(|| {
+            config
+                .service_tier
+                .map(|service_tier| service_tier.request_value().to_string())
+        });
         config.notices.fast_default_opt_out = self.chat_widget.fast_default_opt_out();
         config
     }
diff --git a/codex-rs/tui/src/app/tests.rs b/codex-rs/tui/src/app/tests.rs
index 5ab85887a7..6d2787460a 100644
--- a/codex-rs/tui/src/app/tests.rs
+++ b/codex-rs/tui/src/app/tests.rs
@@ -4484,7 +4484,7 @@ async fn fresh_session_config_uses_current_service_tier() {
     let config = app.fresh_session_config();
 
     assert_eq!(
-        config.service_tier,
+        config.service_tier_id,
         Some(
             codex_protocol::config_types::ServiceTier::Fast
                 .request_value()
@@ -4493,6 +4493,19 @@ async fn fresh_session_config_uses_current_service_tier() {
     );
 }
 
+#[tokio::test]
+async fn fresh_session_config_preserves_custom_service_tier_id() {
+    let mut app = make_test_app().await;
+    app.config.service_tier_id = Some("experimental-tier-id".to_string());
+
+    let config = app.fresh_session_config();
+
+    assert_eq!(
+        (config.service_tier, config.service_tier_id),
+        (None, Some("experimental-tier-id".to_string()))
+    );
+}
+
 #[tokio::test]
 async fn backtrack_selection_with_duplicate_history_targets_unique_turn() {
     let (mut app, _app_event_rx, mut op_rx) = make_test_app_with_channels().await;
diff --git a/codex-rs/tui/src/chatwidget.rs b/codex-rs/tui/src/chatwidget.rs
index 9227f29bee..d9ff88a0fa 100644
--- a/codex-rs/tui/src/chatwidget.rs
+++ b/codex-rs/tui/src/chatwidget.rs
@@ -762,7 +762,8 @@ pub(crate) struct ChatWidget {
     active_cell_revision: u64,
     config: Config,
     raw_output_mode: bool,
-    /// Runtime value resolved by core. `config.service_tier` remains the explicit user choice.
+    /// Runtime value resolved by core. `config.service_tier_id`/`config.service_tier`
+    /// remain the explicit user choice.
     effective_service_tier: Option<ServiceTier>,
     /// The unmasked collaboration mode settings (always Default mode).
     ///
@@ -4900,9 +4901,10 @@ impl ChatWidget {
 
         let current_cwd = Some(config.cwd.to_path_buf());
         let effective_service_tier = config
-            .service_tier
+            .service_tier_id
             .as_deref()
-            .and_then(ServiceTier::from_request_value);
+            .and_then(ServiceTier::from_request_value)
+            .or(config.service_tier);
         let current_terminal_info = terminal_info();
         let runtime_keymap = RuntimeKeymap::from_config(&config.tui_keymap).ok();
         let default_keymap = RuntimeKeymap::defaults();
@@ -5891,10 +5893,14 @@ impl ChatWidget {
             .personality
             .filter(|_| self.config.features.enabled(Feature::Personality))
             .filter(|_| self.current_model_supports_personality());
-        let service_tier = match self.config.service_tier.clone() {
-            Some(service_tier) => Some(Some(service_tier)),
-            None if self.config.notices.fast_default_opt_out == Some(true) => Some(None),
-            None => None,
+        let service_tier = match (
+            self.config.service_tier_id.clone(),
+            self.config.service_tier,
+        ) {
+            (Some(service_tier_id), _) => Some(Some(service_tier_id)),
+            (None, Some(service_tier)) => Some(Some(service_tier.request_value().to_string())),
+            (None, None) if self.config.notices.fast_default_opt_out == Some(true) => Some(None),
+            (None, None) => None,
         };
         let permission_profile = self.config.permissions.permission_profile();
         let op = AppCommand::user_turn(
@@ -9304,7 +9310,8 @@ impl ChatWidget {
     /// Set Fast mode in the widget's config copy.
     #[cfg_attr(not(test), allow(dead_code))]
     pub(crate) fn set_service_tier(&mut self, service_tier: Option<ServiceTier>) {
-        self.config.service_tier =
+        self.config.service_tier = service_tier;
+        self.config.service_tier_id =
             service_tier.map(|service_tier| service_tier.request_value().to_string());
         self.effective_service_tier = service_tier;
     }
@@ -9315,9 +9322,10 @@ impl ChatWidget {
 
     pub(crate) fn configured_service_tier(&self) -> Option<ServiceTier> {
         self.config
-            .service_tier
+            .service_tier_id
             .as_deref()
             .and_then(service_tier_from_value)
+            .or(self.config.service_tier)
     }
 
     pub(crate) fn fast_default_opt_out(&self) -> Option<bool> {
diff --git a/codex-rs/tui/src/chatwidget/tests/helpers.rs b/codex-rs/tui/src/chatwidget/tests/helpers.rs
index 9e12a2b45c..fc535fa16d 100644
--- a/codex-rs/tui/src/chatwidget/tests/helpers.rs
+++ b/codex-rs/tui/src/chatwidget/tests/helpers.rs
@@ -184,9 +184,10 @@ pub(super) async fn make_chatwidget_manual(
     let current_collaboration_mode = base_mode;
     let active_collaboration_mask = collaboration_modes::default_mask(model_catalog.as_ref());
     let effective_service_tier = cfg
-        .service_tier
+        .service_tier_id
         .as_deref()
-        .and_then(ServiceTier::from_request_value);
+        .and_then(ServiceTier::from_request_value)
+        .or(cfg.service_tier);
     let mut widget = ChatWidget {
         app_event_tx,
         codex_op_target: super::CodexOpTarget::Direct(op_tx),