diff --git a/codex-rs/config/src/config_toml.rs b/codex-rs/config/src/config_toml.rs
index 0a82eaf53..989aab169 100644
--- a/codex-rs/config/src/config_toml.rs
+++ b/codex-rs/config/src/config_toml.rs
@@ -41,7 +41,6 @@ use codex_protocol::config_types::ForcedLoginMethod;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
-use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::TrustLevel;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::config_types::WebSearchMode;
@@ -307,8 +306,9 @@ pub struct ConfigToml {
     /// Optionally specify a personality for the model
     pub personality: Option<Personality>,
 
-    /// Optional explicit service tier preference for new turns (`fast` or `flex`).
-    pub service_tier: Option<ServiceTier>,
+    /// Optional explicit service tier request id for new turns (for example
+    /// `priority` or `flex`; legacy `fast` also works).
+    pub service_tier: 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 fab78a128..420eb4614 100644
--- a/codex-rs/config/src/profile_toml.rs
+++ b/codex-rs/config/src/profile_toml.rs
@@ -12,7 +12,6 @@ use crate::types::WindowsToml;
 use codex_features::FeaturesToml;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
-use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::config_types::WebSearchMode;
 use codex_protocol::openai_models::ReasoningEffort;
@@ -24,8 +23,9 @@ use codex_protocol::protocol::AskForApproval;
 #[schemars(deny_unknown_fields)]
 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 request id for new turns (for example
+    /// `priority` or `flex`; legacy `fast` also works).
+    pub service_tier: 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 ecbd73093..edbf45694 100644
--- a/codex-rs/core/config.schema.json
+++ b/codex-rs/core/config.schema.json
@@ -670,12 +670,8 @@
           "$ref": "#/definitions/SandboxMode"
         },
         "service_tier": {
-          "allOf": [
-            {
-              "$ref": "#/definitions/ServiceTier"
-            }
-          ],
-          "description": "Optional explicit service tier preference for new turns (`fast` or `flex`)."
+          "description": "Optional explicit service tier request id for new turns (for example `priority` or `flex`; legacy `fast` also works).",
+          "type": "string"
         },
         "tools": {
           "$ref": "#/definitions/ToolsToml"
@@ -2221,13 +2217,6 @@
       },
       "type": "object"
     },
-    "ServiceTier": {
-      "enum": [
-        "fast",
-        "flex"
-      ],
-      "type": "string"
-    },
     "SessionPickerViewMode": {
       "description": "Preferred layout for the resume/fork session picker.",
       "enum": [
@@ -4502,12 +4491,8 @@
       "description": "Sandbox configuration to apply if `sandbox` is `WorkspaceWrite`."
     },
     "service_tier": {
-      "allOf": [
-        {
-          "$ref": "#/definitions/ServiceTier"
-        }
-      ],
-      "description": "Optional explicit service tier preference for new turns (`fast` or `flex`)."
+      "description": "Optional explicit service tier request id for new turns (for example `priority` or `flex`; legacy `fast` also works).",
+      "type": "string"
     },
     "shell_environment_policy": {
       "allOf": [
diff --git a/codex-rs/core/src/config/config_tests.rs b/codex-rs/core/src/config/config_tests.rs
index 25f6697c7..3059bd530 100644
--- a/codex-rs/core/src/config/config_tests.rs
+++ b/codex-rs/core/src/config/config_tests.rs
@@ -7297,6 +7297,54 @@ async fn legacy_fast_service_tier_override_uses_priority_request_value() -> std:
     Ok(())
 }
 
+#[tokio::test]
+async fn config_toml_service_tier_accepts_arbitrary_string() -> std::io::Result<()> {
+    let mut fixture = create_test_fixture()?;
+    fixture.cfg.service_tier = Some("experimental-tier-id".to_string());
+    let cwd = fixture.cwd_path();
+    let codex_home = fixture.codex_home();
+
+    let config = Config::load_from_base_config_with_overrides(
+        fixture.cfg,
+        ConfigOverrides {
+            cwd: Some(cwd),
+            ..Default::default()
+        },
+        codex_home,
+    )
+    .await?;
+
+    assert_eq!(
+        config.service_tier,
+        Some("experimental-tier-id".to_string())
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn config_toml_legacy_fast_service_tier_uses_priority_request_value() -> std::io::Result<()> {
+    let mut fixture = create_test_fixture()?;
+    fixture.cfg.service_tier = Some("fast".to_string());
+    let cwd = fixture.cwd_path();
+    let codex_home = fixture.codex_home();
+
+    let config = Config::load_from_base_config_with_overrides(
+        fixture.cfg,
+        ConfigOverrides {
+            cwd: Some(cwd),
+            ..Default::default()
+        },
+        codex_home,
+    )
+    .await?;
+
+    assert_eq!(
+        config.service_tier,
+        Some(ServiceTier::Fast.request_value().to_string())
+    );
+    Ok(())
+}
+
 #[tokio::test]
 async fn fast_default_opt_out_notice_config_is_respected() -> std::io::Result<()> {
     let fixture = create_test_fixture()?;
diff --git a/codex-rs/core/src/config/mod.rs b/codex-rs/core/src/config/mod.rs
index e27002aef..b1277eda4 100644
--- a/codex-rs/core/src/config/mod.rs
+++ b/codex-rs/core/src/config/mod.rs
@@ -2734,10 +2734,7 @@ impl Config {
                 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) {
diff --git a/codex-rs/core/src/session/config_lock.rs b/codex-rs/core/src/session/config_lock.rs
index 10f224264..3e8c36734 100644
--- a/codex-rs/core/src/session/config_lock.rs
+++ b/codex-rs/core/src/session/config_lock.rs
@@ -109,10 +109,7 @@ fn save_session_resolved_fields(sc: &SessionConfiguration, lock_config: &mut Con
     lock_config.model = Some(sc.collaboration_mode.model().to_string());
     lock_config.model_reasoning_effort = sc.collaboration_mode.reasoning_effort();
     lock_config.model_reasoning_summary = sc.model_reasoning_summary;
-    lock_config.service_tier = sc
-        .service_tier
-        .as_deref()
-        .and_then(codex_protocol::config_types::ServiceTier::from_request_value);
+    lock_config.service_tier = 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();