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 1329684c4611..e2f7fc87bf74 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
@@ -11274,6 +11274,14 @@
           "defaultReasoningEffort": {
             "$ref": "#/definitions/v2/ReasoningEffort"
           },
+          "defaultServiceTier": {
+            "default": null,
+            "description": "Catalog default service tier id for this model, when one is configured.",
+            "type": [
+              "string",
+              "null"
+            ]
+          },
           "description": {
             "type": "string"
           },
diff --git a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
index 0fa156f01686..2c7340bcd8ef 100644
--- a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
+++ b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
@@ -7803,6 +7803,14 @@
         "defaultReasoningEffort": {
           "$ref": "#/definitions/ReasoningEffort"
         },
+        "defaultServiceTier": {
+          "default": null,
+          "description": "Catalog default service tier id for this model, when one is configured.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "description": {
           "type": "string"
         },
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ModelListResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ModelListResponse.json
index c0221805eb08..32ec2e76796d 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ModelListResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ModelListResponse.json
@@ -43,6 +43,14 @@
         "defaultReasoningEffort": {
           "$ref": "#/definitions/ReasoningEffort"
         },
+        "defaultServiceTier": {
+          "default": null,
+          "description": "Catalog default service tier id for this model, when one is configured.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "description": {
           "type": "string"
         },
diff --git a/codex-rs/app-server-protocol/schema/typescript/v2/Model.ts b/codex-rs/app-server-protocol/schema/typescript/v2/Model.ts
index 2354ffbf9e38..e9dbf15b0b31 100644
--- a/codex-rs/app-server-protocol/schema/typescript/v2/Model.ts
+++ b/codex-rs/app-server-protocol/schema/typescript/v2/Model.ts
@@ -12,4 +12,8 @@ export type Model = { id: string, model: string, upgrade: string | null, upgrade
 /**
  * Deprecated: use `serviceTiers` instead.
  */
-additionalSpeedTiers: Array<string>, serviceTiers: Array<ModelServiceTier>, isDefault: boolean, };
+additionalSpeedTiers: Array<string>, serviceTiers: Array<ModelServiceTier>,
+/**
+ * Catalog default service tier id for this model, when one is configured.
+ */
+defaultServiceTier: string | null, isDefault: boolean, };
diff --git a/codex-rs/app-server-protocol/src/protocol/v2/model.rs b/codex-rs/app-server-protocol/src/protocol/v2/model.rs
index cd139e9c4b49..7f97d791a603 100644
--- a/codex-rs/app-server-protocol/src/protocol/v2/model.rs
+++ b/codex-rs/app-server-protocol/src/protocol/v2/model.rs
@@ -98,6 +98,9 @@ pub struct Model {
     pub additional_speed_tiers: Vec<String>,
     #[serde(default)]
     pub service_tiers: Vec<ModelServiceTier>,
+    /// Catalog default service tier id for this model, when one is configured.
+    #[serde(default)]
+    pub default_service_tier: Option<String>,
     // Only one model should be marked as default.
     pub is_default: bool,
 }
diff --git a/codex-rs/app-server/README.md b/codex-rs/app-server/README.md
index 8ce55410a1a8..ab9b5c4d4e25 100644
--- a/codex-rs/app-server/README.md
+++ b/codex-rs/app-server/README.md
@@ -188,7 +188,7 @@ Example with notification opt-out:
 - `fs/watch` — subscribe this connection to filesystem change notifications for an absolute file or directory path and caller-provided `watchId`; returns the canonicalized `path`.
 - `fs/unwatch` — stop sending notifications for a prior `fs/watch`; returns `{}`.
 - `fs/changed` — notification emitted when watched paths change, including the `watchId` and `changedPaths`.
-- `model/list` — list available models (set `includeHidden: true` to include entries with `hidden: true`), with reasoning effort options, `additionalSpeedTiers`, optional legacy `upgrade` model ids, optional `upgradeInfo` metadata (`model`, `upgradeCopy`, `modelLink`, `migrationMarkdown`), and optional `availabilityNux` metadata.
+- `model/list` — list available models (set `includeHidden: true` to include entries with `hidden: true`), with reasoning effort options, `additionalSpeedTiers`, `serviceTiers`, optional `defaultServiceTier`, optional legacy `upgrade` model ids, optional `upgradeInfo` metadata (`model`, `upgradeCopy`, `modelLink`, `migrationMarkdown`), and optional `availabilityNux` metadata.
 - `modelProvider/capabilities/read` — read provider-level capabilities for the currently configured model provider.
 - `experimentalFeature/list` — list feature flags with stage metadata (`beta`, `underDevelopment`, `stable`, etc.), enabled/default-enabled state, and cursor pagination. Pass `threadId` when showing feature state for an existing loaded thread so `enabled` is computed from that thread's refreshed config, including project-local config for the thread's cwd; if omitted, the server uses its default config resolution context. For non-beta flags, `displayName`/`description`/`announcement` are `null`.
 - `permissionProfile/list` — beta; list available permission profile ids with optional display `description` text, using cursor pagination. Pass `cwd` when the caller needs project-local `[permissions.<id>]` entries to be included in the current catalog view.
diff --git a/codex-rs/app-server/src/models.rs b/codex-rs/app-server/src/models.rs
index 4d75a2058063..e53f13df84e0 100644
--- a/codex-rs/app-server/src/models.rs
+++ b/codex-rs/app-server/src/models.rs
@@ -53,6 +53,7 @@ fn model_from_preset(preset: ModelPreset) -> Model {
                 description: service_tier.description,
             })
             .collect(),
+        default_service_tier: preset.default_service_tier,
         is_default: preset.is_default,
     }
 }
diff --git a/codex-rs/app-server/tests/common/models_cache.rs b/codex-rs/app-server/tests/common/models_cache.rs
index be7d5d047f9d..0a41daa7c67e 100644
--- a/codex-rs/app-server/tests/common/models_cache.rs
+++ b/codex-rs/app-server/tests/common/models_cache.rs
@@ -30,6 +30,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         priority,
         additional_speed_tiers: preset.additional_speed_tiers.clone(),
         service_tiers: preset.service_tiers.clone(),
+        default_service_tier: preset.default_service_tier.clone(),
         upgrade: preset.upgrade.as_ref().map(Into::into),
         base_instructions: "base instructions".to_string(),
         model_messages: None,
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 d22bdd2a188a..94dcca4377bb 100644
--- a/codex-rs/app-server/tests/suite/v2/model_list.rs
+++ b/codex-rs/app-server/tests/suite/v2/model_list.rs
@@ -69,6 +69,7 @@ fn model_from_preset(preset: &ModelPreset) -> Model {
                 description: service_tier.description.clone(),
             })
             .collect(),
+        default_service_tier: preset.default_service_tier.clone(),
         is_default: preset.is_default,
     }
 }
diff --git a/codex-rs/app-server/tests/suite/v2/thread_settings_update.rs b/codex-rs/app-server/tests/suite/v2/thread_settings_update.rs
index 1bc3d742742b..aef2e961c082 100644
--- a/codex-rs/app-server/tests/suite/v2/thread_settings_update.rs
+++ b/codex-rs/app-server/tests/suite/v2/thread_settings_update.rs
@@ -22,6 +22,7 @@ use codex_app_server_protocol::TurnStartParams;
 use codex_app_server_protocol::TurnStartResponse;
 use codex_app_server_protocol::UserInput as V2UserInput;
 use codex_core::test_support::all_model_presets;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use core_test_support::responses;
 use pretty_assertions::assert_eq;
 use serde_json::Value;
@@ -136,7 +137,7 @@ async fn thread_settings_update_while_turn_is_active_emits_notification() -> Res
 }
 
 #[tokio::test]
-async fn thread_settings_update_clears_service_tier() -> Result<()> {
+async fn thread_settings_update_null_service_tier_uses_default() -> Result<()> {
     let server = create_mock_responses_server_sequence_unchecked(vec![
         create_final_assistant_message_sse_response("done")?,
     ])
@@ -181,7 +182,10 @@ async fn thread_settings_update_clears_service_tier() -> Result<()> {
     let clear_updated = read_thread_settings_updated(&mut mcp).await?;
     assert_eq!(clear_updated.thread_id, thread.id);
     assert_eq!(clear_updated.thread_settings.model, model_id);
-    assert_eq!(clear_updated.thread_settings.service_tier, None);
+    assert_eq!(
+        clear_updated.thread_settings.service_tier.as_deref(),
+        Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE)
+    );
 
     start_text_turn(&mut mcp, thread.id).await?;
     timeout(
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 75c124e8931f..ecd8ad8e3d64 100644
--- a/codex-rs/app-server/tests/suite/v2/thread_start.rs
+++ b/codex-rs/app-server/tests/suite/v2/thread_start.rs
@@ -28,6 +28,7 @@ use codex_core::config::set_project_trust_level;
 use codex_exec_server::LOCAL_FS;
 use codex_git_utils::resolve_root_git_project_for_trust;
 use codex_login::REFRESH_TOKEN_URL_OVERRIDE_ENV_VAR;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::TrustLevel;
 use codex_protocol::openai_models::ReasoningEffort;
 use pretty_assertions::assert_eq;
@@ -484,7 +485,7 @@ model_reasoning_effort = "high"
 }
 
 #[tokio::test]
-async fn thread_start_accepts_arbitrary_service_tier_id() -> Result<()> {
+async fn thread_start_drops_unsupported_service_tier_id() -> Result<()> {
     let server = create_mock_responses_server_repeating_assistant("Done").await;
 
     let codex_home = TempDir::new()?;
@@ -508,7 +509,39 @@ async fn thread_start_accepts_arbitrary_service_tier_id() -> Result<()> {
     .await??;
     let ThreadStartResponse { service_tier, .. } = to_response::<ThreadStartResponse>(resp)?;
 
-    assert_eq!(service_tier, Some(service_tier_id));
+    // Unsupported catalog ids are dropped at session config time instead of echoed back.
+    assert_eq!(service_tier, None);
+    Ok(())
+}
+
+#[tokio::test]
+async fn thread_start_accepts_default_service_tier() -> Result<()> {
+    let server = create_mock_responses_server_repeating_assistant("Done").await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml_without_approval_policy(codex_home.path(), &server.uri())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let req_id = mcp
+        .send_thread_start_request(ThreadStartParams {
+            service_tier: Some(Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())),
+            ..Default::default()
+        })
+        .await?;
+
+    let resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(req_id)),
+    )
+    .await??;
+    let ThreadStartResponse { service_tier, .. } = to_response::<ThreadStartResponse>(resp)?;
+
+    assert_eq!(
+        service_tier,
+        Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())
+    );
     Ok(())
 }
 
diff --git a/codex-rs/codex-api/tests/models_integration.rs b/codex-rs/codex-api/tests/models_integration.rs
index d2b31180b907..b1f6f6d4bbf6 100644
--- a/codex-rs/codex-api/tests/models_integration.rs
+++ b/codex-rs/codex-api/tests/models_integration.rs
@@ -76,6 +76,7 @@ async fn models_client_hits_models_endpoint() {
             priority: 1,
             additional_speed_tiers: Vec::new(),
             service_tiers: Vec::new(),
+            default_service_tier: None,
             upgrade: None,
             base_instructions: "base instructions".to_string(),
             model_messages: None,
diff --git a/codex-rs/config/src/config_toml.rs b/codex-rs/config/src/config_toml.rs
index 43f75ffc289e..a4d384b5f39e 100644
--- a/codex-rs/config/src/config_toml.rs
+++ b/codex-rs/config/src/config_toml.rs
@@ -359,7 +359,7 @@ pub struct ConfigToml {
     pub personality: Option<Personality>,
 
     /// Optional explicit service tier request id for new turns (for example
-    /// `priority` or `flex`; legacy `fast` also works).
+    /// `default`, `priority`, or `flex`; legacy `fast` also works).
     pub service_tier: Option<String>,
 
     /// Base URL for requests to ChatGPT (as opposed to the OpenAI API).
diff --git a/codex-rs/config/src/profile_toml.rs b/codex-rs/config/src/profile_toml.rs
index 6cf35be68e2d..5f4c8d62f910 100644
--- a/codex-rs/config/src/profile_toml.rs
+++ b/codex-rs/config/src/profile_toml.rs
@@ -24,7 +24,7 @@ use codex_protocol::protocol::AskForApproval;
 pub struct ConfigProfile {
     pub model: Option<String>,
     /// Optional explicit service tier request id for new turns (for example
-    /// `priority` or `flex`; legacy `fast` also works).
+    /// `default`, `priority`, or `flex`; legacy `fast` also works).
     pub service_tier: Option<String>,
     /// The key in the `model_providers` map identifying the
     /// [`ModelProviderInfo`] to use.
diff --git a/codex-rs/core/config.schema.json b/codex-rs/core/config.schema.json
index c2f6218fd250..a71f84f27779 100644
--- a/codex-rs/core/config.schema.json
+++ b/codex-rs/core/config.schema.json
@@ -686,7 +686,7 @@
           "$ref": "#/definitions/SandboxMode"
         },
         "service_tier": {
-          "description": "Optional explicit service tier request id for new turns (for example `priority` or `flex`; legacy `fast` also works).",
+          "description": "Optional explicit service tier request id for new turns (for example `default`, `priority`, or `flex`; legacy `fast` also works).",
           "type": "string"
         },
         "tools": {
@@ -4783,7 +4783,7 @@
       "description": "Sandbox configuration to apply if `sandbox` is `WorkspaceWrite`."
     },
     "service_tier": {
-      "description": "Optional explicit service tier request id for new turns (for example `priority` or `flex`; legacy `fast` also works).",
+      "description": "Optional explicit service tier request id for new turns (for example `default`, `priority`, or `flex`; legacy `fast` also works).",
       "type": "string"
     },
     "shell_environment_policy": {
diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index e3569e6d18a2..cbffab7b33b0 100644
--- a/codex-rs/core/src/client.rs
+++ b/codex-rs/core/src/client.rs
@@ -748,8 +748,7 @@ impl ModelClient {
             prompt.output_schema_strict,
         );
         let prompt_cache_key = Some(self.state.thread_id.to_string());
-        let service_tier =
-            service_tier.filter(|service_tier| model_info.supports_service_tier(service_tier));
+        let service_tier = model_info.service_tier_for_request(service_tier);
         let request = ResponsesApiRequest {
             model: model_info.slug.clone(),
             instructions: instructions.clone(),
diff --git a/codex-rs/core/src/config/config_tests.rs b/codex-rs/core/src/config/config_tests.rs
index 19fd762da916..66ca4daa37b4 100644
--- a/codex-rs/core/src/config/config_tests.rs
+++ b/codex-rs/core/src/config/config_tests.rs
@@ -70,6 +70,7 @@ use codex_model_provider_info::OLLAMA_OSS_PROVIDER_ID;
 use codex_model_provider_info::WireApi;
 use codex_models_manager::bundled_models_response;
 use codex_network_proxy::NetworkMode;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::models::ActivePermissionProfile;
 use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_DANGER_FULL_ACCESS;
@@ -8277,7 +8278,7 @@ alpha = "one\ntwo"
 }
 
 #[tokio::test]
-async fn explicit_null_service_tier_override_sets_fast_default_opt_out() -> std::io::Result<()> {
+async fn explicit_null_service_tier_override_maps_to_default_service_tier() -> std::io::Result<()> {
     let fixture = create_test_fixture()?;
 
     let config = Config::load_from_base_config_with_overrides(
@@ -8291,8 +8292,33 @@ async fn explicit_null_service_tier_override_sets_fast_default_opt_out() -> std:
     )
     .await?;
 
-    assert_eq!(config.service_tier, None);
-    assert_eq!(config.notices.fast_default_opt_out, Some(true));
+    assert_eq!(
+        config.service_tier,
+        Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())
+    );
+    assert_eq!(config.notices.fast_default_opt_out, None);
+    Ok(())
+}
+
+#[tokio::test]
+async fn default_service_tier_override_uses_default_request_value() -> 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("default".to_string())),
+            ..Default::default()
+        },
+        fixture.codex_home(),
+    )
+    .await?;
+
+    assert_eq!(
+        config.service_tier,
+        Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())
+    );
     Ok(())
 }
 
diff --git a/codex-rs/core/src/config/edit.rs b/codex-rs/core/src/config/edit.rs
index 5418d3b2689e..f467c76cf976 100644
--- a/codex-rs/core/src/config/edit.rs
+++ b/codex-rs/core/src/config/edit.rs
@@ -40,8 +40,6 @@ pub enum ConfigEdit {
     SetNoticeHideFullAccessWarning(bool),
     /// Toggle the Windows world-writable directories warning acknowledgement flag.
     SetNoticeHideWorldWritableWarning(bool),
-    /// Toggle the opt-out marker for Codex-managed fast defaults.
-    SetNoticeFastDefaultOptOut(bool),
     /// Toggle the rate limit model nudge acknowledgement flag.
     SetNoticeHideRateLimitModelNudge(bool),
     /// Toggle the model migration prompt acknowledgement flag.
@@ -552,6 +550,8 @@ impl ConfigDocument {
             ConfigEdit::SetServiceTier { service_tier } => Ok(self.write_profile_value(
                 &["service_tier"],
                 service_tier.as_ref().map(|service_tier| {
+                    // Keep the legacy config spelling stable. Runtime values use
+                    // `priority`, but config.toml continues to store it as `fast`.
                     let config_value = match ServiceTier::from_request_value(service_tier) {
                         Some(ServiceTier::Fast) => "fast",
                         Some(ServiceTier::Flex) => "flex",
@@ -574,11 +574,6 @@ impl ConfigDocument {
                 &[NOTICE_TABLE_KEY, "hide_world_writable_warning"],
                 value(*acknowledged),
             )),
-            ConfigEdit::SetNoticeFastDefaultOptOut(opted_out) => Ok(self.write_value(
-                Scope::Global,
-                &[NOTICE_TABLE_KEY, "fast_default_opt_out"],
-                value(*opted_out),
-            )),
             ConfigEdit::SetNoticeHideRateLimitModelNudge(acknowledged) => Ok(self.write_value(
                 Scope::Global,
                 &[NOTICE_TABLE_KEY, "hide_rate_limit_model_nudge"],
@@ -1182,12 +1177,6 @@ impl ConfigEditsBuilder {
         self
     }
 
-    pub fn set_fast_default_opt_out(mut self, opted_out: bool) -> Self {
-        self.edits
-            .push(ConfigEdit::SetNoticeFastDefaultOptOut(opted_out));
-        self
-    }
-
     pub fn set_hide_rate_limit_model_nudge(mut self, acknowledged: bool) -> Self {
         self.edits
             .push(ConfigEdit::SetNoticeHideRateLimitModelNudge(acknowledged));
diff --git a/codex-rs/core/src/config/edit_tests.rs b/codex-rs/core/src/config/edit_tests.rs
index 751411afdf01..30f82db29ba2 100644
--- a/codex-rs/core/src/config/edit_tests.rs
+++ b/codex-rs/core/src/config/edit_tests.rs
@@ -4,6 +4,7 @@ use codex_config::types::McpServerOAuthConfig;
 use codex_config::types::McpServerToolConfig;
 use codex_config::types::McpServerTransportConfig;
 use codex_config::types::SessionPickerViewMode;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::openai_models::ReasoningEffort;
 use pretty_assertions::assert_eq;
@@ -34,6 +35,20 @@ model_reasoning_effort = "high"
     assert_eq!(contents, expected);
 }
 
+#[test]
+fn set_service_tier_saves_default_as_default() {
+    let tmp = tempdir().expect("tmpdir");
+    let codex_home = tmp.path();
+
+    ConfigEditsBuilder::new(codex_home)
+        .set_service_tier(Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string()))
+        .apply_blocking()
+        .expect("persist");
+
+    let contents = std::fs::read_to_string(codex_home.join(CONFIG_TOML_FILE)).expect("read config");
+    assert_eq!(contents, "service_tier = \"default\"\n");
+}
+
 #[test]
 fn set_service_tier_saves_priority_as_fast() {
     let tmp = tempdir().expect("tmpdir");
diff --git a/codex-rs/core/src/config/mod.rs b/codex-rs/core/src/config/mod.rs
index ff09f26c2845..305ae3b75c43 100644
--- a/codex-rs/core/src/config/mod.rs
+++ b/codex-rs/core/src/config/mod.rs
@@ -81,6 +81,7 @@ use codex_protocol::config_types::AutoCompactTokenLimitScope;
 use codex_protocol::config_types::ForcedLoginMethod;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::SandboxMode;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::ShellEnvironmentPolicy;
@@ -552,6 +553,7 @@ pub struct Config {
     pub model: Option<String>,
 
     /// Effective service tier request id preference for new turns.
+    /// `default` means the user explicitly selected standard routing.
     pub service_tier: Option<String>,
 
     /// Model used specifically for review sessions.
@@ -3169,15 +3171,10 @@ impl Config {
         let forced_login_method = cfg.forced_login_method;
 
         let model = model.or(config_profile.model).or(cfg.model);
-        let mut notices = cfg.notice.unwrap_or_default();
+        let notices = cfg.notice.unwrap_or_default();
         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
-            }
+            Some(None) => Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string()),
             None => config_profile.service_tier.or(cfg.service_tier),
         };
         let service_tier = service_tier.and_then(|service_tier| {
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index 55fd5444c6d2..c12c29e7e174 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -72,7 +72,6 @@ use codex_otel::current_span_trace_id;
 use codex_otel::current_span_w3c_trace_context;
 use codex_otel::set_parent_from_w3c_trace_context;
 use codex_protocol::ThreadId;
-use codex_protocol::account::PlanType as AccountPlanType;
 use codex_protocol::approvals::ElicitationRequestEvent;
 use codex_protocol::approvals::ExecPolicyAmendment;
 use codex_protocol::approvals::NetworkPolicyAmendment;
@@ -80,6 +79,7 @@ use codex_protocol::approvals::NetworkPolicyRuleAction;
 use codex_protocol::config_types::ApprovalsReviewer;
 use codex_protocol::config_types::AutoCompactTokenLimitScope;
 use codex_protocol::config_types::ModeKind;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::Settings;
 use codex_protocol::config_types::WebSearchMode;
 use codex_protocol::dynamic_tools::DynamicToolResponse;
@@ -320,7 +320,6 @@ use codex_otel::TelemetryAuthMode;
 use codex_protocol::config_types::CollaborationMode;
 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::models::ContentItem;
 use codex_protocol::models::ResponseInputItem;
@@ -589,14 +588,10 @@ impl Codex {
                 developer_instructions: None,
             },
         };
-        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(),
-            config.notices.fast_default_opt_out.unwrap_or(false),
-            account_plan_type,
             config.features.enabled(Feature::FastMode),
+            &model_info,
         );
         let session_configuration = SessionConfiguration {
             provider: config.model_provider.clone(),
@@ -798,17 +793,16 @@ impl Codex {
 
 fn get_service_tier(
     configured_service_tier: Option<String>,
-    fast_default_opt_out: bool,
-    account_plan_type: Option<AccountPlanType>,
     fast_mode_enabled: bool,
+    model_info: &ModelInfo,
 ) -> Option<String> {
-    if configured_service_tier.is_some() || fast_default_opt_out || !fast_mode_enabled {
-        return configured_service_tier;
+    if !fast_mode_enabled {
+        return None;
     }
-
-    account_plan_type
-        .is_some_and(is_enterprise_default_service_tier_plan)
-        .then_some(ServiceTier::Fast.request_value().to_string())
+    configured_service_tier.filter(|service_tier| {
+        service_tier == SERVICE_TIER_DEFAULT_REQUEST_VALUE
+            || model_info.supports_service_tier(service_tier)
+    })
 }
 
 fn session_permission_profile_state_from_config(
@@ -817,12 +811,6 @@ fn session_permission_profile_state_from_config(
     Ok(config.permissions.permission_profile_state().clone())
 }
 
-fn is_enterprise_default_service_tier_plan(plan_type: AccountPlanType) -> bool {
-    plan_type == AccountPlanType::Enterprise
-        || plan_type.is_business_like()
-        || plan_type.is_team_like()
-}
-
 #[cfg(test)]
 pub(crate) fn completed_session_loop_termination() -> SessionLoopTermination {
     futures::future::ready(()).boxed().shared()
diff --git a/codex-rs/core/src/session/session.rs b/codex-rs/core/src/session/session.rs
index 7ae5ff241d4e..cac4806bedbc 100644
--- a/codex-rs/core/src/session/session.rs
+++ b/codex-rs/core/src/session/session.rs
@@ -5,6 +5,7 @@ use crate::goals::GoalRuntimeState;
 use crate::skills::SkillError;
 use crate::state::ActiveTurn;
 use codex_protocol::SessionId;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::permissions::FileSystemPath;
 use codex_protocol::permissions::FileSystemSpecialPath;
@@ -223,12 +224,15 @@ impl SessionConfiguration {
         if let Some(service_tier) = updates.service_tier.clone() {
             // TODO(aibrahim): Remove once v2 clients no longer send the legacy
             // "fast" service tier value.
-            next_configuration.service_tier = service_tier.map(|service_tier| {
-                ServiceTier::from_request_value(&service_tier)
-                    .map_or(service_tier, |service_tier| {
-                        service_tier.request_value().to_string()
-                    })
-            });
+            next_configuration.service_tier = match service_tier {
+                Some(service_tier) => Some(
+                    ServiceTier::from_request_value(&service_tier)
+                        .map_or(service_tier, |service_tier| {
+                            service_tier.request_value().to_string()
+                        }),
+                ),
+                None => Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string()),
+            };
         }
         if let Some(personality) = updates.personality {
             next_configuration.personality = Some(personality);
diff --git a/codex-rs/core/src/session/tests.rs b/codex-rs/core/src/session/tests.rs
index e669becc4c9a..d4d0c286608f 100644
--- a/codex-rs/core/src/session/tests.rs
+++ b/codex-rs/core/src/session/tests.rs
@@ -32,7 +32,7 @@ use codex_models_manager::test_support::get_model_offline_for_tests;
 use codex_protocol::AgentPath;
 use codex_protocol::SessionId;
 use codex_protocol::ThreadId;
-use codex_protocol::account::PlanType as AccountPlanType;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::TrustLevel;
 use codex_protocol::exec_output::ExecToolCallOutput;
@@ -43,6 +43,7 @@ use codex_protocol::models::FunctionCallOutputBody;
 use codex_protocol::models::FunctionCallOutputPayload;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::models::SandboxEnforcement;
+use codex_protocol::openai_models::ModelServiceTier;
 use codex_protocol::permissions::FileSystemAccessMode;
 use codex_protocol::permissions::FileSystemPath;
 use codex_protocol::permissions::FileSystemSandboxEntry;
@@ -3390,92 +3391,143 @@ fn session_telemetry(
     )
 }
 
+fn model_with_default_service_tier(default_service_tier: Option<&str>) -> ModelInfo {
+    let mut model_info = model_info::model_info_from_slug("gpt-5.4");
+    model_info.service_tiers = vec![ModelServiceTier {
+        id: ServiceTier::Fast.request_value().to_string(),
+        name: "Fast".to_string(),
+        description: "Priority processing.".to_string(),
+    }];
+    model_info.default_service_tier = default_service_tier.map(str::to_string);
+    model_info
+}
+
 #[test]
-fn get_service_tier_defaults_enterprise_accounts_to_fast() {
+fn get_service_tier_does_not_use_model_default_when_absent_and_fast_mode_enabled() {
+    let model_info = model_with_default_service_tier(Some(ServiceTier::Fast.request_value()));
+
     assert_eq!(
         get_service_tier(
             /*configured_service_tier*/ None,
-            /*fast_default_opt_out*/ false,
-            Some(AccountPlanType::Enterprise),
             /*fast_mode_enabled*/ true,
+            &model_info,
         ),
-        Some(ServiceTier::Fast.request_value().to_string())
+        None
     );
+}
+
+#[test]
+fn get_service_tier_does_not_use_model_default_when_fast_mode_disabled() {
+    let model_info = model_with_default_service_tier(Some(ServiceTier::Fast.request_value()));
+
     assert_eq!(
         get_service_tier(
             /*configured_service_tier*/ None,
-            /*fast_default_opt_out*/ false,
-            Some(AccountPlanType::EnterpriseCbpUsageBased),
-            /*fast_mode_enabled*/ true,
+            /*fast_mode_enabled*/ false,
+            &model_info,
         ),
-        Some(ServiceTier::Fast.request_value().to_string())
+        None
     );
+}
+
+#[test]
+fn get_service_tier_keeps_supported_explicit_tier() {
+    let model_info = model_with_default_service_tier(Some(ServiceTier::Fast.request_value()));
+
     assert_eq!(
         get_service_tier(
-            /*configured_service_tier*/ None,
-            /*fast_default_opt_out*/ false,
-            Some(AccountPlanType::Business),
+            Some(ServiceTier::Fast.request_value().to_string()),
             /*fast_mode_enabled*/ true,
+            &model_info,
         ),
         Some(ServiceTier::Fast.request_value().to_string())
     );
+}
+
+#[test]
+fn get_service_tier_does_not_default_when_model_has_no_default() {
+    let model_info = model_with_default_service_tier(/*default_service_tier*/ None);
+
     assert_eq!(
         get_service_tier(
             /*configured_service_tier*/ None,
-            /*fast_default_opt_out*/ false,
-            Some(AccountPlanType::Team),
             /*fast_mode_enabled*/ true,
+            &model_info,
         ),
-        Some(ServiceTier::Fast.request_value().to_string())
+        None
     );
+}
+
+#[test]
+fn get_service_tier_drops_unsupported_configured_tier_when_fast_mode_enabled() {
+    let model_info = model_with_default_service_tier(Some(ServiceTier::Fast.request_value()));
+
     assert_eq!(
         get_service_tier(
-            /*configured_service_tier*/ None,
-            /*fast_default_opt_out*/ false,
-            Some(AccountPlanType::SelfServeBusinessUsageBased),
+            Some("unsupported".to_string()),
             /*fast_mode_enabled*/ true,
+            &model_info,
         ),
-        Some(ServiceTier::Fast.request_value().to_string())
+        None
     );
-}
-
-#[test]
-fn get_service_tier_respects_fast_default_opt_out() {
     assert_eq!(
         get_service_tier(
-            /*configured_service_tier*/ None,
-            /*fast_default_opt_out*/ true,
-            Some(AccountPlanType::Enterprise),
+            Some(ServiceTier::Flex.request_value().to_string()),
             /*fast_mode_enabled*/ true,
+            &model_info,
         ),
         None
     );
+    assert_eq!(
+        get_service_tier(
+            Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string()),
+            /*fast_mode_enabled*/ true,
+            &model_info,
+        ),
+        Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())
+    );
 }
 
 #[test]
-fn get_service_tier_does_not_default_non_enterprise_or_disabled_fast_mode() {
+fn get_service_tier_ignores_configured_tier_when_fast_mode_disabled() {
+    let model_info = model_with_default_service_tier(Some(ServiceTier::Fast.request_value()));
+
     assert_eq!(
         get_service_tier(
-            /*configured_service_tier*/ None,
-            /*fast_default_opt_out*/ false,
-            Some(AccountPlanType::Pro),
-            /*fast_mode_enabled*/ true,
+            Some(ServiceTier::Fast.request_value().to_string()),
+            /*fast_mode_enabled*/ false,
+            &model_info,
+        ),
+        None
+    );
+    assert_eq!(
+        get_service_tier(
+            Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string()),
+            /*fast_mode_enabled*/ false,
+            &model_info,
+        ),
+        None
+    );
+    assert_eq!(
+        get_service_tier(
+            Some("unsupported".to_string()),
+            /*fast_mode_enabled*/ false,
+            &model_info,
         ),
         None
     );
     assert_eq!(
         get_service_tier(
             /*configured_service_tier*/ None,
-            /*fast_default_opt_out*/ false,
-            Some(AccountPlanType::Enterprise),
             /*fast_mode_enabled*/ false,
+            &model_info,
         ),
         None
     );
 }
 
 #[tokio::test]
-async fn session_settings_null_service_tier_update_clears_service_tier() {
+async fn session_settings_null_service_tier_update_uses_default_service_tier() {
     let session_configuration = make_session_configuration_for_tests().await;
 
     let updated = session_configuration
@@ -3485,7 +3537,10 @@ async fn session_settings_null_service_tier_update_clears_service_tier() {
         })
         .expect("null service tier update should apply");
 
-    assert_eq!(updated.service_tier, None);
+    assert_eq!(
+        updated.service_tier,
+        Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())
+    );
 }
 
 #[tokio::test]
diff --git a/codex-rs/core/src/session/turn_context.rs b/codex-rs/core/src/session/turn_context.rs
index 818a8b7bbe3a..de380831358e 100644
--- a/codex-rs/core/src/session/turn_context.rs
+++ b/codex-rs/core/src/session/turn_context.rs
@@ -473,9 +473,11 @@ impl Session {
         );
 
         let mut per_turn_config = per_turn_config;
-        per_turn_config.service_tier = per_turn_config
-            .service_tier
-            .filter(|service_tier| model_info.supports_service_tier(service_tier));
+        per_turn_config.service_tier = get_service_tier(
+            per_turn_config.service_tier,
+            per_turn_config.features.enabled(Feature::FastMode),
+            &model_info,
+        );
         let per_turn_config = Arc::new(per_turn_config);
         let turn_metadata_state = Arc::new(TurnMetadataState::new(
             session_id.to_string(),
diff --git a/codex-rs/core/src/tools/handlers/multi_agents_spec_tests.rs b/codex-rs/core/src/tools/handlers/multi_agents_spec_tests.rs
index 6f9eac1fff11..d90cdc95e052 100644
--- a/codex-rs/core/src/tools/handlers/multi_agents_spec_tests.rs
+++ b/codex-rs/core/src/tools/handlers/multi_agents_spec_tests.rs
@@ -26,6 +26,7 @@ fn model_preset(id: &str, show_in_picker: bool) -> ModelPreset {
             name: "Fast".to_string(),
             description: "1.5x speed, increased usage".to_string(),
         }],
+        default_service_tier: None,
         is_default: false,
         upgrade: None,
         show_in_picker,
diff --git a/codex-rs/core/tests/common/test_codex.rs b/codex-rs/core/tests/common/test_codex.rs
index 5f5c7ee85a86..a39875829171 100644
--- a/codex-rs/core/tests/common/test_codex.rs
+++ b/codex-rs/core/tests/common/test_codex.rs
@@ -28,7 +28,6 @@ use codex_login::CodexAuth;
 use codex_model_provider_info::ModelProviderInfo;
 use codex_model_provider_info::built_in_model_providers;
 use codex_models_manager::bundled_models_response;
-use codex_protocol::config_types::ServiceTier;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::openai_models::ModelsResponse;
 use codex_protocol::protocol::AskForApproval;
@@ -667,13 +666,13 @@ impl TestCodex {
     pub async fn submit_turn_with_service_tier(
         &self,
         prompt: &str,
-        service_tier: Option<ServiceTier>,
+        service_tier: Option<&str>,
     ) -> Result<()> {
         self.submit_turn_with_permission_profile_context(
             prompt,
             AskForApproval::Never,
             PermissionProfile::Disabled,
-            Some(service_tier.map(|service_tier| service_tier.request_value().to_string())),
+            Some(service_tier.map(str::to_string)),
             /*environments*/ None,
         )
         .await
diff --git a/codex-rs/core/tests/suite/agent_websocket.rs b/codex-rs/core/tests/suite/agent_websocket.rs
index 6e985eebe0b6..6fe697d2b164 100644
--- a/codex-rs/core/tests/suite/agent_websocket.rs
+++ b/codex-rs/core/tests/suite/agent_websocket.rs
@@ -269,7 +269,7 @@ 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::Fast))
+    test.submit_turn_with_service_tier("hello", Some(ServiceTier::Fast.request_value()))
         .await?;
 
     assert_eq!(server.handshakes().len(), 1);
@@ -385,7 +385,7 @@ 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::Fast))
+    test.submit_turn_with_service_tier("first", Some(ServiceTier::Fast.request_value()))
         .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 009dfda60f52..7e77fb79d362 100644
--- a/codex-rs/core/tests/suite/model_switching.rs
+++ b/codex-rs/core/tests/suite/model_switching.rs
@@ -4,6 +4,7 @@ use codex_features::Feature;
 use codex_login::CodexAuth;
 use codex_models_manager::manager::RefreshStrategy;
 use codex_protocol::config_types::ReasoningSummary;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::openai_models::ConfigShellToolType;
@@ -113,6 +114,7 @@ fn test_model_info(
         priority: 1,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: None,
@@ -289,7 +291,7 @@ 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::Fast))
+    test.submit_turn_with_service_tier("fast turn", Some(ServiceTier::Fast.request_value()))
         .await?;
     test.submit_turn_with_service_tier("standard turn", /*service_tier*/ None)
         .await?;
@@ -334,7 +336,7 @@ async fn flex_service_tier_is_applied_to_http_turn() -> Result<()> {
         });
     let test = builder.build(&server).await?;
 
-    test.submit_turn_with_service_tier("flex turn", Some(ServiceTier::Flex))
+    test.submit_turn_with_service_tier("flex turn", Some(ServiceTier::Flex.request_value()))
         .await?;
 
     let request = resp_mock.single_request();
@@ -367,7 +369,85 @@ async fn unsupported_service_tier_is_omitted_from_http_turn() -> Result<()> {
         });
     let test = builder.build(&server).await?;
 
-    test.submit_turn_with_service_tier("fast turn", Some(ServiceTier::Fast))
+    test.submit_turn_with_service_tier("fast turn", Some(ServiceTier::Fast.request_value()))
+        .await?;
+
+    let request = resp_mock.single_request();
+    let body = request.body_json();
+    assert_eq!(body.get("service_tier"), None);
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn default_service_tier_override_is_omitted_from_http_turn() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let model_slug = "test-default-tier-model";
+    let mut model = test_model_info(
+        model_slug,
+        model_slug,
+        "has catalog default service tier",
+        default_input_modalities(),
+    );
+    model.service_tiers = vec![ModelServiceTier {
+        id: ServiceTier::Fast.request_value().to_string(),
+        name: "fast".to_string(),
+        description: "Fast processing.".to_string(),
+    }];
+    model.default_service_tier = Some(ServiceTier::Fast.request_value().to_string());
+    let resp_mock = mount_sse_once(&server, sse_completed("resp-1")).await;
+
+    let mut builder = test_codex()
+        .with_model(model_slug)
+        .with_config(move |config| {
+            config.model_catalog = Some(ModelsResponse {
+                models: vec![model],
+            });
+        });
+    let test = builder.build(&server).await?;
+
+    test.submit_turn_with_service_tier("default turn", Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE))
+        .await?;
+
+    let request = resp_mock.single_request();
+    let body = request.body_json();
+    assert_eq!(body.get("service_tier"), None);
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn null_service_tier_override_is_omitted_from_http_turn_with_catalog_default() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let model_slug = "test-null-default-tier-model";
+    let mut model = test_model_info(
+        model_slug,
+        model_slug,
+        "has catalog default service tier",
+        default_input_modalities(),
+    );
+    model.service_tiers = vec![ModelServiceTier {
+        id: ServiceTier::Fast.request_value().to_string(),
+        name: "fast".to_string(),
+        description: "Fast processing.".to_string(),
+    }];
+    model.default_service_tier = Some(ServiceTier::Fast.request_value().to_string());
+    let resp_mock = mount_sse_once(&server, sse_completed("resp-1")).await;
+
+    let mut builder = test_codex()
+        .with_model(model_slug)
+        .with_config(move |config| {
+            config.model_catalog = Some(ModelsResponse {
+                models: vec![model],
+            });
+        });
+    let test = builder.build(&server).await?;
+
+    test.submit_turn_with_service_tier("standard turn", /*service_tier*/ None)
         .await?;
 
     let request = resp_mock.single_request();
@@ -864,6 +944,7 @@ async fn model_switch_to_smaller_model_updates_token_context_window() -> Result<
         priority: 1,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: None,
diff --git a/codex-rs/core/tests/suite/models_cache_ttl.rs b/codex-rs/core/tests/suite/models_cache_ttl.rs
index 3b729738df38..6d11b47cba7a 100644
--- a/codex-rs/core/tests/suite/models_cache_ttl.rs
+++ b/codex-rs/core/tests/suite/models_cache_ttl.rs
@@ -347,6 +347,7 @@ fn test_remote_model(slug: &str, priority: i32) -> ModelInfo {
         priority,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: None,
diff --git a/codex-rs/core/tests/suite/personality.rs b/codex-rs/core/tests/suite/personality.rs
index bbe8178f8d8c..96695769c9a1 100644
--- a/codex-rs/core/tests/suite/personality.rs
+++ b/codex-rs/core/tests/suite/personality.rs
@@ -562,6 +562,7 @@ async fn remote_model_friendly_personality_instructions_with_feature() -> anyhow
         priority: 1,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: Some(ModelMessages {
@@ -671,6 +672,7 @@ async fn user_turn_personality_remote_model_template_includes_update_message() -
         priority: 1,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: Some(ModelMessages {
diff --git a/codex-rs/core/tests/suite/remote_models.rs b/codex-rs/core/tests/suite/remote_models.rs
index e143cb8e3ed9..18ebd732b7ff 100644
--- a/codex-rs/core/tests/suite/remote_models.rs
+++ b/codex-rs/core/tests/suite/remote_models.rs
@@ -475,6 +475,7 @@ async fn remote_models_remote_model_uses_unified_exec() -> Result<()> {
         priority: 1,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: None,
@@ -722,6 +723,7 @@ async fn remote_models_apply_remote_base_instructions() -> Result<()> {
         priority: 1,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: remote_base.to_string(),
         model_messages: None,
@@ -1203,6 +1205,7 @@ fn test_remote_model_with_policy(
         priority,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: None,
diff --git a/codex-rs/core/tests/suite/rmcp_client.rs b/codex-rs/core/tests/suite/rmcp_client.rs
index 210c287c2f47..c00da7723653 100644
--- a/codex-rs/core/tests/suite/rmcp_client.rs
+++ b/codex-rs/core/tests/suite/rmcp_client.rs
@@ -1275,6 +1275,7 @@ async fn stdio_image_responses_are_sanitized_for_text_only_model() -> anyhow::Re
                 priority: 1,
                 additional_speed_tiers: Vec::new(),
                 service_tiers: Vec::new(),
+                default_service_tier: None,
                 upgrade: None,
                 base_instructions: "base instructions".to_string(),
                 model_messages: None,
diff --git a/codex-rs/core/tests/suite/spawn_agent_description.rs b/codex-rs/core/tests/suite/spawn_agent_description.rs
index 9a7d70adebb1..fbeb90fba50c 100644
--- a/codex-rs/core/tests/suite/spawn_agent_description.rs
+++ b/codex-rs/core/tests/suite/spawn_agent_description.rs
@@ -63,6 +63,7 @@ fn test_model_info(
         priority: 1,
         additional_speed_tiers: Vec::new(),
         service_tiers,
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: None,
diff --git a/codex-rs/core/tests/suite/view_image.rs b/codex-rs/core/tests/suite/view_image.rs
index 6306b88b8f19..867ba108588b 100644
--- a/codex-rs/core/tests/suite/view_image.rs
+++ b/codex-rs/core/tests/suite/view_image.rs
@@ -1357,6 +1357,7 @@ async fn view_image_tool_returns_unsupported_message_for_text_only_model() -> an
         priority: 1,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
         model_messages: None,
diff --git a/codex-rs/model-provider/src/amazon_bedrock/catalog.rs b/codex-rs/model-provider/src/amazon_bedrock/catalog.rs
index d6fe1de09532..8fec7f835e47 100644
--- a/codex-rs/model-provider/src/amazon_bedrock/catalog.rs
+++ b/codex-rs/model-provider/src/amazon_bedrock/catalog.rs
@@ -55,6 +55,7 @@ fn gpt_5_4_cmb_bedrock_model(priority: i32) -> ModelInfo {
             name: SPEED_TIER_FAST.to_string(),
             description: "Fastest inference with increased plan usage".to_string(),
         }],
+        default_service_tier: None,
         availability_nux: None,
         upgrade: None,
         base_instructions: BASE_INSTRUCTIONS.to_string(),
@@ -96,6 +97,7 @@ fn bedrock_oss_model(slug: &str, display_name: &str, priority: i32) -> ModelInfo
         priority,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         availability_nux: None,
         upgrade: None,
         base_instructions: BASE_INSTRUCTIONS.to_string(),
diff --git a/codex-rs/models-manager/src/model_info.rs b/codex-rs/models-manager/src/model_info.rs
index 774dd3eacafc..17b843440aa7 100644
--- a/codex-rs/models-manager/src/model_info.rs
+++ b/codex-rs/models-manager/src/model_info.rs
@@ -77,6 +77,7 @@ pub fn model_info_from_slug(slug: &str) -> ModelInfo {
         priority: 99,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         availability_nux: None,
         upgrade: None,
         base_instructions: BASE_INSTRUCTIONS.to_string(),
diff --git a/codex-rs/protocol/src/config_types.rs b/codex-rs/protocol/src/config_types.rs
index b4b4759c9b71..550dbdaf3a98 100644
--- a/codex-rs/protocol/src/config_types.rs
+++ b/codex-rs/protocol/src/config_types.rs
@@ -432,6 +432,12 @@ pub enum ServiceTier {
     Flex,
 }
 
+/// Request/config sentinel for explicit standard routing.
+///
+/// This is not a catalog service tier id. It means the user intentionally
+/// selected no service tier, so model catalog defaults should not apply.
+pub const SERVICE_TIER_DEFAULT_REQUEST_VALUE: &str = "default";
+
 impl ServiceTier {
     pub const fn request_value(self) -> &'static str {
         match self {
diff --git a/codex-rs/protocol/src/openai_models.rs b/codex-rs/protocol/src/openai_models.rs
index d51e70ddf16f..d0a58b502463 100644
--- a/codex-rs/protocol/src/openai_models.rs
+++ b/codex-rs/protocol/src/openai_models.rs
@@ -17,6 +17,7 @@ use ts_rs::TS;
 
 use crate::config_types::Personality;
 use crate::config_types::ReasoningSummary;
+use crate::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use crate::config_types::ServiceTier;
 use crate::config_types::Verbosity;
 
@@ -147,6 +148,9 @@ pub struct ModelPreset {
     /// Service tiers this model can run with.
     #[serde(default)]
     pub service_tiers: Vec<ModelServiceTier>,
+    /// Catalog default service tier id for this model.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub default_service_tier: Option<String>,
     /// Whether this is the default model for new users.
     pub is_default: bool,
     /// recommended upgrade model
@@ -270,6 +274,8 @@ pub struct ModelInfo {
     pub additional_speed_tiers: Vec<String>,
     #[serde(default)]
     pub service_tiers: Vec<ModelServiceTier>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub default_service_tier: Option<String>,
     pub availability_nux: Option<ModelAvailabilityNux>,
     pub upgrade: Option<ModelInfoUpgrade>,
     pub base_instructions: String,
@@ -455,6 +461,7 @@ impl From<ModelInfo> for ModelPreset {
             supports_personality,
             additional_speed_tiers: info.additional_speed_tiers,
             service_tiers: info.service_tiers,
+            default_service_tier: info.default_service_tier,
             is_default: false, // default is the highest priority available model
             upgrade: info.upgrade.as_ref().map(|upgrade| ModelUpgrade {
                 id: upgrade.model.clone(),
@@ -493,6 +500,13 @@ impl ModelInfo {
             .iter()
             .any(|tier| tier.id == service_tier)
     }
+
+    pub fn service_tier_for_request(&self, service_tier: Option<String>) -> Option<String> {
+        service_tier.filter(|service_tier| {
+            service_tier != SERVICE_TIER_DEFAULT_REQUEST_VALUE
+                && self.supports_service_tier(service_tier)
+        })
+    }
 }
 
 impl ModelPreset {
@@ -576,6 +590,7 @@ mod tests {
             priority: 1,
             additional_speed_tiers: Vec::new(),
             service_tiers: Vec::new(),
+            default_service_tier: None,
             availability_nux: None,
             upgrade: None,
             base_instructions: "base".to_string(),
@@ -846,6 +861,7 @@ mod tests {
                 message: "Try Spark.".to_string(),
             }),
             additional_speed_tiers: vec![SPEED_TIER_FAST.to_string()],
+            default_service_tier: Some(ServiceTier::Fast.request_value().to_string()),
             service_tiers: Vec::new(),
             ..test_model(/*spec*/ None)
         });
@@ -857,6 +873,10 @@ mod tests {
             })
         );
         assert!(preset.supports_fast_mode());
+        assert_eq!(
+            preset.default_service_tier,
+            Some(ServiceTier::Fast.request_value().to_string())
+        );
     }
 
     #[test]
@@ -872,4 +892,60 @@ mod tests {
 
         assert!(preset.supports_fast_mode());
     }
+
+    #[test]
+    fn service_tier_for_request_omits_explicit_default_tier() {
+        let model = ModelInfo {
+            default_service_tier: Some(ServiceTier::Fast.request_value().to_string()),
+            service_tiers: vec![ModelServiceTier {
+                id: ServiceTier::Fast.request_value().to_string(),
+                name: "Fast".to_string(),
+                description: "Priority processing.".to_string(),
+            }],
+            ..test_model(/*spec*/ None)
+        };
+
+        assert_eq!(
+            model.service_tier_for_request(Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())),
+            None
+        );
+    }
+
+    #[test]
+    fn service_tier_for_request_filters_unsupported_tiers() {
+        let model = ModelInfo {
+            default_service_tier: Some(ServiceTier::Fast.request_value().to_string()),
+            service_tiers: vec![ModelServiceTier {
+                id: ServiceTier::Fast.request_value().to_string(),
+                name: "Fast".to_string(),
+                description: "Priority processing.".to_string(),
+            }],
+            ..test_model(/*spec*/ None)
+        };
+
+        assert_eq!(
+            model.service_tier_for_request(Some(ServiceTier::Fast.request_value().to_string())),
+            Some(ServiceTier::Fast.request_value().to_string())
+        );
+        assert_eq!(
+            model.service_tier_for_request(Some("unsupported".to_string())),
+            None
+        );
+        assert_eq!(model.service_tier_for_request(/*service_tier*/ None), None);
+    }
+
+    #[test]
+    fn service_tier_for_request_does_not_apply_catalog_default() {
+        let model = ModelInfo {
+            default_service_tier: Some(ServiceTier::Fast.request_value().to_string()),
+            service_tiers: vec![ModelServiceTier {
+                id: ServiceTier::Fast.request_value().to_string(),
+                name: "Fast".to_string(),
+                description: "Priority processing.".to_string(),
+            }],
+            ..test_model(/*spec*/ None)
+        };
+
+        assert_eq!(model.service_tier_for_request(/*service_tier*/ None), None);
+    }
 }
diff --git a/codex-rs/tools/src/tool_config_tests.rs b/codex-rs/tools/src/tool_config_tests.rs
index 8ce68acbbad1..033bf07b10b0 100644
--- a/codex-rs/tools/src/tool_config_tests.rs
+++ b/codex-rs/tools/src/tool_config_tests.rs
@@ -22,6 +22,7 @@ fn model_with_shell_type(shell_type: ConfigShellToolType) -> ModelInfo {
         priority: 0,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         availability_nux: None,
         upgrade: None,
         base_instructions: String::new(),
diff --git a/codex-rs/tui/src/app/event_dispatch.rs b/codex-rs/tui/src/app/event_dispatch.rs
index 51615f6a956f..bdd7b46917d2 100644
--- a/codex-rs/tui/src/app/event_dispatch.rs
+++ b/codex-rs/tui/src/app/event_dispatch.rs
@@ -767,6 +767,8 @@ impl App {
                 self.chat_widget.set_model(&model);
                 self.sync_active_thread_model_setting(app_server, model)
                     .await;
+                self.sync_active_thread_service_tier_to_cached_session()
+                    .await;
             }
             AppEvent::UpdatePersonality(personality) => {
                 self.on_update_personality(personality);
@@ -1327,9 +1329,6 @@ impl App {
                     profile,
                     service_tier.as_deref(),
                 );
-                if service_tier.is_none() {
-                    self.config.notices.fast_default_opt_out = Some(true);
-                }
                 match crate::config_update::write_config_batch(app_server.request_handle(), edits)
                     .await
                 {
diff --git a/codex-rs/tui/src/app/session_lifecycle.rs b/codex-rs/tui/src/app/session_lifecycle.rs
index 6c8789443402..bf5ffdad36b2 100644
--- a/codex-rs/tui/src/app/session_lifecycle.rs
+++ b/codex-rs/tui/src/app/session_lifecycle.rs
@@ -656,7 +656,6 @@ 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();
-        config.notices.fast_default_opt_out = self.chat_widget.fast_default_opt_out();
         config
     }
     pub(super) async fn resume_target_session(
diff --git a/codex-rs/tui/src/app/side.rs b/codex-rs/tui/src/app/side.rs
index 492639f1397f..04831e16e495 100644
--- a/codex-rs/tui/src/app/side.rs
+++ b/codex-rs/tui/src/app/side.rs
@@ -464,7 +464,6 @@ impl App {
         }
         fork_config.model_reasoning_effort = self.chat_widget.current_reasoning_effort();
         fork_config.service_tier = self.chat_widget.configured_service_tier();
-        fork_config.notices.fast_default_opt_out = self.chat_widget.fast_default_opt_out();
         fork_config.ephemeral = true;
         fork_config.developer_instructions = Some(Self::side_developer_instructions(
             fork_config.developer_instructions.as_deref(),
diff --git a/codex-rs/tui/src/app_server_session.rs b/codex-rs/tui/src/app_server_session.rs
index 9eca24e051ba..4597c6091388 100644
--- a/codex-rs/tui/src/app_server_session.rs
+++ b/codex-rs/tui/src/app_server_session.rs
@@ -6,6 +6,7 @@
 use crate::bottom_pane::FeedbackAudience;
 use crate::legacy_core::config::Config;
 use crate::permission_compat::legacy_compatible_permission_profile;
+use crate::service_tier_resolution;
 use crate::session_state::MessageHistoryMetadata;
 use crate::session_state::ThreadSessionState;
 use crate::status::StatusAccountDisplay;
@@ -107,6 +108,7 @@ use codex_app_server_protocol::UserInput;
 use codex_otel::TelemetryAuthMode;
 use codex_protocol::ThreadId;
 use codex_protocol::approvals::GuardianAssessmentEvent;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::models::ActivePermissionProfile;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::models::ResponseItem;
@@ -164,6 +166,8 @@ pub(crate) struct AppServerSession {
     remote_cwd_override: Option<PathBuf>,
     thread_params_mode: ThreadParamsMode,
     thread_settings_update_supported: bool,
+    default_model: Option<String>,
+    available_models: Vec<ModelPreset>,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -205,6 +209,8 @@ impl AppServerSession {
             remote_cwd_override: None,
             thread_params_mode,
             thread_settings_update_supported: true,
+            default_model: None,
+            available_models: Vec::new(),
         }
     }
 
@@ -254,6 +260,8 @@ impl AppServerSession {
             })
             .or_else(|| available_models.first().map(|model| model.model.clone()))
             .wrap_err("model/list returned no models for TUI bootstrap")?;
+        self.default_model = Some(default_model.clone());
+        self.available_models = available_models.clone();
 
         let (
             account_email,
@@ -365,12 +373,13 @@ impl AppServerSession {
         session_start_source: Option<ThreadStartSource>,
     ) -> Result<AppServerStartedThread> {
         let request_id = self.next_request_id();
+        let session_config = self.session_config_with_effective_service_tier(config);
         let response: ThreadStartResponse = self
             .client
             .request_typed(ClientRequest::ThreadStart {
                 request_id,
                 params: thread_start_params_from_config(
-                    config,
+                    &session_config,
                     self.thread_params_mode(),
                     self.remote_cwd_override.as_deref(),
                     session_start_source,
@@ -389,12 +398,13 @@ impl AppServerSession {
         thread_id: ThreadId,
     ) -> Result<AppServerStartedThread> {
         let request_id = self.next_request_id();
+        let session_config = self.session_config_with_effective_service_tier(&config);
         let response: ThreadResumeResponse = self
             .client
             .request_typed(ClientRequest::ThreadResume {
                 request_id,
                 params: thread_resume_params_from_config(
-                    config.clone(),
+                    session_config,
                     thread_id,
                     self.thread_params_mode(),
                     self.remote_cwd_override.as_deref(),
@@ -420,12 +430,13 @@ impl AppServerSession {
         thread_id: ThreadId,
     ) -> Result<AppServerStartedThread> {
         let request_id = self.next_request_id();
+        let session_config = self.session_config_with_effective_service_tier(&config);
         let response: ThreadForkResponse = self
             .client
             .request_typed(ClientRequest::ThreadFork {
                 request_id,
                 params: thread_fork_params_from_config(
-                    config.clone(),
+                    session_config,
                     thread_id,
                     self.thread_params_mode(),
                     self.remote_cwd_override.as_deref(),
@@ -448,6 +459,32 @@ impl AppServerSession {
         self.thread_params_mode
     }
 
+    fn session_config_with_effective_service_tier(&self, config: &Config) -> Config {
+        let Some(model) = config.model.as_deref().or(self.default_model.as_deref()) else {
+            return config.clone();
+        };
+        let mut session_config = config.clone();
+        match service_tier_resolution::service_tier_update_for_core(
+            config,
+            model,
+            &self.available_models,
+        ) {
+            Some(Some(service_tier)) => {
+                session_config.service_tier = Some(service_tier);
+                session_config.notices.fast_default_opt_out = None;
+            }
+            Some(None) => {
+                session_config.service_tier = Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string());
+                session_config.notices.fast_default_opt_out = None;
+            }
+            None => {
+                session_config.service_tier = None;
+                session_config.notices.fast_default_opt_out = None;
+            }
+        }
+        session_config
+    }
+
     async fn fork_parent_title_from_app_server(
         &mut self,
         forked_from_id: Option<&str>,
@@ -1159,6 +1196,7 @@ fn model_preset_from_api_model(model: ApiModel) -> ModelPreset {
                 description: service_tier.description,
             })
             .collect(),
+        default_service_tier: model.default_service_tier,
         is_default: model.is_default,
         upgrade,
         show_in_picker: !model.hidden,
@@ -1219,11 +1257,10 @@ fn config_request_overrides_from_config(
 }
 
 fn service_tier_override_from_config(config: &Config) -> Option<Option<String>> {
-    config
-        .service_tier
-        .clone()
-        .map(Some)
-        .or_else(|| (config.notices.fast_default_opt_out == Some(true)).then_some(None))
+    config.service_tier.clone().map(Some).or_else(|| {
+        (config.notices.fast_default_opt_out == Some(true))
+            .then(|| Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string()))
+    })
 }
 
 fn sandbox_mode_from_permission_profile(
diff --git a/codex-rs/tui/src/chatwidget/constructor.rs b/codex-rs/tui/src/chatwidget/constructor.rs
index 56bdc6bf6898..ef90c85d4c4c 100644
--- a/codex-rs/tui/src/chatwidget/constructor.rs
+++ b/codex-rs/tui/src/chatwidget/constructor.rs
@@ -64,7 +64,11 @@ impl ChatWidget {
         let active_cell = Some(Self::placeholder_session_header_cell(&config));
 
         let current_cwd = Some(config.cwd.to_path_buf());
-        let effective_service_tier = config.service_tier.clone();
+        let effective_service_tier = crate::service_tier_resolution::effective_service_tier(
+            &config,
+            &header_model,
+            &model_catalog.try_list_models().unwrap_or_default(),
+        );
         let current_terminal_info = terminal_info();
         let runtime_keymap = RuntimeKeymap::from_config(&config.tui_keymap).ok();
         let default_keymap = RuntimeKeymap::defaults();
diff --git a/codex-rs/tui/src/chatwidget/input_submission.rs b/codex-rs/tui/src/chatwidget/input_submission.rs
index 2fd376f139bc..2c960ee3d52d 100644
--- a/codex-rs/tui/src/chatwidget/input_submission.rs
+++ b/codex-rs/tui/src/chatwidget/input_submission.rs
@@ -331,11 +331,7 @@ 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 = self.service_tier_update_for_core();
         let active_permission_profile = self.config.permissions.active_permission_profile();
         let op = AppCommand::user_turn(
             items,
diff --git a/codex-rs/tui/src/chatwidget/service_tiers.rs b/codex-rs/tui/src/chatwidget/service_tiers.rs
index fd11048cb242..7338673aa340 100644
--- a/codex-rs/tui/src/chatwidget/service_tiers.rs
+++ b/codex-rs/tui/src/chatwidget/service_tiers.rs
@@ -4,14 +4,16 @@ use super::ChatWidget;
 use crate::app_command::AppCommand;
 use crate::app_event::AppEvent;
 use crate::bottom_pane::slash_commands::ServiceTierCommand;
+use crate::service_tier_resolution;
 use codex_features::Feature;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::openai_models::SPEED_TIER_FAST;
 
 impl ChatWidget {
     pub(crate) fn set_service_tier(&mut self, service_tier: Option<String>) {
-        self.config.service_tier = service_tier.clone();
-        self.effective_service_tier = service_tier;
+        self.config.service_tier = service_tier;
+        self.refresh_effective_service_tier();
         self.refresh_model_dependent_surfaces();
     }
 
@@ -23,8 +25,12 @@ impl ChatWidget {
         self.config.service_tier.clone()
     }
 
-    pub(crate) fn fast_default_opt_out(&self) -> Option<bool> {
-        self.config.notices.fast_default_opt_out
+    pub(crate) fn service_tier_update_for_core(&self) -> Option<Option<String>> {
+        service_tier_resolution::service_tier_update_for_core(
+            &self.config,
+            self.current_model(),
+            &self.model_catalog.try_list_models().unwrap_or_default(),
+        )
     }
 
     pub(crate) fn should_show_fast_status(&self, model: &str, service_tier: Option<&str>) -> bool {
@@ -50,7 +56,7 @@ impl ChatWidget {
             return;
         };
         let next_tier = if self.current_service_tier() == Some(fast_tier.id.as_str()) {
-            None
+            Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())
         } else {
             Some(fast_tier.id)
         };
@@ -59,7 +65,7 @@ impl ChatWidget {
 
     pub(crate) fn toggle_service_tier_from_ui(&mut self, command: ServiceTierCommand) {
         let next_tier = if self.current_service_tier() == Some(command.id.as_str()) {
-            None
+            Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())
         } else {
             Some(command.id)
         };
@@ -98,9 +104,6 @@ impl ChatWidget {
     }
 
     fn set_service_tier_selection(&mut self, service_tier: Option<String>) {
-        if service_tier.is_none() {
-            self.config.notices.fast_default_opt_out = Some(true);
-        }
         self.set_service_tier(service_tier.clone());
         self.app_event_tx
             .send(AppEvent::CodexOp(AppCommand::override_turn_context(
@@ -144,4 +147,12 @@ impl ChatWidget {
             .into_iter()
             .find(|tier| tier.name.eq_ignore_ascii_case(SPEED_TIER_FAST))
     }
+
+    pub(super) fn refresh_effective_service_tier(&mut self) {
+        self.effective_service_tier = service_tier_resolution::effective_service_tier(
+            &self.config,
+            self.current_model(),
+            &self.model_catalog.try_list_models().unwrap_or_default(),
+        );
+    }
 }
diff --git a/codex-rs/tui/src/chatwidget/settings.rs b/codex-rs/tui/src/chatwidget/settings.rs
index 0df36f6bf890..1fddad6275bd 100644
--- a/codex-rs/tui/src/chatwidget/settings.rs
+++ b/codex-rs/tui/src/chatwidget/settings.rs
@@ -66,6 +66,7 @@ impl ChatWidget {
             }
         }
         if feature == Feature::FastMode {
+            self.refresh_effective_service_tier();
             self.sync_service_tier_commands();
         }
         if feature == Feature::Personality {
@@ -238,6 +239,7 @@ impl ChatWidget {
         {
             mask.model = Some(model.to_string());
         }
+        self.refresh_effective_service_tier();
         self.refresh_model_dependent_surfaces();
     }
 
@@ -519,6 +521,7 @@ impl ChatWidget {
         settings.collaboration_mode.settings.model = settings.model;
         settings.collaboration_mode.settings.reasoning_effort = settings.effort;
         self.set_effective_collaboration_mode(settings.collaboration_mode);
+        self.refresh_effective_service_tier();
         self.refresh_status_surfaces();
         self.sync_service_tier_commands();
         self.sync_personality_command_enabled();
diff --git a/codex-rs/tui/src/chatwidget/tests.rs b/codex-rs/tui/src/chatwidget/tests.rs
index 77aa742c0161..5d27e0ddc596 100644
--- a/codex-rs/tui/src/chatwidget/tests.rs
+++ b/codex-rs/tui/src/chatwidget/tests.rs
@@ -144,6 +144,7 @@ pub(super) use codex_protocol::approvals::GuardianUserAuthorization;
 pub(super) use codex_protocol::config_types::CollaborationMode;
 pub(super) use codex_protocol::config_types::ModeKind;
 pub(super) use codex_protocol::config_types::Personality;
+pub(super) use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 pub(super) use codex_protocol::config_types::ServiceTier;
 pub(super) use codex_protocol::models::ActivePermissionProfile;
 pub(super) use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_WORKSPACE;
diff --git a/codex-rs/tui/src/chatwidget/tests/helpers.rs b/codex-rs/tui/src/chatwidget/tests/helpers.rs
index 79e65173f93c..63487f18b9e8 100644
--- a/codex-rs/tui/src/chatwidget/tests/helpers.rs
+++ b/codex-rs/tui/src/chatwidget/tests/helpers.rs
@@ -246,15 +246,14 @@ pub(crate) fn set_chatgpt_auth(chat: &mut ChatWidget) {
 }
 
 fn test_model_info(slug: &str, priority: i32, supports_fast_mode: bool) -> ModelInfo {
-    let service_tiers = if supports_fast_mode {
-        vec![json!({
+    let mut service_tiers = Vec::new();
+    if supports_fast_mode {
+        service_tiers.push(json!({
             "id": ServiceTier::Fast.request_value(),
             "name": "fast",
             "description": "Fastest inference with increased plan usage"
-        })]
-    } else {
-        Vec::new()
-    };
+        }));
+    }
     serde_json::from_value(json!({
         "slug": slug,
         "display_name": slug,
@@ -267,6 +266,7 @@ fn test_model_info(slug: &str, priority: i32, supports_fast_mode: bool) -> Model
         "priority": priority,
         "additional_speed_tiers": [],
         "service_tiers": service_tiers,
+        "default_service_tier": null,
         "availability_nux": null,
         "upgrade": null,
         "base_instructions": "base instructions",
diff --git a/codex-rs/tui/src/chatwidget/tests/popups_and_settings.rs b/codex-rs/tui/src/chatwidget/tests/popups_and_settings.rs
index 2a25415c032b..09d911ad16d0 100644
--- a/codex-rs/tui/src/chatwidget/tests/popups_and_settings.rs
+++ b/codex-rs/tui/src/chatwidget/tests/popups_and_settings.rs
@@ -2279,6 +2279,7 @@ async fn model_picker_hides_show_in_picker_false_models_from_cache() {
         supports_personality: false,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         is_default: false,
         upgrade: None,
         show_in_picker,
@@ -2500,6 +2501,7 @@ async fn single_reasoning_option_skips_selection() {
         supports_personality: false,
         additional_speed_tiers: Vec::new(),
         service_tiers: Vec::new(),
+        default_service_tier: None,
         is_default: false,
         upgrade: None,
         show_in_picker: true,
diff --git a/codex-rs/tui/src/chatwidget/tests/slash_commands.rs b/codex-rs/tui/src/chatwidget/tests/slash_commands.rs
index c8c0f6058425..74d965ae5cc7 100644
--- a/codex-rs/tui/src/chatwidget/tests/slash_commands.rs
+++ b/codex-rs/tui/src/chatwidget/tests/slash_commands.rs
@@ -91,8 +91,19 @@ fn next_add_to_history_event(rx: &mut tokio::sync::mpsc::UnboundedReceiver<AppEv
 async fn service_tier_commands_lowercase_catalog_names() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(Some("gpt-5.4")).await;
     let mut preset = get_available_model(&chat, "gpt-5.4");
-    let expected_description = preset.service_tiers[0].description.clone();
-    preset.service_tiers[0].name = "Fast".to_string();
+    let expected_description = preset
+        .service_tiers
+        .iter()
+        .find(|tier| tier.id == ServiceTier::Fast.request_value())
+        .expect("fast tier")
+        .description
+        .clone();
+    preset
+        .service_tiers
+        .iter_mut()
+        .find(|tier| tier.id == ServiceTier::Fast.request_value())
+        .expect("fast tier")
+        .name = "Fast".to_string();
     chat.model_catalog = std::sync::Arc::new(ModelCatalog::new(vec![preset]));
 
     assert_eq!(
@@ -2135,6 +2146,47 @@ async fn user_turn_carries_service_tier_after_fast_toggle() {
     }
 }
 
+#[tokio::test]
+async fn model_switch_recomputes_catalog_default_service_tier() {
+    let (mut chat, _rx, mut op_rx) = make_chatwidget_manual(Some("gpt-5.3-codex")).await;
+    chat.thread_id = Some(ThreadId::new());
+    set_chatgpt_auth(&mut chat);
+    set_fast_mode_test_catalog(&mut chat);
+    chat.set_feature_enabled(Feature::FastMode, /*enabled*/ true);
+
+    let mut models = chat.model_catalog.try_list_models().expect("test catalog");
+    let default_model = models
+        .iter_mut()
+        .find(|model| model.model == "gpt-5.4")
+        .expect("gpt-5.4 test model");
+    default_model.default_service_tier = Some(ServiceTier::Fast.request_value().to_string());
+    chat.model_catalog = std::sync::Arc::new(ModelCatalog::new(models));
+    chat.refresh_effective_service_tier();
+
+    assert_eq!(chat.current_service_tier(), None);
+
+    chat.set_model("gpt-5.4");
+    assert_eq!(
+        chat.current_service_tier(),
+        Some(ServiceTier::Fast.request_value())
+    );
+
+    chat.set_model("gpt-5.3-codex");
+    assert_eq!(chat.current_service_tier(), None);
+
+    chat.bottom_pane
+        .set_composer_text("hello".to_string(), Vec::new(), Vec::new());
+    chat.handle_key_event(KeyEvent::from(KeyCode::Enter));
+
+    match next_submit_op(&mut op_rx) {
+        Op::UserTurn {
+            service_tier: Some(Some(service_tier)),
+            ..
+        } if service_tier == SERVICE_TIER_DEFAULT_REQUEST_VALUE => {}
+        other => panic!("expected Op::UserTurn with default service tier override, got {other:?}"),
+    }
+}
+
 #[tokio::test]
 async fn queued_fast_slash_applies_before_next_queued_message() {
     let (mut chat, mut rx, mut op_rx) = make_chatwidget_manual(Some("gpt-5.4")).await;
@@ -2194,18 +2246,20 @@ async fn user_turn_sends_standard_override_after_fast_is_turned_off() {
         events.iter().any(|event| matches!(
             event,
             AppEvent::CodexOp(Op::OverrideTurnContext {
-                service_tier: Some(None),
+                service_tier: Some(Some(service_tier)),
                 ..
-            })
+            }) if service_tier == SERVICE_TIER_DEFAULT_REQUEST_VALUE
         )),
-        "expected fast-mode off override app event; events: {events:?}"
+        "expected fast-mode off default service tier app event; events: {events:?}"
     );
     assert!(
         events.iter().any(|event| matches!(
             event,
-            AppEvent::PersistServiceTierSelection { service_tier: None }
+            AppEvent::PersistServiceTierSelection {
+                service_tier: Some(service_tier)
+            } if service_tier == SERVICE_TIER_DEFAULT_REQUEST_VALUE
         )),
-        "expected fast-mode opt-out persistence app event; events: {events:?}"
+        "expected default service tier persistence app event; events: {events:?}"
     );
 
     chat.bottom_pane
@@ -2214,10 +2268,10 @@ async fn user_turn_sends_standard_override_after_fast_is_turned_off() {
 
     match next_submit_op(&mut op_rx) {
         Op::UserTurn {
-            service_tier: Some(None),
+            service_tier: Some(Some(service_tier)),
             ..
-        } => {}
-        other => panic!("expected Op::UserTurn with standard service tier override, got {other:?}"),
+        } if service_tier == SERVICE_TIER_DEFAULT_REQUEST_VALUE => {}
+        other => panic!("expected Op::UserTurn with default service tier override, got {other:?}"),
     }
 }
 
diff --git a/codex-rs/tui/src/config_update.rs b/codex-rs/tui/src/config_update.rs
index cac79f648e3a..f3dc0dc5fc27 100644
--- a/codex-rs/tui/src/config_update.rs
+++ b/codex-rs/tui/src/config_update.rs
@@ -13,6 +13,7 @@ use codex_app_server_protocol::MergeStrategy;
 use codex_app_server_protocol::RequestId;
 use codex_app_server_protocol::SkillsConfigWriteParams;
 use codex_app_server_protocol::SkillsConfigWriteResponse;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use color_eyre::eyre::Result;
 use color_eyre::eyre::WrapErr;
@@ -75,26 +76,22 @@ pub(crate) fn build_service_tier_selection_edits(
     let service_tier_edit = service_tier.map_or_else(
         || clear_config_value(profile_scoped_key_path(profile, "service_tier")),
         |service_tier| {
-            let config_value =
+            let config_value = if service_tier == SERVICE_TIER_DEFAULT_REQUEST_VALUE {
+                SERVICE_TIER_DEFAULT_REQUEST_VALUE
+            } else {
                 match codex_protocol::config_types::ServiceTier::from_request_value(service_tier) {
                     Some(codex_protocol::config_types::ServiceTier::Fast) => "fast",
                     Some(codex_protocol::config_types::ServiceTier::Flex) => "flex",
                     None => service_tier,
-                };
+                }
+            };
             replace_config_value(
                 profile_scoped_key_path(profile, "service_tier"),
                 serde_json::json!(config_value),
             )
         },
     );
-    let mut edits = vec![service_tier_edit];
-    if service_tier.is_none() {
-        edits.push(replace_config_value(
-            "notice.fast_default_opt_out",
-            serde_json::json!(true),
-        ));
-    }
-    edits
+    vec![service_tier_edit]
 }
 
 pub(crate) async fn write_config_batch(
diff --git a/codex-rs/tui/src/lib.rs b/codex-rs/tui/src/lib.rs
index 40cd121c7135..cbd19702bf0e 100644
--- a/codex-rs/tui/src/lib.rs
+++ b/codex-rs/tui/src/lib.rs
@@ -164,6 +164,7 @@ mod render;
 mod resize_reflow_cap;
 mod resume_picker;
 mod selection_list;
+mod service_tier_resolution;
 mod session_log;
 mod session_resume;
 mod session_state;
diff --git a/codex-rs/tui/src/service_tier_resolution.rs b/codex-rs/tui/src/service_tier_resolution.rs
new file mode 100644
index 000000000000..1e65fd3fb07e
--- /dev/null
+++ b/codex-rs/tui/src/service_tier_resolution.rs
@@ -0,0 +1,64 @@
+use crate::legacy_core::config::Config;
+use codex_features::Feature;
+use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
+use codex_protocol::openai_models::ModelPreset;
+
+pub(crate) fn configured_service_tier(config: &Config) -> Option<String> {
+    config.service_tier.clone().or_else(|| {
+        (config.notices.fast_default_opt_out == Some(true))
+            .then(|| SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string())
+    })
+}
+
+pub(crate) fn effective_service_tier(
+    config: &Config,
+    model: &str,
+    models: &[ModelPreset],
+) -> Option<String> {
+    if !config.features.enabled(Feature::FastMode) {
+        return None;
+    }
+
+    let configured = configured_service_tier(config);
+    let Some(preset) = models.iter().find(|preset| preset.model == model) else {
+        return configured;
+    };
+
+    match configured.as_deref() {
+        Some(service_tier) if service_tier == SERVICE_TIER_DEFAULT_REQUEST_VALUE => configured,
+        Some(service_tier) if model_supports_service_tier(preset, service_tier) => configured,
+        Some(_) => None,
+        None => preset
+            .default_service_tier
+            .clone()
+            .filter(|service_tier| model_supports_service_tier(preset, service_tier)),
+    }
+}
+
+pub(crate) fn service_tier_update_for_core(
+    config: &Config,
+    model: &str,
+    models: &[ModelPreset],
+) -> Option<Option<String>> {
+    if !config.features.enabled(Feature::FastMode) {
+        return None;
+    }
+
+    let effective = effective_service_tier(config, model, models);
+    if let Some(service_tier) = effective {
+        return Some(Some(service_tier));
+    }
+
+    if !models.iter().any(|preset| preset.model == model) {
+        return None;
+    }
+
+    Some(Some(SERVICE_TIER_DEFAULT_REQUEST_VALUE.to_string()))
+}
+
+pub(crate) fn model_supports_service_tier(model: &ModelPreset, service_tier: &str) -> bool {
+    model
+        .service_tiers
+        .iter()
+        .any(|tier| tier.id == service_tier)
+}