add fast mode toggle (#13212)

- add a local Fast mode setting in codex-core (similar to how model id
is currently stored on disk locally)
- send `service_tier=priority` on requests when Fast is enabled
- add `/fast` in the TUI and persist it locally
- feature flag

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

@@ -62,6 +62,7 @@ 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;
@@ -185,6 +186,9 @@ pub struct Config {
     /// Optional override of model selection.
     pub model: Option<String>,
 
+    /// Effective service tier preference for new turns.
+    pub service_tier: Option<ServiceTier>,
+
     /// Model used specifically for review sessions.
     pub review_model: Option<String>,
 
@@ -1184,6 +1188,9 @@ pub struct ConfigToml {
     /// Optionally specify a personality for the model
     pub personality: Option<Personality>,
 
+    /// Optional explicit service tier preference for new turns.
+    pub service_tier: Option<ServiceTier>,
+
     /// Base URL for requests to ChatGPT (as opposed to the OpenAI API).
     pub chatgpt_base_url: Option<String>,
 
@@ -1948,6 +1955,14 @@ impl Config {
         let forced_login_method = cfg.forced_login_method;
 
         let model = model.or(config_profile.model).or(cfg.model);
+        let service_tier = if features.enabled(Feature::FastMode) {
+            config_profile
+                .service_tier
+                .or(cfg.service_tier)
+                .filter(|tier| matches!(tier, ServiceTier::Fast))
+        } else {
+            None
+        };
 
         let compact_prompt = compact_prompt.or(cfg.compact_prompt).and_then(|value| {
             let trimmed = value.trim();
@@ -2094,6 +2109,7 @@ impl Config {
 
         let config = Self {
             model,
+            service_tier,
             review_model,
             model_context_window: cfg.model_context_window,
             model_auto_compact_token_limit: cfg.model_auto_compact_token_limit,
@@ -4878,6 +4894,7 @@ model_verbosity = "high"
                 review_model: None,
                 model_context_window: None,
                 model_auto_compact_token_limit: None,
+                service_tier: None,
                 model_provider_id: "openai".to_string(),
                 model_provider: fixture.openai_provider.clone(),
                 permissions: Permissions {
@@ -5007,6 +5024,7 @@ model_verbosity = "high"
             review_model: None,
             model_context_window: None,
             model_auto_compact_token_limit: None,
+            service_tier: None,
             model_provider_id: "openai-custom".to_string(),
             model_provider: fixture.openai_custom_provider.clone(),
             permissions: Permissions {
@@ -5134,6 +5152,7 @@ model_verbosity = "high"
             review_model: None,
             model_context_window: None,
             model_auto_compact_token_limit: None,
+            service_tier: None,
             model_provider_id: "openai".to_string(),
             model_provider: fixture.openai_provider.clone(),
             permissions: Permissions {
@@ -5247,6 +5266,7 @@ model_verbosity = "high"
             review_model: None,
             model_context_window: None,
             model_auto_compact_token_limit: None,
+            service_tier: None,
             model_provider_id: "openai".to_string(),
             model_provider: fixture.openai_provider.clone(),
             permissions: Permissions {
@@ -5635,6 +5655,33 @@ trust_level = "untrusted"
         Ok(())
     }
 
+    #[test]
+    fn legacy_standard_service_tier_loads_as_default_none() -> anyhow::Result<()> {
+        let codex_home = TempDir::new()?;
+        let cfg = toml::from_str::<ConfigToml>(
+            r#"
+service_tier = "standard"
+
+[features]
+fast_mode = true
+"#,
+        )
+        .expect("TOML deserialization should succeed");
+
+        let config = Config::load_from_base_config_with_overrides(
+            cfg,
+            ConfigOverrides {
+                cwd: Some(codex_home.path().to_path_buf()),
+                ..Default::default()
+            },
+            codex_home.path().to_path_buf(),
+        )?;
+
+        assert_eq!(config.service_tier, None);
+
+        Ok(())
+    }
+
     #[test]
     fn derive_sandbox_policy_falls_back_to_constraint_value_for_implicit_defaults()
     -> anyhow::Result<()> {