2- Use string service tiers in session protocol (#20971)

## Summary
- break service tier session/op/app-server protocol fields from the
closed enum to string tier ids
- send the service tier string directly through model requests, prewarm,
compaction, memories, and TUI/app-server turn starts
- regenerate app-server protocol JSON/TypeScript schemas, removing the
standalone ServiceTier TS enum

## Verification
- just fmt
- cargo check -p codex-core -p codex-app-server -p codex-tui
- just write-app-server-schema

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/config_types.rs

@@ -355,6 +355,23 @@ pub enum ServiceTier {
     Flex,
 }
 
+impl ServiceTier {
+    pub const fn request_value(self) -> &'static str {
+        match self {
+            Self::Fast => "priority",
+            Self::Flex => "flex",
+        }
+    }
+
+    pub fn from_request_value(value: &str) -> Option<Self> {
+        match value {
+            "fast" | "priority" => Some(Self::Fast),
+            "flex" => Some(Self::Flex),
+            _ => None,
+        }
+    }
+}
+
 #[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq, Display, JsonSchema, TS)]
 #[serde(rename_all = "lowercase")]
 #[strum(serialize_all = "lowercase")]

codex-rs/protocol/src/protocol.rs

@@ -22,7 +22,6 @@ use crate::config_types::CollaborationMode;
 use crate::config_types::ModeKind;
 use crate::config_types::Personality;
 use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
-use crate::config_types::ServiceTier;
 use crate::config_types::WindowsSandboxLevel;
 use crate::dynamic_tools::DynamicToolCallOutputContentItem;
 use crate::dynamic_tools::DynamicToolCallRequest;
@@ -514,7 +513,7 @@ pub enum Op {
         /// Use `Some(Some(_))` to set a specific tier, `Some(None)` to clear the
         /// preference, or `None` to leave the existing value unchanged.
         #[serde(skip_serializing_if = "Option::is_none")]
-        service_tier: Option<Option<ServiceTier>>,
+        service_tier: Option<Option<String>>,
 
         /// EXPERIMENTAL - set a pre-set collaboration mode.
         /// Takes precedence over model, effort, and developer instructions if set.
@@ -575,7 +574,7 @@ pub enum Op {
         /// explicitly clear the tier for this turn, or `None` to keep the existing
         /// session preference.
         #[serde(default, skip_serializing_if = "Option::is_none")]
-        service_tier: Option<Option<ServiceTier>>,
+        service_tier: Option<Option<String>>,
 
         // The JSON schema to use for the final assistant message
         final_output_json_schema: Option<Value>,
@@ -652,7 +651,7 @@ pub enum Op {
         /// Use `Some(Some(_))` to set a specific tier, `Some(None)` to clear the
         /// preference, or `None` to leave the existing value unchanged.
         #[serde(skip_serializing_if = "Option::is_none")]
-        service_tier: Option<Option<ServiceTier>>,
+        service_tier: Option<Option<String>>,
 
         /// EXPERIMENTAL - set a pre-set collaboration mode.
         /// Takes precedence over model, effort, and developer instructions if set.
@@ -3476,7 +3475,7 @@ pub struct SessionConfiguredEvent {
     pub model_provider_id: String,
 
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub service_tier: Option<ServiceTier>,
+    pub service_tier: Option<String>,
 
     /// When to escalate for approval for execution
     pub approval_policy: AskForApproval,
@@ -3542,7 +3541,7 @@ impl<'de> Deserialize<'de> for SessionConfiguredEvent {
             thread_name: Option<String>,
             model: String,
             model_provider_id: String,
-            service_tier: Option<ServiceTier>,
+            service_tier: Option<String>,
             approval_policy: AskForApproval,
             #[serde(default)]
             approvals_reviewer: ApprovalsReviewer,