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/app-server/tests/suite/v2/thread_start.rs

@@ -28,7 +28,6 @@ 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::ServiceTier;
 use codex_protocol::config_types::TrustLevel;
 use codex_protocol::openai_models::ReasoningEffort;
 use pretty_assertions::assert_eq;
@@ -402,7 +401,7 @@ model_reasoning_effort = "high"
 }
 
 #[tokio::test]
-async fn thread_start_accepts_flex_service_tier() -> Result<()> {
+async fn thread_start_accepts_arbitrary_service_tier_id() -> Result<()> {
     let server = create_mock_responses_server_repeating_assistant("Done").await;
 
     let codex_home = TempDir::new()?;
@@ -411,9 +410,10 @@ async fn thread_start_accepts_flex_service_tier() -> Result<()> {
     let mut mcp = McpProcess::new(codex_home.path()).await?;
     timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
 
+    let service_tier_id = "experimental-tier-id".to_string();
     let req_id = mcp
         .send_thread_start_request(ThreadStartParams {
-            service_tier: Some(Some(ServiceTier::Flex)),
+            service_tier: Some(Some(service_tier_id.clone())),
             ..Default::default()
         })
         .await?;
@@ -425,7 +425,7 @@ async fn thread_start_accepts_flex_service_tier() -> Result<()> {
     .await??;
     let ThreadStartResponse { service_tier, .. } = to_response::<ThreadStartResponse>(resp)?;
 
-    assert_eq!(service_tier, Some(ServiceTier::Flex));
+    assert_eq!(service_tier, Some(service_tier_id));
     Ok(())
 }
 

codex-rs/app-server/tests/suite/v2/turn_start.rs

@@ -358,6 +358,71 @@ async fn turn_start_emits_thread_scoped_warning_notification_for_trimmed_skills(
     Ok(())
 }
 
+#[tokio::test]
+async fn turn_start_sends_service_tier_id_to_model_request() -> Result<()> {
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp-1"),
+        responses::ev_assistant_message("msg-1", "Done"),
+        responses::ev_completed("resp-1"),
+    ]);
+    let response_mock = responses::mount_sse_once(&server, body).await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(
+        codex_home.path(),
+        &server.uri(),
+        "never",
+        &BTreeMap::default(),
+    )?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let thread_req = mcp
+        .send_thread_start_request(ThreadStartParams {
+            model: Some("mock-model".to_string()),
+            ..Default::default()
+        })
+        .await?;
+    let thread_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(thread_req)),
+    )
+    .await??;
+    let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(thread_resp)?;
+
+    let service_tier_id = "experimental-tier-id".to_string();
+    let turn_req = mcp
+        .send_turn_start_request(TurnStartParams {
+            thread_id: thread.id,
+            service_tier: Some(Some(service_tier_id.clone())),
+            input: vec![V2UserInput::Text {
+                text: "Hello".to_string(),
+                text_elements: Vec::new(),
+            }],
+            ..Default::default()
+        })
+        .await?;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(turn_req)),
+    )
+    .await??;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("turn/completed"),
+    )
+    .await??;
+
+    assert_eq!(
+        response_mock.single_request().body_json()["service_tier"],
+        json!(service_tier_id)
+    );
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn thread_start_omits_empty_instruction_overrides_from_model_request() -> Result<()> {
     let server = responses::start_mock_server().await;