Support asynchronous host service-tier reads and acceptance (#60075)

## What changed

Make `AgentServiceTier` reads asynchronous and have updates return a receipt for local acceptance. Await acceptance during root startup and settings updates outside state and registry locks, sharing the receipt with explicit same-tier retries.

Await shared-tier reads during child preparation, startup prewarm, and step capture. Allow cancellation while capturing a step, and keep host propagation independent of the acceptance wait.

## Testing

Extend the host-child WebSocket test to cover delayed acceptance, same-tier retries without duplicate updates, accessible settings while acceptance is pending, and tier inheritance during prewarm and subsequent steps.

GitOrigin-RevId: 0e2e1899834c37684e904c4a36e66344a0c6c7f3

codex-rs/core/src/agent/child_config.rs

@@ -282,7 +282,8 @@ pub(crate) async fn apply_spawn_agent_service_tier(
     session: &Session,
     config: &mut Config,
 ) -> Result<(), String> {
-    let Some(service_tier) = crate::agent::host::service_tier(session, config.service_tier.clone())
+    let Some(service_tier) =
+        crate::agent::host::service_tier(session, config.service_tier.clone()).await
     else {
         config.service_tier = None;
         return Ok(());

codex-rs/core/src/agent/host/mod.rs

@@ -22,6 +22,7 @@ pub use mcp_metadata::AgentMcpMetadataRecipient;
 pub use ownership::AgentRuntimeOwnership;
 pub(crate) use ownership::AgentRuntimeServices;
 pub use service_tier::AgentServiceTier;
+pub(crate) use service_tier::ServiceTierAcceptance;
 pub(crate) use service_tier::host_child_uses_shared_service_tier;
 pub(crate) use service_tier::propagate_config_update;
 pub(crate) use service_tier::service_tier;

codex-rs/core/src/agent/host/service_tier.rs

@@ -4,21 +4,32 @@ use crate::agent::api::AgentConfigUpdate;
 use crate::agent::host::AgentRuntimeServices;
 use crate::session::session::Session;
 use codex_protocol::SessionId;
+use futures::FutureExt;
+use futures::future::BoxFuture;
+use futures::future::Shared;
 
-/// The host's local view of the service tier shared within an agent session.
+pub(crate) type ServiceTierAcceptance = Shared<BoxFuture<'static, ()>>;
+
+/// The host's view of the service tier shared within an agent session.
 ///
 /// Install an `Arc<dyn AgentServiceTier>` in thread extension data before startup. Host-owned
 /// children with a provider reject explicit per-turn tier overrides. Host-owned runtimes without
 /// a provider retain their configured tier and do not propagate shared tier updates.
 /// Controller-owned runtimes use their controller.
-/// Both methods must be cheap, synchronous and non-reentrant: Core may call them while
-/// holding configuration or registration locks. They must perform no network I/O. The host publishes
-/// committed root settings into its cache and arranges propagation to other runtimes.
-/// Children read the latest cached value at the existing preparation and step boundaries;
+/// Core awaits reads during preparation and step capture. A host may answer from a cache.
+/// Hosts own cross-runtime propagation and recovery. Until an update reaches a runtime,
+/// its `current` may return the prior tier.
+/// Children read the latest value at the existing preparation and step boundaries;
 /// updates do not change requests already in flight. `None` restores the default tier.
 pub trait AgentServiceTier: Send + Sync {
-    fn current(&self) -> Option<String>;
-    fn update(&self, service_tier: Option<String>);
+    fn current(&self) -> BoxFuture<'_, Option<String>>;
+
+    /// Hand off the committed tier in call order and return a receipt for local acceptance.
+    /// Core calls this method under its commit locks: the handoff must be fast and nonblocking.
+    /// Drive the update independently of the receipt: dropping the receipt only cancels the wait.
+    /// Core awaits it outside the state and registry locks. Explicit settings updates that
+    /// request the same committed tier share this receipt until the tier changes.
+    fn update(&self, service_tier: Option<String>) -> BoxFuture<'static, ()>;
 }
 
 pub(crate) fn host_child_uses_shared_service_tier(session: &Session) -> bool {
@@ -26,34 +37,39 @@ pub(crate) fn host_child_uses_shared_service_tier(session: &Session) -> bool {
         && session.session_id() != SessionId::from(session.thread_id())
 }
 
-pub(crate) fn service_tier(
+pub(crate) async fn service_tier(
     session: &Session,
     configured_service_tier: Option<String>,
 ) -> Option<String> {
     match &session.agent_runtime {
         AgentRuntimeServices::Controller => session.services.agent_control.service_tier(),
-        AgentRuntimeServices::Host(services) => services
-            .service_tier
-            .as_ref()
-            .map_or(configured_service_tier, |provider| provider.current()),
+        AgentRuntimeServices::Host(services) => match &services.service_tier {
+            Some(provider) => provider.current().await,
+            None => configured_service_tier,
+        },
     }
 }
 
-pub(crate) fn propagate_config_update(session: &Session, update: AgentConfigUpdate) {
+pub(crate) fn propagate_config_update(
+    session: &Session,
+    update: AgentConfigUpdate,
+) -> Option<ServiceTierAcceptance> {
     match &session.agent_runtime {
-        AgentRuntimeServices::Controller => session
-            .services
-            .agent_control
-            .propagate_config_update(update),
+        AgentRuntimeServices::Controller => {
+            session
+                .services
+                .agent_control
+                .propagate_config_update(update);
+            None
+        }
         AgentRuntimeServices::Host(services) => {
             // Only the root can publish shared settings. Compare session and thread IDs
             // because a detached child may have no structural parent ID.
-            if session.session_id() == SessionId::from(session.thread_id())
-                && let Some(provider) = &services.service_tier
-            {
-                let AgentConfigUpdate::ServiceTier(tier) = update;
-                provider.update(tier);
+            if session.session_id() != SessionId::from(session.thread_id()) {
+                return None;
             }
+            let AgentConfigUpdate::ServiceTier(tier) = update;
+            Some(services.service_tier.as_ref()?.update(tier).shared())
         }
     }
 }