Add a root-only option for background compaction (#59598)

## What changed

Add the disabled-by-default `async_compaction_root_only` feature flag. When enabled, it restricts background compaction to root sessions, excluding non-root agents. Expose the flag in the configuration schema.

## Testing

Add a parameterized test covering root session eligibility and eligibility for review subagents and guardian sessions with the flag enabled and disabled.

GitOrigin-RevId: b4cce3f24f7f418ae61a82a7aef310f849cfd532

codex-rs/core/config.schema.json

@@ -895,6 +895,9 @@
             "async_compaction": {
               "$ref": "#/definitions/FeatureToml_for_AsyncCompactionConfigToml"
             },
+            "async_compaction_root_only": {
+              "type": "boolean"
+            },
             "auth_elicitation": {
               "type": "boolean"
             },
@@ -7830,6 +7833,9 @@
         "async_compaction": {
           "$ref": "#/definitions/FeatureToml_for_AsyncCompactionConfigToml"
         },
+        "async_compaction_root_only": {
+          "type": "boolean"
+        },
         "auth_elicitation": {
           "type": "boolean"
         },

codex-rs/core/src/session/async_compaction.rs

@@ -39,11 +39,11 @@ pub(crate) struct AsyncCompactionState {
 }
 
 pub(super) fn enabled(turn_context: &TurnContext) -> bool {
-    turn_context
-        .config
-        .features
-        .enabled(Feature::AsyncCompaction)
-        && !turn_context.config.features.enabled(Feature::TokenBudget)
+    let features = &turn_context.config.features;
+    features.enabled(Feature::AsyncCompaction)
+        && !(features.enabled(Feature::AsyncCompactionRootOnly)
+            && turn_context.session_source.is_non_root_agent())
+        && !features.enabled(Feature::TokenBudget)
         && matches!(
             turn_context.provider.capabilities().remote_compaction,
             RemoteCompactionSupport::V2

codex-rs/core/src/session/async_compaction_tests.rs

@@ -1,4 +1,4 @@
-//! Cancellation prevents new speculation without cancelling session-owned work.
+//! Background compaction eligibility and cancellation.
 
 use super::*;
 use codex_analytics::CompactionReason;
@@ -8,6 +8,9 @@ use codex_protocol::models::AudioReference;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::InputModality;
+use codex_protocol::protocol::InternalSessionSource;
+use codex_protocol::protocol::SessionSource;
+use codex_protocol::protocol::SubAgentSource;
 use codex_protocol::protocol::TokenUsage;
 use core_test_support::responses;
 use pretty_assertions::assert_eq;
@@ -218,3 +221,24 @@ async fn cancelled_turn_skips_launch_but_preserves_existing_work(trigger: Compac
     }
     shutdown(&sess).await;
 }
+
+#[test_case::test_case(false; "all agents")]
+#[test_case::test_case(true; "root only")]
+#[tokio::test]
+async fn root_only_controls_child_eligibility(root_only: bool) {
+    let (_, mut turn) = crate::session::tests::make_session_and_context().await;
+    let features = &mut Arc::make_mut(&mut turn.config).features;
+    features.enable(Feature::AsyncCompaction).unwrap();
+    features
+        .set_enabled(Feature::AsyncCompactionRootOnly, root_only)
+        .unwrap();
+
+    assert!(enabled(&turn));
+    for source in [
+        SessionSource::SubAgent(SubAgentSource::Review),
+        SessionSource::Internal(InternalSessionSource::Guardian),
+    ] {
+        turn.session_source = source;
+        assert_eq!(enabled(&turn), !root_only);
+    }
+}

codex-rs/features/src/lib.rs

@@ -492,6 +492,8 @@ pub enum Feature {
     RemoteCompactionV2,
     /// Prepare remote compaction in the background before the foreground limit is reached.
     AsyncCompaction,
+    /// Restrict background compaction to the root session.
+    AsyncCompactionRootOnly,
     /// Include retained images in the remote compaction context budget.
     CompactionImageBudget,
     /// Retain client-authored developer messages across compacted context windows.
@@ -2347,6 +2349,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::AsyncCompactionRootOnly,
+        key: "async_compaction_root_only",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CompactionImageBudget,
         key: "compaction_image_budget",