Preserve multi-agent settings across config representations (#35656)

## Why

`features.multi_agent_v2` can be represented as either a legacy boolean toggle
or a table with an `enabled` field and nested settings. Layering or editing
configs that mix these forms could replace one form with the other and discard
the enabled state or nested settings.

## What changed

- Normalize boolean toggles to the table's `enabled` field when merging config
  layers, applying CLI overrides, and editing user or profile config.
- Preserve nested multi-agent settings when toggling the feature, while keeping
  ordinary replacement semantics for unrelated paths.
- Attribute normalized `enabled` values to the layer that supplied the boolean
  toggle so config write results report overrides correctly.

## Testing

Added coverage for layered config, CLI overrides, config edits, app-server
writes, and origin metadata using both root and profile feature paths.

GitOrigin-RevId: 38b248c949b9ea5d6340a73d754f91c1834ac486

codex-rs/app-server/src/config_manager_service.rs

@@ -583,10 +583,33 @@ fn apply_merge(
         ));
     };
 
-    if matches!(strategy, MergeStrategy::Upsert)
-        && (shell_environment_policy_representation_switch(root, segments, value)
-            || (matches!(value_at_path(root, segments), Some(TomlValue::Table(_)))
-                && matches!(value, TomlValue::Table(_))))
+    let multi_agent_v2_feature_depth = match segments {
+        [features, feature, ..] if features == "features" && feature == "multi_agent_v2" => Some(2),
+        [profiles, _, features, feature, ..]
+            if profiles == "profiles" && features == "features" && feature == "multi_agent_v2" =>
+        {
+            Some(4)
+        }
+        _ => None,
+    };
+    let preserves_multi_agent_v2_feature_config =
+        multi_agent_v2_feature_depth.is_some_and(|feature_depth| {
+            match value_at_path(root, &segments[..feature_depth]) {
+                Some(TomlValue::Boolean(_)) => {
+                    segments.len() > feature_depth || matches!(value, TomlValue::Table(_))
+                }
+                Some(TomlValue::Table(_)) => {
+                    segments.len() == feature_depth && matches!(value, TomlValue::Boolean(_))
+                }
+                _ => false,
+            }
+        });
+
+    if preserves_multi_agent_v2_feature_config
+        || matches!(strategy, MergeStrategy::Upsert)
+            && (shell_environment_policy_representation_switch(root, segments, value)
+                || (matches!(value_at_path(root, segments), Some(TomlValue::Table(_)))
+                    && matches!(value, TomlValue::Table(_))))
     {
         let overlay = sparse_overlay(segments, value);
         merge_toml_values(root, &overlay);
@@ -741,6 +764,24 @@ fn value_at_semantic_path<'a>(root: &'a TomlValue, segments: &[String]) -> Optio
     shell_environment_filter_entry(root, segments)
         .map(|(_, value)| value)
         .or_else(|| value_at_path(root, segments))
+        .or_else(|| {
+            let (field, parents) = segments.split_last()?;
+            if field != "enabled" {
+                return None;
+            }
+            let is_multi_agent_v2_feature = match parents {
+                [features, feature] => features == "features" && feature == "multi_agent_v2",
+                [profiles, _, features, feature] => {
+                    profiles == "profiles" && features == "features" && feature == "multi_agent_v2"
+                }
+                _ => false,
+            };
+            if !is_multi_agent_v2_feature {
+                return None;
+            }
+            let feature = value_at_path(root, parents)?;
+            matches!(feature, TomlValue::Boolean(_)).then_some(feature)
+        })
 }
 
 fn override_message(layer: &ConfigLayerSource) -> String {

codex-rs/app-server/src/config_manager_service_tests.rs

@@ -1037,6 +1037,63 @@ async fn write_value_reports_managed_override() {
     assert_eq!(overridden.effective_value, serde_json::json!("never"));
 }
 
+/// Legacy managed feature toggles own their normalized enabled origin and override metadata.
+#[tokio::test]
+async fn multi_agent_v2_boolean_layer_owns_enabled_origin_and_overrides() {
+    let tmp = tempdir().expect("tempdir");
+    let user_path = tmp.path().join(CONFIG_TOML_FILE);
+    std::fs::write(
+        &user_path,
+        "[features.multi_agent_v2]\nenabled = true\nsubagent_usage_hint_text = \"keep\"\n",
+    )
+    .expect("user config");
+
+    let managed_path = tmp.path().join("managed_config.toml");
+    std::fs::write(&managed_path, "[features]\nmulti_agent_v2 = false\n").expect("managed config");
+    let managed_file = AbsolutePathBuf::try_from(managed_path.clone()).expect("managed file");
+    let service = ConfigManager::new_for_tests(
+        tmp.path().to_path_buf(),
+        vec![],
+        LoaderOverrides::with_managed_config_path_for_tests(managed_path),
+        CloudConfigBundleLoader::default(),
+    );
+
+    let read = service
+        .read(ConfigReadParams {
+            include_layers: false,
+            cwd: None,
+        })
+        .await
+        .expect("read config");
+    assert_eq!(
+        read.origins
+            .get("features.multi_agent_v2.enabled")
+            .expect("enabled origin")
+            .name,
+        ApiConfigLayerSource::LegacyManagedConfigTomlFromFile {
+            file: managed_file.clone(),
+        },
+    );
+
+    let result = service
+        .write_value(ConfigValueWriteParams {
+            file_path: Some(user_path.display().to_string()),
+            key_path: "features.multi_agent_v2.enabled".to_string(),
+            value: serde_json::json!(true),
+            merge_strategy: MergeStrategy::Upsert,
+            expected_version: None,
+        })
+        .await
+        .expect("write config");
+    assert_eq!(result.status, WriteStatus::OkOverridden);
+    let overridden = result.overridden_metadata.expect("overridden metadata");
+    assert_eq!(
+        overridden.overriding_layer.name,
+        ApiConfigLayerSource::LegacyManagedConfigTomlFromFile { file: managed_file }
+    );
+    assert_eq!(overridden.effective_value, serde_json::json!(false));
+}
+
 #[tokio::test]
 async fn upsert_merges_tables_replace_overwrites() -> Result<()> {
     let tmp = tempdir().expect("tempdir");
@@ -1229,6 +1286,69 @@ no_memories_if_mcp_or_web_search = false
             serde_json::json!({"disable_on_external_context": true}),
             r#"[memories]
 disable_on_external_context = true
+"#,
+        ),
+        (
+            r#"[features]
+multi_agent_v2 = true
+"#,
+            "features.multi_agent_v2.subagent_usage_hint_text",
+            serde_json::json!("Delegate carefully."),
+            r#"[features.multi_agent_v2]
+enabled = true
+subagent_usage_hint_text = "Delegate carefully."
+"#,
+        ),
+        (
+            r#"[features]
+multi_agent_v2 = true
+"#,
+            "features.multi_agent_v2",
+            serde_json::json!({"subagent_usage_hint_text": "Delegate carefully."}),
+            r#"[features.multi_agent_v2]
+enabled = true
+subagent_usage_hint_text = "Delegate carefully."
+"#,
+        ),
+        (
+            r#"[features.multi_agent_v2]
+enabled = true
+subagent_usage_hint_text = "Delegate carefully."
+"#,
+            "features.multi_agent_v2",
+            serde_json::json!(false),
+            r#"[features.multi_agent_v2]
+enabled = false
+subagent_usage_hint_text = "Delegate carefully."
+"#,
+        ),
+        (
+            r#"[features.multi_agent_v2]
+enabled = true
+subagent_usage_hint_text = "Delegate carefully."
+"#,
+            "features.multi_agent_v2",
+            serde_json::Value::Null,
+            "",
+        ),
+        (
+            r#"[desktop.features.multi_agent_v2]
+custom = true
+"#,
+            "desktop.features.multi_agent_v2",
+            serde_json::json!(false),
+            r#"[desktop.features]
+multi_agent_v2 = false
+"#,
+        ),
+        (
+            r#"[desktop.features]
+multi_agent_v2 = true
+"#,
+            "desktop.features.multi_agent_v2",
+            serde_json::json!({"custom": true}),
+            r#"[desktop.features.multi_agent_v2]
+custom = true
 "#,
         ),
     ];