Add an opt-in feature for subagent model context defaults (#52659)

## What changed

Add the disabled-by-default `subagent_default_context_limits` feature and expose it in the configuration schema. When enabled, shared configuration for spawning and resuming subagents clears inherited `model_context_window` and `model_auto_compact_token_limit` overrides so subagents use their model's default context window and auto-compaction threshold.

GitOrigin-RevId: 5168ae2b4fbe72c533d8c6152922a8bceb2c5cbd

codex-rs/core/config.schema.json

@@ -1178,6 +1178,9 @@
             "step_model_switching": {
               "type": "boolean"
             },
+            "subagent_default_context_limits": {
+              "type": "boolean"
+            },
             "system_proxy_fallback": {
               "type": "boolean"
             },
@@ -7472,6 +7475,9 @@
         "step_model_switching": {
           "type": "boolean"
         },
+        "subagent_default_context_limits": {
+          "type": "boolean"
+        },
         "system_proxy_fallback": {
           "type": "boolean"
         },

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

@@ -9,6 +9,7 @@ use crate::config::Config;
 use crate::session::session::Session;
 use crate::session::step_context::StepContext;
 use crate::session::turn_context::TurnContext;
+use codex_features::Feature;
 use codex_models_manager::manager::RefreshStrategy;
 use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::models::BaseInstructions;
@@ -131,6 +132,13 @@ pub(crate) fn build_agent_resume_config(turn: &TurnContext) -> Result<Config, St
 fn build_agent_shared_config(turn: &TurnContext) -> Result<Config, String> {
     let base_config = turn.config.clone();
     let mut config = (*base_config).clone();
+    if config
+        .features
+        .enabled(Feature::SubagentDefaultContextLimits)
+    {
+        config.model_context_window = None;
+        config.model_auto_compact_token_limit = None;
+    }
     // Preserve activation for history forks without freezing the parent's model-owned prompts.
     // Fresh child startup restores configured preferences from the retained snapshot.
     config.token_budget = turn.configured_token_budget.clone();

codex-rs/features/src/lib.rs

@@ -225,6 +225,8 @@ pub enum Feature {
     ModelCatalogInContext,
     /// Inherit client-defined dynamic tools in fresh V2 subagents.
     MultiAgentV2DynamicTools,
+    /// Use each subagent model's default context window and auto-compaction threshold.
+    SubagentDefaultContextLimits,
     /// Keep sampling through reasoning and commentary boundaries when agent mail arrives.
     /// Pending mail is delivered at the next normal input boundary instead.
     DeferMailboxPreemption,
@@ -1464,6 +1466,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::SubagentDefaultContextLimits,
+        key: "subagent_default_context_limits",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::DeferMailboxPreemption,
         key: "defer_mailbox_preemption",