diff --git a/codex-rs/app-server/tests/suite/v2/turn_start.rs b/codex-rs/app-server/tests/suite/v2/turn_start.rs
index bc3c4750bd..e50ee65f76 100644
--- a/codex-rs/app-server/tests/suite/v2/turn_start.rs
+++ b/codex-rs/app-server/tests/suite/v2/turn_start.rs
@@ -1673,7 +1673,10 @@ async fn turn_start_emits_spawn_agent_item_with_effective_inherited_model_metada
     const PARENT_PROMPT: &str = "spawn a child and continue";
     const SPAWN_CALL_ID: &str = "spawn-call-1";
     const INHERITED_MODEL: &str = "gpt-5.2-codex";
-    const INHERITED_REASONING_EFFORT: ReasoningEffort = ReasoningEffort::XHigh;
+    // thread/start only sets the model here; the session keeps the resolved collaboration-mode
+    // effort for that thread, so fork-context children should inherit `Medium`, not the model
+    // catalog default.
+    const INHERITED_REASONING_EFFORT: ReasoningEffort = ReasoningEffort::Medium;
     const REQUESTED_MODEL: &str = "gpt-5.1";
     const REQUESTED_REASONING_EFFORT: ReasoningEffort = ReasoningEffort::Low;
 
diff --git a/codex-rs/core/src/tools/handlers/multi_agents_common.rs b/codex-rs/core/src/tools/handlers/multi_agents_common.rs
index dd68465a7f..4778f6b9f8 100644
--- a/codex-rs/core/src/tools/handlers/multi_agents_common.rs
+++ b/codex-rs/core/src/tools/handlers/multi_agents_common.rs
@@ -225,7 +225,11 @@ fn build_agent_shared_config(turn: &TurnContext) -> Result<Config, FunctionCallE
     let mut config = (*base_config).clone();
     config.model = Some(turn.model_info.slug.clone());
     config.model_provider = turn.provider.clone();
-    config.model_reasoning_effort = turn.reasoning_effort;
+    // Forked children must preserve the spawning turn's effective model settings, including a
+    // model catalog default effort, so their config snapshot matches the actual request shape.
+    config.model_reasoning_effort = turn
+        .reasoning_effort
+        .or(turn.model_info.default_reasoning_level);
     config.model_reasoning_summary = Some(turn.reasoning_summary);
     config.developer_instructions = turn.developer_instructions.clone();
     config.compact_prompt = turn.compact_prompt.clone();