diff --git a/codex-rs/app-server/src/request_processors/thread_processor_tests.rs b/codex-rs/app-server/src/request_processors/thread_processor_tests.rs
index 61a31f9c4d..0929686c1b 100644
--- a/codex-rs/app-server/src/request_processors/thread_processor_tests.rs
+++ b/codex-rs/app-server/src/request_processors/thread_processor_tests.rs
@@ -62,6 +62,9 @@ mod thread_processor_behavior_tests {
     use codex_model_provider_info::ModelProviderInfo;
     use codex_model_provider_info::WireApi;
     use codex_protocol::ThreadId;
+    use codex_protocol::config_types::CollaborationMode;
+    use codex_protocol::config_types::ModeKind;
+    use codex_protocol::config_types::Settings;
     use codex_protocol::openai_models::ReasoningEffort;
     use codex_protocol::permissions::FileSystemAccessMode;
     use codex_protocol::permissions::FileSystemPath;
@@ -652,7 +655,16 @@ mod thread_processor_behavior_tests {
             cwd,
             ephemeral: false,
             reasoning_effort: None,
+            reasoning_summary: None,
             personality: None,
+            collaboration_mode: CollaborationMode {
+                mode: ModeKind::Default,
+                settings: Settings {
+                    model: "gpt-5".to_string(),
+                    reasoning_effort: None,
+                    developer_instructions: None,
+                },
+            },
             session_source: SessionSource::Cli,
             thread_source: None,
         };
diff --git a/codex-rs/core/src/codex_thread.rs b/codex-rs/core/src/codex_thread.rs
index be5b0ccb88..28a2154cdb 100644
--- a/codex-rs/core/src/codex_thread.rs
+++ b/codex-rs/core/src/codex_thread.rs
@@ -61,7 +61,9 @@ pub struct ThreadConfigSnapshot {
     pub cwd: AbsolutePathBuf,
     pub ephemeral: bool,
     pub reasoning_effort: Option<ReasoningEffort>,
+    pub reasoning_summary: Option<ReasoningSummary>,
     pub personality: Option<Personality>,
+    pub collaboration_mode: CollaborationMode,
     pub session_source: SessionSource,
     pub thread_source: Option<ThreadSource>,
 }
@@ -251,11 +253,29 @@ impl CodexThread {
             .await
     }
 
-    /// Validate persistent turn context overrides without committing them.
-    pub async fn validate_turn_context_overrides(
+    /// Preview persistent turn context overrides without committing them.
+    pub async fn preview_turn_context_overrides(
         &self,
         overrides: CodexThreadTurnContextOverrides,
-    ) -> ConstraintResult<()> {
+    ) -> ConstraintResult<ThreadConfigSnapshot> {
+        let updates = self.turn_context_settings_update(overrides).await;
+        self.codex.session.preview_settings(&updates).await
+    }
+
+    /// Apply persistent turn context overrides and return the effective state.
+    pub async fn update_turn_context_overrides(
+        &self,
+        overrides: CodexThreadTurnContextOverrides,
+    ) -> ConstraintResult<ThreadConfigSnapshot> {
+        let updates = self.turn_context_settings_update(overrides).await;
+        self.codex.session.update_settings(updates).await?;
+        Ok(self.config_snapshot().await)
+    }
+
+    async fn turn_context_settings_update(
+        &self,
+        overrides: CodexThreadTurnContextOverrides,
+    ) -> SessionSettingsUpdate {
         let CodexThreadTurnContextOverrides {
             cwd,
             approval_policy,
@@ -281,7 +301,7 @@ impl CodexThread {
                 .with_updates(model, effort, /*developer_instructions*/ None)
         };
 
-        let updates = SessionSettingsUpdate {
+        SessionSettingsUpdate {
             cwd,
             approval_policy,
             approvals_reviewer,
@@ -294,8 +314,7 @@ impl CodexThread {
             service_tier,
             personality,
             ..Default::default()
-        };
-        self.codex.session.validate_settings(&updates).await
+        }
     }
 
     /// Use sparingly: this is intended to be removed soon.
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index 3ef9466812..62222261c5 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -1375,12 +1375,15 @@ impl Session {
         Ok(())
     }
 
-    pub(crate) async fn validate_settings(
+    pub(crate) async fn preview_settings(
         &self,
         updates: &SessionSettingsUpdate,
-    ) -> ConstraintResult<()> {
+    ) -> ConstraintResult<ThreadConfigSnapshot> {
         let state = self.state.lock().await;
-        state.session_configuration.apply(updates).map(|_| ())
+        state
+            .session_configuration
+            .apply(updates)
+            .map(|configuration| configuration.thread_config_snapshot())
     }
 
     pub(crate) async fn set_session_startup_prewarm(
diff --git a/codex-rs/core/src/session/session.rs b/codex-rs/core/src/session/session.rs
index 5b1b8c83e4..4e0ca4ed31 100644
--- a/codex-rs/core/src/session/session.rs
+++ b/codex-rs/core/src/session/session.rs
@@ -145,7 +145,9 @@ impl SessionConfiguration {
             cwd: self.cwd.clone(),
             ephemeral: self.original_config_do_not_use.ephemeral,
             reasoning_effort: self.collaboration_mode.reasoning_effort(),
+            reasoning_summary: self.model_reasoning_summary,
             personality: self.personality,
+            collaboration_mode: self.collaboration_mode.clone(),
             session_source: self.session_source.clone(),
             thread_source: self.thread_source,
         }