diff --git a/codex-rs/core/src/compact.rs b/codex-rs/core/src/compact.rs
index 189dd5df40..7155486919 100644
--- a/codex-rs/core/src/compact.rs
+++ b/codex-rs/core/src/compact.rs
@@ -103,10 +103,7 @@ pub(crate) async fn build_compaction_initial_context(
             step_context,
         } => {
             let items = sess
-                .build_initial_context_with_world_state(
-                    step_context.turn.as_ref(),
-                    world_state.as_ref(),
-                )
+                .build_initial_context_with_world_state(step_context, world_state.as_ref())
                 .await;
             (
                 items.into_iter().map(ResponseItemEnvelope::new).collect(),
diff --git a/codex-rs/core/src/compact_tests.rs b/codex-rs/core/src/compact_tests.rs
index 72fa09a1e8..0ab5c93e5f 100644
--- a/codex-rs/core/src/compact_tests.rs
+++ b/codex-rs/core/src/compact_tests.rs
@@ -26,7 +26,7 @@ async fn process_compacted_history_with_test_session(
             .expect("world state should build"),
     );
     let initial_context = session
-        .build_initial_context_with_world_state(&turn_context, world_state.as_ref())
+        .build_initial_context_with_world_state(&step_context, world_state.as_ref())
         .await;
     let initial_context_injection = InitialContextInjection::BeforeLastUserMessage {
         world_state,
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index 0aae16f6a3..ed0b6981a1 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -3977,11 +3977,15 @@ impl Session {
             .collect()
     }
 
+    /// `step_context` and `world_state` must come from the same captured step.
+    /// If more callers need this pair, bundle them into a captured-context struct
+    /// so callers cannot mix settings and WorldState from different steps.
     pub(crate) async fn build_initial_context_with_world_state(
         &self,
-        turn_context: &TurnContext,
+        step_context: &StepContext,
         world_state: &WorldState,
     ) -> Vec<ResponseItem> {
+        let turn_context = step_context.turn.as_ref();
         let mut developer_sections = Vec::<RenderedFragment>::with_capacity(8);
         let mut contextual_user_sections = Vec::<RenderedFragment>::with_capacity(2);
         let mut separate_developer_sections = Vec::<RenderedFragment>::new();
@@ -4071,7 +4075,11 @@ impl Session {
         }
         // This is full-context metadata. Steady-state context diffs should not re-emit it.
         if turn_context.config.features.enabled(Feature::TokenBudget)
-            && turn_context.model_context_window().is_some()
+            && step_context
+                .settings
+                .model_info
+                .resolved_context_window()
+                .is_some()
         {
             // Keep the legacy bridge hint when native Notes is disabled. A failed
             // native request must not fall back to the bridge.
@@ -4291,7 +4299,7 @@ impl Session {
         };
         let (window_number, window_ids) = window;
         let context_items = self
-            .build_initial_context_with_world_state(turn_context, world_state.as_ref())
+            .build_initial_context_with_world_state(step_context, world_state.as_ref())
             .await
             .into_iter()
             .map(ResponseItemEnvelope::new)
@@ -4350,7 +4358,7 @@ impl Session {
         // Full initial context resets the baseline; later turns persist only its changes.
         let (mut context_items, world_state_item) = if should_inject_full_context {
             let context_items = self
-                .build_initial_context_with_world_state(turn_context, world_state.as_ref())
+                .build_initial_context_with_world_state(step_context, world_state.as_ref())
                 .await;
             let snapshot = world_state.snapshot();
             self.state
diff --git a/codex-rs/core/src/session/multi_agents.rs b/codex-rs/core/src/session/multi_agents.rs
index 666f32cce2..19d0cb95e5 100644
--- a/codex-rs/core/src/session/multi_agents.rs
+++ b/codex-rs/core/src/session/multi_agents.rs
@@ -1,6 +1,6 @@
 use crate::config::MultiAgentV2Config;
 use crate::context::MultiAgentRoleInstructions;
-use crate::session::turn_context::TurnContext;
+use crate::session::step_context::StepContext;
 use codex_protocol::config_types::MultiAgentMode;
 use codex_protocol::openai_models::MultiAgentRoleMessages;
 use codex_protocol::openai_models::ReasoningEffort;
@@ -65,15 +65,17 @@ pub(crate) struct ResolvedMultiAgentV2UsageHints {
 }
 
 pub(super) fn usage_hint_text(
-    turn_context: &TurnContext,
+    step_context: &StepContext,
     session_source: &SessionSource,
 ) -> Option<MultiAgentRoleInstructions> {
+    let turn_context = step_context.turn.as_ref();
     if turn_context.multi_agent_version != MultiAgentVersion::V2 {
         return None;
     }
 
-    let catalog = turn_context
-        .model_info()
+    let catalog = step_context
+        .settings
+        .model_info
         .model_messages
         .as_ref()
         .and_then(|messages| messages.multi_agent.as_ref())
@@ -152,13 +154,15 @@ pub(crate) fn resolve_usage_hints(
     }
 }
 
-pub(crate) fn effective_multi_agent_mode(turn_context: &TurnContext) -> Option<MultiAgentMode> {
+pub(crate) fn effective_multi_agent_mode(step_context: &StepContext) -> Option<MultiAgentMode> {
+    let turn_context = step_context.turn.as_ref();
+    let settings = &step_context.settings;
     if turn_context.multi_agent_version != MultiAgentVersion::V2 {
         return None;
     }
 
-    let catalog_mode = turn_context
-        .model_info()
+    let catalog_mode = settings
+        .model_info
         .model_messages
         .as_ref()
         .and_then(|messages| messages.multi_agent.as_ref())
@@ -174,7 +178,7 @@ pub(crate) fn effective_multi_agent_mode(turn_context: &TurnContext) -> Option<M
     // of an effort-derived built-in policy.
     let multi_agent_mode = match mode_hint_text {
         Some(hint_text) => MultiAgentMode::Custom(hint_text.to_string()),
-        None => match turn_context.effective_reasoning_effort() {
+        None => match settings.effective_reasoning_effort() {
             Some(ReasoningEffort::Ultra) => catalog_mode
                 .and_then(|messages| messages.proactive.clone())
                 .map(MultiAgentMode::Custom)
diff --git a/codex-rs/core/src/session/step_settings.rs b/codex-rs/core/src/session/step_settings.rs
index 49502e8a6e..d9d99b9b14 100644
--- a/codex-rs/core/src/session/step_settings.rs
+++ b/codex-rs/core/src/session/step_settings.rs
@@ -90,6 +90,12 @@ impl ResolvedStepSettings {
             .cloned()
     }
 
+    pub(crate) fn effective_collaboration_mode(&self) -> CollaborationMode {
+        let mut collaboration_mode = self.selected.collaboration_mode.clone();
+        collaboration_mode.settings.model = self.model_info.slug.clone();
+        collaboration_mode
+    }
+
     pub(crate) fn approval_policy(&self) -> AskForApproval {
         self.selected.approval_policy.value()
     }
diff --git a/codex-rs/core/src/session/tests.rs b/codex-rs/core/src/session/tests.rs
index 3b2d0bfb1f..ca6fa99501 100644
--- a/codex-rs/core/src/session/tests.rs
+++ b/codex-rs/core/src/session/tests.rs
@@ -74,6 +74,7 @@ use codex_protocol::models::FunctionCallOutputPayload;
 use codex_protocol::models::ImageDetail;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::models::SandboxEnforcement;
+use codex_protocol::openai_models::ModelInstructionsVariables;
 use codex_protocol::openai_models::ModelServiceTier;
 use codex_protocol::openai_models::ToolMode;
 use codex_protocol::permissions::FileSystemAccessMode;
@@ -6416,9 +6417,13 @@ async fn build_initial_context(
     session: &Session,
     turn_context: &Arc<TurnContext>,
 ) -> Vec<ResponseItem> {
-    let world_state = build_world_state_from_turn_context(session, turn_context).await;
+    let step_context = StepContext::for_test(Arc::clone(turn_context));
+    let world_state = session
+        .build_world_state_for_step(&step_context)
+        .await
+        .expect("world state should build");
     session
-        .build_initial_context_with_world_state(turn_context.as_ref(), &world_state)
+        .build_initial_context_with_world_state(&step_context, &world_state)
         .await
 }
 
@@ -10123,8 +10128,9 @@ async fn build_initial_context_reuses_in_flight_recommendation_prewarm() {
     // Keep the OnceCell initializer unpolled while first-thread context joins its in-flight fetch.
     // This does not depend on how quickly the HTTP server returns its response.
     let world_state = WorldState::default();
+    let step_context = StepContext::for_test(Arc::clone(&turn_context));
     let initial_context =
-        session.build_initial_context_with_world_state(&turn_context, &world_state);
+        session.build_initial_context_with_world_state(&step_context, &world_state);
     tokio::pin!(initial_context);
     assert!(futures::poll!(initial_context.as_mut()).is_pending());
 
@@ -10802,6 +10808,103 @@ async fn record_context_updates_and_set_reference_context_item_persists_split_fi
     );
 }
 
+#[tokio::test]
+async fn build_initial_context_uses_retained_step_after_model_change() {
+    let (session, mut turn_context, _rx_event) =
+        make_session_and_context_with_auth_and_config_and_rx(
+            CodexAuth::from_api_key("Test API Key"),
+            Vec::new(),
+            |config| {
+                config.features.enable(Feature::Personality).unwrap();
+                config.features.enable(Feature::TokenBudget).unwrap();
+                config.personality = Some(Personality::Friendly);
+            },
+        )
+        .await;
+    update_turn_settings_for_test(Arc::get_mut(&mut turn_context).unwrap(), |settings| {
+        let model_info = Arc::make_mut(&mut settings.model_info);
+        model_info.slug = "model-a".to_string();
+        model_info.context_window = None;
+        model_info.max_context_window = None;
+        let messages = model_info.model_messages.as_mut().unwrap();
+        messages.instructions_template = Some("A instructions: {{ personality }}".to_string());
+        messages.instructions_variables = Some(ModelInstructionsVariables {
+            personality_default: Some("default".to_string()),
+            personality_friendly: Some("friendly".to_string()),
+            personality_pragmatic: Some("pragmatic".to_string()),
+        });
+    });
+    session
+        .set_previous_turn_settings(Some(PreviousTurnSettings {
+            model: "base-model".to_string(),
+            comp_hash: None,
+            realtime_active: None,
+        }))
+        .await;
+    let step_a = session
+        .capture_step_context(Arc::clone(&turn_context), &CancellationToken::new())
+        .await
+        .unwrap();
+    let world_a = Arc::new(session.build_world_state_for_step(&step_a).await.unwrap());
+    let retained = crate::compact::InitialContextInjection::BeforeLastUserMessage {
+        world_state: Arc::clone(&world_a),
+        step_context: Arc::clone(&step_a),
+    };
+    let (initial_a, _) =
+        crate::compact::build_compaction_initial_context(&session, &retained).await;
+
+    let mut selected_b = step_a.settings.selected().clone();
+    selected_b.collaboration_mode.settings.model = "model-b".to_string();
+    selected_b.personality = Some(Personality::Pragmatic);
+    let mut model_b = step_a.settings.model_info.as_ref().clone();
+    model_b.slug = "model-b".to_string();
+    model_b.context_window = Some(128_000);
+    model_b
+        .model_messages
+        .as_mut()
+        .unwrap()
+        .instructions_template = Some("B instructions: {{ personality }}".to_string());
+    turn_context
+        .current_settings
+        .store(Arc::new(ResolvedStepSettings::new(
+            Arc::new(selected_b),
+            Arc::new(model_b),
+            /*fast_mode_enabled*/ false,
+        )));
+    let step_b = session
+        .capture_step_context(Arc::clone(&turn_context), &CancellationToken::new())
+        .await
+        .unwrap();
+    let world_b = session.build_world_state_for_step(&step_b).await.unwrap();
+    let initial_b = session
+        .build_initial_context_with_world_state(&step_b, &world_b)
+        .await;
+    let (restored_a, restored_world) =
+        crate::compact::build_compaction_initial_context(&session, &retained).await;
+
+    assert_eq!(restored_a, initial_a);
+    assert!(Arc::ptr_eq(restored_world.as_ref().unwrap(), &world_a));
+    let initial_a = initial_a
+        .into_iter()
+        .map(ResponseItemEnvelope::into_item)
+        .collect::<Vec<_>>();
+    let a_text = developer_input_texts(&initial_a).join("\n");
+    let b_text = developer_input_texts(&initial_b).join("\n");
+    assert!(a_text.contains("A instructions: friendly"));
+    assert!(!a_text.contains("<context_window>"));
+    assert!(b_text.contains("B instructions: pragmatic"));
+    assert!(!b_text.contains("A instructions:"));
+    assert!(
+        b_text.contains("<context_window>"),
+        "full-context metadata must use B's window even though the turn started without one"
+    );
+    assert_eq!(
+        step_b.environments.to_selections(),
+        step_a.environments.to_selections()
+    );
+    assert!(Arc::ptr_eq(&step_b.turn.config, &step_a.turn.config));
+}
+
 #[tokio::test]
 async fn build_initial_context_prepends_model_switch_message() {
     let (session, turn_context) = make_session_and_context().await;
diff --git a/codex-rs/core/src/session/turn_context.rs b/codex-rs/core/src/session/turn_context.rs
index f224c7ed28..ea3bfd157d 100644
--- a/codex-rs/core/src/session/turn_context.rs
+++ b/codex-rs/core/src/session/turn_context.rs
@@ -369,16 +369,6 @@ impl TurnContext {
         self.initial_settings.personality()
     }
 
-    /// Legacy: returns the frozen initial-turn collaboration-mode developer instructions.
-    /// Step-scoped consumers should use their captured `StepContext::settings`.
-    pub(crate) fn collaboration_mode_developer_instructions(&self) -> &Option<String> {
-        &self
-            .initial_settings
-            .selected_collaboration_mode()
-            .settings
-            .developer_instructions
-    }
-
     pub(crate) fn skills_snapshot(&self) -> Arc<HostSkillsSnapshot> {
         let Some(snapshot) = self.extension_data.get::<HostSkillsSnapshot>() else {
             unreachable!("every turn has a host skills snapshot");
@@ -389,14 +379,7 @@ impl TurnContext {
     /// Legacy: returns the frozen initial-turn collaboration mode with the resolved model slug.
     /// Step-scoped consumers should use their captured `StepContext::settings`.
     pub(crate) fn collaboration_mode(&self) -> CollaborationMode {
-        CollaborationMode {
-            mode: self.mode(),
-            settings: Settings {
-                model: self.model_info().slug.clone(),
-                reasoning_effort: self.reasoning_effort().cloned(),
-                developer_instructions: self.collaboration_mode_developer_instructions().clone(),
-            },
-        }
+        self.initial_settings.effective_collaboration_mode()
     }
 
     pub(crate) fn plugin_attribution_for_command(
diff --git a/codex-rs/core/src/session/world_state.rs b/codex-rs/core/src/session/world_state.rs
index 1253a9fcfe..3ed069258f 100644
--- a/codex-rs/core/src/session/world_state.rs
+++ b/codex-rs/core/src/session/world_state.rs
@@ -37,13 +37,14 @@ impl Session {
         step_context: &StepContext,
     ) -> CodexResult<WorldState> {
         let turn_context = step_context.turn.as_ref();
+        let settings = &step_context.settings;
+        let model_info = settings.model_info.as_ref();
+        let personality = settings.personality();
         tracing::trace!(
             selected_capability_root_count = step_context.selected_capability_roots.len(),
             "building step world state"
         );
-        let model_instructions = turn_context
-            .model_info()
-            .get_model_instructions(turn_context.personality());
+        let model_instructions = model_info.get_model_instructions(personality);
         let model_instructions = if !turn_context.config.update_plan_enabled
             && turn_context.config.model_catalog.is_none()
             && (turn_context.config.base_instructions.is_none()
@@ -77,8 +78,8 @@ impl Session {
                 base_instructions,
             )
         };
-        let personality_is_baked = turn_context.model_info().supports_personality()
-            && base_instructions == model_instructions;
+        let personality_is_baked =
+            model_info.supports_personality() && base_instructions == model_instructions;
         let environment_subagents = if turn_context.config.include_environment_context {
             self.services
                 .agent_control
@@ -92,22 +93,21 @@ impl Session {
         };
         let mut world_state = WorldState::default();
         world_state.add_section(ModelInstructionsState::new(
-            &turn_context.model_info().slug,
+            &model_info.slug,
             previous_model.as_deref(),
             model_instructions,
         ));
         if self.features.enabled(Feature::Personality) {
-            let personality_instructions = turn_context.personality().and_then(|personality| {
-                turn_context
-                    .model_info()
+            let personality_instructions = personality.and_then(|personality| {
+                model_info
                     .model_messages
                     .as_ref()
                     .and_then(|messages| messages.get_personality_message(Some(personality)))
                     .filter(|message| !message.is_empty())
             });
             world_state.add_section(PersonalityState::new(
-                &turn_context.model_info().slug,
-                turn_context.personality(),
+                &model_info.slug,
+                personality,
                 previous_context
                     .as_ref()
                     .map(|previous| previous.model.as_str())
@@ -172,12 +172,12 @@ impl Session {
             let cwd = environment
                 .and_then(|environment| environment.cwd().to_abs_path().ok())
                 .unwrap_or_else(|| turn_context.cwd.clone());
-            let model_messages = turn_context.model_info().model_messages.as_ref();
+            let model_messages = model_info.model_messages.as_ref();
             world_state.add_section(PermissionsState::new(
                 &permission_profile,
-                step_context.settings.approval_policy(),
+                settings.approval_policy(),
                 ApprovalPromptContext::new(
-                    step_context.settings.approvals_reviewer(),
+                    settings.approvals_reviewer(),
                     model_messages.and_then(|messages| messages.approvals.as_ref()),
                     model_messages.and_then(|messages| messages.permissions.as_ref()),
                 ),
@@ -197,9 +197,8 @@ impl Session {
         }
         if turn_context.config.include_collaboration_mode_instructions {
             world_state.add_section(CollaborationModeState::from_collaboration_mode(
-                &turn_context.collaboration_mode(),
-                turn_context
-                    .model_info()
+                &settings.effective_collaboration_mode(),
+                model_info
                     .model_messages
                     .as_ref()
                     .and_then(|messages| messages.collaboration_modes.as_ref()),
@@ -266,12 +265,12 @@ impl Session {
                 false
             };
         let apps_usage_instructions_available =
-            apps_available && turn_context.model_info().include_apps_usage_instructions;
+            apps_available && model_info.include_apps_usage_instructions;
         world_state.add_section(AppsInstructionsState::new(
             apps_usage_instructions_available,
         ));
-        let plugins_usage_instructions_available = step_context.mcp.plugins_available()
-            && turn_context.model_info().include_plugin_usage_instructions;
+        let plugins_usage_instructions_available =
+            step_context.mcp.plugins_available() && model_info.include_plugin_usage_instructions;
         world_state.add_section(PluginsInstructionsState::new(
             plugins_usage_instructions_available,
         ));
@@ -314,10 +313,10 @@ impl Session {
             }
         }
         let mut multi_agent_mode = MultiAgentModeState::new(
-            super::multi_agents::effective_multi_agent_mode(turn_context),
+            super::multi_agents::effective_multi_agent_mode(step_context),
         );
         if let Some(usage_hint_text) =
-            super::multi_agents::usage_hint_text(turn_context, &turn_context.session_source)
+            super::multi_agents::usage_hint_text(step_context, &turn_context.session_source)
         {
             let usage_hint = MultiAgentUsageHintState::new(usage_hint_text);
             multi_agent_mode = multi_agent_mode.with_usage_hint(&usage_hint);
diff --git a/codex-rs/core/src/thread_manager_tests.rs b/codex-rs/core/src/thread_manager_tests.rs
index a9d7077e98..7103c17145 100644
--- a/codex-rs/core/src/thread_manager_tests.rs
+++ b/codex-rs/core/src/thread_manager_tests.rs
@@ -7,6 +7,7 @@ use crate::mcp::McpEnvironmentScope;
 use crate::mcp::McpThreadIdentity;
 use crate::rollout::RolloutRecorder;
 use crate::session::session::SessionSettingsUpdate;
+use crate::session::step_context::StepContext;
 use crate::session::tests::build_world_state_from_turn_context;
 use crate::session::tests::make_session_and_context;
 use crate::tasks::InterruptedTurnHistoryMarker;
@@ -598,8 +599,9 @@ async fn ignores_session_prefix_messages_when_truncating() {
     let (session, turn_context) = make_session_and_context().await;
     let turn_context = Arc::new(turn_context);
     let world_state = build_world_state_from_turn_context(&session, &turn_context).await;
+    let step_context = StepContext::for_test(turn_context);
     let mut items = session
-        .build_initial_context_with_world_state(&turn_context, &world_state)
+        .build_initial_context_with_world_state(&step_context, &world_state)
         .await;
     items.push(user_msg("feature request"));
     items.push(assistant_msg("ack"));
@@ -1201,10 +1203,11 @@ async fn spawn_internal_session_preserves_parent_lineage_without_forking_history
     let reviewer_turn = reviewer.thread.session.new_default_turn().await;
     let reviewer_world_state =
         build_world_state_from_turn_context(&reviewer.thread.session, &reviewer_turn).await;
+    let reviewer_step = StepContext::for_test(Arc::clone(&reviewer_turn));
     let reviewer_context = reviewer
         .thread
         .session
-        .build_initial_context_with_world_state(&reviewer_turn, &reviewer_world_state)
+        .build_initial_context_with_world_state(&reviewer_step, &reviewer_world_state)
         .await;
     assert!(
         !serde_json::to_string(&reviewer_context)
diff --git a/codex-rs/core/src/thread_rollout_truncation_tests.rs b/codex-rs/core/src/thread_rollout_truncation_tests.rs
index 9bdef1a08c..88d496eaa1 100644
--- a/codex-rs/core/src/thread_rollout_truncation_tests.rs
+++ b/codex-rs/core/src/thread_rollout_truncation_tests.rs
@@ -1,4 +1,5 @@
 use super::*;
+use crate::session::step_context::StepContext;
 use crate::session::tests::build_world_state_from_turn_context;
 use crate::session::tests::make_session_and_context;
 use codex_protocol::AgentPath;
@@ -364,8 +365,9 @@ async fn ignores_session_prefix_messages_when_truncating_rollout_from_start() {
     let (session, turn_context) = make_session_and_context().await;
     let turn_context = Arc::new(turn_context);
     let world_state = build_world_state_from_turn_context(&session, &turn_context).await;
+    let step_context = StepContext::for_test(turn_context);
     let mut items = session
-        .build_initial_context_with_world_state(&turn_context, &world_state)
+        .build_initial_context_with_world_state(&step_context, &world_state)
         .await;
     items.push(user_msg("feature request"));
     items.push(assistant_msg("ack"));
diff --git a/codex-rs/core/tests/suite/step_settings.rs b/codex-rs/core/tests/suite/step_settings.rs
index b487f88f05..19ad6daed2 100644
--- a/codex-rs/core/tests/suite/step_settings.rs
+++ b/codex-rs/core/tests/suite/step_settings.rs
@@ -9,12 +9,19 @@ use codex_login::CodexAuth;
 use codex_models_manager::bundled_models_response;
 use codex_models_manager::manager::RefreshStrategy;
 use codex_protocol::config_types::ApprovalsReviewer;
+use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::ServiceTier;
+use codex_protocol::openai_models::ApprovalMessages;
+use codex_protocol::openai_models::CollaborationModeMessages;
 use codex_protocol::openai_models::ConfirmationPolicies;
 use codex_protocol::openai_models::ModelInfo;
+use codex_protocol::openai_models::ModelInstructionsVariables;
 use codex_protocol::openai_models::ModelTokenBudgetConfig;
 use codex_protocol::openai_models::ModelsResponse;
+use codex_protocol::openai_models::MultiAgentMessages;
+use codex_protocol::openai_models::MultiAgentModeMessages;
+use codex_protocol::openai_models::MultiAgentRoleMessages;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::openai_models::ReasoningEffortPreset;
 use codex_protocol::openai_models::ToolMessage;
@@ -344,6 +351,21 @@ async fn settings_updates_preserve_turn_identity_and_target(target: SettingsTarg
         Some("original-turn-state".to_string())
     );
     assert_eq!(requests[2].header(TURN_STATE_HEADER), None);
+    let expected_switch_counts = match target {
+        SettingsTarget::Thread => vec![0, 0, 1],
+        SettingsTarget::Turn => vec![0, 1, 2],
+    };
+    assert_eq!(
+        requests
+            .iter()
+            .map(|request| request
+                .message_input_texts("developer")
+                .iter()
+                .filter(|text| text.contains("<model_switch>"))
+                .count())
+            .collect::<Vec<_>>(),
+        expected_switch_counts
+    );
 
     Ok(())
 }
@@ -365,7 +387,7 @@ enum TokenBudgetScenario {
 #[test_case(TokenBudgetScenario::InitialWindowOnly)]
 #[test_case(TokenBudgetScenario::DestinationWithoutGuidance)]
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn active_model_switch_resolves_token_budget_from_original_preferences(
+async fn active_model_switch_updates_core_context_from_captured_settings(
     scenario: TokenBudgetScenario,
 ) -> Result<()> {
     skip_if_no_network!(Ok(()));
@@ -411,6 +433,15 @@ async fn active_model_switch_resolves_token_budget_from_original_preferences(
                 .features
                 .enable(Feature::TokenBudget)
                 .expect("enable token-budget feature");
+            config
+                .features
+                .enable(Feature::Personality)
+                .expect("enable personality");
+            config
+                .features
+                .enable(Feature::MultiAgentV2)
+                .expect("enable multi-agent V2");
+            config.personality = Some(Personality::Pragmatic);
             if context_window_model.is_some() {
                 config.model_context_window = None;
             }
@@ -426,11 +457,38 @@ async fn active_model_switch_resolves_token_budget_from_original_preferences(
                     model.context_window = (slug == context_window_model).then_some(128_000);
                     model.max_context_window = None;
                 }
-                model
-                    .model_messages
-                    .as_mut()
-                    .expect("model messages")
-                    .token_budget = (initial_model || destination_has_guidance).then(|| ModelTokenBudgetConfig {
+                let messages = model.model_messages.as_mut().expect("model messages");
+                messages.instructions_template = Some(format!(
+                    "Instructions for {slug}. {{{{ personality }}}}"
+                ));
+                messages.instructions_variables = Some(ModelInstructionsVariables {
+                    personality_default: Some(format!("Default {slug} personality.")),
+                    personality_friendly: Some(format!("Friendly {slug} personality.")),
+                    personality_pragmatic: Some(format!("Pragmatic {slug} personality.")),
+                });
+                messages.collaboration_modes = Some(CollaborationModeMessages {
+                    default: Some(format!("Default collaboration for {slug}.")),
+                    plan: None,
+                });
+                messages.multi_agent = Some(MultiAgentMessages {
+                    role: Some(MultiAgentRoleMessages {
+                        root: Some(format!("Root role for {slug}.")),
+                        subagent: None,
+                    }),
+                    mode: Some(MultiAgentModeMessages {
+                        explicit: Some(format!("Delegation policy for {slug}.")),
+                        proactive: None,
+                        hint_text: None,
+                    }),
+                });
+                messages.approvals = Some(ApprovalMessages {
+                    on_request: Some(format!("Approval instructions for {slug}.")),
+                    on_request_auto_review: None,
+                    never: None,
+                    unless_trusted: None,
+                });
+                messages.token_budget =
+                    (initial_model || destination_has_guidance).then(|| ModelTokenBudgetConfig {
                     enabled: false,
                     use_history_notes_extension: false,
                     reminder_threshold_tokens: if initial_model { 8_000 } else { 2_000 },
@@ -487,6 +545,64 @@ async fn active_model_switch_resolves_token_budget_from_original_preferences(
 
     let requests = response_mock.requests();
     assert_eq!(requests.len(), 3);
+    assert_eq!(
+        requests
+            .iter()
+            .map(|request| request.body_json()["model"].clone())
+            .collect::<Vec<_>>(),
+        vec![json!(MODEL_A), json!(MODEL_B), json!(MODEL_B)]
+    );
+    let initial_instructions =
+        format!("Instructions for {MODEL_A}. Pragmatic {MODEL_A} personality.");
+    assert_eq!(requests[0].instructions_text(), initial_instructions);
+    assert!(!requests[0].body_contains_text("<model_switch>"));
+    for text in [
+        format!("Root role for {MODEL_A}."),
+        format!("Delegation policy for {MODEL_A}."),
+    ] {
+        assert!(requests[0].body_contains_text(&text));
+    }
+    for pair in requests.windows(/*size*/ 2) {
+        assert!(
+            pair[1].input().starts_with(&pair[0].input()),
+            "context updates must be append-only"
+        );
+        assert_eq!(pair[1].instructions_text(), initial_instructions);
+        assert_eq!(pair[1].body_json()["tools"], pair[0].body_json()["tools"]);
+        assert_eq!(
+            pair[1].message_input_texts("user"),
+            pair[0].message_input_texts("user")
+        );
+    }
+    for request in &requests[1..] {
+        let developer_texts = request.message_input_texts("developer");
+        let switches = developer_texts
+            .iter()
+            .filter(|text| text.contains("<model_switch>"))
+            .collect::<Vec<_>>();
+        assert_eq!(switches.len(), 1);
+        assert!(switches[0].contains(&format!(
+            "Instructions for {MODEL_B}. Pragmatic {MODEL_B} personality."
+        )));
+        assert!(
+            !request.body_contains_text("<personality_spec>"),
+            "personality is included in the model-switch instructions"
+        );
+        for text in [
+            format!("Default collaboration for {MODEL_B}."),
+            format!("Approval instructions for {MODEL_B}."),
+            format!("Root role for {MODEL_B}."),
+            format!("Delegation policy for {MODEL_B}."),
+        ] {
+            assert_eq!(
+                developer_texts
+                    .iter()
+                    .filter(|message| message.contains(&text))
+                    .count(),
+                1
+            );
+        }
+    }
     let initial_guidance = format!("Use {MODEL_A} token-budget guidance.");
     let initial_guidance_expected =
         !explicit_default_template && context_window_model != Some(MODEL_B);
@@ -532,6 +648,89 @@ async fn active_model_switch_resolves_token_budget_from_original_preferences(
     Ok(())
 }
 
+#[test_case(Some(ReasoningEffort::Ultra); "selected effort")]
+#[test_case(None; "captured model default effort")]
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn active_model_switch_updates_multi_agent_policy_from_captured_effort(
+    effort: Option<ReasoningEffort>,
+) -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let use_model_default = effort.is_none();
+    let server = start_mock_server().await;
+    let response_mock = mount_sse_sequence(
+        &server,
+        vec![
+            paused_response("resp-1", "pause-before-effort-switch"),
+            sse_completed("resp-2"),
+        ],
+    )
+    .await;
+    let test = step_settings_test()
+        .with_config(move |config| {
+            config
+                .features
+                .enable(Feature::MultiAgentV2)
+                .expect("enable multi-agent V2");
+            for model in &mut config
+                .model_catalog
+                .as_mut()
+                .expect("controlled model catalog")
+                .models
+            {
+                model
+                    .model_messages
+                    .as_mut()
+                    .expect("model messages")
+                    .multi_agent = None;
+                model
+                    .supported_reasoning_levels
+                    .push(ReasoningEffortPreset {
+                        effort: ReasoningEffort::Ultra,
+                        description: "Ultra".to_string(),
+                    });
+                model.default_reasoning_level =
+                    Some(if model.slug == MODEL_B && use_model_default {
+                        ReasoningEffort::Ultra
+                    } else {
+                        ReasoningEffort::Low
+                    });
+            }
+        })
+        .build_with_auto_env(&server)
+        .await?;
+    let request = start_paused_turn(&test.codex).await?;
+    assert_eq!(
+        submit_turn_settings(
+            &test.codex,
+            &request.turn_id,
+            TurnSettingsUpdate {
+                model: Some(MODEL_B.to_string()),
+                effort: Some(effort),
+                ..Default::default()
+            },
+        )
+        .await?,
+        TurnSettingsUpdateOutcome::Applied
+    );
+    answer_paused_turn(&test.codex, &request.turn_id).await?;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let requests = response_mock.requests();
+    assert_eq!(requests.len(), 2);
+    assert_eq!(requests[0].body_json()["model"], MODEL_A);
+    assert_eq!(requests[1].body_json()["model"], MODEL_B);
+    assert_eq!(requests[1].body_json()["reasoning"]["effort"], "xhigh");
+    let proactive_text = "Proactive multi-agent delegation is active.";
+    assert!(!requests[0].body_contains_text(proactive_text));
+    assert!(requests[1].body_contains_text(proactive_text));
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn mcp_confirmation_policy_follows_step_model_changes() -> Result<()> {
     skip_if_no_network!(Ok(()));