diff --git a/codex-rs/app-server/tests/suite/v2/guardian_sync_session_tests.rs b/codex-rs/app-server/tests/suite/v2/guardian_sync_session_tests.rs
index a226cd900b..4012493637 100644
--- a/codex-rs/app-server/tests/suite/v2/guardian_sync_session_tests.rs
+++ b/codex-rs/app-server/tests/suite/v2/guardian_sync_session_tests.rs
@@ -124,6 +124,12 @@ async fn managed_reviewers_reuse_fork_and_resume_after_parent_shutdown(
     // The seed review finished; pause the parent before it requests concurrent approvals.
     timeout(TIMEOUT, server.wait_for_request_count(/*count*/ 3)).await?;
     let seed: Value = serde_json::from_slice(&server.requests().await[1])?;
+    assert!(
+        !seed["tools"]
+            .to_string()
+            .contains(&format!("mcp__{TEST_SERVER_NAME}")),
+        "reviewers must not inherit the parent's configured MCP tools"
+    );
     let reviewer_id = seed["client_metadata"]["thread_id"]
         .as_str()
         .expect("reviewer ID")
diff --git a/codex-rs/core/src/codex_delegate.rs b/codex-rs/core/src/codex_delegate.rs
index 83908c5173..d5795df0ff 100644
--- a/codex-rs/core/src/codex_delegate.rs
+++ b/codex-rs/core/src/codex_delegate.rs
@@ -56,6 +56,7 @@ pub(crate) async fn run_codex_thread_interactive(
     parent_environments: TurnEnvironmentSnapshot,
     cancel_token: CancellationToken,
     subagent_source: SubAgentSource,
+    isolation: codex_extension_api::SessionIsolation,
     initial_history: Option<InitialHistory>,
     git_enrichment_policy: GitEnrichmentPolicy,
     windows_sandbox_proxy_settings_mode: codex_sandboxing::WindowsSandboxProxySettingsMode,
@@ -79,7 +80,7 @@ pub(crate) async fn run_codex_thread_interactive(
     };
     let session_source = SessionSource::SubAgent(subagent_source.clone());
     let is_guardian_reviewer = crate::guardian::is_basic_session_source(&session_source);
-    let extensions = if is_guardian_reviewer {
+    let extensions = if isolation == codex_extension_api::SessionIsolation::Isolated {
         codex_extension_api::empty_extension_registry()
     } else {
         Arc::clone(&parent_session.services.extensions)
@@ -87,6 +88,7 @@ pub(crate) async fn run_codex_thread_interactive(
     // Inline delegates never register with ThreadManager or receive on_thread_ready.
     // Bind their standalone spawn path before inherited extensions run.
     let mut thread_extension_init = codex_extension_api::ExtensionDataInit::default();
+    thread_extension_init.insert(isolation);
     thread_extension_init.insert(crate::guardian::GuardianReviewSessionHost::default());
     let (session, io) = Session::spawn(SessionSpawnArgs {
         config,
@@ -213,6 +215,7 @@ pub(crate) async fn run_codex_thread_one_shot(
         parent_environments,
         child_cancel.clone(),
         subagent_source,
+        codex_extension_api::SessionIsolation::Inherit,
         initial_history,
         GitEnrichmentPolicy::Fresh,
         codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile,
diff --git a/codex-rs/core/src/codex_delegate_tests.rs b/codex-rs/core/src/codex_delegate_tests.rs
index f79ff413fb..5c31a2531b 100644
--- a/codex-rs/core/src/codex_delegate_tests.rs
+++ b/codex-rs/core/src/codex_delegate_tests.rs
@@ -202,6 +202,7 @@ async fn run_codex_thread_interactive_respects_pre_cancelled_spawn() {
             parent_environments,
             cancel_token,
             SubAgentSource::Review,
+            codex_extension_api::SessionIsolation::Inherit,
             /*initial_history*/ None,
             crate::session::GitEnrichmentPolicy::Fresh,
             codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile,
@@ -217,7 +218,7 @@ async fn run_codex_thread_interactive_respects_pre_cancelled_spawn() {
 }
 
 #[tokio::test]
-async fn guardian_delegates_do_not_inherit_parent_extensions() {
+async fn delegate_isolation_does_not_depend_on_attribution() {
     let (mut parent_session, parent_ctx, _rx_events) =
         crate::session::tests::make_session_and_context_with_rx().await;
     let thread_starts = Arc::new(AtomicUsize::new(0));
@@ -229,13 +230,25 @@ async fn guardian_delegates_do_not_inherit_parent_extensions() {
         .services
         .extensions = Arc::new(extensions.build());
 
-    for (subagent_source, expected_thread_starts, expected_thread_source) in [
+    for (subagent_source, isolation, expected_thread_starts, expected_thread_source) in [
         (
             SubAgentSource::Other(crate::guardian::GUARDIAN_REVIEWER_NAME.to_string()),
+            codex_extension_api::SessionIsolation::Isolated,
             0,
             ThreadSource::GuardianReview,
         ),
-        (SubAgentSource::Review, 1, ThreadSource::Subagent),
+        (
+            SubAgentSource::Review,
+            codex_extension_api::SessionIsolation::Isolated,
+            0,
+            ThreadSource::Subagent,
+        ),
+        (
+            SubAgentSource::Review,
+            codex_extension_api::SessionIsolation::Inherit,
+            1,
+            ThreadSource::Subagent,
+        ),
     ] {
         let mut config = parent_ctx.config.as_ref().clone();
         config.permissions.approval_policy = Constrained::allow_only(AskForApproval::Never);
@@ -248,6 +261,7 @@ async fn guardian_delegates_do_not_inherit_parent_extensions() {
             parent_ctx.environments.clone(),
             CancellationToken::new(),
             subagent_source,
+            isolation,
             /*initial_history*/ None,
             crate::session::GitEnrichmentPolicy::Fresh,
             codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile,
@@ -291,6 +305,7 @@ async fn run_codex_thread_interactive_rejects_approval_policy_that_can_prompt()
         parent_environments,
         CancellationToken::new(),
         SubAgentSource::Review,
+        codex_extension_api::SessionIsolation::Inherit,
         /*initial_history*/ None,
         crate::session::GitEnrichmentPolicy::Fresh,
         codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile,
diff --git a/codex-rs/core/src/guardian/review_session_factory.rs b/codex-rs/core/src/guardian/review_session_factory.rs
index b58db97bc9..aa4ff2dd23 100644
--- a/codex-rs/core/src/guardian/review_session_factory.rs
+++ b/codex-rs/core/src/guardian/review_session_factory.rs
@@ -128,6 +128,7 @@ impl ReviewerSessionFactory for PreparedSession {
                     self.context.environments().clone(),
                     cancellation.clone(),
                     SubAgentSource::Other(GUARDIAN_REVIEWER_NAME.to_owned()),
+                    codex_extension_api::SessionIsolation::Isolated,
                     initial_history,
                     GitEnrichmentPolicy::Skip,
                     codex_sandboxing::WindowsSandboxProxySettingsMode::Preserve,
diff --git a/codex-rs/core/src/guardian/review_session_threads.rs b/codex-rs/core/src/guardian/review_session_threads.rs
index 660b5bcc8b..e2cc14ca28 100644
--- a/codex-rs/core/src/guardian/review_session_threads.rs
+++ b/codex-rs/core/src/guardian/review_session_threads.rs
@@ -67,7 +67,10 @@ impl ManagedReviewerThreads {
         config.permissions.approval_policy = Constrained::allow_only(AskForApproval::Never);
         config.model_provider.supports_websockets &=
             parent.services.model_client.responses_websocket_enabled();
+        let mut thread_extension_init = codex_extension_api::ExtensionDataInit::default();
+        thread_extension_init.insert(codex_extension_api::SessionIsolation::Isolated);
         let options = StartThreadOptions {
+            thread_extension_init,
             session_source: Some(SessionSource::Internal(InternalSessionSource::Guardian)),
             thread_source: Some(ThreadSource::GuardianReview),
             environments: Some(context.environments().to_selections()),
diff --git a/codex-rs/core/src/session/mcp.rs b/codex-rs/core/src/session/mcp.rs
index d4505eace2..adc4740b98 100644
--- a/codex-rs/core/src/session/mcp.rs
+++ b/codex-rs/core/src/session/mcp.rs
@@ -135,12 +135,7 @@ impl Session {
             )
             .await;
         let mcp_config = self
-            .project_selected_environment_mcp_servers(
-                &session_source,
-                config,
-                &environments,
-                mcp_projection,
-            )
+            .project_selected_environment_mcp_servers(config, &environments, mcp_projection)
             .await
             .config;
         let local_process_cwd = environments
@@ -312,7 +307,6 @@ impl Session {
             .await;
         let mcp_projection = self
             .project_selected_environment_mcp_servers(
-                &desired.session_source,
                 &desired.config,
                 &desired.environments,
                 mcp_projection,
diff --git a/codex-rs/core/src/session/mcp_runtime.rs b/codex-rs/core/src/session/mcp_runtime.rs
index c3c329e89c..e14034de7b 100644
--- a/codex-rs/core/src/session/mcp_runtime.rs
+++ b/codex-rs/core/src/session/mcp_runtime.rs
@@ -143,13 +143,12 @@ impl Session {
     /// Adds effective executor-owned configuration from this exact thread snapshot.
     pub(super) fn project_selected_environment_mcp_servers<'a>(
         &'a self,
-        session_source: &'a SessionSource,
         config: &'a Config,
         environments: &'a TurnEnvironmentSnapshot,
         mut projection: McpRuntimeProjection,
     ) -> BoxFuture<'a, McpRuntimeProjection> {
         Box::pin(async move {
-            if crate::guardian::is_basic_session_source(session_source) {
+            if self.isolation == codex_extension_api::SessionIsolation::Isolated {
                 return projection;
             }
 
@@ -306,7 +305,6 @@ impl Session {
     ) {
         let mcp_projection = self
             .project_selected_environment_mcp_servers(
-                &desired.session_source,
                 &desired.config,
                 &desired.environments,
                 mcp_projection,
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index d997aa1527..98bfa792b2 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -580,7 +580,11 @@ impl Session {
         config
             .startup_warnings
             .extend(user_instruction_provider_warnings);
-        let exec_policy = if crate::guardian::is_basic_session_source(&session_source) {
+        let isolation = thread_extension_init
+            .get::<codex_extension_api::SessionIsolation>()
+            .map(|policy| *policy)
+            .unwrap_or_default();
+        let exec_policy = if isolation == codex_extension_api::SessionIsolation::Isolated {
             let managed_policy = config
                 .config_layer_stack
                 .requirements()
diff --git a/codex-rs/core/src/session/session.rs b/codex-rs/core/src/session/session.rs
index fa3835d601..ffe66620c4 100644
--- a/codex-rs/core/src/session/session.rs
+++ b/codex-rs/core/src/session/session.rs
@@ -58,6 +58,7 @@ pub(crate) struct Session {
     /// session.
     pub(super) features: ManagedFeatures,
     pub(crate) guardian_context_mode: GuardianContextMode,
+    pub(super) isolation: codex_extension_api::SessionIsolation,
     pub(crate) windows_sandbox_proxy_settings_mode:
         codex_sandboxing::WindowsSandboxProxySettingsMode,
     pub(super) multi_agent_version: OnceLock<MultiAgentVersion>,
@@ -860,6 +861,10 @@ impl Session {
         // Publish the already resolved model before extensions make startup decisions.
         // Turn construction refreshes this attachment when the selected model changes.
         thread_extension_init.insert(model_info);
+        let isolation = thread_extension_init
+            .get::<codex_extension_api::SessionIsolation>()
+            .map(|policy| *policy)
+            .unwrap_or_default();
         let mcp_thread_init = thread_extension_init.clone();
         let thread_extension_data = codex_extension_api::ExtensionData::new_with_init(
             thread_id.to_string(),
@@ -1582,6 +1587,7 @@ impl Session {
                 managed_network_proxy_refresh_lock: Semaphore::new(/*permits*/ 1),
                 features: config.features.clone(),
                 guardian_context_mode,
+                isolation,
                 windows_sandbox_proxy_settings_mode,
                 multi_agent_version,
                 mcp_refresh: McpRefresh::new(),
diff --git a/codex-rs/core/src/session/tests.rs b/codex-rs/core/src/session/tests.rs
index be539ca4f4..2089299b59 100644
--- a/codex-rs/core/src/session/tests.rs
+++ b/codex-rs/core/src/session/tests.rs
@@ -6701,6 +6701,7 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
         managed_network_proxy_refresh_lock: Semaphore::new(/*permits*/ 1),
         features: config.features.clone(),
         guardian_context_mode: GuardianContextMode::from_features(&config.features),
+        isolation: codex_extension_api::SessionIsolation::Inherit,
         windows_sandbox_proxy_settings_mode:
             codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile,
         multi_agent_version: OnceLock::from(config.multi_agent_version_from_features()),
@@ -8874,6 +8875,7 @@ where
         managed_network_proxy_refresh_lock: Semaphore::new(/*permits*/ 1),
         features: config.features.clone(),
         guardian_context_mode: GuardianContextMode::from_features(&config.features),
+        isolation: codex_extension_api::SessionIsolation::Inherit,
         windows_sandbox_proxy_settings_mode:
             codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile,
         multi_agent_version: OnceLock::from(config.multi_agent_version_from_features()),
diff --git a/codex-rs/core/src/session/tests/guardian_tests.rs b/codex-rs/core/src/session/tests/guardian_tests.rs
index 81f836e001..d51009d986 100644
--- a/codex-rs/core/src/session/tests/guardian_tests.rs
+++ b/codex-rs/core/src/session/tests/guardian_tests.rs
@@ -1257,6 +1257,8 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
         /*state_db*/ None,
     ));
 
+    let mut thread_extension_init = codex_extension_api::ExtensionDataInit::default();
+    thread_extension_init.insert(codex_extension_api::SessionIsolation::Isolated);
     let (session, io) = Session::spawn(SessionSpawnArgs {
         config,
         allow_provider_model_fallback: false,
@@ -1291,7 +1293,7 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
         user_shell_override: None,
         parent_trace: None,
         environment_selections: Vec::new(),
-        thread_extension_init: codex_extension_api::ExtensionDataInit::default(),
+        thread_extension_init,
         client_mcp_extensions: ClientMcpExtensions::default(),
         reserved_thread_id: None,
         analytics_events_client: None,
diff --git a/codex-rs/core/src/thread_manager.rs b/codex-rs/core/src/thread_manager.rs
index b202e15fc6..7fc5f3ac72 100644
--- a/codex-rs/core/src/thread_manager.rs
+++ b/codex-rs/core/src/thread_manager.rs
@@ -1932,13 +1932,26 @@ impl ThreadManagerState {
             environments,
             inherited_environments: captured_environments,
             user_instructions: supplied_user_instructions,
-            thread_extension_init,
+            mut thread_extension_init,
             client_mcp_extensions,
             reserved_thread_id,
             disabled_plugin_ids,
         } = options;
         let inherited_environments = captured_environments.or(inherited_environments);
         let session_source = session_source.unwrap_or_else(|| self.session_source.clone());
+        // Older callers and saved reviewers identify isolation through their source.
+        // New internal callers supply an explicit runtime policy before startup.
+        let isolation = thread_extension_init
+            .get::<codex_extension_api::SessionIsolation>()
+            .map(|policy| *policy)
+            .unwrap_or_else(|| {
+                if crate::guardian::is_basic_session_source(&session_source) {
+                    codex_extension_api::SessionIsolation::Isolated
+                } else {
+                    codex_extension_api::SessionIsolation::Inherit
+                }
+            });
+        thread_extension_init.insert(isolation);
         let environments = environments.unwrap_or_else(|| {
             default_thread_environment_selections(
                 self.environment_manager.as_ref(),
@@ -1989,7 +2002,7 @@ impl ThreadManagerState {
             extensions,
             mcp_manager,
             multi_agent_version,
-        ) = if crate::guardian::is_basic_session_source(&session_source) {
+        ) = if isolation == codex_extension_api::SessionIsolation::Isolated {
             (
                 LoadedUserInstructions::default(),
                 None,
diff --git a/codex-rs/ext/extension-api/src/lib.rs b/codex-rs/ext/extension-api/src/lib.rs
index f92d36bc00..0fab8a01a6 100644
--- a/codex-rs/ext/extension-api/src/lib.rs
+++ b/codex-rs/ext/extension-api/src/lib.rs
@@ -1,10 +1,13 @@
 mod capabilities;
 mod contributors;
 mod registry;
+mod session_isolation;
 mod state;
 mod turn_admission;
 mod user_instructions;
 
+pub use session_isolation::SessionIsolation;
+
 pub use capabilities::AgentSpawnFuture;
 pub use capabilities::AgentSpawner;
 pub use capabilities::ConversationHistorySnapshot;
diff --git a/codex-rs/ext/extension-api/src/session_isolation.rs b/codex-rs/ext/extension-api/src/session_isolation.rs
new file mode 100644
index 0000000000..8318517a54
--- /dev/null
+++ b/codex-rs/ext/extension-api/src/session_isolation.rs
@@ -0,0 +1,15 @@
+//! Host-supplied isolation for internal runtimes, independent of their attribution.
+//! Isolation only removes inherited capabilities; it never grants review authority.
+
+/// Runtime policy supplied through `ExtensionDataInit` before session startup.
+/// The host captures this value once so later extension-state changes cannot alter it.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub enum SessionIsolation {
+    /// Use the host's ordinary instruction providers, extensions and execution rules.
+    #[default]
+    Inherit,
+    /// Start without inherited instruction providers or extensions, retain only managed
+    /// execution rules, and omit executor-discovered MCP servers. Explicitly supplied
+    /// instructions and permissions remain the responsibility of the internal caller.
+    Isolated,
+}