diff --git a/codex-rs/app-server/src/config_manager.rs b/codex-rs/app-server/src/config_manager.rs
index 10739fb0ef..d5f2c2b89a 100644
--- a/codex-rs/app-server/src/config_manager.rs
+++ b/codex-rs/app-server/src/config_manager.rs
@@ -399,6 +399,10 @@ impl ConfigManager {
             cwd,
         )
         .await
+        .and_then(|config| {
+            config.validate_windows_mxc_requirement()?;
+            Ok(config)
+        })
     }
 
     /// Reload sources using the task's session flags before materializing config.
@@ -437,6 +441,10 @@ impl ConfigManager {
             Some(cwd.to_path_buf()),
         )
         .await
+        .and_then(|config| {
+            config.validate_windows_mxc_requirement()?;
+            Ok(config)
+        })
     }
 
     #[instrument(level = "trace", skip_all)]
diff --git a/codex-rs/app-server/src/in_process_bootstrap.rs b/codex-rs/app-server/src/in_process_bootstrap.rs
index a21c3b0b96..17b37b9d6d 100644
--- a/codex-rs/app-server/src/in_process_bootstrap.rs
+++ b/codex-rs/app-server/src/in_process_bootstrap.rs
@@ -133,6 +133,7 @@ pub(super) async fn configure(
     let policy_config = config_manager
         .load_startup_config(Some(config.cwd.to_path_buf()))
         .await?;
+    config.validate_windows_mxc_requirement_from(&policy_config)?;
     let auth_config = codex_login::AuthConfig {
         chatgpt_base_url: Some(policy_config.chatgpt_base_url.clone()),
         forced_login_method: policy_config.forced_login_method,
diff --git a/codex-rs/app-server/src/lib.rs b/codex-rs/app-server/src/lib.rs
index e170dd222e..87798b2ffa 100644
--- a/codex-rs/app-server/src/lib.rs
+++ b/codex-rs/app-server/src/lib.rs
@@ -579,6 +579,7 @@ pub async fn run_main_with_transport_options(
             })?
         }
     };
+    config.validate_windows_mxc_requirement()?;
     config.auth_config().validate()?;
     let auth_manager =
         AuthManager::shared_from_config(&config, /*enable_codex_api_key_env*/ false)
diff --git a/codex-rs/app-server/src/request_processors/config_processor.rs b/codex-rs/app-server/src/request_processors/config_processor.rs
index 914abf4fd8..1343329ae1 100644
--- a/codex-rs/app-server/src/request_processors/config_processor.rs
+++ b/codex-rs/app-server/src/request_processors/config_processor.rs
@@ -1246,6 +1246,7 @@ client_id = "mcp-client"
                     codex_config::WindowsSandboxImplementationToml::Unelevated,
                 ]),
                 allow_mxc: None,
+                require_mxc: None,
             }),
             ..ConfigRequirementsToml::default()
         });
diff --git a/codex-rs/app-server/tests/suite/v2/account.rs b/codex-rs/app-server/tests/suite/v2/account.rs
index 6b06bbb7a8..94a3eb2e45 100644
--- a/codex-rs/app-server/tests/suite/v2/account.rs
+++ b/codex-rs/app-server/tests/suite/v2/account.rs
@@ -413,10 +413,30 @@ async fn logout_account_succeeds_when_config_reload_fails() -> Result<()> {
     Ok(())
 }
 
+#[test_case(false, false; "local_auth_before_cloud")]
+#[test_case(true, false; "local_mxc_after_cloud_selection")]
+#[test_case(false, true; "cloud_mxc_without_default_recovery")]
 #[tokio::test]
-async fn startup_enforces_local_auth_requirements_before_cloud_fetch() -> Result<()> {
+async fn startup_enforces_requirements_at_the_correct_load_stage(
+    local_mxc: bool,
+    cloud_mxc: bool,
+) -> Result<()> {
     let codex_home = TempDir::new()?;
     let mock_server = MockServer::start().await;
+    let require_mxc = "[windows]\nrequire_mxc = true\n";
+    let fetch_cloud = local_mxc || cloud_mxc;
+    if fetch_cloud {
+        let bundle = codex_config::test_support::CloudConfigBundleFixture::enterprise_requirement(
+            if cloud_mxc { require_mxc } else { "" },
+        )
+        .add_enterprise_config(format!("[features]\nprefer_mxc = {local_mxc}\n"))
+        .into_bundle();
+        Mock::given(path("/backend-api/wham/config/bundle"))
+            .respond_with(ResponseTemplate::new(200).set_body_json(bundle))
+            .expect(1..)
+            .mount(&mock_server)
+            .await;
+    }
     create_config_toml(
         codex_home.path(),
         CreateConfigTomlParams {
@@ -426,7 +446,13 @@ async fn startup_enforces_local_auth_requirements_before_cloud_fetch() -> Result
     )?;
     std::fs::write(
         codex_home.path().join("requirements.toml"),
-        "allowed_login_methods = [\"api\"]\n",
+        if local_mxc {
+            require_mxc
+        } else if cloud_mxc {
+            ""
+        } else {
+            "allowed_login_methods = [\"api\"]\n"
+        },
     )?;
     write_chatgpt_auth(
         codex_home.path(),
@@ -441,20 +467,30 @@ async fn startup_enforces_local_auth_requirements_before_cloud_fetch() -> Result
     let mut mcp = TestAppServer::builder()
         .with_codex_home(codex_home.path())
         .without_auto_env()
-        .build_initialized_with_timeout(DEFAULT_READ_TIMEOUT)
+        .build()
         .await?;
 
-    assert!(
+    if cfg!(windows) && fetch_cloud && (!local_mxc || !codex_sandboxing::windows_mxc_available()) {
+        assert!(
+            !timeout(DEFAULT_READ_TIMEOUT, mcp.wait_for_exit())
+                .await??
+                .success()
+        );
+    } else {
+        timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+        if !fetch_cloud {
+            assert_eq!(read_account(&mut mcp).await?.account, None);
+        }
+    }
+    assert_eq!(
         mock_server
             .received_requests()
             .await
             .expect("recorded requests")
             .is_empty(),
-        "disallowed ChatGPT auth must not fetch cloud requirements"
+        !fetch_cloud,
     );
 
-    assert_eq!(read_account(&mut mcp).await?.account, None);
-
     Ok(())
 }
 
diff --git a/codex-rs/cli/src/debug_sandbox.rs b/codex-rs/cli/src/debug_sandbox.rs
index e4cf88b4ff..c0efcdb1f6 100644
--- a/codex-rs/cli/src/debug_sandbox.rs
+++ b/codex-rs/cli/src/debug_sandbox.rs
@@ -754,7 +754,9 @@ async fn build_debug_sandbox_config_with_loader_overrides(
             .codex_home(codex_home.clone())
             .fallback_cwd(Some(codex_home));
     }
-    builder.build().await
+    let config = builder.build().await?;
+    config.validate_windows_mxc_requirement()?;
+    Ok(config)
 }
 
 fn config_uses_permission_profiles(config: &Config) -> bool {
diff --git a/codex-rs/config/src/config_requirements.rs b/codex-rs/config/src/config_requirements.rs
index 250b3b3c15..464c55e94a 100644
--- a/codex-rs/config/src/config_requirements.rs
+++ b/codex-rs/config/src/config_requirements.rs
@@ -872,6 +872,8 @@ pub struct WindowsRequirementsToml {
     pub allowed_sandbox_implementations: Option<Vec<WindowsSandboxImplementationToml>>,
     /// False blocks both explicit MXC configuration and automatic selection.
     pub allow_mxc: Option<bool>,
+    /// Require MXC as the selected local Windows backend.
+    pub require_mxc: Option<bool>,
 }
 
 #[derive(Deserialize, Debug, Clone, Copy, PartialEq, Eq)]
@@ -883,7 +885,9 @@ pub enum WindowsSandboxImplementationToml {
 
 impl WindowsRequirementsToml {
     pub fn is_empty(&self) -> bool {
-        self.allowed_sandbox_implementations.is_none() && self.allow_mxc.is_none()
+        self.allowed_sandbox_implementations.is_none()
+            && self.allow_mxc.is_none()
+            && self.require_mxc.is_none()
     }
 }
 
@@ -1927,6 +1931,7 @@ impl TryFrom<ConfigRequirementsWithSources> for ConfigRequirements {
                     WindowsRequirementsToml {
                         allowed_sandbox_implementations: Some(implementations),
                         allow_mxc: _,
+                        require_mxc: _,
                     },
                 source: requirement_source,
             }) => {
@@ -1973,6 +1978,7 @@ impl TryFrom<ConfigRequirementsWithSources> for ConfigRequirements {
                     WindowsRequirementsToml {
                         allowed_sandbox_implementations: None,
                         allow_mxc: _,
+                        require_mxc: _,
                     },
                 ..
             })
@@ -2850,6 +2856,7 @@ mod tests {
         let windows = WindowsRequirementsToml {
             allowed_sandbox_implementations: Some(vec![WindowsSandboxImplementationToml::Elevated]),
             allow_mxc: None,
+            require_mxc: Some(true),
         };
         let enforce_residency = ResidencyRequirement::Us;
         let enforce_source = source.clone();
@@ -3804,6 +3811,9 @@ allowed_approvals_reviewers = ["user"]
 
     #[test]
     fn deserialize_allowed_windows_sandbox_implementations() -> Result<()> {
+        let requirements: ConfigRequirementsToml = from_str("[windows]\nrequire_mxc = true")?;
+        assert!(!requirements.is_empty());
+
         let toml_str = r#"
             [windows]
             allowed_sandbox_implementations = ["elevated"]
diff --git a/codex-rs/core/src/config/config_tests.rs b/codex-rs/core/src/config/config_tests.rs
index 2046b5e98c..4edb8adaa8 100644
--- a/codex-rs/core/src/config/config_tests.rs
+++ b/codex-rs/core/src/config/config_tests.rs
@@ -11094,8 +11094,12 @@ async fn explicit_sandbox_mode_falls_back_when_disallowed_by_requirements() -> s
     Ok(())
 }
 
+#[test_case::test_case(false; "optional")]
+#[test_case::test_case(true; "required")]
 #[tokio::test]
-async fn local_mxc_preference_preserves_configured_backend() -> anyhow::Result<()> {
+async fn local_mxc_preference_preserves_configured_backend(
+    require_mxc: bool,
+) -> anyhow::Result<()> {
     use codex_sandboxing::SandboxType::WindowsMxc;
     use codex_sandboxing::SandboxType::WindowsRestrictedToken as RestrictedToken;
 
@@ -11122,16 +11126,53 @@ async fn local_mxc_preference_preserves_configured_backend() -> anyhow::Result<(
             codex_home.path().join(CONFIG_TOML_FILE),
             toml::to_string(&cfg)?,
         )?;
-        let mut config = ConfigBuilder::without_managed_config_for_tests()
+        let loaded = ConfigBuilder::without_managed_config_for_tests()
             .codex_home(codex_home.path().to_path_buf())
             .fallback_cwd(Some(codex_home.path().to_path_buf()))
             .cloud_config_bundle(
                 CloudConfigBundleFixture::loader_with_enterprise_requirement(format!(
-                    "[windows]\nallow_mxc = {allow_mxc}\n"
+                    "[windows]\nallow_mxc = {allow_mxc}\nrequire_mxc = {require_mxc}\n"
                 )),
             )
             .build()
             .await?;
+        let mut config = Config::rebuild_with_session_layers(
+            &loaded.config_layer_stack,
+            loaded.cwd.to_path_buf(),
+            &loaded.config_layer_stack,
+            loaded.codex_home.clone(),
+            /*default_zsh_path*/ None,
+        )
+        .await?;
+        let mut caller_config = config.clone();
+        caller_config.config_layer_stack = ConfigLayerStack::default();
+        caller_config.prefer_mxc = false;
+        caller_config.permissions.windows_sandbox_type = RestrictedToken;
+        assert_eq!(
+            caller_config
+                .validate_windows_mxc_requirement_from(&config)
+                .is_err(),
+            cfg!(windows) && require_mxc,
+        );
+        let result = config.validate_windows_mxc_requirement();
+        caller_config.config_layer_stack = config.config_layer_stack.clone();
+        assert_eq!(
+            config
+                .resolve_runtime_refresh(&caller_config, RuntimeConfigRefresh::Mcp)
+                .is_err(),
+            result.is_err(),
+        );
+        if cfg!(windows)
+            && require_mxc
+            && mode != "mxc"
+            && !(prefer && binding && allow_mxc && codex_sandboxing::windows_mxc_available())
+        {
+            let error = result.expect_err("MXC must already be selected");
+            assert_eq!(error.kind(), std::io::ErrorKind::InvalidInput);
+            assert!(error.to_string().contains("windows.require_mxc = true"));
+            continue;
+        }
+        result?;
         assert_eq!(
             config_allows_mxc(
                 &config
diff --git a/codex-rs/core/src/config/runtime_refresh.rs b/codex-rs/core/src/config/runtime_refresh.rs
index 995117dedf..abcf4fbcae 100644
--- a/codex-rs/core/src/config/runtime_refresh.rs
+++ b/codex-rs/core/src/config/runtime_refresh.rs
@@ -1,5 +1,6 @@
 //! Resolves refreshable MCP inputs without rebuilding unrelated session configuration.
 //! Existing sessions refresh ordinary MCP state without gaining enterprise authority.
+//! Validate the retained backend after applying the refreshable requirements.
 
 use crate::config::Config;
 use crate::config::ManagedFeatures;
@@ -168,6 +169,7 @@ impl Config {
         if enterprise_retired {
             config.disable_mcp_enterprise_auth();
         }
+        config.validate_windows_mxc_requirement()?;
         Ok(config)
     }
 
diff --git a/codex-rs/core/src/config/windows_sandbox_config.rs b/codex-rs/core/src/config/windows_sandbox_config.rs
index 8df89df767..0b681f1d8a 100644
--- a/codex-rs/core/src/config/windows_sandbox_config.rs
+++ b/codex-rs/core/src/config/windows_sandbox_config.rs
@@ -1,6 +1,7 @@
 //! Configured Windows sandbox modes and the effective local backend.
 //! Keep the configured backend for remote inheritance; apply the local rollout
 //! using the preference resolved during config loading.
+//! Validate MXC requirements only after bootstrap has loaded the complete config.
 
 use super::Config;
 use super::EffectivePermissionSelection;
@@ -19,6 +20,35 @@ use codex_protocol::models::PermissionProfile;
 use codex_sandboxing::SandboxType;
 
 impl Config {
+    /// Reject a non-MXC selection without changing the final configuration.
+    /// Call after cloud configuration loads and outside config-recovery fallbacks.
+    pub fn validate_windows_mxc_requirement(&self) -> std::io::Result<()> {
+        self.validate_windows_mxc_requirement_from(self)
+    }
+
+    /// Check this execution config against separately loaded managed requirements.
+    pub fn validate_windows_mxc_requirement_from(
+        &self,
+        policy_config: &Self,
+    ) -> std::io::Result<()> {
+        if cfg!(windows)
+            && policy_config
+                .config_layer_stack
+                .requirements_toml()
+                .windows
+                .as_ref()
+                .and_then(|windows| windows.require_mxc)
+                == Some(true)
+            && self.effective_local_windows_sandbox_type() != SandboxType::WindowsMxc
+        {
+            return Err(std::io::Error::new(
+                std::io::ErrorKind::InvalidInput,
+                "windows.require_mxc = true requires MXC to be selected on this Windows host",
+            ));
+        }
+        Ok(())
+    }
+
     /// Configured backend, without the local rollout preference. Remote executors inherit this.
     pub fn windows_sandbox_type_from_config(&self) -> SandboxType {
         self.permissions.windows_sandbox_type
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index 4714566430..0c667d1be5 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -553,6 +553,7 @@ impl Session {
     }
 
     async fn spawn_internal(args: SessionSpawnArgs) -> CodexResult<(Arc<Self>, SessionIo)> {
+        args.config.validate_windows_mxc_requirement()?;
         let SessionSpawnArgs {
             startup,
             config,
diff --git a/codex-rs/tui/src/debug_config.rs b/codex-rs/tui/src/debug_config.rs
index 2840e99609..f1fc03a58f 100644
--- a/codex-rs/tui/src/debug_config.rs
+++ b/codex-rs/tui/src/debug_config.rs
@@ -975,6 +975,7 @@ interrupt_message = false
             windows: Some(WindowsRequirementsToml {
                 allowed_sandbox_implementations: None,
                 allow_mxc: None,
+                require_mxc: None,
             }),
             additional_developer_instructions: None,
             guardian_policy_config: Some("Use the managed guardian policy.".to_string()),