diff --git a/codex-rs/network-proxy/src/credentialed_routes.rs b/codex-rs/network-proxy/src/credentialed_routes.rs
index 008d4d8f79..2b4af8ae51 100644
--- a/codex-rs/network-proxy/src/credentialed_routes.rs
+++ b/codex-rs/network-proxy/src/credentialed_routes.rs
@@ -132,11 +132,6 @@ impl CredentialedRoutesReloader {
         }
     }
 
-    pub async fn replace_base_state(&self, base_state: ConfigState) -> Result<ConfigState> {
-        *self.base_state.write().await = base_state;
-        self.reload_now().await
-    }
-
     pub async fn current_routes(&self) -> CredentialedRoutesConfig {
         self.credentialed_routes.read().await.clone()
     }
@@ -193,6 +188,19 @@ impl ConfigReloader for CredentialedRoutesReloader {
             self.build_state(&credentialed_routes).await
         })
     }
+
+    fn replace_base_state(&self, base_state: ConfigState) -> ConfigReloaderFuture<'_, ConfigState> {
+        Box::pin(async move {
+            let credentialed_routes = self.credentialed_routes.read().await.clone();
+            let state = build_config_state_with_credentialed_routes(
+                base_state.config.clone(),
+                base_state.constraints.clone(),
+                &credentialed_routes,
+            )?;
+            *self.base_state.write().await = base_state;
+            Ok(state)
+        })
+    }
 }
 
 fn build_config_state_with_credentialed_routes(
diff --git a/codex-rs/network-proxy/src/credentialed_routes_tests.rs b/codex-rs/network-proxy/src/credentialed_routes_tests.rs
index c3104951eb..bfd928380e 100644
--- a/codex-rs/network-proxy/src/credentialed_routes_tests.rs
+++ b/codex-rs/network-proxy/src/credentialed_routes_tests.rs
@@ -157,3 +157,39 @@ async fn failed_route_state_build_keeps_the_last_working_routes() {
         CredentialedRoutesConfig::default()
     );
 }
+
+#[tokio::test]
+async fn failed_base_state_replacement_keeps_the_last_working_base() {
+    let base_state = build_config_state(
+        NetworkProxyConfig::default(),
+        NetworkProxyConstraints::default(),
+    )
+    .unwrap();
+    let credentialed_routes =
+        proxy_config(vec![route("connector_123", "https://api.example.com/v1")]);
+    let reloader = CredentialedRoutesReloader::new(
+        base_state,
+        credentialed_routes,
+        Arc::new(|| async { Ok(CredentialedRoutesConfig::default()) }),
+    );
+    let mut constrained_config = NetworkProxyConfig::default();
+    constrained_config
+        .network
+        .set_allowed_domains(vec!["existing.example.com".to_string()]);
+    let constrained_state = build_config_state(
+        constrained_config,
+        NetworkProxyConstraints {
+            allowed_domains: Some(vec!["existing.example.com".to_string()]),
+            ..NetworkProxyConstraints::default()
+        },
+    )
+    .unwrap();
+
+    assert!(
+        reloader
+            .replace_base_state(constrained_state)
+            .await
+            .is_err()
+    );
+    assert!(reloader.reload_now().await.is_ok());
+}
diff --git a/codex-rs/network-proxy/src/proxy.rs b/codex-rs/network-proxy/src/proxy.rs
index c3685a4310..47f08c5dc7 100644
--- a/codex-rs/network-proxy/src/proxy.rs
+++ b/codex-rs/network-proxy/src/proxy.rs
@@ -655,27 +655,8 @@ impl NetworkProxy {
     }
 
     pub async fn replace_config_state(&self, new_state: ConfigState) -> Result<()> {
-        let current_cfg = self.state.current_cfg().await?;
-        anyhow::ensure!(
-            new_state.config.network.enabled == current_cfg.network.enabled,
-            "cannot update network.enabled on a running proxy"
-        );
-        anyhow::ensure!(
-            new_state.config.network.proxy_url == current_cfg.network.proxy_url,
-            "cannot update network.proxy_url on a running proxy"
-        );
-        anyhow::ensure!(
-            new_state.config.network.socks_url == current_cfg.network.socks_url,
-            "cannot update network.socks_url on a running proxy"
-        );
-        anyhow::ensure!(
-            new_state.config.network.enable_socks5 == current_cfg.network.enable_socks5,
-            "cannot update network.enable_socks5 on a running proxy"
-        );
-        anyhow::ensure!(
-            new_state.config.network.enable_socks5_udp == current_cfg.network.enable_socks5_udp,
-            "cannot update network.enable_socks5_udp on a running proxy"
-        );
+        self.ensure_runtime_config_compatible(&new_state.config)
+            .await?;
 
         let settings = NetworkProxyRuntimeSettings::from_config(&new_state.config)?;
         self.state.replace_config_state(new_state).await?;
@@ -687,6 +668,41 @@ impl NetworkProxy {
         Ok(())
     }
 
+    pub async fn replace_base_config_state(&self, base_state: ConfigState) -> Result<()> {
+        self.ensure_runtime_config_compatible(&base_state.config)
+            .await?;
+        let new_state = self.state.replace_base_state(base_state).await?;
+        self.replace_config_state(new_state).await
+    }
+
+    async fn ensure_runtime_config_compatible(
+        &self,
+        new_cfg: &config::NetworkProxyConfig,
+    ) -> Result<()> {
+        let current_cfg = self.state.current_cfg().await?;
+        anyhow::ensure!(
+            new_cfg.network.enabled == current_cfg.network.enabled,
+            "cannot update network.enabled on a running proxy"
+        );
+        anyhow::ensure!(
+            new_cfg.network.proxy_url == current_cfg.network.proxy_url,
+            "cannot update network.proxy_url on a running proxy"
+        );
+        anyhow::ensure!(
+            new_cfg.network.socks_url == current_cfg.network.socks_url,
+            "cannot update network.socks_url on a running proxy"
+        );
+        anyhow::ensure!(
+            new_cfg.network.enable_socks5 == current_cfg.network.enable_socks5,
+            "cannot update network.enable_socks5 on a running proxy"
+        );
+        anyhow::ensure!(
+            new_cfg.network.enable_socks5_udp == current_cfg.network.enable_socks5_udp,
+            "cannot update network.enable_socks5_udp on a running proxy"
+        );
+        Ok(())
+    }
+
     fn runtime_settings(&self) -> NetworkProxyRuntimeSettings {
         self.runtime_settings
             .read()
diff --git a/codex-rs/network-proxy/src/runtime.rs b/codex-rs/network-proxy/src/runtime.rs
index e721d0de70..5e82e00137 100644
--- a/codex-rs/network-proxy/src/runtime.rs
+++ b/codex-rs/network-proxy/src/runtime.rs
@@ -177,6 +177,11 @@ pub trait ConfigReloader: Send + Sync {
 
     /// Force a reload, regardless of whether a change was detected.
     fn reload_now(&self) -> ConfigReloaderFuture<'_, ConfigState>;
+
+    /// Rebuild state after the caller replaces the proxy's base configuration.
+    fn replace_base_state(&self, base_state: ConfigState) -> ConfigReloaderFuture<'_, ConfigState> {
+        Box::pin(async move { Ok(base_state) })
+    }
 }
 
 pub type ConfigReloaderFuture<'a, T> = Pin<Box<dyn Future<Output = Result<T>> + Send + 'a>>;
@@ -352,6 +357,10 @@ impl NetworkProxyState {
         Ok(())
     }
 
+    pub(crate) async fn replace_base_state(&self, base_state: ConfigState) -> Result<ConfigState> {
+        self.reloader.replace_base_state(base_state).await
+    }
+
     pub async fn host_blocked(&self, host: &str, port: u16) -> Result<HostBlockDecision> {
         self.reload_if_needed().await?;
         let host = match Host::parse(host) {