diff --git a/codex-rs/app-server/src/config_manager_service_tests.rs b/codex-rs/app-server/src/config_manager_service_tests.rs
index 7133ea9d0a..d07489e454 100644
--- a/codex-rs/app-server/src/config_manager_service_tests.rs
+++ b/codex-rs/app-server/src/config_manager_service_tests.rs
@@ -493,6 +493,49 @@ enabled = false
     Ok(())
 }
 
+#[tokio::test]
+async fn write_value_rejects_unknown_mcp_field_hidden_by_cloud_config() -> Result<()> {
+    let tmp = tempdir().expect("tempdir");
+    let config_path = tmp.path().join(CONFIG_TOML_FILE);
+    std::fs::write(&config_path, "")?;
+    let service = ConfigManager::new_for_tests(
+        tmp.path().to_path_buf(),
+        vec![],
+        LoaderOverrides::without_managed_config_for_tests(),
+        CloudConfigBundleFixture::loader_with_enterprise_config(
+            r#"[mcp_servers.docs]
+command = "cloud-docs-server"
+enabled = false
+"#,
+        ),
+    );
+
+    let error = service
+        .write_value(ConfigValueWriteParams {
+            file_path: Some(config_path.display().to_string()),
+            key_path: "mcp_servers.docs.typo".to_string(),
+            value: serde_json::json!(true),
+            merge_strategy: MergeStrategy::Replace,
+            expected_version: None,
+        })
+        .await
+        .expect_err("the cloud transport must not hide an unknown user-layer field");
+
+    assert_eq!(
+        error.write_error_code(),
+        Some(ConfigWriteErrorCode::ConfigValidationError)
+    );
+    assert!(
+        error
+            .to_string()
+            .contains("unknown configuration field `mcp_servers.docs.typo`"),
+        "{error}"
+    );
+    assert_eq!(std::fs::read_to_string(config_path)?, "");
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn read_includes_origins_and_layers() {
     let tmp = tempdir().expect("tempdir");
diff --git a/codex-rs/config/src/config_toml_layer.rs b/codex-rs/config/src/config_toml_layer.rs
index 53d114eae2..077d5ecddd 100644
--- a/codex-rs/config/src/config_toml_layer.rs
+++ b/codex-rs/config/src/config_toml_layer.rs
@@ -1,7 +1,9 @@
 use crate::config_toml::ConfigToml;
 use crate::mcp_types::RawMcpServerConfig;
+use crate::strict_config::ignored_toml_value_field;
 use codex_utils_absolute_path::AbsolutePathBufGuard;
 use serde::Deserialize;
+use serde::de::Error as _;
 use std::collections::HashMap;
 use std::path::Path;
 use toml::Value as TomlValue;
@@ -17,6 +19,13 @@ impl ParsedConfigTomlLayer {
     pub fn parse(raw: TomlValue, base_dir: &Path) -> Result<Self, toml::de::Error> {
         let parts = split_config_toml_layer(raw.clone());
         let _guard = AbsolutePathBufGuard::new(base_dir);
+        if let Some(ignored_field) =
+            ignored_toml_value_field::<ConfigTomlComposableSections>(parts.composable.clone())
+        {
+            return Err(toml::de::Error::custom(format!(
+                "unknown configuration field `{ignored_field}`"
+            )));
+        }
         let _: ConfigTomlComposableSections = parts.composable.try_into()?;
         let self_contained_config = parts.self_contained.try_into()?;