diff --git a/codex-rs/core/src/client_common.rs b/codex-rs/core/src/client_common.rs
index 54ad70daaa..1e98b25d11 100644
--- a/codex-rs/core/src/client_common.rs
+++ b/codex-rs/core/src/client_common.rs
@@ -8,6 +8,7 @@ use codex_tools::ToolSpec;
 use futures::Stream;
 use serde_json::Value;
 use std::pin::Pin;
+use std::sync::Arc;
 use std::task::Context;
 use std::task::Poll;
 use tokio::sync::mpsc;
@@ -21,7 +22,7 @@ pub struct Prompt {
 
     /// Tools available to the model, including additional tools sourced from
     /// external MCP servers.
-    pub(crate) tools: Vec<ToolSpec>,
+    pub(crate) tools: Arc<[ToolSpec]>,
 
     /// Whether parallel tool calls are permitted for this prompt.
     pub(crate) parallel_tool_calls: bool,
@@ -39,7 +40,7 @@ impl Default for Prompt {
     fn default() -> Self {
         Self {
             input: Vec::new(),
-            tools: Vec::new(),
+            tools: Arc::default(),
             parallel_tool_calls: false,
             base_instructions: BaseInstructions::default(),
             output_schema: None,
diff --git a/codex-rs/core/src/tools/router.rs b/codex-rs/core/src/tools/router.rs
index 83edd0463a..b3e60a5986 100644
--- a/codex-rs/core/src/tools/router.rs
+++ b/codex-rs/core/src/tools/router.rs
@@ -67,7 +67,7 @@ pub(crate) fn tool_log_payload<'a>(
 
 pub struct ToolRouter {
     registry: ToolRegistry,
-    model_visible_specs: Vec<ToolSpec>,
+    model_visible_specs: Arc<[ToolSpec]>,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -102,12 +102,12 @@ impl ToolRouter {
     pub(crate) fn from_parts(registry: ToolRegistry, model_visible_specs: Vec<ToolSpec>) -> Self {
         Self {
             registry,
-            model_visible_specs,
+            model_visible_specs: model_visible_specs.into(),
         }
     }
 
-    pub(crate) fn model_visible_specs(&self) -> Vec<ToolSpec> {
-        self.model_visible_specs.clone()
+    pub(crate) fn model_visible_specs(&self) -> Arc<[ToolSpec]> {
+        Arc::clone(&self.model_visible_specs)
     }
 
     pub(crate) fn deferred_tool_namespaces(&self) -> BTreeMap<String, String> {
diff --git a/codex-rs/core/src/tools/router_tests.rs b/codex-rs/core/src/tools/router_tests.rs
index 690fb9ca2b..f354c75475 100644
--- a/codex-rs/core/src/tools/router_tests.rs
+++ b/codex-rs/core/src/tools/router_tests.rs
@@ -471,9 +471,11 @@ async fn specs_filter_deferred_dynamic_tools() -> anyhow::Result<()> {
         Vec::new(),
         &dynamic_tools,
     );
+    let visible_specs = router.model_visible_specs();
 
+    assert!(Arc::ptr_eq(&visible_specs, &router.model_visible_specs()));
     assert_eq!(
-        namespace_function_names(&router.model_visible_specs(), "codex_app"),
+        namespace_function_names(&visible_specs, "codex_app"),
         vec![visible_tool.to_string()]
     );
     assert_eq!(
@@ -481,6 +483,11 @@ async fn specs_filter_deferred_dynamic_tools() -> anyhow::Result<()> {
         BTreeMap::from([("codex_app".to_string(), "Codex app tools.".to_string())])
     );
 
+    let updated_router = test_tool_router(step_context.as_ref(), Vec::new(), Vec::new(), &[]);
+    let updated_specs = updated_router.model_visible_specs();
+    assert!(!Arc::ptr_eq(&visible_specs, &updated_specs));
+    assert!(namespace_function_names(&updated_specs, "codex_app").is_empty());
+
     Ok(())
 }
 
diff --git a/codex-rs/core/src/tools/spec_plan_tests.rs b/codex-rs/core/src/tools/spec_plan_tests.rs
index c144b6da30..3b225ac4b0 100644
--- a/codex-rs/core/src/tools/spec_plan_tests.rs
+++ b/codex-rs/core/src/tools/spec_plan_tests.rs
@@ -80,7 +80,7 @@ struct ToolPlanProbe {
 
 impl ToolPlanProbe {
     fn from_router(router: ToolRouter) -> Self {
-        let visible_specs = router.model_visible_specs();
+        let visible_specs = router.model_visible_specs().to_vec();
         let visible_names = visible_specs
             .iter()
             .map(|spec| spec.name().to_string())