diff --git a/codex-rs/code-mode/src/service.rs b/codex-rs/code-mode/src/service.rs
index 9f2df24793f4..ece08c598e03 100644
--- a/codex-rs/code-mode/src/service.rs
+++ b/codex-rs/code-mode/src/service.rs
@@ -348,7 +348,9 @@ impl runtime::SessionRuntimeDelegate for ProtocolDelegate {
 }
 
 fn runtime_request(request: ExecuteRequest) -> runtime::CreateCellRequest {
+    let idempotency_key = format!("{}:{}", request.tool_call_id, request.source);
     runtime::CreateCellRequest {
+        idempotency_key,
         tool_call_id: request.tool_call_id,
         enabled_tools: request
             .enabled_tools
diff --git a/codex-rs/code-mode/src/service_contract_tests.rs b/codex-rs/code-mode/src/service_contract_tests.rs
index 8c07da9a8f3d..924adaa3949c 100644
--- a/codex-rs/code-mode/src/service_contract_tests.rs
+++ b/codex-rs/code-mode/src/service_contract_tests.rs
@@ -287,9 +287,12 @@ async fn observed_natural_completion_wins_over_termination() {
         }
     );
     tokio::time::timeout(Duration::from_secs(1), async {
+        let mut probe_index = 0;
         loop {
+            probe_index += 1;
             let response = service
                 .execute(ExecuteRequest {
+                    tool_call_id: format!("probe-{probe_index}"),
                     yield_time_ms: Some(60_000),
                     ..execute_request(r#"text(String(load("finished")));"#)
                 })
diff --git a/codex-rs/code-mode/src/session_runtime/mod.rs b/codex-rs/code-mode/src/session_runtime/mod.rs
index b66b38a18805..5357ab7a776a 100644
--- a/codex-rs/code-mode/src/session_runtime/mod.rs
+++ b/codex-rs/code-mode/src/session_runtime/mod.rs
@@ -10,6 +10,7 @@ use std::time::Duration;
 
 use serde_json::Value as JsonValue;
 use tokio::sync::Mutex;
+use tokio::sync::OnceCell;
 use tokio_util::sync::CancellationToken;
 use tokio_util::task::TaskTracker;
 
@@ -55,12 +56,19 @@ pub(crate) struct SessionRuntime<D: SessionRuntimeDelegate> {
 struct Inner<D: SessionRuntimeDelegate> {
     stored_values: Mutex<HashMap<String, JsonValue>>,
     cells: Mutex<HashMap<CellId, RegisteredCell>>,
+    created_cells: Mutex<HashMap<String, Arc<OnceCell<IdempotentCell>>>>,
     cell_tasks: TaskTracker,
     shutdown_token: CancellationToken,
     delegate: Arc<D>,
     next_cell_id: AtomicU64,
 }
 
+#[derive(Clone)]
+struct IdempotentCell {
+    id: CellId,
+    kind: CellKind,
+}
+
 #[derive(Clone)]
 enum RegisteredCell {
     Continuing(CellHandle),
@@ -88,6 +96,7 @@ impl<D: SessionRuntimeDelegate> SessionRuntime<D> {
             inner: Arc::new(Inner {
                 stored_values: Mutex::new(HashMap::new()),
                 cells: Mutex::new(HashMap::new()),
+                created_cells: Mutex::new(HashMap::new()),
                 cell_tasks: TaskTracker::new(),
                 shutdown_token: CancellationToken::new(),
                 delegate,
@@ -102,7 +111,7 @@ impl<D: SessionRuntimeDelegate> SessionRuntime<D> {
 
     pub(crate) async fn create_cell(&self, request: CreateCellRequest) -> Result<Cell, Error> {
         let id = self
-            .start_cell(
+            .create_idempotent_cell(
                 request,
                 CellExecutionPolicy::ContinueWhenUnblocked,
                 CellKind::Continuing,
@@ -116,7 +125,7 @@ impl<D: SessionRuntimeDelegate> SessionRuntime<D> {
         request: CreateCellRequest,
     ) -> Result<PausableCell, Error> {
         let id = self
-            .start_cell(
+            .create_idempotent_cell(
                 request,
                 CellExecutionPolicy::PauseAtPendingFrontier,
                 CellKind::Pausable,
@@ -254,6 +263,37 @@ impl<D: SessionRuntimeDelegate> SessionRuntime<D> {
         Ok(cell_id)
     }
 
+    async fn create_idempotent_cell(
+        &self,
+        request: CreateCellRequest,
+        execution_policy: CellExecutionPolicy,
+        kind: CellKind,
+    ) -> Result<CellId, Error> {
+        let key = request.idempotency_key.clone();
+        let created_cell = {
+            let mut created_cells = self.inner.created_cells.lock().await;
+            Arc::clone(
+                created_cells
+                    .entry(key)
+                    .or_insert_with(|| Arc::new(OnceCell::new())),
+            )
+        };
+        let existing = created_cell
+            .get_or_try_init(|| async move {
+                let id = self.start_cell(request, execution_policy, kind).await?;
+                Ok::<IdempotentCell, Error>(IdempotentCell { id, kind })
+            })
+            .await?;
+        if existing.kind != kind {
+            return Err(Error::WrongCellKind {
+                cell_id: existing.id.clone(),
+                expected: kind,
+                actual: existing.kind,
+            });
+        }
+        Ok(existing.id.clone())
+    }
+
     async fn continuing_handle(&self, cell_id: &CellId) -> Result<CellHandle, Error> {
         self.handle_for_kind(cell_id, CellKind::Continuing).await
     }
diff --git a/codex-rs/code-mode/src/session_runtime/tests.rs b/codex-rs/code-mode/src/session_runtime/tests.rs
index f908293a738c..ca6069841f82 100644
--- a/codex-rs/code-mode/src/session_runtime/tests.rs
+++ b/codex-rs/code-mode/src/session_runtime/tests.rs
@@ -117,6 +117,7 @@ async fn default_policy_resolves_tools_before_the_first_observation() {
     let runtime = SessionRuntime::new(Arc::new(ImmediateToolDelegate { invocations_tx }));
     let cell = runtime
         .create_cell(CreateCellRequest {
+            idempotency_key: "default-policy".to_string(),
             tool_call_id: "call-1".to_string(),
             enabled_tools: vec![tool_definition("first"), tool_definition("second")],
             source: r#"
@@ -153,6 +154,31 @@ text("done");
     runtime.shutdown().await.unwrap();
 }
 
+#[tokio::test]
+async fn concurrent_create_retries_return_the_same_cell_for_an_idempotency_key() {
+    let runtime = SessionRuntime::new(Arc::new(RecordingDelegate));
+    let source = "await new Promise(() => {});";
+    let (first, retry) = tokio::join!(
+        runtime.create_cell(execute_request(source)),
+        runtime.create_cell(execute_request(source)),
+    );
+    let first = first.unwrap();
+    let retry = retry.unwrap();
+
+    assert_eq!(retry, first);
+    assert_eq!(runtime.inner.cells.lock().await.len(), 1);
+    assert_eq!(
+        runtime.create_pausable_cell(execute_request(source)).await,
+        Err(Error::WrongCellKind {
+            cell_id: first.id().clone(),
+            expected: CellKind::Pausable,
+            actual: CellKind::Continuing,
+        })
+    );
+
+    runtime.shutdown().await.unwrap();
+}
+
 #[tokio::test]
 async fn pausable_cell_supports_a_synchronous_host_driver() {
     let (invocations_tx, mut invocations_rx) = mpsc::unbounded_channel();
@@ -163,6 +189,7 @@ async fn pausable_cell_supports_a_synchronous_host_driver() {
     }));
     let cell = runtime
         .create_pausable_cell(CreateCellRequest {
+            idempotency_key: "synchronous-driver".to_string(),
             tool_call_id: "call-1".to_string(),
             enabled_tools: vec![tool_definition("first"), tool_definition("second")],
             source: r#"
@@ -267,6 +294,7 @@ async fn pending_frontier_reports_only_authoritatively_outstanding_parallel_tool
     }));
     let cell = runtime
         .create_pausable_cell(CreateCellRequest {
+            idempotency_key: "authoritative-outstanding".to_string(),
             tool_call_id: "call-1".to_string(),
             enabled_tools: vec![tool_definition("first"), tool_definition("second")],
             source: r#"
@@ -344,6 +372,7 @@ async fn pausable_cell_drains_a_parallel_host_frontier_without_duplicate_output(
     }));
     let cell = runtime
         .create_pausable_cell(CreateCellRequest {
+            idempotency_key: "parallel-drain".to_string(),
             tool_call_id: "call-1".to_string(),
             enabled_tools: vec![tool_definition("first"), tool_definition("second")],
             source: r#"
@@ -432,7 +461,10 @@ async fn cell_capabilities_reject_ids_of_the_other_kind() {
         .await
         .unwrap();
     let pausable = runtime
-        .create_pausable_cell(execute_request("await new Promise(() => {});"))
+        .create_pausable_cell(CreateCellRequest {
+            idempotency_key: "pausable-cell".to_string(),
+            ..execute_request("await new Promise(() => {});")
+        })
         .await
         .unwrap();
 
@@ -465,6 +497,7 @@ async fn pending_observation_waits_for_resumed_work_to_reach_a_new_frontier() {
     }));
     let cell = runtime
         .create_pausable_cell(CreateCellRequest {
+            idempotency_key: "pending-observation".to_string(),
             tool_call_id: "call-1".to_string(),
             enabled_tools: vec![tool_definition("blocked")],
             source: r#"
@@ -561,6 +594,7 @@ async fn termination_rejects_a_waiting_store_commit_before_the_next_cell_can_loa
 
     let reader = runtime
         .create_cell(CreateCellRequest {
+            idempotency_key: "reader".to_string(),
             tool_call_id: "reader".to_string(),
             enabled_tools: Vec::new(),
             source: r#"text(String(load("candidate")));"#.to_string(),
@@ -581,6 +615,7 @@ async fn termination_rejects_a_waiting_store_commit_before_the_next_cell_can_loa
 
 fn execute_request(source: &str) -> CreateCellRequest {
     CreateCellRequest {
+        idempotency_key: format!("call-1:{source}"),
         tool_call_id: "call-1".to_string(),
         enabled_tools: Vec::new(),
         source: source.to_string(),
diff --git a/codex-rs/code-mode/src/session_runtime/types.rs b/codex-rs/code-mode/src/session_runtime/types.rs
index 4b45ac6321a1..34d532e3bdfb 100644
--- a/codex-rs/code-mode/src/session_runtime/types.rs
+++ b/codex-rs/code-mode/src/session_runtime/types.rs
@@ -162,6 +162,7 @@ pub(crate) enum ImageDetail {
 ///
 /// The owning session assigns the cell ID when it admits the request.
 pub(crate) struct CreateCellRequest {
+    pub(crate) idempotency_key: String,
     pub(crate) tool_call_id: String,
     pub(crate) enabled_tools: Vec<ToolDefinition>,
     pub(crate) source: String,