diff --git a/codex-rs/code-mode-protocol/src/runtime.rs b/codex-rs/code-mode-protocol/src/runtime.rs
index 2848b9ed7e16..a0c2bf3fe86e 100644
--- a/codex-rs/code-mode-protocol/src/runtime.rs
+++ b/codex-rs/code-mode-protocol/src/runtime.rs
@@ -14,6 +14,7 @@ pub const DEFAULT_MAX_OUTPUT_TOKENS_PER_EXEC_CALL: usize = 10_000;
 
 #[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
 pub struct CreateCellRequest {
+    pub idempotency_key: String,
     pub tool_call_id: String,
     pub enabled_tools: Vec<ToolDefinition>,
     pub source: String,
@@ -21,6 +22,7 @@ pub struct CreateCellRequest {
 
 #[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
 pub struct ObserveRequest {
+    pub idempotency_key: String,
     pub cell_id: CellId,
     pub yield_time_ms: u64,
 }
diff --git a/codex-rs/code-mode-protocol/src/runtime_tests.rs b/codex-rs/code-mode-protocol/src/runtime_tests.rs
index 997cb6d13e85..e0e615cf81e1 100644
--- a/codex-rs/code-mode-protocol/src/runtime_tests.rs
+++ b/codex-rs/code-mode-protocol/src/runtime_tests.rs
@@ -34,6 +34,7 @@ fn image(image_url: &str, detail: Option<ImageDetail>) -> FunctionCallOutputCont
 #[test]
 fn requests_round_trip_with_exact_tool_and_namespace_fields() {
     let create = CreateCellRequest {
+        idempotency_key: "thread-3:response-call-7".to_string(),
         tool_call_id: "response-call-7".to_string(),
         enabled_tools: vec![
             ToolDefinition {
@@ -61,6 +62,7 @@ fn requests_round_trip_with_exact_tool_and_namespace_fields() {
     assert_json_round_trip(
         &create,
         json!({
+            "idempotency_key": "thread-3:response-call-7",
             "tool_call_id": "response-call-7",
             "enabled_tools": [
                 {
@@ -89,10 +91,15 @@ fn requests_round_trip_with_exact_tool_and_namespace_fields() {
 
     assert_json_round_trip(
         &ObserveRequest {
+            idempotency_key: "thread-3:wait-call-2".to_string(),
             cell_id: CellId::new("cell-a7".to_string()),
             yield_time_ms: 250,
         },
-        json!({"cell_id": "cell-a7", "yield_time_ms": 250}),
+        json!({
+            "idempotency_key": "thread-3:wait-call-2",
+            "cell_id": "cell-a7",
+            "yield_time_ms": 250,
+        }),
     );
 }
 
diff --git a/codex-rs/code-mode/src/cell_actor/conversions.rs b/codex-rs/code-mode/src/cell_actor/conversions.rs
index 324002a79afc..807423f20fc3 100644
--- a/codex-rs/code-mode/src/cell_actor/conversions.rs
+++ b/codex-rs/code-mode/src/cell_actor/conversions.rs
@@ -12,6 +12,7 @@ use crate::session_runtime::ToolKind as CellToolKind;
 
 pub(super) fn runtime_request(request: CellRequest) -> CreateCellRequest {
     CreateCellRequest {
+        idempotency_key: request.idempotency_key,
         tool_call_id: request.tool_call_id,
         enabled_tools: request
             .enabled_tools
diff --git a/codex-rs/code-mode/src/cell_actor/tests.rs b/codex-rs/code-mode/src/cell_actor/tests.rs
index 5933f15c6cad..c236aabe36d8 100644
--- a/codex-rs/code-mode/src/cell_actor/tests.rs
+++ b/codex-rs/code-mode/src/cell_actor/tests.rs
@@ -121,6 +121,7 @@ fn spawn_cell_actor_harness_with_policy<H: CellHost>(
     let (runtime_tx, runtime_pause_tx, runtime_terminate_handle) = spawn_runtime(
         HashMap::new(),
         CreateCellRequest {
+            idempotency_key: "cell-actor-harness".to_string(),
             tool_call_id: "call-1".to_string(),
             enabled_tools: Vec::new(),
             source: "await new Promise(() => {});".to_string(),
diff --git a/codex-rs/code-mode/src/runtime/mod.rs b/codex-rs/code-mode/src/runtime/mod.rs
index 03e0bfa3c8e2..c0c9a7fb54e5 100644
--- a/codex-rs/code-mode/src/runtime/mod.rs
+++ b/codex-rs/code-mode/src/runtime/mod.rs
@@ -351,6 +351,7 @@ mod tests {
 
     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/service.rs b/codex-rs/code-mode/src/service.rs
index 68119590fd19..dbfbc4fa39ee 100644
--- a/codex-rs/code-mode/src/service.rs
+++ b/codex-rs/code-mode/src/service.rs
@@ -22,6 +22,7 @@ use codex_code_mode_protocol::TerminateOutcome;
 use codex_code_mode_protocol::ToolInvocationFuture;
 use serde_json::Value as JsonValue;
 use tokio::sync::Mutex;
+use tokio::sync::watch;
 use tokio_util::sync::CancellationToken;
 
 use crate::session_runtime as runtime;
@@ -94,10 +95,17 @@ impl CodeModeSessionProvider for InProcessCodeModeSessionProvider {
 }
 
 pub struct CodeModeService {
-    runtime: SessionRuntime<ProtocolDelegate>,
+    runtime: Arc<SessionRuntime<ProtocolDelegate>>,
+    observations: Mutex<HashMap<String, ObservationRecord>>,
+    #[cfg(test)]
     pending_generations: Mutex<HashMap<runtime::CellId, runtime::PendingGeneration>>,
 }
 
+struct ObservationRecord {
+    request: ObserveRequest,
+    result_rx: watch::Receiver<Option<Result<ObserveOutcome, String>>>,
+}
+
 impl CodeModeService {
     pub fn new() -> Self {
         Self::with_delegate(Arc::new(NoopCodeModeSessionDelegate))
@@ -105,7 +113,9 @@ impl CodeModeService {
 
     pub fn with_delegate(delegate: Arc<dyn CodeModeSessionDelegate>) -> Self {
         Self {
-            runtime: SessionRuntime::new(Arc::new(ProtocolDelegate { delegate })),
+            runtime: Arc::new(SessionRuntime::new(Arc::new(ProtocolDelegate { delegate }))),
+            observations: Mutex::new(HashMap::new()),
+            #[cfg(test)]
             pending_generations: Mutex::new(HashMap::new()),
         }
     }
@@ -130,44 +140,51 @@ impl CodeModeService {
     }
 
     pub async fn observe(&self, request: ObserveRequest) -> Result<ObserveOutcome, String> {
-        self.begin_observe(request).await.await
+        let idempotency_key = request.idempotency_key.clone();
+        let mut result_rx = {
+            let mut observations = self.observations.lock().await;
+            if let Some(existing) = observations.get(&idempotency_key) {
+                if existing.request != request {
+                    return Err(format!(
+                        "observation idempotency key `{idempotency_key}` was reused for a different request"
+                    ));
+                }
+                existing.result_rx.clone()
+            } else {
+                let (result_tx, result_rx) = watch::channel(None);
+                observations.insert(
+                    idempotency_key,
+                    ObservationRecord {
+                        request: request.clone(),
+                        result_rx: result_rx.clone(),
+                    },
+                );
+                let runtime = Arc::clone(&self.runtime);
+                tokio::spawn(async move {
+                    let result = begin_observe_runtime(runtime, request).await.await;
+                    result_tx.send_replace(Some(result));
+                });
+                result_rx
+            }
+        };
+
+        result_rx
+            .wait_for(Option::is_some)
+            .await
+            .map_err(|_| "observation ended before producing a result".to_string())?;
+
+        result_rx
+            .borrow()
+            .clone()
+            .ok_or_else(|| "observation ended before producing a result".to_string())?
     }
 
+    #[allow(dead_code)]
     async fn begin_observe(
         &self,
         request: ObserveRequest,
     ) -> CodeModeSessionResultFuture<'static, ObserveOutcome> {
-        let ObserveRequest {
-            cell_id,
-            yield_time_ms,
-        } = request;
-        let runtime_cell_id = runtime_cell_id(&cell_id);
-        let cell = match self.runtime.cell(&runtime_cell_id).await {
-            Ok(cell) => cell,
-            Err(runtime::Error::MissingCell(_) | runtime::Error::ClosedCell(_)) => {
-                return missing_observation(cell_id);
-            }
-            Err(error) => return Box::pin(async move { Err(error.to_string()) }),
-        };
-        match self
-            .runtime
-            .begin_wait(&cell, Duration::from_millis(yield_time_ms))
-            .await
-        {
-            Ok(pending_event) => Box::pin(async move {
-                match pending_event.event().await {
-                    Ok(event) => Ok(observe_outcome(&cell_id, event)),
-                    Err(runtime::Error::MissingCell(_) | runtime::Error::ClosedCell(_)) => {
-                        Ok(ObserveOutcome::Missing { cell_id })
-                    }
-                    Err(error) => Err(error.to_string()),
-                }
-            }),
-            Err(runtime::Error::MissingCell(_) | runtime::Error::ClosedCell(_)) => {
-                missing_observation(cell_id)
-            }
-            Err(error) => Box::pin(async move { Err(error.to_string()) }),
-        }
+        begin_observe_runtime(Arc::clone(&self.runtime), request).await
     }
 
     pub async fn terminate(&self, cell_id: CellId) -> Result<TerminateOutcome, String> {
@@ -179,6 +196,7 @@ impl CodeModeService {
             }
             Err(error) => Err(error.to_string()),
         };
+        #[cfg(test)]
         self.pending_generations
             .lock()
             .await
@@ -254,11 +272,49 @@ impl CodeModeService {
             .shutdown()
             .await
             .map_err(|error| error.to_string());
+        #[cfg(test)]
         self.pending_generations.lock().await.clear();
         result
     }
 }
 
+async fn begin_observe_runtime(
+    runtime: Arc<SessionRuntime<ProtocolDelegate>>,
+    request: ObserveRequest,
+) -> CodeModeSessionResultFuture<'static, ObserveOutcome> {
+    let ObserveRequest {
+        idempotency_key: _,
+        cell_id,
+        yield_time_ms,
+    } = request;
+    let runtime_cell_id = runtime_cell_id(&cell_id);
+    let cell = match runtime.cell(&runtime_cell_id).await {
+        Ok(cell) => cell,
+        Err(runtime::Error::MissingCell(_) | runtime::Error::ClosedCell(_)) => {
+            return missing_observation(cell_id);
+        }
+        Err(error) => return Box::pin(async move { Err(error.to_string()) }),
+    };
+    match runtime
+        .begin_wait(&cell, Duration::from_millis(yield_time_ms))
+        .await
+    {
+        Ok(pending_event) => Box::pin(async move {
+            match pending_event.event().await {
+                Ok(event) => Ok(observe_outcome(&cell_id, event)),
+                Err(runtime::Error::MissingCell(_) | runtime::Error::ClosedCell(_)) => {
+                    Ok(ObserveOutcome::Missing { cell_id })
+                }
+                Err(error) => Err(error.to_string()),
+            }
+        }),
+        Err(runtime::Error::MissingCell(_) | runtime::Error::ClosedCell(_)) => {
+            missing_observation(cell_id)
+        }
+        Err(error) => Box::pin(async move { Err(error.to_string()) }),
+    }
+}
+
 impl Default for CodeModeService {
     fn default() -> Self {
         Self::new()
@@ -349,9 +405,8 @@ impl runtime::SessionRuntimeDelegate for ProtocolDelegate {
 }
 
 fn runtime_request(request: CreateCellRequest) -> runtime::CreateCellRequest {
-    let idempotency_key = format!("{}:{}", request.tool_call_id, request.source);
     runtime::CreateCellRequest {
-        idempotency_key,
+        idempotency_key: request.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 fddb9bbe96ef..3a507cafe8bc 100644
--- a/codex-rs/code-mode/src/service_contract_tests.rs
+++ b/codex-rs/code-mode/src/service_contract_tests.rs
@@ -156,6 +156,7 @@ fn cell_id(value: &str) -> CellId {
 
 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(),
@@ -174,6 +175,7 @@ async fn execute_with_yield_time(
     let cell_id = service.create_cell(request).await.unwrap();
     service
         .observe(ObserveRequest {
+            idempotency_key: format!("observe:{cell_id}"),
             cell_id,
             yield_time_ms,
         })
@@ -214,6 +216,74 @@ async fn next_event(events_rx: &mut mpsc::UnboundedReceiver<DelegateEvent>) -> D
         .expect("delegate event channel closed")
 }
 
+#[tokio::test]
+async fn create_retry_returns_the_cell_from_the_original_ambiguous_request() {
+    let service = CodeModeService::new();
+    let request = execute_request("await new Promise(() => {});");
+
+    let original_cell_id = service.create_cell(request.clone()).await.unwrap();
+    let retry_cell_id = service.create_cell(request).await.unwrap();
+
+    assert_eq!(retry_cell_id, original_cell_id);
+    service.terminate(original_cell_id).await.unwrap();
+}
+
+#[tokio::test]
+async fn cancelled_observation_is_replayed_by_its_idempotency_key() {
+    let (delegate, mut events_rx) = BlockingDelegate::new();
+    let service = Arc::new(CodeModeService::with_delegate(delegate.clone()));
+    let created_cell_id = service
+        .create_cell(CreateCellRequest {
+            idempotency_key: "ambiguous-observation-cell".to_string(),
+            enabled_tools: vec![blocking_tool()],
+            source: r#"await tools.block({}); text("done");"#.to_string(),
+            ..execute_request("")
+        })
+        .await
+        .unwrap();
+    assert_eq!(next_event(&mut events_rx).await, DelegateEvent::ToolStarted);
+    let request = ObserveRequest {
+        idempotency_key: "lost-observation-response".to_string(),
+        cell_id: created_cell_id,
+        yield_time_ms: 60_000,
+    };
+
+    let first_attempt = tokio::spawn({
+        let service = Arc::clone(&service);
+        let request = request.clone();
+        async move { service.observe(request).await }
+    });
+    tokio::time::timeout(Duration::from_secs(1), async {
+        loop {
+            if service
+                .observations
+                .lock()
+                .await
+                .contains_key("lost-observation-response")
+            {
+                break;
+            }
+            tokio::task::yield_now().await;
+        }
+    })
+    .await
+    .expect("observation registration timed out");
+    first_attempt.abort();
+    assert!(first_attempt.await.unwrap_err().is_cancelled());
+    delegate.release_tool();
+
+    assert_eq!(
+        service.observe(request).await,
+        Ok(ObserveOutcome::Completed {
+            cell_id: cell_id("1"),
+            content_items: vec![FunctionCallOutputContentItem::InputText {
+                text: "done".to_string(),
+            }],
+            error_text: None,
+        })
+    );
+}
+
 #[tokio::test]
 async fn yields_and_resumes() {
     let service = CodeModeService::new();
@@ -239,6 +309,7 @@ async fn yields_and_resumes() {
     assert_eq!(
         service
             .observe(ObserveRequest {
+                idempotency_key: "observe-after-yield".to_string(),
                 cell_id: cell_id("1"),
                 yield_time_ms: 60_000,
             })
@@ -326,7 +397,7 @@ async fn observed_natural_completion_wins_over_termination() {
             let response = create_and_observe_to_pending(
                 &service,
                 CreateCellRequest {
-                    tool_call_id: format!("completion-probe-{probe_generation}"),
+                    idempotency_key: format!("completion-probe-{probe_generation}"),
                     ..execute_request(r#"text(String(load("finished")));"#)
                 },
             )
@@ -547,6 +618,7 @@ async fn second_observer_is_rejected_without_displacing_the_first() {
 
     let first_observer = service
         .begin_observe(ObserveRequest {
+            idempotency_key: "first-observer".to_string(),
             cell_id: cell_id("1"),
             yield_time_ms: 60_000,
         })
@@ -554,6 +626,7 @@ async fn second_observer_is_rejected_without_displacing_the_first() {
     assert_eq!(
         service
             .observe(ObserveRequest {
+                idempotency_key: "second-observer".to_string(),
                 cell_id: cell_id("1"),
                 yield_time_ms: 60_000,
             })
@@ -598,6 +671,7 @@ async fn natural_completion_cleans_up_callbacks_before_responding() {
     assert_eq!(
         service
             .observe(ObserveRequest {
+                idempotency_key: "observe-created-cell".to_string(),
                 cell_id: created_cell_id,
                 yield_time_ms: 60_000,
             })
diff --git a/codex-rs/code-mode/src/service_tests.rs b/codex-rs/code-mode/src/service_tests.rs
index 78ca0426c1ef..14a579a8eb26 100644
--- a/codex-rs/code-mode/src/service_tests.rs
+++ b/codex-rs/code-mode/src/service_tests.rs
@@ -64,6 +64,7 @@ impl CodeModeSessionDelegate for ReleasableToolDelegate {
 
 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(),
@@ -97,6 +98,7 @@ async fn execute_with_yield_time(
     let cell_id = service.create_cell(request).await.unwrap();
     service
         .observe(ObserveRequest {
+            idempotency_key: format!("observe:{cell_id}"),
             cell_id,
             yield_time_ms,
         })
@@ -152,6 +154,7 @@ async fn stored_values_are_shared_between_cells_but_not_sessions() {
     let write_response = execute(
         &first_session,
         CreateCellRequest {
+            idempotency_key: "write-shared-value".to_string(),
             source: r#"store("key", "visible");"#.to_string(),
             ..execute_request("")
         },
@@ -161,6 +164,7 @@ async fn stored_values_are_shared_between_cells_but_not_sessions() {
     let same_session = execute(
         &first_session,
         CreateCellRequest {
+            idempotency_key: "read-shared-value".to_string(),
             source: r#"text(String(load("key")));"#.to_string(),
             ..execute_request("")
         },
@@ -169,6 +173,7 @@ async fn stored_values_are_shared_between_cells_but_not_sessions() {
     let other_session = execute(
         &second_session,
         CreateCellRequest {
+            idempotency_key: "read-other-session-value".to_string(),
             source: r#"text(String(load("key")));"#.to_string(),
             ..execute_request("")
         },
@@ -980,6 +985,7 @@ async fn observe_reports_missing_cell_separately_from_runtime_results() {
 
     let response = service
         .observe(ObserveRequest {
+            idempotency_key: "observe-missing".to_string(),
             cell_id: cell_id("missing"),
             yield_time_ms: 1,
         })
diff --git a/codex-rs/core/src/tools/code_mode/execute_handler.rs b/codex-rs/core/src/tools/code_mode/execute_handler.rs
index 8410e08c33b9..98fd326f6a33 100644
--- a/codex-rs/core/src/tools/code_mode/execute_handler.rs
+++ b/codex-rs/core/src/tools/code_mode/execute_handler.rs
@@ -40,11 +40,13 @@ impl CodeModeExecuteHandler {
         let enabled_tools =
             codex_tools::collect_code_mode_tool_definitions(&self.nested_tool_specs);
         let started_at = std::time::Instant::now();
+        let idempotency_key = format!("{}:{call_id}", exec.session.thread_id());
         let cell_id = exec
             .session
             .services
             .code_mode_service
             .create_cell(codex_code_mode::CreateCellRequest {
+                idempotency_key: idempotency_key.clone(),
                 tool_call_id: call_id.clone(),
                 enabled_tools,
                 source: args.code.clone(),
@@ -77,6 +79,7 @@ impl CodeModeExecuteHandler {
             .services
             .code_mode_service
             .observe(codex_code_mode::ObserveRequest {
+                idempotency_key,
                 cell_id: cell_id.clone(),
                 yield_time_ms: args
                     .yield_time_ms
diff --git a/codex-rs/core/src/tools/code_mode/wait_handler.rs b/codex-rs/core/src/tools/code_mode/wait_handler.rs
index 04e162bb6e00..ce98d736c323 100644
--- a/codex-rs/core/src/tools/code_mode/wait_handler.rs
+++ b/codex-rs/core/src/tools/code_mode/wait_handler.rs
@@ -66,6 +66,7 @@ impl CodeModeWaitHandler {
         let ToolInvocation {
             session,
             turn,
+            call_id,
             tool_name,
             payload,
             ..
@@ -76,6 +77,7 @@ impl CodeModeWaitHandler {
                 if tool_name.namespace.is_none() && tool_name.name.as_str() == WAIT_TOOL_NAME =>
             {
                 let args: ExecWaitArgs = parse_arguments(&arguments)?;
+                let idempotency_key = format!("{}:{call_id}", session.thread_id());
                 let exec = ExecContext { session, turn };
                 let started_at = std::time::Instant::now();
                 let cell_id = codex_code_mode::CellId::new(args.cell_id);
@@ -99,6 +101,7 @@ impl CodeModeWaitHandler {
                         .services
                         .code_mode_service
                         .observe(codex_code_mode::ObserveRequest {
+                            idempotency_key,
                             cell_id,
                             yield_time_ms: args.yield_time_ms,
                         })