diff --git a/codex-rs/codex-api/src/sse/chat.rs b/codex-rs/codex-api/src/sse/chat.rs
index 5e48c57bd..21adfa571 100644
--- a/codex-rs/codex-api/src/sse/chat.rs
+++ b/codex-rs/codex-api/src/sse/chat.rs
@@ -10,6 +10,7 @@ use eventsource_stream::Eventsource;
 use futures::Stream;
 use futures::StreamExt;
 use std::collections::HashMap;
+use std::collections::HashSet;
 use std::time::Duration;
 use tokio::sync::mpsc;
 use tokio::time::Instant;
@@ -41,12 +42,17 @@ pub async fn process_chat_sse<S>(
 
     #[derive(Default, Debug)]
     struct ToolCallState {
+        id: Option<String>,
         name: Option<String>,
         arguments: String,
     }
 
-    let mut tool_calls: HashMap<String, ToolCallState> = HashMap::new();
-    let mut tool_call_order: Vec<String> = Vec::new();
+    let mut tool_calls: HashMap<usize, ToolCallState> = HashMap::new();
+    let mut tool_call_order: Vec<usize> = Vec::new();
+    let mut tool_call_order_seen: HashSet<usize> = HashSet::new();
+    let mut tool_call_index_by_id: HashMap<String, usize> = HashMap::new();
+    let mut next_tool_call_index = 0usize;
+    let mut last_tool_call_index: Option<usize> = None;
     let mut assistant_item: Option<ResponseItem> = None;
     let mut reasoning_item: Option<ResponseItem> = None;
     let mut completed_sent = false;
@@ -149,15 +155,40 @@ pub async fn process_chat_sse<S>(
 
                 if let Some(tool_call_values) = delta.get("tool_calls").and_then(|c| c.as_array()) {
                     for tool_call in tool_call_values {
-                        let id = tool_call
-                            .get("id")
-                            .and_then(|i| i.as_str())
-                            .map(str::to_string)
-                            .unwrap_or_else(|| format!("tool-call-{}", tool_call_order.len()));
+                        let mut index = tool_call
+                            .get("index")
+                            .and_then(serde_json::Value::as_u64)
+                            .map(|i| i as usize);
 
-                        let call_state = tool_calls.entry(id.clone()).or_default();
-                        if !tool_call_order.contains(&id) {
-                            tool_call_order.push(id.clone());
+                        let mut call_id_for_lookup = None;
+                        if let Some(call_id) = tool_call.get("id").and_then(|i| i.as_str()) {
+                            call_id_for_lookup = Some(call_id.to_string());
+                            if let Some(existing) = tool_call_index_by_id.get(call_id) {
+                                index = Some(*existing);
+                            }
+                        }
+
+                        if index.is_none() && call_id_for_lookup.is_none() {
+                            index = last_tool_call_index;
+                        }
+
+                        let index = index.unwrap_or_else(|| {
+                            while tool_calls.contains_key(&next_tool_call_index) {
+                                next_tool_call_index += 1;
+                            }
+                            let idx = next_tool_call_index;
+                            next_tool_call_index += 1;
+                            idx
+                        });
+
+                        let call_state = tool_calls.entry(index).or_default();
+                        if tool_call_order_seen.insert(index) {
+                            tool_call_order.push(index);
+                        }
+
+                        if let Some(id) = tool_call.get("id").and_then(|i| i.as_str()) {
+                            call_state.id.get_or_insert_with(|| id.to_string());
+                            tool_call_index_by_id.entry(id.to_string()).or_insert(index);
                         }
 
                         if let Some(func) = tool_call.get("function") {
@@ -171,6 +202,8 @@ pub async fn process_chat_sse<S>(
                                 call_state.arguments.push_str(arguments);
                             }
                         }
+
+                        last_tool_call_index = Some(index);
                     }
                 }
             }
@@ -224,13 +257,25 @@ pub async fn process_chat_sse<S>(
                         .await;
                 }
 
-                for call_id in tool_call_order.drain(..) {
-                    let state = tool_calls.remove(&call_id).unwrap_or_default();
+                for index in tool_call_order.drain(..) {
+                    let Some(state) = tool_calls.remove(&index) else {
+                        continue;
+                    };
+                    tool_call_order_seen.remove(&index);
+                    let ToolCallState {
+                        id,
+                        name,
+                        arguments,
+                    } = state;
+                    let Some(name) = name else {
+                        debug!("Skipping tool call at index {index} because name is missing");
+                        continue;
+                    };
                     let item = ResponseItem::FunctionCall {
                         id: None,
-                        name: state.name.unwrap_or_default(),
-                        arguments: state.arguments,
-                        call_id: call_id.clone(),
+                        name,
+                        arguments,
+                        call_id: id.unwrap_or_else(|| format!("tool-call-{index}")),
                     };
                     let _ = tx_event.send(Ok(ResponseEvent::OutputItemDone(item))).await;
                 }
@@ -335,6 +380,59 @@ mod tests {
         out
     }
 
+    #[tokio::test]
+    async fn concatenates_tool_call_arguments_across_deltas() {
+        let delta_name = json!({
+            "choices": [{
+                "delta": {
+                    "tool_calls": [{
+                        "id": "call_a",
+                        "index": 0,
+                        "function": { "name": "do_a" }
+                    }]
+                }
+            }]
+        });
+
+        let delta_args_1 = json!({
+            "choices": [{
+                "delta": {
+                    "tool_calls": [{
+                        "index": 0,
+                        "function": { "arguments": "{ \"foo\":" }
+                    }]
+                }
+            }]
+        });
+
+        let delta_args_2 = json!({
+            "choices": [{
+                "delta": {
+                    "tool_calls": [{
+                        "index": 0,
+                        "function": { "arguments": "1}" }
+                    }]
+                }
+            }]
+        });
+
+        let finish = json!({
+            "choices": [{
+                "finish_reason": "tool_calls"
+            }]
+        });
+
+        let body = build_body(&[delta_name, delta_args_1, delta_args_2, finish]);
+        let events = collect_events(&body).await;
+        assert_matches!(
+            &events[..],
+            [
+                ResponseEvent::OutputItemDone(ResponseItem::FunctionCall { call_id, name, arguments, .. }),
+                ResponseEvent::Completed { .. }
+            ] if call_id == "call_a" && name == "do_a" && arguments == "{ \"foo\":1}"
+        );
+    }
+
     #[tokio::test]
     async fn emits_multiple_tool_calls() {
         let delta_a = json!({
@@ -367,50 +465,74 @@ mod tests {
 
         let body = build_body(&[delta_a, delta_b, finish]);
         let events = collect_events(&body).await;
-        assert_eq!(events.len(), 3);
-
         assert_matches!(
-            &events[0],
-            ResponseEvent::OutputItemDone(ResponseItem::FunctionCall { call_id, name, arguments, .. })
-            if call_id == "call_a" && name == "do_a" && arguments == "{\"foo\":1}"
+            &events[..],
+            [
+                ResponseEvent::OutputItemDone(ResponseItem::FunctionCall { call_id: call_a, name: name_a, arguments: args_a, .. }),
+                ResponseEvent::OutputItemDone(ResponseItem::FunctionCall { call_id: call_b, name: name_b, arguments: args_b, .. }),
+                ResponseEvent::Completed { .. }
+            ] if call_a == "call_a" && name_a == "do_a" && args_a == "{\"foo\":1}" && call_b == "call_b" && name_b == "do_b" && args_b == "{\"bar\":2}"
         );
-        assert_matches!(
-            &events[1],
-            ResponseEvent::OutputItemDone(ResponseItem::FunctionCall { call_id, name, arguments, .. })
-            if call_id == "call_b" && name == "do_b" && arguments == "{\"bar\":2}"
-        );
-        assert_matches!(events[2], ResponseEvent::Completed { .. });
     }
 
     #[tokio::test]
-    async fn concatenates_tool_call_arguments_across_deltas() {
-        let delta_name = json!({
+    async fn emits_tool_calls_for_multiple_choices() {
+        let payload = json!({
+            "choices": [
+                {
+                    "delta": {
+                        "tool_calls": [{
+                            "id": "call_a",
+                            "index": 0,
+                            "function": { "name": "do_a", "arguments": "{}" }
+                        }]
+                    },
+                    "finish_reason": "tool_calls"
+                },
+                {
+                    "delta": {
+                        "tool_calls": [{
+                            "id": "call_b",
+                            "index": 0,
+                            "function": { "name": "do_b", "arguments": "{}" }
+                        }]
+                    },
+                    "finish_reason": "tool_calls"
+                }
+            ]
+        });
+
+        let body = build_body(&[payload]);
+        let events = collect_events(&body).await;
+        assert_matches!(
+            &events[..],
+            [
+                ResponseEvent::OutputItemDone(ResponseItem::FunctionCall { call_id: call_a, name: name_a, arguments: args_a, .. }),
+                ResponseEvent::OutputItemDone(ResponseItem::FunctionCall { call_id: call_b, name: name_b, arguments: args_b, .. }),
+                ResponseEvent::Completed { .. }
+            ] if call_a == "call_a" && name_a == "do_a" && args_a == "{}" && call_b == "call_b" && name_b == "do_b" && args_b == "{}"
+        );
+    }
+
+    #[tokio::test]
+    async fn merges_tool_calls_by_index_when_id_missing_on_subsequent_deltas() {
+        let delta_with_id = json!({
             "choices": [{
                 "delta": {
                     "tool_calls": [{
+                        "index": 0,
                         "id": "call_a",
-                        "function": { "name": "do_a" }
+                        "function": { "name": "do_a", "arguments": "{ \"foo\":" }
                     }]
                 }
             }]
         });
 
-        let delta_args_1 = json!({
+        let delta_without_id = json!({
             "choices": [{
                 "delta": {
                     "tool_calls": [{
-                        "id": "call_a",
-                        "function": { "arguments": "{ \"foo\":" }
-                    }]
-                }
-            }]
-        });
-
-        let delta_args_2 = json!({
-            "choices": [{
-                "delta": {
-                    "tool_calls": [{
-                        "id": "call_a",
+                        "index": 0,
                         "function": { "arguments": "1}" }
                     }]
                 }
@@ -423,7 +545,7 @@ mod tests {
             }]
         });
 
-        let body = build_body(&[delta_name, delta_args_1, delta_args_2, finish]);
+        let body = build_body(&[delta_with_id, delta_without_id, finish]);
         let events = collect_events(&body).await;
         assert_matches!(
             &events[..],