diff --git a/crates/token_proxy_core/src/proxy/codex_compat.test.rs b/crates/token_proxy_core/src/proxy/codex_compat.test.rs
index 180e558..71dfd07 100644
--- a/crates/token_proxy_core/src/proxy/codex_compat.test.rs
+++ b/crates/token_proxy_core/src/proxy/codex_compat.test.rs
@@ -125,3 +125,45 @@ fn responses_request_to_codex_strips_prompt_cache_retention() {
     assert!(value.get("previous_response_id").is_none());
     assert!(value.get("safety_identifier").is_none());
 }
+
+#[test]
+fn responses_request_to_codex_preserves_parallel_tool_calls_false() {
+    let input = json!({
+        "model": "gpt-5",
+        "input": [
+            {
+                "type": "message",
+                "role": "user",
+                "content": [{ "type": "input_text", "text": "hi" }]
+            }
+        ],
+        "parallel_tool_calls": false
+    });
+    let bytes = Bytes::from(input.to_string());
+    let output = responses_request_to_codex(&bytes, Some("gpt-5-codex")).expect("convert");
+    let value: serde_json::Value = serde_json::from_slice(&output).expect("json");
+    assert_eq!(value["parallel_tool_calls"], json!(false));
+}
+
+#[test]
+fn responses_request_to_codex_strips_output_parts_from_function_call_output() {
+    let input = json!({
+        "model": "gpt-5",
+        "input": [
+            {
+                "type": "function_call_output",
+                "call_id": "call_1",
+                "output": "ok",
+                "output_parts": [
+                    { "type": "text", "text": "ok" }
+                ]
+            }
+        ]
+    });
+    let bytes = Bytes::from(input.to_string());
+    let output = responses_request_to_codex(&bytes, Some("gpt-5-codex")).expect("convert");
+    let value: serde_json::Value = serde_json::from_slice(&output).expect("json");
+    let input_items = value["input"].as_array().expect("input array");
+    assert_eq!(input_items.len(), 1);
+    assert!(input_items[0].get("output_parts").is_none());
+}
diff --git a/crates/token_proxy_core/src/proxy/codex_compat/request.rs b/crates/token_proxy_core/src/proxy/codex_compat/request.rs
index 429f9fa..f9ccc06 100644
--- a/crates/token_proxy_core/src/proxy/codex_compat/request.rs
+++ b/crates/token_proxy_core/src/proxy/codex_compat/request.rs
@@ -413,7 +413,9 @@ fn normalize_responses_payload(object: &mut Map<String, Value>, model_hint: Opti
     object.insert("model".to_string(), Value::String(model.to_string()));
     object.insert("stream".to_string(), Value::Bool(true));
     object.insert("store".to_string(), Value::Bool(false));
-    object.insert("parallel_tool_calls".to_string(), Value::Bool(true));
+    if !object.contains_key("parallel_tool_calls") {
+        object.insert("parallel_tool_calls".to_string(), Value::Bool(true));
+    }
     object.insert(
         "include".to_string(),
         json!(["reasoning.encrypted_content"]),
@@ -441,12 +443,34 @@ fn normalize_responses_payload(object: &mut Map<String, Value>, model_hint: Opti
             "role": "user",
             "content": [json!({"type":"input_text","text": text})]
         })],
-        Some(Value::Array(items)) => items.clone(),
+        Some(Value::Array(items)) => sanitize_responses_input_for_codex(items),
         _ => Vec::new(),
     };
     object.insert("input".to_string(), Value::Array(input));
 }
 
+fn sanitize_responses_input_for_codex(items: &[Value]) -> Vec<Value> {
+    items
+        .iter()
+        .map(sanitize_responses_input_item_for_codex)
+        .collect()
+}
+
+fn sanitize_responses_input_item_for_codex(item: &Value) -> Value {
+    let Some(object) = item.as_object() else {
+        return item.clone();
+    };
+    if object.get("type").and_then(Value::as_str) != Some("function_call_output") {
+        return item.clone();
+    }
+    let mut sanitized = object.clone();
+    // Claude -> Responses may carry structured tool output in `output_parts`.
+    // Codex only needs the flattened `output` string here; forwarding the extra field
+    // breaks composition without adding value.
+    sanitized.remove("output_parts");
+    Value::Object(sanitized)
+}
+
 fn rewrite_input_function_names(input: &mut Value, tool_map: &ToolNameMap) {
     let Some(items) = input.as_array_mut() else {
         return;