diff --git a/codex-rs/mcp-server/src/codex_tool_runner.rs b/codex-rs/mcp-server/src/codex_tool_runner.rs
index ced01539bb..ff660167d5 100644
--- a/codex-rs/mcp-server/src/codex_tool_runner.rs
+++ b/codex-rs/mcp-server/src/codex_tool_runner.rs
@@ -61,7 +61,7 @@ pub async fn run_codex_tool_session(
                 is_error: Some(true),
                 structured_content: None,
             };
-            outgoing.send_response(id.clone(), result.into()).await;
+            outgoing.send_response(id.clone(), result).await;
             return;
         }
     };
@@ -235,9 +235,7 @@ async fn run_codex_tool_session_inner(
                             is_error: None,
                             structured_content: None,
                         };
-                        outgoing
-                            .send_response(request_id.clone(), result.into())
-                            .await;
+                        outgoing.send_response(request_id.clone(), result).await;
                         // unregister the id so we don't keep it in the map
                         running_requests_id_to_codex_uuid
                             .lock()
@@ -296,9 +294,7 @@ async fn run_codex_tool_session_inner(
                     // structured way.
                     structured_content: None,
                 };
-                outgoing
-                    .send_response(request_id.clone(), result.into())
-                    .await;
+                outgoing.send_response(request_id.clone(), result).await;
                 break;
             }
         }
diff --git a/codex-rs/mcp-server/src/error_code.rs b/codex-rs/mcp-server/src/error_code.rs
new file mode 100644
index 0000000000..1ffd889d40
--- /dev/null
+++ b/codex-rs/mcp-server/src/error_code.rs
@@ -0,0 +1,2 @@
+pub(crate) const INVALID_REQUEST_ERROR_CODE: i64 = -32600;
+pub(crate) const INTERNAL_ERROR_CODE: i64 = -32603;
diff --git a/codex-rs/mcp-server/src/lib.rs b/codex-rs/mcp-server/src/lib.rs
index f6d0838efc..abbe3b94a0 100644
--- a/codex-rs/mcp-server/src/lib.rs
+++ b/codex-rs/mcp-server/src/lib.rs
@@ -18,6 +18,7 @@ use tracing_subscriber::EnvFilter;
 mod codex_tool_config;
 mod codex_tool_runner;
 mod conversation_loop;
+mod error_code;
 mod exec_approval;
 mod json_to_toml;
 pub mod mcp_protocol;
diff --git a/codex-rs/mcp-server/src/message_processor.rs b/codex-rs/mcp-server/src/message_processor.rs
index a3349aeb35..98beb15460 100644
--- a/codex-rs/mcp-server/src/message_processor.rs
+++ b/codex-rs/mcp-server/src/message_processor.rs
@@ -7,6 +7,7 @@ use crate::codex_tool_config::CodexToolCallParam;
 use crate::codex_tool_config::CodexToolCallReplyParam;
 use crate::codex_tool_config::create_tool_for_codex_tool_call_param;
 use crate::codex_tool_config::create_tool_for_codex_tool_call_reply_param;
+use crate::error_code::INVALID_REQUEST_ERROR_CODE;
 use crate::mcp_protocol::ToolCallRequestParams;
 use crate::mcp_protocol::ToolCallResponse;
 use crate::mcp_protocol::ToolCallResponseResult;
@@ -191,7 +192,7 @@ impl MessageProcessor {
         if self.initialized {
             // Already initialised: send JSON-RPC error response.
             let error = JSONRPCErrorError {
-                code: -32600, // Invalid Request
+                code: INVALID_REQUEST_ERROR_CODE,
                 message: "initialize called more than once".to_string(),
                 data: None,
             };
@@ -230,9 +231,6 @@ impl MessageProcessor {
     where
         T: ModelContextProtocolRequest,
     {
-        // result has `Serialized` instance so should never fail
-        #[expect(clippy::unwrap_used)]
-        let result = serde_json::to_value(result).unwrap();
         self.outgoing.send_response(id, result).await;
     }
 
@@ -533,9 +531,7 @@ impl MessageProcessor {
                     is_error: Some(true),
                     structured_content: None,
                 };
-                outgoing
-                    .send_response(request_id, serde_json::to_value(result).unwrap_or_default())
-                    .await;
+                outgoing.send_response(request_id, result).await;
                 return;
             }
         };
diff --git a/codex-rs/mcp-server/src/outgoing_message.rs b/codex-rs/mcp-server/src/outgoing_message.rs
index e7b0b9b63c..3a77cbf2a0 100644
--- a/codex-rs/mcp-server/src/outgoing_message.rs
+++ b/codex-rs/mcp-server/src/outgoing_message.rs
@@ -18,6 +18,8 @@ use tokio::sync::mpsc;
 use tokio::sync::oneshot;
 use tracing::warn;
 
+use crate::error_code::INTERNAL_ERROR_CODE;
+
 /// Sends messages to the client and manages request callbacks.
 pub(crate) struct OutgoingMessageSender {
     next_request_id: AtomicI64,
@@ -74,9 +76,24 @@ impl OutgoingMessageSender {
         }
     }
 
-    pub(crate) async fn send_response(&self, id: RequestId, result: Result) {
-        let outgoing_message = OutgoingMessage::Response(OutgoingResponse { id, result });
-        let _ = self.sender.send(outgoing_message).await;
+    pub(crate) async fn send_response<T: Serialize>(&self, id: RequestId, response: T) {
+        match serde_json::to_value(response) {
+            Ok(result) => {
+                let outgoing_message = OutgoingMessage::Response(OutgoingResponse { id, result });
+                let _ = self.sender.send(outgoing_message).await;
+            }
+            Err(err) => {
+                self.send_error(
+                    id,
+                    JSONRPCErrorError {
+                        code: INTERNAL_ERROR_CODE,
+                        message: format!("failed to serialize response: {err}"),
+                        data: None,
+                    },
+                )
+                .await;
+            }
+        }
     }
 
     pub(crate) async fn send_event_as_notification(