From 9923aecf293b09c115d8ff001df1366ffbf4f8ee Mon Sep 17 00:00:00 2001
From: Justin Spahr-Summers <justin@anthropic.com>
Date: Thu, 23 Jan 2025 17:29:46 +0000
Subject: [PATCH 1/2] Report SSE error codes

---
 src/client/sse.ts | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/client/sse.ts b/src/client/sse.ts
index 932e7e20..d98a30ad 100644
--- a/src/client/sse.ts
+++ b/src/client/sse.ts
@@ -1,6 +1,6 @@
+import { EventSource, type EventSourceInit } from "eventsource";
 import { Transport } from "../shared/transport.js";
 import { JSONRPCMessage, JSONRPCMessageSchema } from "../types.js";
-import { EventSource, type EventSourceInit } from "eventsource";
 
 /**
  * Client transport for SSE: this will connect to a server using Server-Sent Events for receiving
@@ -42,7 +42,10 @@ export class SSEClientTransport implements Transport {
       this._abortController = new AbortController();
 
       this._eventSource.onerror = (event) => {
-        const error = new Error(`SSE error: ${JSON.stringify(event)}`);
+        const message = `SSE error: ${event.message}`;
+        const error = new Error(message);
+        Object.assign(error, { code: event.code });
+
         reject(error);
         this.onerror?.(error);
       };

From 63f8f07a22f4a356ef8bcc2907bed9e2375616f5 Mon Sep 17 00:00:00 2001
From: Justin Spahr-Summers <justin@anthropic.com>
Date: Fri, 24 Jan 2025 11:26:12 +0000
Subject: [PATCH 2/2] Add `SseError` class for easier introspection

---
 src/client/sse.ts | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/src/client/sse.ts b/src/client/sse.ts
index d98a30ad..14921f57 100644
--- a/src/client/sse.ts
+++ b/src/client/sse.ts
@@ -1,7 +1,17 @@
-import { EventSource, type EventSourceInit } from "eventsource";
+import { EventSource, type ErrorEvent, type EventSourceInit } from "eventsource";
 import { Transport } from "../shared/transport.js";
 import { JSONRPCMessage, JSONRPCMessageSchema } from "../types.js";
 
+export class SseError extends Error {
+  constructor(
+    public readonly code: number | undefined,
+    message: string | undefined,
+    public readonly event: ErrorEvent,
+  ) {
+    super(`SSE error: ${message}`);
+  }
+}
+
 /**
  * Client transport for SSE: this will connect to a server using Server-Sent Events for receiving
  * messages and make separate POST requests for sending messages.
@@ -42,10 +52,7 @@ export class SSEClientTransport implements Transport {
       this._abortController = new AbortController();
 
       this._eventSource.onerror = (event) => {
-        const message = `SSE error: ${event.message}`;
-        const error = new Error(message);
-        Object.assign(error, { code: event.code });
-
+        const error = new SseError(event.code, event.message, event);
         reject(error);
         this.onerror?.(error);
       };