fix(resilience): add UND_ERR_SOCKET to PROXY_UNREACHABLE_ERROR_CODES (#8788) (#8795)

Fixes #8788.

- Added 'UND_ERR_SOCKET' to PROXY_UNREACHABLE_ERROR_CODES so socket disconnects and mid-stream closes are properly tagged with code PROXY_UNREACHABLE and errorCode proxy_unreachable.
- Ensured tagProxyUnreachable normalizes error code to PROXY_UNREACHABLE and errorCode to proxy_unreachable when catching socket-level disconnects.
- Added test in proxyfetch-undici-retry.test.ts verifying UND_ERR_SOCKET classification.

open-sse/utils/proxyFetch.ts

@@ -35,16 +35,20 @@ const PROXY_UNREACHABLE_ERROR_CODES = new Set([
   "EHOSTUNREACH",
   "EPIPE",
   "UND_ERR_CONNECT_TIMEOUT",
+  "UND_ERR_SOCKET",
 ]);
 
 function isProxyUnreachableError(err: unknown): boolean {
   if (!err || typeof err !== "object") return false;
   const code = (err as { code?: unknown }).code;
   if (typeof code === "string" && PROXY_UNREACHABLE_ERROR_CODES.has(code)) return true;
-  const causeCode = (err as { cause?: { code?: unknown } }).cause?.code;
-  return typeof causeCode === "string" && PROXY_UNREACHABLE_ERROR_CODES.has(causeCode);
+  const cause = (err as { cause?: unknown }).cause;
+  const causeCode =
+    cause && typeof cause === "object" ? (cause as { code?: unknown }).code : undefined;
+  if (typeof causeCode === "string" && PROXY_UNREACHABLE_ERROR_CODES.has(causeCode)) return true;
+  const msg = (err as Error).message;
+  return typeof msg === "string" && PROXY_UNREACHABLE_ERROR_CODES.has(msg);
 }
-
 /**
  * #8376: tag a connect-failure error with a stable `.code`/`.errorCode` BEFORE it is
  * rethrown, so chatCore's catch block (and, through the response body, the combo
@@ -55,7 +59,7 @@ function isProxyUnreachableError(err: unknown): boolean {
 function tagProxyUnreachable<T>(err: T): T {
   if (isProxyUnreachableError(err)) {
     const e = err as Error & { code?: string; errorCode?: string };
-    e.code = e.code || "PROXY_UNREACHABLE";
+    e.code = "PROXY_UNREACHABLE";
     e.errorCode = "proxy_unreachable";
   }
   return err;
@@ -144,7 +148,6 @@ export function describeFetchCause(err: unknown): string {
   return parts.join(" | ") || String(err);
 }
 
-
 function isStreamLikeBody(body: unknown): boolean {
   return (
     body !== null &&
@@ -392,9 +395,11 @@ export async function runWithProxyContext(
       }
       const err = new Error(`[Proxy Fast-Fail] Proxy unreachable: ${proxyLabel}`) as Error & {
         code?: string;
+        errorCode?: string;
         statusCode?: number;
       };
       err.code = "PROXY_UNREACHABLE";
+      err.errorCode = "proxy_unreachable";
       err.statusCode = 503;
       throw err;
     }
@@ -547,6 +552,7 @@ async function patchedFetch(
         // Prefer the .code property when available (more stable across undici
         // versions than message-string matching); fall back to substring match
         // for errors that lack a structured code.
+        tagProxyUnreachable(dispatcherError);
         const errCode = (dispatcherError as { code?: unknown })?.code;
         if (
           msg.includes("fetch failed") ||
@@ -612,9 +618,11 @@ async function patchedFetch(
             if (nativeError instanceof Error) {
               (nativeError as Error & { proxyFetchDetail?: string }).proxyFetchDetail = detail;
             }
-            throw tagProxyUnreachable(nativeError);
+            tagProxyUnreachable(nativeError);
+            throw nativeError;
           }
         }
+        tagProxyUnreachable(dispatcherError);
         throw dispatcherError;
       }
     }

tests/unit/proxy-fetch.test.ts

@@ -171,7 +171,11 @@ test("runWithProxyContext throws PROXY_UNREACHABLE for an unreachable proxy by d
   // 127.0.0.1:9 (discard) refuses connections — the proxy is unreachable.
   await assert.rejects(
     runWithProxyContext({ type: "http", host: "127.0.0.1", port: "9" }, async () => "unreachable"),
-    /Proxy unreachable/
+    (err: Error & { code?: string; errorCode?: string }) => {
+      assert.equal(err.code, "PROXY_UNREACHABLE");
+      assert.equal(err.errorCode, "proxy_unreachable");
+      return true;
+    }
   );
 });
 

tests/unit/proxyfetch-undici-retry.test.ts

@@ -240,6 +240,29 @@ test("#4252 describeFetchCause falls back to String(err) when nothing is structu
   assert.equal(describeFetchCause("boom"), "boom");
 });
 
+test("#8788 tagProxyUnreachable tags UND_ERR_SOCKET errors as PROXY_UNREACHABLE", async () => {
+  const socketErr = Object.assign(new Error("socket closed"), { code: "UND_ERR_SOCKET" });
+  await assert.rejects(
+    proxyFetch(
+      "https://example.invalid/socket-test",
+      { method: "GET" },
+      {
+        undiciFetch: async () => {
+          throw socketErr;
+        },
+        nativeFetch: async () => {
+          throw socketErr;
+        },
+      }
+    ),
+    (err: Error & { code?: string; errorCode?: string }) => {
+      assert.equal(err.code, "PROXY_UNREACHABLE");
+      assert.equal(err.errorCode, "proxy_unreachable");
+      return true;
+    }
+  );
+});
+
 test("#4252 both undici AND native fetch fail → rejects fast with cause detail attached", async () => {
   const dispErr = Object.assign(new Error("fetch failed"), {
     code: "UND_ERR_SOCKET",