fix(client): surface underlying network error via Error.cause on probe failures (#2657)

.changeset/brave-donkeys-listen.md

@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/core-internal': patch
+'@modelcontextprotocol/client': patch
+---
+
+`SdkError` now accepts standard `ErrorOptions`, and version-negotiation probe failures surface the underlying network error via `Error.cause` (#2657).

packages/client/src/client/probeClassifier.ts

@@ -311,9 +311,14 @@ function classifyNetworkError(error: unknown, context: ProbeClassifierContext):
     }
     return {
         kind: 'error',
-        error: new SdkError(SdkErrorCode.EraNegotiationFailed, `Version negotiation probe failed: ${describeError(error)}`, {
-            cause: error
-        })
+        error: new SdkError(
+            SdkErrorCode.EraNegotiationFailed,
+            `Version negotiation probe failed: ${describeError(error)}`,
+            // Keep data.cause for existing consumers while also exposing the
+            // standard Error.cause chain (#2657).
+            { cause: error },
+            { cause: error }
+        )
     };
 }
 

packages/client/test/client/probeClassifier.test.ts

@@ -270,6 +270,17 @@ describe('row: network outage → typed connect error (Node)', () => {
         const verdict = classify({ kind: 'network-error', error: new TypeError('fetch failed') }, { environment: 'node' });
         expect(verdict.kind).toBe('error');
     });
+
+    test('the underlying network error is reachable via Error.cause (#2657)', () => {
+        const cause = Object.assign(new Error('fetch failed'), { code: 'ECONNREFUSED' });
+        const verdict = classify({ kind: 'network-error', error: cause });
+        expect(verdict.kind).toBe('error');
+        if (verdict.kind === 'error') {
+            expect(verdict.error.cause).toBe(cause);
+            // The legacy data.cause slot stays populated too.
+            expect(((verdict.error as SdkError).data as { cause?: unknown }).cause).toBe(cause);
+        }
+    });
 });
 
 describe('row: timeout — transport-aware verdict', () => {

packages/core-internal/src/errors/sdkErrors.ts

@@ -147,9 +147,10 @@ export class SdkError extends Error {
     constructor(
         public readonly code: SdkErrorCode,
         message: string,
-        public readonly data?: unknown
+        public readonly data?: unknown,
+        options?: ErrorOptions
     ) {
-        super(message);
+        super(message, options);
         this.name = 'SdkError';
         stampErrorBrands(this, new.target);
     }