fix(core): forward cause from the SdkError data slot to Error.cause

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

@@ -149,12 +149,25 @@ export class SdkError extends Error {
         message: string,
         public readonly data?: unknown
     ) {
-        super(message);
+        // If a call site passes an underlying error via the `data` slot as
+        // `{ cause }`, forward it to `Error` so the standard `.cause` chain
+        // stays unbroken — loggers and error trackers (pino, Sentry, …) walk
+        // `.cause` to reach the root cause. The full `data` object is still
+        // retained on `this.data`. See modelcontextprotocol/typescript-sdk#2657.
+        super(message, dataHasCause(data) ? { cause: data.cause } : undefined);
         this.name = 'SdkError';
         stampErrorBrands(this, new.target);
     }
 }
 
+/**
+ * Narrow an {@linkcode SdkError.data | data} value to one that carries a
+ * `cause`, so it can be forwarded to the `Error` constructor's `ErrorOptions`.
+ */
+function dataHasCause(data: unknown): data is { cause: unknown } {
+    return typeof data === 'object' && data !== null && 'cause' in data;
+}
+
 /**
  * Typed shape for HTTP error data carried by {@linkcode SdkHttpError}.
  */

packages/core-internal/test/errors/sdkErrorCause.test.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect } from 'vitest';
+import { SdkError, SdkErrorCode, SdkHttpError } from '../../src/index';
+
+describe('SdkError cause forwarding (#2657)', () => {
+    it('forwards a `cause` from the data slot to Error.cause', () => {
+        const root = new Error('getaddrinfo ENOTFOUND does-not-resolve.invalid');
+        const error = new SdkError(SdkErrorCode.EraNegotiationFailed, 'Version negotiation probe failed: fetch failed', {
+            cause: root
+        });
+
+        // The standard `.cause` chain must reach the underlying error, so
+        // pino/Sentry-style walkers surface the real failure.
+        expect(error.cause).toBe(root);
+    });
+
+    it('still keeps the full object on `data` (does not move it out)', () => {
+        const root = new Error('boom');
+        const error = new SdkError(SdkErrorCode.EraNegotiationFailed, 'msg', { cause: root, extra: 1 });
+
+        expect(error.cause).toBe(root);
+        expect(error.data).toEqual({ cause: root, extra: 1 });
+    });
+
+    it('leaves cause undefined when data carries none', () => {
+        const error = new SdkError(SdkErrorCode.NotConnected, 'Transport is not connected', {
+            status: 401
+        });
+
+        expect(error.cause).toBeUndefined();
+        expect(error.data).toEqual({ status: 401 });
+    });
+
+    it('leaves cause undefined when there is no data', () => {
+        const error = new SdkError(SdkErrorCode.NotConnected, 'Transport is not connected');
+
+        expect(error.cause).toBeUndefined();
+    });
+
+    it('does not treat an HTTP data payload as a cause', () => {
+        const error = new SdkHttpError(SdkErrorCode.ClientHttpAuthentication, 'Unauthorized', {
+            status: 401,
+            statusText: 'Unauthorized'
+        });
+
+        expect(error.cause).toBeUndefined();
+        expect(error.status).toBe(401);
+    });
+});