diff --git a/.changeset/fix-double-onerror-startOrAuthSse.md b/.changeset/fix-double-onerror-startOrAuthSse.md
new file mode 100644
index 000000000..69b9c9f9f
--- /dev/null
+++ b/.changeset/fix-double-onerror-startOrAuthSse.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Fix double `onerror` invocation when `_startOrAuthSse` fails. The internal catch block fired `onerror` then threw, and all callers already `.catch(onerror)`, causing every failure to fire twice. Removed the redundant internal call.
diff --git a/packages/client/src/client/streamableHttp.ts b/packages/client/src/client/streamableHttp.ts
index 56cbb4d98..e5982199e 100644
--- a/packages/client/src/client/streamableHttp.ts
+++ b/packages/client/src/client/streamableHttp.ts
@@ -294,8 +294,7 @@ export class StreamableHTTPClientTransport implements Transport {
 
             this._handleSseStream(response.body, options, true);
         } catch (error) {
-            this.onerror?.(error as Error);
-            throw error;
+            throw error as Error;
         }
     }
 
diff --git a/packages/client/test/client/streamableHttp.test.ts b/packages/client/test/client/streamableHttp.test.ts
index 4a23e6db4..971ca37fc 100644
--- a/packages/client/test/client/streamableHttp.test.ts
+++ b/packages/client/test/client/streamableHttp.test.ts
@@ -1131,6 +1131,43 @@ describe('StreamableHTTPClientTransport', () => {
             expect(fetchMock.mock.calls[0]![1]?.method).toBe('POST');
         });
 
+        it('should fire onerror exactly once when _startOrAuthSse fails (not double-fire from catch + caller)', async () => {
+            transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'));
+
+            const errorSpy = vi.fn();
+            transport.onerror = errorSpy;
+
+            const fetchMock = globalThis.fetch as Mock;
+
+            // POST returns 202, which triggers _startOrAuthSse when the outbound
+            // message is an initialized notification (streamableHttp.ts:642)
+            fetchMock.mockResolvedValueOnce({
+                ok: true,
+                status: 202,
+                headers: new Headers(),
+                text: async () => ''
+            });
+
+            // The subsequent GET (_startOrAuthSse) fails with a non-ok status
+            fetchMock.mockResolvedValueOnce({
+                ok: false,
+                status: 500,
+                statusText: 'Internal Server Error',
+                headers: new Headers(),
+                text: async () => 'server error'
+            });
+
+            await transport.start();
+            // Sending an initialized notification triggers the _startOrAuthSse path
+            await transport.send({ jsonrpc: '2.0', method: 'notifications/initialized' });
+
+            // Let the fire-and-forget _startOrAuthSse().catch() settle
+            await vi.runAllTimersAsync();
+
+            expect(errorSpy).toHaveBeenCalledTimes(1);
+            expect(errorSpy.mock.calls[0]![0].message).toContain('Failed to open SSE stream');
+        });
+
         it('should not throw JSON parse error on priming events with empty data', async () => {
             transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'));