Fix JSON parse error on SSE events with empty data

Priming events for resumability (SEP-1699) have an event ID but empty
data. The client was attempting JSON.parse("") which throws "Unexpected
end of JSON input".

Skip processing for all events with empty data, not just message events.
This handles priming events, keep-alives, and any other events that may
have empty data fields.

Author: felixweinberger

src/client/streamableHttp.test.ts

@@ -865,6 +865,57 @@ describe('StreamableHTTPClientTransport', () => {
             const reconnectHeaders = fetchMock.mock.calls[1][1]?.headers as Headers;
             expect(reconnectHeaders.get('last-event-id')).toBe('event-123');
         });
+
+        it('should not throw JSON parse error on priming events with empty data', async () => {
+            transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'));
+
+            const errorSpy = vi.fn();
+            transport.onerror = errorSpy;
+
+            const resumptionTokenSpy = vi.fn();
+
+            // Create a stream that sends a priming event (ID only, empty data) then a real message
+            const streamWithPrimingEvent = new ReadableStream({
+                start(controller) {
+                    // Send a priming event with ID but empty data - this should NOT cause a JSON parse error
+                    controller.enqueue(new TextEncoder().encode('id: priming-123\ndata: \n\n'));
+                    // Send a real message
+                    controller.enqueue(
+                        new TextEncoder().encode('id: msg-456\ndata: {"jsonrpc":"2.0","result":{"tools":[]},"id":"req-1"}\n\n')
+                    );
+                    controller.close();
+                }
+            });
+
+            const fetchMock = global.fetch as Mock;
+            fetchMock.mockResolvedValueOnce({
+                ok: true,
+                status: 200,
+                headers: new Headers({ 'content-type': 'text/event-stream' }),
+                body: streamWithPrimingEvent
+            });
+
+            await transport.start();
+            transport.send(
+                {
+                    jsonrpc: '2.0',
+                    method: 'tools/list',
+                    id: 'req-1',
+                    params: {}
+                },
+                { resumptionToken: undefined, onresumptiontoken: resumptionTokenSpy }
+            );
+
+            await vi.advanceTimersByTimeAsync(50);
+
+            // No JSON parse errors should have occurred
+            expect(errorSpy).not.toHaveBeenCalledWith(
+                expect.objectContaining({ message: expect.stringContaining('Unexpected end of JSON') })
+            );
+            // Resumption token callback should have been called for both events with IDs
+            expect(resumptionTokenSpy).toHaveBeenCalledWith('priming-123');
+            expect(resumptionTokenSpy).toHaveBeenCalledWith('msg-456');
+        });
     });
 
     it('invalidates all credentials on InvalidClientError during auth', async () => {

src/client/streamableHttp.ts

@@ -338,6 +338,11 @@ export class StreamableHTTPClientTransport implements Transport {
                         onresumptiontoken?.(event.id);
                     }
 
+                    // Skip events with no data (priming events, keep-alives)
+                    if (!event.data) {
+                        continue;
+                    }
+
                     if (!event.event || event.event === 'message') {
                         try {
                             const message = JSONRPCMessageSchema.parse(JSON.parse(event.data));