fix(client): suppress onerror when SSE disconnect will be handled by reconnect

.changeset/fix-sse-disconnect-noise.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Suppress `onerror` when an SSE stream disconnects but reconnection will be scheduled. Previously `onerror` fired unconditionally on every stream disconnect, producing `"SSE stream disconnected: TypeError: terminated"` noise every few minutes on long-lived connections even though the transport recovered transparently.

packages/client/src/client/streamableHttp.ts

@@ -445,9 +445,6 @@ export class StreamableHTTPClientTransport implements Transport {
                     );
                 }
             } catch (error) {
-                // Handle stream errors - likely a network disconnect
-                this.onerror?.(new Error(`SSE stream disconnected: ${error}`));
-
                 // Attempt to reconnect if the stream disconnects unexpectedly and we aren't closing
                 // Reconnect if: already reconnectable (GET stream) OR received a priming event (POST stream with event ID)
                 // BUT don't reconnect if we already received a response - the request is complete
@@ -467,6 +464,9 @@ export class StreamableHTTPClientTransport implements Transport {
                     } catch (error) {
                         this.onerror?.(new Error(`Failed to reconnect: ${error instanceof Error ? error.message : String(error)}`));
                     }
+                } else {
+                    // Stream disconnected and reconnection will not happen; surface the error
+                    this.onerror?.(new Error(`SSE stream disconnected: ${error}`));
                 }
             }
         };

packages/client/test/client/streamableHttp.test.ts

@@ -832,12 +832,9 @@ describe('StreamableHTTPClientTransport', () => {
             await vi.advanceTimersByTimeAsync(20); // Trigger reconnection timeout
 
             // ASSERT
-            expect(errorSpy).toHaveBeenCalledWith(
-                expect.objectContaining({
-                    message: expect.stringContaining('SSE stream disconnected: Error: Network failure')
-                })
-            );
-            // THE KEY ASSERTION: A second fetch call proves reconnection was attempted.
+            // onerror is NOT called: reconnection will handle the disconnect transparently
+            expect(errorSpy).not.toHaveBeenCalled();
+            // A second fetch call proves reconnection was attempted.
             expect(fetchMock).toHaveBeenCalledTimes(2);
             expect(fetchMock.mock.calls[0]![1]?.method).toBe('GET');
             expect(fetchMock.mock.calls[1]![1]?.method).toBe('GET');