mcp: prevent MemoryEventStore.After panic on index past the end

mcp/event.go

@@ -342,6 +342,15 @@ func (s *MemoryEventStore) After(_ context.Context, sessionID, streamID string,
 			return nil, fmt.Errorf("MemoryEventStore.After: index %d, stream ID %v, session %q: %w",
 				index, streamID, sessionID, ErrEventsPurged)
 		}
+		// An index at or beyond the latest stored event has nothing after it, so
+		// there is nothing to replay. The boundary start == dl.first+len(dl.data)
+		// (resume from the last event seen) already yields an empty slice; guard
+		// the strictly-greater case too, otherwise dl.data[start-dl.first:]
+		// panics with a slice-bounds-out-of-range on an index past the end (for
+		// example a client-supplied Last-Event-ID beyond any event ever sent).
+		if start-dl.first > len(dl.data) {
+			return nil, nil
+		}
 		return slices.Clone(dl.data[start-dl.first:]), nil
 	}
 

mcp/event_test.go

@@ -299,7 +299,13 @@ func TestMemoryEventStoreAfter(t *testing.T) {
 		{"S1", "1", 0, []string{"d2", "d3"}, ""},
 		{"S1", "1", 1, []string{"d3"}, ""},
 		{"S1", "1", 2, nil, ""},
+		// An index past the latest stored event (highest here is 2) must yield an
+		// empty replay, not panic with a slice-bounds-out-of-range. This is
+		// reachable from a client-supplied Last-Event-ID beyond any event sent.
+		{"S1", "1", 3, nil, ""},
+		{"S1", "1", 100, nil, ""},
 		{"S1", "2", 0, nil, ""},
+		{"S1", "2", 5, nil, ""}, // past the end of stream "2" (highest is 0)
 		{"S1", "3", 0, nil, "unknown stream ID"},
 		{"S2", "0", 0, nil, "unknown session ID"},
 	} {