diff --git a/weed/filer/reader_at_prefetch_window_test.go b/weed/filer/reader_at_prefetch_window_test.go
new file mode 100644
index 000000000..982bc40cb
--- /dev/null
+++ b/weed/filer/reader_at_prefetch_window_test.go
@@ -0,0 +1,74 @@
+package filer
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"strings"
+	"sync/atomic"
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/util/chunk_cache"
+	util_http "github.com/seaweedfs/seaweedfs/weed/util/http"
+)
+
+// A sequential reader that consumes a chunk in several small ReadAt calls, as
+// the S3 GET path does with its 256 KiB copy buffer over 4 MiB chunks, must
+// keep its read-ahead at prefetchCount chunks. Each ReadAt triggers a
+// prefetch; if chunks already being fetched are not counted toward the
+// window, every call starts prefetchCount more downloads and the read-ahead
+// runs to the ReaderCache downloader limit, buffering most of the object.
+func TestChunkReadAtSequentialPrefetchStaysWithinWindow(t *testing.T) {
+	const chunkSize = 64 << 10
+	const chunkCount = 64
+	const sliceSize = 16 << 10
+
+	var readerChunk atomic.Int64
+	var maxLead atomic.Int64
+	rc := NewReaderCache(256, (*chunk_cache.TieredChunkCache)(nil), func(context.Context, string) ([]string, error) {
+		return []string{"unused"}, nil
+	}, nil)
+	defer rc.destroy()
+	rc.fetchChunkDataFn = func(_ context.Context, buffer []byte, _ []string, _ []byte, _ bool, _ bool, _ int64, fileId string, _ util_http.RefreshUrlsFunc) (int, error) {
+		idx, err := strconv.Atoi(strings.TrimPrefix(fileId, "chunk"))
+		if err != nil {
+			return 0, err
+		}
+		lead := int64(idx) - readerChunk.Load()
+		for {
+			cur := maxLead.Load()
+			if lead <= cur || maxLead.CompareAndSwap(cur, lead) {
+				break
+			}
+		}
+		return len(buffer), nil
+	}
+
+	views := NewIntervalList[*ChunkView]()
+	for i := 0; i < chunkCount; i++ {
+		views.AppendInterval(&Interval[*ChunkView]{
+			StartOffset: int64(i * chunkSize),
+			StopOffset:  int64((i + 1) * chunkSize),
+			Value: &ChunkView{
+				FileId:     fmt.Sprintf("chunk%d", i),
+				ViewSize:   chunkSize,
+				ViewOffset: int64(i * chunkSize),
+				ChunkSize:  chunkSize,
+			},
+		})
+	}
+	reader := NewChunkReaderAtFromClient(context.Background(), rc, views, chunkSize*chunkCount, DefaultPrefetchCount)
+	defer reader.ReleaseStream()
+
+	buf := make([]byte, sliceSize)
+	for off := int64(0); off < chunkSize*chunkCount; off += sliceSize {
+		readerChunk.Store(off / chunkSize)
+		if _, err := reader.ReadAt(buf, off); err != nil && off+sliceSize < chunkSize*chunkCount {
+			t.Fatalf("ReadAt(%d): %v", off, err)
+		}
+	}
+
+	if got := maxLead.Load(); got > DefaultPrefetchCount {
+		t.Fatalf("read-ahead reached %d chunks past the reader, want at most %d", got, DefaultPrefetchCount)
+	}
+}
diff --git a/weed/filer/reader_cache.go b/weed/filer/reader_cache.go
index 1c2ba7252..cd6049bee 100644
--- a/weed/filer/reader_cache.go
+++ b/weed/filer/reader_cache.go
@@ -92,8 +92,10 @@ func (rc *ReaderCache) MaybeCache(chunkViews *Interval[*ChunkView], count int) {
 		return
 	}
 
-	cached := 0
-	for x := chunkViews; x != nil && cached < count; x = x.Next {
+	// The window is the next count chunks, whether or not a download for them
+	// is already running: counting only newly started downloads would push the
+	// window count chunks further on every call.
+	for x, i := chunkViews, 0; x != nil && i < count; x, i = x.Next, i+1 {
 		chunkView := x.Value
 		if _, found := rc.downloaders[chunkView.FileId]; found {
 			continue
@@ -124,7 +126,6 @@ func (rc *ReaderCache) MaybeCache(chunkViews *Interval[*ChunkView], count int) {
 		rc.downloaders[chunkView.FileId] = cacher
 		go cacher.startCaching()
 		started = append(started, cacher)
-		cached++
 	}
 
 	return