diff --git a/sio/anyio/file.go b/sio/anyio/file.go
index 941776965..eadd9c77f 100644
--- a/sio/anyio/file.go
+++ b/sio/anyio/file.go
@@ -70,9 +70,11 @@ func FileType(ctx context.Context, sctx *super.Context, engine storage.Engine, p
 		return nil, err
 	}
 	defer r.Close()
-	var b [1]byte
-	if _, err := r.ReadAt(b[:], 0); err != nil {
-		// r can't seek so it's a fifo or pipe.
+	rs, ok := r.(io.ReadSeekCloser)
+	if !ok {
+		return nil, nil
+	}
+	if _, err := rs.Seek(0, io.SeekCurrent); err != nil {
 		return nil, nil
 	}
 	f, err := NewFile(sctx, r, path, opts)
@@ -80,6 +82,10 @@ func FileType(ctx context.Context, sctx *super.Context, engine storage.Engine, p
 		return nil, err
 	}
 	defer f.Close()
+	// On BSD/macOS, open("/dev/stdin") dups fd 0 rather than re-opening the
+	// file, so it shares stdin's file offset. Reset to 0 so a re-open reads
+	// from the start instead of resuming where the last read left off.
+	defer rs.Seek(0, io.SeekStart)
 	switch r := f.Reader.(type) {
 	case *arrowio.Reader:
 		return r.Type(), nil
diff --git a/sio/anyio/ztests/sup-stdin.yaml b/sio/anyio/ztests/sup-stdin.yaml
new file mode 100644
index 000000000..c6a1820a1
--- /dev/null
+++ b/sio/anyio/ztests/sup-stdin.yaml
@@ -0,0 +1,13 @@
+script: |
+  super -s -c 'from "stdio:stdin"' < f
+
+inputs:
+  - name: f 
+    data: &f |
+      {x:1}
+      {y:2}
+      {z:3}
+
+outputs:
+  - name: stdout
+    data: *f
diff --git a/sio/fjsonio/stream.go b/sio/fjsonio/stream.go
index 26c7c1588..3a32c5c19 100644
--- a/sio/fjsonio/stream.go
+++ b/sio/fjsonio/stream.go
@@ -74,9 +74,6 @@ func (s *stream) close() error {
 	// drain channel
 	for range s.ch {
 	}
-	if closer, ok := s.r.(io.Closer); ok {
-		return closer.Close()
-	}
 	return nil
 }