diff --git a/net/edf/register.go b/net/edf/register.go
index 2006bbe3..8eeab258 100644
--- a/net/edf/register.go
+++ b/net/edf/register.go
@@ -60,7 +60,12 @@ func RegisterTypeOf(v any) error {
 
 		fenc := func(value reflect.Value, b *lib.Buffer, _ *stateEncode) error {
 			v := value.Interface().(Marshaler)
-			buf := b.Extend(4)
+			// Record the offset for the length prefix instead of using the
+			// slice returned by Extend. If MarshalEDF triggers buffer
+			// reallocation, the Extend slice becomes stale but the offset
+			// remains valid against the new backing array.
+			lenPrefixOffset := b.Len()
+			b.Extend(4)
 			l := b.Len()
 			if err := v.MarshalEDF(b); err != nil {
 				return err
@@ -70,7 +75,7 @@ func RegisterTypeOf(v any) error {
 			if int64(lenBinary) > int64(math.MaxUint32-1) {
 				return ErrBinaryTooLong
 			}
-			binary.BigEndian.PutUint32(buf, uint32(lenBinary))
+			binary.BigEndian.PutUint32(b.B[lenPrefixOffset:], uint32(lenBinary))
 			return nil
 		}
 		encoders.Store(tov, regEncoder(name, fenc))
diff --git a/net/edf/register_marshaler_test.go b/net/edf/register_marshaler_test.go
new file mode 100644
index 00000000..d8334e11
--- /dev/null
+++ b/net/edf/register_marshaler_test.go
@@ -0,0 +1,77 @@
+package edf
+
+// Verifies that Encode/Decode roundtrip works correctly when MarshalEDF
+// writes enough data to trigger lib.Buffer reallocation. A small initial
+// buffer capacity (64 bytes) guarantees that MarshalEDF's Write call
+// triggers append-reallocation for any non-trivial payload.
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"ergo.services/ergo/gen"
+	"ergo.services/ergo/lib"
+)
+
+type largePayload struct {
+	Data []byte
+}
+
+func (lp largePayload) MarshalEDF(w io.Writer) error {
+	_, err := w.Write(lp.Data)
+	return err
+}
+
+func (lp *largePayload) UnmarshalEDF(data []byte) error {
+	lp.Data = make([]byte, len(data))
+	copy(lp.Data, data)
+	return nil
+}
+
+func TestMarshalerBufferReallocation(t *testing.T) {
+	if err := RegisterTypeOf(largePayload{}); err != nil && err != gen.ErrTaken {
+		t.Fatal(err)
+	}
+
+	// Small payload that fits without reallocation, and a large payload
+	// that forces the buffer to grow.
+	sizes := []struct {
+		name string
+		n    int
+	}{
+		{"small", 100},
+		{"large", lib.DefaultBufferLength * 2},
+	}
+
+	for _, tc := range sizes {
+		t.Run(tc.name, func(t *testing.T) {
+			data := make([]byte, tc.n)
+			for i := range data {
+				data[i] = byte(i % 251)
+			}
+			original := largePayload{Data: data}
+
+			// Small capacity forces MarshalEDF to trigger buffer reallocation.
+			buf := &lib.Buffer{B: make([]byte, 0, 64)}
+
+			if err := Encode(original, buf, Options{}); err != nil {
+				t.Fatalf("Encode: %v", err)
+			}
+
+			decoded, _, err := Decode(buf.B, Options{})
+			if err != nil {
+				t.Fatalf("Decode: %v (encoded %d bytes)", err, buf.Len())
+			}
+
+			got, ok := decoded.(largePayload)
+			if !ok {
+				t.Fatalf("expected largePayload, got %T", decoded)
+			}
+			if !bytes.Equal(got.Data, original.Data) {
+				t.Fatalf("roundtrip mismatch: got %d bytes, want %d bytes",
+					len(got.Data), len(original.Data))
+			}
+		})
+	}
+}