diff --git a/mli.go b/mli.go
index daadb8b..594af0d 100644
--- a/mli.go
+++ b/mli.go
@@ -30,6 +30,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"io"
 	"strconv"
 )
 
@@ -357,3 +358,53 @@ func validateLength(length int, minLength, maxLength int64) error {
 	}
 	return nil
 }
+
+// EncodeWriter encodes the length into the MLI selected by key and writes it to w.
+// length must exclude the MLI itself. Returns [ErrLength] when length is negative
+// or outside the selected format's range.
+func EncodeWriter(w io.Writer, key string, length int) error {
+	b, err := Encode(key, length)
+	if err != nil {
+		return err
+	}
+	_, err = w.Write(b)
+	return err
+}
+
+// DecodeReader reads the MLI bytes from r for the given key and returns the decoded message length.
+// Returns [ErrByteSize] if the reader doesn't provide enough bytes.
+// Returns [ErrLength] if the decoded value is invalid for the MLI type.
+func DecodeReader(r io.Reader, key string) (int, error) {
+	size := mliSize(key)
+	if size == 0 {
+		return 0, fmt.Errorf("Invalid MLI type provided")
+	}
+
+	b := make([]byte, size)
+	if _, err := io.ReadFull(r, b); err != nil {
+		return 0, ErrByteSize
+	}
+
+	return Decode(key, &b)
+}
+
+func mliSize(key string) int {
+	switch key {
+	case MLI2I:
+		return Size2I
+	case MLI2E:
+		return Size2E
+	case MLI4I:
+		return Size4I
+	case MLI4E:
+		return Size4E
+	case MLI2EE:
+		return Size2EE
+	case MLI2BCD2:
+		return Size2BCD2
+	case MLIA4E:
+		return SizeA4E
+	default:
+		return 0
+	}
+}
diff --git a/mli_test.go b/mli_test.go
index de6d23f..477cdf1 100644
--- a/mli_test.go
+++ b/mli_test.go
@@ -14,6 +14,7 @@
 package simplemli
 
 import (
+	"bytes"
 	"encoding/hex"
 	"errors"
 	"testing"
@@ -361,3 +362,117 @@ func FuzzDecode(f *testing.F) {
 		}
 	})
 }
+
+func TestEncodeWriter(t *testing.T) {
+	tests := []struct {
+		name   string
+		key    string
+		length int
+	}{
+		{name: "2I", key: MLI2I, length: 43},
+		{name: "2E", key: MLI2E, length: 43},
+		{name: "4I", key: MLI4I, length: 49},
+		{name: "4E", key: MLI4E, length: 34},
+		{name: "2EE", key: MLI2EE, length: 56},
+		{name: "2BCD2", key: MLI2BCD2, length: 284},
+		{name: "A4E", key: MLIA4E, length: 43},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			err := EncodeWriter(&buf, tt.key, tt.length)
+			if err != nil {
+				t.Fatalf("EncodeWriter(%q, %d) unexpected error: %v", tt.key, tt.length, err)
+			}
+
+			expected, err := Encode(tt.key, tt.length)
+			if err != nil {
+				t.Fatalf("Encode(%q, %d) unexpected error: %v", tt.key, tt.length, err)
+			}
+
+			if !bytes.Equal(buf.Bytes(), expected) {
+				t.Errorf("EncodeWriter output mismatch: got %x, want %x", buf.Bytes(), expected)
+			}
+		})
+	}
+}
+
+func TestEncodeWriterErrors(t *testing.T) {
+	t.Run("Invalid key", func(t *testing.T) {
+		var buf bytes.Buffer
+		err := EncodeWriter(&buf, "Invalid", 10)
+		if err == nil {
+			t.Error("Expected error for invalid key")
+		}
+	})
+
+	t.Run("Negative length", func(t *testing.T) {
+		var buf bytes.Buffer
+		err := EncodeWriter(&buf, MLI2I, -1)
+		if !errors.Is(err, ErrLength) {
+			t.Errorf("Expected ErrLength, got %v", err)
+		}
+	})
+}
+
+func TestDecodeReader(t *testing.T) {
+	tests := []struct {
+		name   string
+		key    string
+		length int
+	}{
+		{name: "2I", key: MLI2I, length: 43},
+		{name: "2E", key: MLI2E, length: 43},
+		{name: "4I", key: MLI4I, length: 49},
+		{name: "4E", key: MLI4E, length: 34},
+		{name: "2EE", key: MLI2EE, length: 56},
+		{name: "2BCD2", key: MLI2BCD2, length: 284},
+		{name: "A4E", key: MLIA4E, length: 43},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			encoded, err := Encode(tt.key, tt.length)
+			if err != nil {
+				t.Fatalf("Encode(%q, %d) unexpected error: %v", tt.key, tt.length, err)
+			}
+
+			reader := bytes.NewReader(encoded)
+			got, err := DecodeReader(reader, tt.key)
+			if err != nil {
+				t.Fatalf("DecodeReader(%q) unexpected error: %v", tt.key, err)
+			}
+
+			if got != tt.length {
+				t.Errorf("DecodeReader(%q) = %d, want %d", tt.key, got, tt.length)
+			}
+		})
+	}
+}
+
+func TestDecodeReaderErrors(t *testing.T) {
+	t.Run("Invalid key", func(t *testing.T) {
+		reader := bytes.NewReader([]byte{0x00, 0x00})
+		_, err := DecodeReader(reader, "Invalid")
+		if err == nil {
+			t.Error("Expected error for invalid key")
+		}
+	})
+
+	t.Run("Insufficient bytes", func(t *testing.T) {
+		reader := bytes.NewReader([]byte{0x00}) // Only 1 byte for 2I which needs 2
+		_, err := DecodeReader(reader, MLI2I)
+		if !errors.Is(err, ErrByteSize) {
+			t.Errorf("Expected ErrByteSize, got %v", err)
+		}
+	})
+
+	t.Run("EOF", func(t *testing.T) {
+		reader := bytes.NewReader([]byte{})
+		_, err := DecodeReader(reader, MLI2I)
+		if !errors.Is(err, ErrByteSize) {
+			t.Errorf("Expected ErrByteSize, got %v", err)
+		}
+	})
+}