diff --git a/internal/buffer/out_message_test.go b/internal/buffer/out_message_test.go
index 81b0192..f647f22 100644
--- a/internal/buffer/out_message_test.go
+++ b/internal/buffer/out_message_test.go
@@ -1,12 +1,16 @@
 package buffer
 
 import (
+	"bytes"
 	"crypto/rand"
 	"fmt"
 	"io"
 	"reflect"
 	"testing"
 	"unsafe"
+
+	"github.com/jacobsa/fuse/internal/fusekernel"
+	"github.com/kylelemons/godebug/pretty"
 )
 
 func toByteSlice(p unsafe.Pointer, n int) []byte {
@@ -86,18 +90,154 @@ func TestMemclr(t *testing.T) {
 	}
 }
 
+func TestOutMessageAppend(t *testing.T) {
+	var om OutMessage
+	om.Reset()
+
+	// Append some payload.
+	const wantPayloadStr = "tacoburrito"
+	wantPayload := []byte(wantPayloadStr)
+	om.Append(wantPayload[:4])
+	om.Append(wantPayload[4:])
+
+	// The result should be a zeroed header followed by the desired payload.
+	const wantLen = int(OutMessageInitialSize) + len(wantPayloadStr)
+
+	if got, want := om.Len(), wantLen; got != want {
+		t.Errorf("om.Len() = %d, want %d", got, want)
+	}
+
+	b := om.Bytes()
+	if got, want := len(b), wantLen; got != want {
+		t.Fatalf("len(om.Bytes()) = %d, want %d", got, want)
+	}
+
+	want := append(
+		make([]byte, OutMessageInitialSize),
+		wantPayload...)
+
+	if !bytes.Equal(b, want) {
+		t.Error("messages differ")
+	}
+}
+
+func TestOutMessageAppendString(t *testing.T) {
+	var om OutMessage
+	om.Reset()
+
+	// Append some payload.
+	const wantPayload = "tacoburrito"
+	om.AppendString(wantPayload[:4])
+	om.AppendString(wantPayload[4:])
+
+	// The result should be a zeroed header followed by the desired payload.
+	const wantLen = int(OutMessageInitialSize) + len(wantPayload)
+
+	if got, want := om.Len(), wantLen; got != want {
+		t.Errorf("om.Len() = %d, want %d", got, want)
+	}
+
+	b := om.Bytes()
+	if got, want := len(b), wantLen; got != want {
+		t.Fatalf("len(om.Bytes()) = %d, want %d", got, want)
+	}
+
+	want := append(
+		make([]byte, OutMessageInitialSize),
+		wantPayload...)
+
+	if !bytes.Equal(b, want) {
+		t.Error("messages differ")
+	}
+}
+
+func TestOutMessageShrinkTo(t *testing.T) {
+	// Set up a buffer with some payload.
+	var om OutMessage
+	om.Reset()
+	om.AppendString("taco")
+	om.AppendString("burrito")
+
+	// Shrink it.
+	om.ShrinkTo(OutMessageInitialSize + uintptr(len("taco")))
+
+	// The result should be a zeroed header followed by "taco".
+	const wantLen = int(OutMessageInitialSize) + len("taco")
+
+	if got, want := om.Len(), wantLen; got != want {
+		t.Errorf("om.Len() = %d, want %d", got, want)
+	}
+
+	b := om.Bytes()
+	if got, want := len(b), wantLen; got != want {
+		t.Fatalf("len(om.Bytes()) = %d, want %d", got, want)
+	}
+
+	want := append(
+		make([]byte, OutMessageInitialSize),
+		"taco"...)
+
+	if !bytes.Equal(b, want) {
+		t.Error("messages differ")
+	}
+}
+
+func TestOutMessageHeader(t *testing.T) {
+	var om OutMessage
+	om.Reset()
+
+	// Fill in the header.
+	want := fusekernel.OutHeader{
+		Len:    0xdeadbeef,
+		Error:  -31231917,
+		Unique: 0xcafebabeba5eba11,
+	}
+
+	h := om.OutHeader()
+	if h == nil {
+		t.Fatal("OutHeader returned nil")
+	}
+
+	*h = want
+
+	// Check that the result is as expected.
+	b := om.Bytes()
+	if len(b) != int(unsafe.Sizeof(want)) {
+		t.Fatalf("unexpected length %d; want %d", len(b), unsafe.Sizeof(want))
+	}
+
+	got := *(*fusekernel.OutHeader)(unsafe.Pointer(&b[0]))
+	if diff := pretty.Compare(got, want); diff != "" {
+		t.Errorf("diff -got +want:\n%s", diff)
+	}
+}
+
 func TestOutMessageReset(t *testing.T) {
 	var om OutMessage
 	h := om.OutHeader()
 
-	const trials = 100
+	const trials = 10
 	for i := 0; i < trials; i++ {
+		// Fill the header with garbage.
 		err := fillWithGarbage(unsafe.Pointer(h), int(unsafe.Sizeof(*h)))
 		if err != nil {
 			t.Fatalf("fillWithGarbage: %v", err)
 		}
 
+		// Ensure a non-zero payload length.
+		if p := om.GrowNoZero(128); p == nil {
+			t.Fatal("GrowNoZero failed")
+		}
+
+		// Reset.
 		om.Reset()
+
+		// Check that the length was updated.
+		if got, want := int(om.Len()), int(OutMessageInitialSize); got != want {
+			t.Fatalf("om.Len() = %d, want %d", got, want)
+		}
+
+		// Check that the header was zeroed.
 		if h.Len != 0 {
 			t.Fatalf("non-zero Len %v", h.Len)
 		}
@@ -114,26 +254,44 @@ func TestOutMessageReset(t *testing.T) {
 
 func TestOutMessageGrow(t *testing.T) {
 	var om OutMessage
-
-	// Overwrite with garbage.
-	err := fillWithGarbage(unsafe.Pointer(&om), int(unsafe.Sizeof(om)))
-	if err != nil {
-		t.Fatalf("fillWithGarbage: %v", err)
-	}
-
-	// Zero the header.
 	om.Reset()
 
-	// Grow to the max size. This should zero the message.
-	if p := om.Grow(MaxReadSize); p == nil {
-		t.Fatal("Grow returned nil")
+	// Set up garbage where the payload will soon be.
+	const payloadSize = 1234
+	{
+		p := om.GrowNoZero(payloadSize)
+		if p == nil {
+			t.Fatal("GrowNoZero failed")
+		}
+
+		err := fillWithGarbage(p, payloadSize)
+		if err != nil {
+			t.Fatalf("fillWithGarbage: %v", err)
+		}
+
+		om.ShrinkTo(OutMessageInitialSize)
+	}
+
+	// Call Grow.
+	if p := om.Grow(payloadSize); p == nil {
+		t.Fatal("Grow failed")
+	}
+
+	// Check the resulting length in two ways.
+	const wantLen = int(payloadSize + OutMessageInitialSize)
+	if got, want := om.Len(), wantLen; got != want {
+		t.Errorf("om.Len() = %d, want %d", got)
 	}
 
-	// Check that everything has been zeroed.
 	b := om.Bytes()
-	for i, x := range b {
+	if got, want := len(b), wantLen; got != want {
+		t.Fatalf("len(om.Len()) = %d, want %d", got)
+	}
+
+	// Check that the payload was zeroed.
+	for i, x := range b[OutMessageInitialSize:] {
 		if x != 0 {
-			t.Fatalf("non-zero byte 0x%02x at offset %d", x, i)
+			t.Fatalf("non-zero byte 0x%02x at payload offset %d", x, i)
 		}
 	}
 }