diff --git a/core/content/helpers.go b/core/content/helpers.go
index 7bbed58a05..3a79876160 100644
--- a/core/content/helpers.go
+++ b/core/content/helpers.go
@@ -188,14 +188,20 @@ func OpenWriter(ctx context.Context, cs Ingester, opts ...WriterOpt) (Writer, er
 // Copy copies data with the expected digest from the reader into the
 // provided content store writer. This copy commits the writer.
 //
-// This is useful when the digest and size are known beforehand. When
-// the size or digest is unknown, these values may be empty.
+// A size of 0 indicates an unknown size, in which case the reader is
+// copied until EOF. For backward compatibility, -1 is also accepted
+// as an unknown size. Other negative sizes are invalid. Positive sizes
+// are enforced, and an empty expected digest disables digest verification.
 //
 // Copy is buffered, so no need to wrap reader in buffered io.
 func Copy(ctx context.Context, cw Writer, or io.Reader, size int64, expected digest.Digest, opts ...Opt) error {
-	if size < 0 {
+	if size < -1 {
 		return fmt.Errorf("invalid content size %d: %w", size, errdefs.ErrInvalidArgument)
 	}
+	if size == -1 {
+		// Writer.Commit uses 0 for an unknown size.
+		size = 0
+	}
 	for i := 0; ; i++ {
 		if i >= 1 {
 			log.G(ctx).WithField("digest", expected).Debugf("retrying copy due to reset")
diff --git a/core/content/helpers_test.go b/core/content/helpers_test.go
index ace6a2818a..893981bc81 100644
--- a/core/content/helpers_test.go
+++ b/core/content/helpers_test.go
@@ -205,10 +205,34 @@ func TestCopy(t *testing.T) {
 			expectedErr: errdefs.ErrFailedPrecondition,
 		},
 		{
-			name: "copy fails when size is negative",
+			name: "copy with unspecified size (zero)",
+			source: copySource{
+				reader: strings.NewReader("content"),
+				size:   0,
+				digest: digest.FromString("content"),
+			},
+			writer: fakeWriter{
+				Buffer: &bytes.Buffer{},
+			},
+			expected: "content",
+		},
+		{
+			name: "copy with unspecified size (-1)",
 			source: copySource{
 				reader: strings.NewReader("content"),
 				size:   -1,
+				digest: digest.FromString("content"),
+			},
+			writer: fakeWriter{
+				Buffer: &bytes.Buffer{},
+			},
+			expected: "content",
+		},
+		{
+			name: "copy with invalid negative size",
+			source: copySource{
+				reader: strings.NewReader("content"),
+				size:   -2,
 			},
 			writer: fakeWriter{
 				Buffer: &bytes.Buffer{},
@@ -230,7 +254,9 @@ func TestCopy(t *testing.T) {
 				assert.ErrorIs(t, err, testcase.expectedErr)
 				return
 			}
-
+			if testcase.source.size == -1 {
+				assert.Zero(t, testcase.writer.committedSize)
+			}
 			assert.NoError(t, err)
 			assert.Equal(t, testcase.source.digest, testcase.writer.committedDigest)
 			assert.Equal(t, testcase.expected, testcase.writer.String())
@@ -273,6 +299,7 @@ type commitFunction func() error
 
 type fakeWriter struct {
 	*bytes.Buffer
+	committedSize   int64
 	committedDigest digest.Digest
 	status          Status
 	commitFunc      commitFunction
@@ -284,6 +311,7 @@ func (f *fakeWriter) Close() error {
 }
 
 func (f *fakeWriter) Commit(ctx context.Context, size int64, expected digest.Digest, opts ...Opt) error {
+	f.committedSize = size
 	f.committedDigest = expected
 	if f.commitFunc == nil {
 		return nil