diff --git a/src/internal/serve/wire.zig b/src/internal/serve/wire.zig
index 46841fd..599d1f9 100644
--- a/src/internal/serve/wire.zig
+++ b/src/internal/serve/wire.zig
@@ -513,8 +513,18 @@ pub fn encodeSyncV2(w: *std.Io.Writer, args: struct {
     try s.write(seq);
     try s.objectField("sync");
     try s.beginObject();
+    // lexicon "bytes" value: the {"$bytes":"<unpadded std base64>"} sentinel
+    // (atmos cbor.AppendJSONBytes) — a bare base64 string is rejected by
+    // conformant decoders (caught live by the d4dd2f0 differential oracle)
     try s.objectField("blocks");
-    try writeBase64String(&s, args.blocks);
+    try s.beginObject();
+    try s.objectField("$bytes");
+    try s.beginWriteRaw();
+    try s.writer.writeByte('"');
+    try writeBase64NoPad(s.writer, args.blocks);
+    try s.writer.writeByte('"');
+    s.endWriteRaw();
+    try s.endObject();
     try s.objectField("did");
     try writeString(&s, args.did);
     try s.objectField("rev");
@@ -534,22 +544,6 @@ pub fn encodeSyncV2(w: *std.Io.Writer, args: struct {
     try w.writeByte('}');
 }
 
-/// base64 std WITH padding, matching Go's []byte JSON encoding
-fn writeBase64String(s: *std.json.Stringify, data: []const u8) !void {
-    try s.beginWriteRaw();
-    try s.writer.writeByte('"');
-    const encoder = std.base64.standard.Encoder;
-    var chunk: [4]u8 = undefined;
-    var i: usize = 0;
-    while (i < data.len) : (i += 3) {
-        const n = @min(3, data.len - i);
-        const out = encoder.encode(&chunk, data[i..][0..n]);
-        try s.writer.writeAll(out);
-    }
-    try s.writer.writeByte('"');
-    s.endWriteRaw();
-}
-
 /// #info advisory (seq-less): {"$type":"message","payload":{"$type":"...#info","message"?,"name"}}
 pub fn encodeInfoV2(w: *std.Io.Writer, name: []const u8, message: ?[]const u8) !void {
     try w.writeAll(message_frame_prefix);