diff --git a/commands/blob/allocate.go b/commands/blob/allocate.go
index 1bf3d03..5ca89b9 100644
--- a/commands/blob/allocate.go
+++ b/commands/blob/allocate.go
@@ -7,6 +7,16 @@ import (
 	"github.com/fil-forge/ucantone/ucan/command"
 )
 
-const MaxBlobSize = 268_435_456
+// MaxBlobSize is the network-wide ceiling on a single blob, in bytes, as
+// shipped to a storage node: the most raw bytes a default-configured piri
+// accepts in one piece. Piri's default piece ceiling is a 256 MiB PADDED
+// piece (piri pkg/pdp/piecesize, DefaultMaxPaddedSize), and Fr32 padding
+// leaves 127/128 of it for data — 266338304 (~254 MiB). Piri's piece config
+// is raise-only (an operator may accept more, never less), so a client
+// sharding uploads at this constant is safe against every conforming node.
+//
+// This measures the stored artifact. An encrypting client must choose its
+// plaintext split below this, leaving room for its envelope framing.
+const MaxBlobSize = (256 << 20) * 127 / 128
 
 var Allocate = binding.Bind[*AllocateArguments, *AllocateOK](command.MustParse("/blob/allocate"))