diff --git a/README.md b/README.md
index 71a8f67..e72c706 100644
--- a/README.md
+++ b/README.md
@@ -153,6 +153,7 @@ However, nothing can stop you from just copying `cp0.json` (and `schema.json` if
 | bytecode.operands[i].name | Operand variable name. Allowed chars are `a-zA-Z0-9_`, must not begin with digit or underscore and must not end with underscore. Required.
 | bytecode.operands[i].loader | Loader function for operand. Must be one of `int`, `uint`, `ref`, `pushint_long`, `subslice`. Loaders are described below. Required.
 | bytecode.operands[i].loader_args | Arguments for loader function, specified below. Optional, no arguments in case of absence.
+| bytecode.operands[i].internal | Internal flag. If true, this operand is used as a subslice length variable, should be used only for serialization/deserialization, humans and implementations do not need them. Optional, default is false.
 | value_flow | Information related to usage of stack and registers by instruction. Optional.
 | value_flow.doc_stack | Free-form description of stack inputs and outputs. Usually the form is `[inputs] - [outputs]` where `[inputs]` are consumed stack values and `outputs` are produced stack values (top of stack is the last value). Optional.
 | value_flow.inputs | Incoming values constraints. Input is unconstrained if absent.
@@ -212,14 +213,16 @@ Special loader which currently is used only in `PUSHINT_LONG` instruction. Loads
         "loader": "uint",
         "loader_args": {
             "size": 2
-        }
+        },
+        "internal": true
     },
     {
         "name": "x",
         "loader": "uint",
         "loader_args": {
             "size": 5
-        }
+        },
+        "internal": true
     },
     {
         "name": "slice",
@@ -235,7 +238,7 @@ Special loader which currently is used only in `PUSHINT_LONG` instruction. Loads
 ]
 ```
 
-Loads subslice of bit length `{bits_length_var} * 8 + bits_padding` and ref count `{refs_length_var} + refs_add`. If `completion_tag` argument with value `true` is passed, remove completion tag from bitstring (trailing `'1' + '0' * x`).
+Loads subslice of bit length `{bits_length_var} * 8 + bits_padding` and ref count `{refs_length_var} + refs_add`. If `completion_tag` argument with value `true` is passed, remove completion tag from bitstring (trailing `'1' + '0' * x`). Length variables are usually `"internal": true` because they should not be showed to user or provided to an implementation.
 
 | Argument | Description
 | -------- | -----------
diff --git a/cp0.json b/cp0.json
index c1e9349..fcf11cd 100644
--- a/cp0.json
+++ b/cp0.json
@@ -3305,7 +3305,8 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 4
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "s",
@@ -3352,14 +3353,16 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 2
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "x",
                         "loader": "uint",
                         "loader_args": {
                             "size": 5
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "slice",
@@ -3423,14 +3426,16 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 3
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "x",
                         "loader": "uint",
                         "loader_args": {
                             "size": 7
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "slice",
@@ -3478,14 +3483,16 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 2
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "x",
                         "loader": "uint",
                         "loader_args": {
                             "size": 7
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "s",
@@ -3538,7 +3545,8 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 4
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "s",
@@ -19073,14 +19081,16 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 2
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "y",
                         "loader": "uint",
                         "loader_args": {
                             "size": 3
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "sss",
@@ -21152,7 +21162,8 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 7
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "s",
@@ -21205,7 +21216,8 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 7
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "s",
@@ -43102,7 +43114,8 @@
                         "loader": "uint",
                         "loader_args": {
                             "size": 4
-                        }
+                        },
+                        "internal": true
                     },
                     {
                         "name": "s",
diff --git a/schema.json b/schema.json
index 0fa4091..b3f73a8 100644
--- a/schema.json
+++ b/schema.json
@@ -623,6 +623,12 @@
                     "type": "object",
                     "title": "Arguments for loader function. Optional, no arguments in case of absence.",
                     "default": {}
+                  },
+                  "internal": {
+                    "type": "boolean",
+                    "title": "Internal flag",
+                    "description": "If true, this operand is used as a subslice length variable, should be used only for serialization/deserialization, humans and implementations do not need them.",
+                    "default": false
                   }
                 },
                 "required": [