Grammar

Line-oriented, one level of nesting, no lookahead. Leading whitespace is insignificant.

3.1 Sigils

Sigil	Means	Appears in
@	block header: @kind key=value key="quoted value"	starts every block
?	the prompt. Repeatable; lines join with a newline.	all
:	subtext / help, rendered smaller under the prompt	all
-	an option row	@choice @gallery @order
_	a field row	@input
%	a scale row	@scale
=	a prefill row: field=value	@example
>	prose line (markdown)	@note
#	comment. Parsed, never rendered, never sent.	anywhere
~~~	literal fence, opens and closes a verbatim body	@code @example

A line beginning with none of the above is treated as prose and appended to the current prompt. This is deliberate: an agent that forgets a sigil gets slightly-wrong rendering, never a crash.

3.2 Shape

doc      = block+ ;
block    = header , line* ;
header   = "@" , kind , { ws , attr } , NL ;
attr     = key , [ "=" , ( bareword | quoted ) ] ;      (* bare attr = true *)
option   = "-" , [ "!" ] , value , { "|" , cell } , NL ;
cell     = label | hint | ( key , "=" , value ) ;       (* img= swatch= font= *)
field    = "_" , name , [ "*" ] , "|" , type , { "|" , cell } , NL ;
scale    = "%" , name , "|" , leftPole , "|" , rightPole , [ "|" , default ] , NL ;
literal  = "~~~" , NL , { any } , "~~~" , NL ;

Pipes are the only column separator. Pad them for readability; the parser trims. A literal pipe inside a label is \|. Trailing key=value cells are unordered, so | img=… | Editorial | and | Editorial | img=… are equivalent.

3.3 Why ~~~ and not backticks

An ANVIL fence is itself delimited by ```. A backtick fence cannot nest inside a backtick fence without asking the model to count backticks correctly, which is exactly the sort of thing that breaks mid-stream.

~~~ is the in-block literal delimiter for that reason, and it is why @code is a necessary block rather than sugar: a code sample that belongs inside a block has no other way in, and content outside the fence is not part of the block — so it is not part of the frozen record either.