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GBNF Guide

GBNF (GGML BNF) is a format for defining formal grammars to constrain model outputs in llama.cpp. For example, you can use it to force the model to generate valid JSON, or speak only in emojis. GBNF grammars are supported in various ways in examples/main and examples/server.

Background

Bakus-Naur Form (BNF) is a notation for describing the syntax of formal languages like programming languages, file formats, and protocols. GBNF is an extension of BNF that primarily adds a few modern regex-like features.

Basics

In GBNF, we define production rules that specify how a non-terminal (rule name) can be replaced with sequences of terminals (characters, specifically Unicode code points) and other non-terminals. The basic format of a production rule is nonterminal ::= sequence....

Example

Before going deeper, let's look at some of the features demonstrated in grammars/chess.gbnf, a small chess notation grammar:

# `root` specifies the pattern for the overall output
root ::= (
    # it must start with the characters "1. " followed by a sequence
    # of characters that match the `move` rule, followed by a space, followed
    # by another move, and then a newline
    "1. " move " " move "\n"

    # it's followed by one or more subsequent moves, numbered with one or two digits
    ([1-9] [0-9]? ". " move " " move "\n")+
)

# `move` is an abstract representation, which can be a pawn, nonpawn, or castle.
# The `[+#]?` denotes the possibility of checking or mate signs after moves
move ::= (pawn | nonpawn | castle) [+#]?

pawn ::= ...
nonpawn ::= ...
castle ::= ...

Non-Terminals and Terminals

Non-terminal symbols (rule names) stand for a pattern of terminals and other non-terminals. They are required to be a dashed lowercase word, like move, castle, or check-mate.

Terminals are actual characters (code points). They can be specified as a sequence like "1" or "O-O" or as ranges like [1-9] or [NBKQR].

Characters and character ranges

Terminals support the full range of Unicode. Unicode characters can be specified directly in the grammar, for example hiragana ::= [ぁ-ゟ], or with escapes: 8-bit (\xXX), 16-bit (\uXXXX) or 32-bit (\UXXXXXXXX).

Character ranges can be negated with ^:

single-line ::= [^\n]+ "\n"`

Sequences and Alternatives

The order of symbols in a sequence matters. For example, in "1. " move " " move "\n", the "1. " must come before the first move, etc.

Alternatives, denoted by |, give different sequences that are acceptable. For example, in move ::= pawn | nonpawn | castle, move can be a pawn move, a nonpawn move, or a castle.

Parentheses () can be used to group sequences, which allows for embedding alternatives in a larger rule or applying repetition and optional symbols (below) to a sequence.

Repetition and Optional Symbols

  • * after a symbol or sequence means that it can be repeated zero or more times (equivalent to {0,}).
  • + denotes that the symbol or sequence should appear one or more times (equivalent to {1,}).
  • ? makes the preceding symbol or sequence optional (equivalent to {0,1}).
  • {m} repeats the precedent symbol or sequence exactly m times
  • {m,} repeats the precedent symbol or sequence at least m times
  • {m,n} repeats the precedent symbol or sequence at between m and n times (included)
  • {0,n} repeats the precedent symbol or sequence at most n times (included)

Comments and newlines

Comments can be specified with #:

# defines optional whitespace
ws ::= [ \t\n]+

Newlines are allowed between rules and between symbols or sequences nested inside parentheses. Additionally, a newline after an alternate marker | will continue the current rule, even outside of parentheses.

The root rule

In a full grammar, the root rule always defines the starting point of the grammar. In other words, it specifies what the entire output must match.

# a grammar for lists
root ::= ("- " item)+
item ::= [^\n]+ "\n"

Next steps

This guide provides a brief overview. Check out the GBNF files in this directory (grammars/) for examples of full grammars. You can try them out with:

./llama-cli -m <model> --grammar-file grammars/some-grammar.gbnf -p 'Some prompt'

llama.cpp can also convert JSON schemas to grammars either ahead of time or at each request, see below.

Troubleshooting

Grammars currently have performance gotchas (see ggerganov#4218).

Efficient optional repetitions

A common pattern is to allow repetitions of a pattern x up to N times.

While semantically correct, the syntax x? x? x?.... x? (with N repetitions) may result in extremely slow sampling. Instead, you can write x{0,N} (or (x (x (x ... (x)?...)?)?)? w/ N-deep nesting in earlier llama.cpp versions).

Using GBNF grammars

You can use GBNF grammars:

JSON Schemas → GBNF

llama.cpp supports converting a subset of https://json-schema.org/ to GBNF grammars:

  • In llama-server:
    • For any completion endpoints, passed as the json_schema body field
    • For the /chat/completions endpoint, passed inside the result_format body field (e.g. {"type", "json_object", "schema": {"items": {}}})
  • In llama-cli, passed as the --json / -j flag
  • To convert to a grammar ahead of time:

Take a look at tests to see which features are likely supported (you'll also find usage examples in ggerganov#5978, ggerganov#6659 & ggerganov#6555).

Here is also a non-exhaustive list of unsupported features: