# GBNF Guide

GBNF (GGML BNF) is a format for defining [formal grammars](https://en.wikipedia.org/wiki/Formal_grammar) 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 `tools/cli`, `tools/completion` and `tools/server`.

## Background

[Backus-Naur Form (BNF)](https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form) 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](https://en.wikipedia.org/wiki/Code_point)) 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](https://en.wikipedia.org/wiki/Code_point)). 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)

## Tokens

Tokens allow grammars to match specific tokenizer tokens rather than character sequences. This is useful for constraining outputs based on special tokens (like `<think>` or `</think>`).

Tokens can be specified in two ways:

1. **Token ID**: Use angle brackets with the token ID in square brackets: `<[token-id]>`. For example, `<[1000]>` matches the token with ID 1000.

2. **Token string**: Use angle brackets with the token text directly: `<token>`. For example, `<think>` will match the token whose text is exactly `<think>`. This only works if the string tokenizes to exactly one token in the vocabulary, otherwise the grammar will fail to parse.

You can negate token matches using the `!` prefix: `!<[1000]>` or `!<think>` matches any token *except* the specified one.

```
# Match a thinking block: <think>...</think>
# Using token strings (requires these to be single tokens in the vocab)
root ::= <think> thinking </think> .*
thinking ::= !</think>*

# Equivalent grammar using explicit token IDs
# Assumes token 1000 = <think>, token 1001 = </think>
root ::= <[1000]> thinking <[1001]> .*
thinking ::= !<[1001]>*
```

## 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 https://github.com/ggml-org/llama.cpp/issues/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:

- In [llama-server](../tools/server)'s completion endpoints, passed as the `grammar` body field
- In [llama-cli](../tools/cli) and [llama-completion](../tools/completion), passed as the `--grammar` & `--grammar-file` flags
- With [test-gbnf-validator](../tests/test-gbnf-validator.cpp), to test them against strings.

## JSON Schemas → GBNF

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

- In [llama-server](../tools/server):
    - For any completion endpoints, passed as the `json_schema` body field
    - For the `/chat/completions` endpoint, passed inside the `response_format` body field (e.g. `{"type", "json_object", "schema": {"items": {}}}` or `{ type: "json_schema", json_schema: {"schema": ...} }`)
- In [llama-cli](../tools/cli) and [llama-completion](../tools/completion), passed as the `--json` / `-j` flag
- To convert to a grammar ahead of time:
    - in CLI, with [examples/json_schema_to_grammar.py](../examples/json_schema_to_grammar.py)

> [!NOTE]
> The JSON schema is only used to constrain the model output and is not injected into the prompt. The model has no visibility into the schema, so if you want it to understand the expected structure, describe it explicitly in your prompt. This does not apply to tool calling, where schemas are injected into the prompt.

Take a look at [tests](../tests/test-json-schema-to-grammar.cpp) to see which features are likely supported (you'll also find usage examples in https://github.com/ggml-org/llama.cpp/pull/5978, https://github.com/ggml-org/llama.cpp/pull/6659 & https://github.com/ggml-org/llama.cpp/pull/6555).

```bash
llama-cli \
  -hfr bartowski/Phi-3-medium-128k-instruct-GGUF \
  -hff Phi-3-medium-128k-instruct-Q8_0.gguf \
  -j '{
    "type": "array",
    "items": {
        "type": "object",
        "properties": {
            "name": {
                "type": "string",
                "minLength": 1,
                "maxLength": 100
            },
            "age": {
                "type": "integer",
                "minimum": 0,
                "maximum": 150
            }
        },
        "required": ["name", "age"],
        "additionalProperties": false
    },
    "minItems": 10,
    "maxItems": 100
  }' \
  -p 'Generate a {name, age}[] JSON array with famous actors of all ages.'
```

<details>

<summary>Show grammar</summary>

You can convert any schema in command-line with:

```bash
examples/json_schema_to_grammar.py name-age-schema.json
```

```
char ::= [^"\\\x7F\x00-\x1F] | [\\] (["\\bfnrt] | "u" [0-9a-fA-F]{4})
item ::= "{" space item-name-kv "," space item-age-kv "}" space
item-age ::= ([0-9] | ([1-8] [0-9] | [9] [0-9]) | "1" ([0-4] [0-9] | [5] "0")) space
item-age-kv ::= "\"age\"" space ":" space item-age
item-name ::= "\"" char{1,100} "\"" space
item-name-kv ::= "\"name\"" space ":" space item-name
root ::= "[" space item ("," space item){9,99} "]" space
space ::= | " " | "\n" [ \t]{0,20}
```

</details>

Here is also a list of known limitations (contributions welcome):

- `additionalProperties` defaults to `false` (produces faster grammars + reduces hallucinations).
- `"additionalProperties": true` may produce keys that contain unescaped newlines.
- Unsupported features are skipped silently. It is currently advised to use the command-line Python converter (see above) to see any warnings, and to inspect the resulting grammar / test it w/ [llama-gbnf-validator](../examples/gbnf-validator/gbnf-validator.cpp).
- Can't mix `properties` w/ `anyOf` / `oneOf` in the same type (https://github.com/ggml-org/llama.cpp/issues/7703)
- [prefixItems](https://json-schema.org/draft/2020-12/json-schema-core#name-prefixitems) is broken (but [items](https://json-schema.org/draft/2020-12/json-schema-core#name-items) works)
- `minimum`, `exclusiveMinimum`, `maximum`, `exclusiveMaximum`: only supported for `"type": "integer"` for now, not `number`
- Nested `$ref`s are broken (https://github.com/ggml-org/llama.cpp/issues/8073)
- [pattern](https://json-schema.org/draft/2020-12/json-schema-validation#name-pattern)s must start with `^` and end with `$`
- Remote `$ref`s not supported in the C++ version (Python & JavaScript versions fetch https refs)
- `string` [formats](https://json-schema.org/draft/2020-12/json-schema-validation#name-defined-formats) lack `uri`, `email`
- No [`patternProperties`](https://json-schema.org/draft/2020-12/json-schema-core#name-patternproperties)

And a non-exhaustive list of other unsupported features that are unlikely to be implemented (hard and/or too slow to support w/ stateless grammars):

- [`uniqueItems`](https://json-schema.org/draft/2020-12/json-schema-validation#name-uniqueitems)
- [`contains`](https://json-schema.org/draft/2020-12/json-schema-core#name-contains) / `minContains`
- `$anchor` (cf. [dereferencing](https://json-schema.org/draft/2020-12/json-schema-core#name-dereferencing))
- [`not`](https://json-schema.org/draft/2020-12/json-schema-core#name-not)
- [Conditionals](https://json-schema.org/draft/2020-12/json-schema-core#name-keywords-for-applying-subsche) `if` / `then` / `else` / `dependentSchemas`

### A word about additionalProperties

> [!WARNING]
> The JSON schemas spec states `object`s accept [additional properties](https://json-schema.org/understanding-json-schema/reference/object#additionalproperties) by default.
> Since this is slow and seems prone to hallucinations, we default to no additional properties.
> You can set `"additionalProperties": true` in the schema of any object to explicitly allow additional properties.

If you're using [Pydantic](https://pydantic.dev/) to generate schemas, you can enable additional properties with the `extra` config on each model class:

```python
# pip install pydantic
import json
from typing import Annotated, List
from pydantic import BaseModel, Extra, Field
class QAPair(BaseModel):
    class Config:
        extra = 'allow'  # triggers additionalProperties: true in the JSON schema
    question: str
    concise_answer: str
    justification: str

class Summary(BaseModel):
    class Config:
        extra = 'allow'
    key_facts: List[Annotated[str, Field(pattern='- .{5,}')]]
    question_answers: List[Annotated[List[QAPair], Field(min_items=5)]]

print(json.dumps(Summary.model_json_schema(), indent=2))
```

<details>
<summary>Show JSON schema & grammar</summary>

```json
{
  "$defs": {
    "QAPair": {
      "additionalProperties": true,
      "properties": {
        "question": {
          "title": "Question",
          "type": "string"
        },
        "concise_answer": {
          "title": "Concise Answer",
          "type": "string"
        },
        "justification": {
          "title": "Justification",
          "type": "string"
        }
      },
      "required": [
        "question",
        "concise_answer",
        "justification"
      ],
      "title": "QAPair",
      "type": "object"
    }
  },
  "additionalProperties": true,
  "properties": {
    "key_facts": {
      "items": {
        "pattern": "^- .{5,}$",
        "type": "string"
      },
      "title": "Key Facts",
      "type": "array"
    },
    "question_answers": {
      "items": {
        "items": {
          "$ref": "#/$defs/QAPair"
        },
        "minItems": 5,
        "type": "array"
      },
      "title": "Question Answers",
      "type": "array"
    }
  },
  "required": [
    "key_facts",
    "question_answers"
  ],
  "title": "Summary",
  "type": "object"
}
```

```
QAPair ::= "{" space QAPair-question-kv "," space QAPair-concise-answer-kv "," space QAPair-justification-kv ( "," space ( QAPair-additional-kv ( "," space QAPair-additional-kv )* ) )? "}" space
QAPair-additional-k ::= ["] ( [c] ([o] ([n] ([c] ([i] ([s] ([e] ([_] ([a] ([n] ([s] ([w] ([e] ([r] char+ | [^"r] char*) | [^"e] char*) | [^"w] char*) | [^"s] char*) | [^"n] char*) | [^"a] char*) | [^"_] char*) | [^"e] char*) | [^"s] char*) | [^"i] char*) | [^"c] char*) | [^"n] char*) | [^"o] char*) | [j] ([u] ([s] ([t] ([i] ([f] ([i] ([c] ([a] ([t] ([i] ([o] ([n] char+ | [^"n] char*) | [^"o] char*) | [^"i] char*) | [^"t] char*) | [^"a] char*) | [^"c] char*) | [^"i] char*) | [^"f] char*) | [^"i] char*) | [^"t] char*) | [^"s] char*) | [^"u] char*) | [q] ([u] ([e] ([s] ([t] ([i] ([o] ([n] char+ | [^"n] char*) | [^"o] char*) | [^"i] char*) | [^"t] char*) | [^"s] char*) | [^"e] char*) | [^"u] char*) | [^"cjq] char* )? ["] space
QAPair-additional-kv ::= QAPair-additional-k ":" space value
QAPair-concise-answer-kv ::= "\"concise_answer\"" space ":" space string
QAPair-justification-kv ::= "\"justification\"" space ":" space string
QAPair-question-kv ::= "\"question\"" space ":" space string
additional-k ::= ["] ( [k] ([e] ([y] ([_] ([f] ([a] ([c] ([t] ([s] char+ | [^"s] char*) | [^"t] char*) | [^"c] char*) | [^"a] char*) | [^"f] char*) | [^"_] char*) | [^"y] char*) | [^"e] char*) | [q] ([u] ([e] ([s] ([t] ([i] ([o] ([n] ([_] ([a] ([n] ([s] ([w] ([e] ([r] ([s] char+ | [^"s] char*) | [^"r] char*) | [^"e] char*) | [^"w] char*) | [^"s] char*) | [^"n] char*) | [^"a] char*) | [^"_] char*) | [^"n] char*) | [^"o] char*) | [^"i] char*) | [^"t] char*) | [^"s] char*) | [^"e] char*) | [^"u] char*) | [^"kq] char* )? ["] space
additional-kv ::= additional-k ":" space value
array ::= "[" space ( value ("," space value)* )? "]" space
boolean ::= ("true" | "false") space
char ::= [^"\\\x7F\x00-\x1F] | [\\] (["\\bfnrt] | "u" [0-9a-fA-F]{4})
decimal-part ::= [0-9]{1,16}
dot ::= [^\x0A\x0D]
integral-part ::= [0] | [1-9] [0-9]{0,15}
key-facts ::= "[" space (key-facts-item ("," space key-facts-item)*)? "]" space
key-facts-item ::= "\"" "- " key-facts-item-1{5,} "\"" space
key-facts-item-1 ::= dot
key-facts-kv ::= "\"key_facts\"" space ":" space key-facts
null ::= "null" space
number ::= ("-"? integral-part) ("." decimal-part)? ([eE] [-+]? integral-part)? space
object ::= "{" space ( string ":" space value ("," space string ":" space value)* )? "}" space
question-answers ::= "[" space (question-answers-item ("," space question-answers-item)*)? "]" space
question-answers-item ::= "[" space question-answers-item-item ("," space question-answers-item-item){4,} "]" space
question-answers-item-item ::= QAPair
question-answers-kv ::= "\"question_answers\"" space ":" space question-answers
root ::= "{" space key-facts-kv "," space question-answers-kv ( "," space ( additional-kv ( "," space additional-kv )* ) )? "}" space
space ::= | " " | "\n" [ \t]{0,20}
string ::= "\"" char* "\"" space
value ::= object | array | string | number | boolean | null
```

</details>

If you're using [Zod](https://zod.dev/), you can make your objects to explicitly allow extra properties w/ `nonstrict()` / `passthrough()` (or explicitly no extra props w/ `z.object(...).strict()` or `z.strictObject(...)`) but note that [zod-to-json-schema](https://github.com/StefanTerdell/zod-to-json-schema) currently always sets `"additionalProperties": false` anyway.

```js
import { z } from 'zod';
import { zodToJsonSchema } from 'zod-to-json-schema';

const Foo = z.object({
  age: z.number().positive(),
  email: z.string().email(),
}).strict();

console.log(zodToJsonSchema(Foo));
```

<details>
<summary>Show JSON schema & grammar</summary>

```json
{
  "type": "object",
  "properties": {
    "age": {
      "type": "number",
      "exclusiveMinimum": 0
    },
    "email": {
      "type": "string",
      "format": "email"
    }
  },
  "required": [
    "age",
    "email"
  ],
  "additionalProperties": false,
  "$schema": "http://json-schema.org/draft-07/schema#"
}
```

```
age-kv ::= "\"age\"" space ":" space number
char ::= [^"\\\x7F\x00-\x1F] | [\\] (["\\bfnrt] | "u" [0-9a-fA-F]{4})
decimal-part ::= [0-9]{1,16}
email-kv ::= "\"email\"" space ":" space string
integral-part ::= [0] | [1-9] [0-9]{0,15}
number ::= ("-"? integral-part) ("." decimal-part)? ([eE] [-+]? integral-part)? space
root ::= "{" space age-kv "," space email-kv "}" space
space ::= | " " | "\n" [ \t]{0,20}
string ::= "\"" char* "\"" space
```

</details>
