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+# 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)
+    - in JavaScript with [json-schema-to-grammar.mjs](../tools/server/public_legacy/json-schema-to-grammar.mjs) (this is used by the [server](../tools/server)'s Web UI)
+
+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 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>