Implementation:Ollama Ollama Llama Json Schema To Grammar

Knowledge Sources	Ollama
Domains	Grammar, Structured Output
Last Updated	2025-02-15 00:00 GMT

Overview

Converts JSON Schema definitions into GBNF (GGML BNF) grammar rules that constrain LLM token generation to produce valid JSON matching the schema.

Description

Implements common_schema_converter which recursively walks a JSON schema (using nlohmann::ordered_json), handling types (object, array, string, number, boolean, null), combinators (oneOf, anyOf, allOf), $ref resolution, enum/const constraints, pattern regexes, and additionalProperties. Uses BuiltinRule definitions for primitive types and a TrieNode-based optimization for string literal alternatives. Builds up named GBNF rules with repetition operators and produces a complete grammar string. The build_repetition helper generates quantified rule expressions.

Usage

Use this to enable structured output and function calling by constraining LLM generation to produce valid JSON conforming to a user-specified schema.

Code Reference

Source Location

• Repository: Ollama
• File: llama/llama.cpp/common/json-schema-to-grammar.cpp
• Lines: 1-1153

Signature

static std::string build_repetition(
    const std::string & item_rule,
    int min_items, int max_items,
    const std::string & separator_rule = "");

static void _build_min_max_int(
    int64_t min_value, int64_t max_value,
    std::stringstream & out, int decimals_left = 16,
    bool top_level = true);

class common_schema_converter {
    // Recursively converts JSON schema to GBNF grammar rules
    // Handles object, array, string, number, boolean, null types
    // Supports $ref, enum, const, oneOf, anyOf, allOf
};

Import

#include "json-schema-to-grammar.h"

I/O Contract

Inputs

Name	Type	Required	Description
schema	nlohmann::ordered_json	Yes	JSON Schema definition to convert
force_gbnf	bool	No	Force GBNF output format (default: false)

Outputs

Name	Type	Description
grammar	std::string	Complete GBNF grammar string that constrains generation to match the schema

Usage Examples

#include "json-schema-to-grammar.h"
#include <nlohmann/json.hpp>

using json = nlohmann::ordered_json;

// Define a JSON schema
json schema = {
    {"type", "object"},
    {"properties", {
        {"name", {{"type", "string"}}},
        {"age", {{"type", "integer"}, {"minimum", 0}}}
    }},
    {"required", {"name", "age"}}
};

// Convert to GBNF grammar
std::string grammar = json_schema_to_grammar(schema);
// Result: GBNF rules that constrain output to valid JSON matching the schema

Related Pages

• Principle:Ollama_Ollama_Structured_Output