JSON Schema is a specification for describing JSON values that has become a critical part of
LLM infrastructure. We recently released github.com/google/jsonschema-go/jsonschema, a comprehensive JSON Schema package for Go. We use it in the official Go SDK for MCP and expect it to become the canonical JSON Schema package for Google’s Go SDKs that work with LLMs.
JSON Schema has been around for many years. Why are we doing this now, and what do LLMs have to do with it?
JSON is a flexible way to describe values. A JSON value can be null, a string, a number, a boolean, a list of values, or a mapping from strings to values. In programming language terms, JSON is dynamically typed. For example, a JSON array can contain a mix of strings, numbers, or any other JSON value. That flexibility can be quite powerful, but sometimes it’s useful to constrain it. Think of JSON Schema as a type system for JSON, although its expressiveness goes well beyond typical type systems. You can write a JSON schema that requires all array elements to be strings, as you could in a typical programming language type system, but you can also constrain the length of the array or insist that its first three elements are strings of length at least five while the remaining elements are numbers.
The ability to describe the shape of JSON values like that has always been useful, but it is vital when trying to coax JSON values out of LLMs, whose output is notoriously hard to constrain. JSON Schema provides an expressive and precise way to tell an LLM how its JSON output should look. That’s particularly useful for generating inputs to tools, which are usually ordinary functions with precise requirements on their input. It also turns out to be useful to describe a tool’s output to the LLM. So frameworks like MCP use JSON Schema to specify both the inputs to and outputs from tools. JSON Schema has become the lingua franca for defining structured interactions with LLMs.
Requirements for a JSON Schema package
Before writing our own package, we took a careful look at the existing JSON Schema packages; we didn’t want to reinvent the wheel. But we couldn’t find one that had all the features that we felt were important:
- Schema creation: A clear, easy-to-use Go API to build schemas in code.
- Serialization: A way to convert a schema to and from its JSON representation.
- Validation: A way to check whether a given JSON value conforms to a schema.
- Inference: A way to generate a JSON Schema from an existing Go type.