JSON Contracts
Register JSON schemas and validate agent or LLM structured outputs so tool calls do not silently drift.
Overview
JSON Contracts is a MCP server for the Build phase that registers JSON schemas and validates structured LLM and agent outputs against them.
What is this MCP server?
- Local JSON contract registry via JSON_CONTRACTS_DIR (default ./json-contracts)
- Validator for structured LLM and agent outputs against registered contracts
- stdio npm transport (json-contracts@0.1.2) for Claude Code, Cursor, and other MCP clients
- Designed for repeatable parse-safe JSON instead of free-form model text
- Server version 0.1.2 via npm identifier json-contracts
- stdio transport; JSON_CONTRACTS_DIR defaults to ./json-contracts
Community signal: 1 GitHub stars.
What problem does it solve?
Agents return JSON-shaped text that often misses fields or invents keys, breaking your integrations with no single source of truth.
Who is it for?
Solo builders shipping agent features who want filesystem-backed JSON Schema contracts and MCP-side validation without a hosted platform.
Skip if: Teams that only need informal prompts with no structured I/O, or shops already standardizing on OpenAPI/Protobuf with separate codegen pipelines only.
What do I get? / Deliverables
After you register contracts and connect the MCP server, agents can validate outputs against your schemas before code paths trust them.
- Validated structured JSON aligned to your registered contracts
- A reusable local contract set agents can reference across sessions
- Fewer integration breaks from malformed agent payloads
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Journey fit
Canonical shelf is Build because contracts are wired into integrations and agent tooling as you ship features. Integrations is where structured tool I/O and schema enforcement matter most for solo builders connecting agents to APIs.
How it compares
MCP schema validator for agent JSON, not a general-purpose JSON linter CLI or hosted observability product.
Common Questions / FAQ
Who is JSON Contracts for?
Indie developers and small teams using MCP-enabled agents who need reliable structured JSON from LLM tool calls.
When should I use JSON Contracts?
When you define or consume fixed JSON shapes from agents and want validation before those payloads drive app logic or tests.
How do I add JSON Contracts to my agent?
Install the npm package json-contracts, set JSON_CONTRACTS_DIR to your contracts folder, and register the stdio MCP server in Claude Code, Cursor, or your client's config.