SchemaFit

SchemaFit is an MIT-licensed, local-first command-line interface (CLI) that acts as a continuous integration (CI) linter for structured-output schemas. It rigorously checks JSON Schemas, tool definitions, and response format specifications against provider-specific constraints before deployment, preventing runtime errors. Key capabilities include identifying unsupported keywords, resolving nesting inconsistencies, flagging required/optional mismatches, and highlighting portability issues across major platforms such as OpenAI, Anthropic, Gemini, Mistral, and Cohere. By encoding each provider's documented constraint surface into versioned rule packs, SchemaFit offers precise, static analysis without requiring API keys or model calls. It pinpoints exact JSON-Pointer paths and keywords for each violation, ensuring developers can fix issues quickly. This tool enables teams to catch schema validation failures during pull requests (PRs) rather than encountering 400 errors in production. A 'repair' function can even generate a best-effort provider-valid variant by stripping or rewriting offending keywords while preserving the original intent. With zero runtime dependencies and static offline operation, SchemaFit is a robust addition to any development pipeline. SchemaFit is ideal for engineering teams and developers building applications that rely on consistent structured output across diverse service providers. It ensures schema integrity, accelerates development cycles by preventing common deployment roadblocks, and maintains inter-provider compatibility.