LLM Tool & Function-Calling Schema Builder

About the LLM Tool & Function-Calling Schema Builder

Design the tools an AI model can call — a name, a description that tells the model when to use it, and the parameters with their types, allowed values, required fields, nested objects and arrays — in a form or directly as JSON Schema. The same tool is exported in every format: OpenAI function tools (Responses and Chat Completions), Anthropic tool definitions with input_schema, Gemini function declarations and Model Context Protocol tools with inputSchema, plus TypeScript types, a Zod schema and a Pydantic model for your own code.

Paste a tool written for any of these APIs to convert it. Before a model sees the tool, the page lists what strict mode refuses — OpenAI’s Structured Outputs and Anthropic’s strict tool use each support only part of JSON Schema — and can make the schema strict-compatible for OpenAI in one click. Paste the arguments a model sent to see exactly why the call would be rejected. Everything runs offline in your browser.

Common uses

• Writing function-calling tools for an assistant or an agent without hand-writing JSON Schema.
• Moving tools between OpenAI, Anthropic, Gemini and an MCP server.
• Finding out why an API refuses a strict tool, or why a model’s arguments fail validation.
• Getting matching TypeScript, Zod or Pydantic types for the code that runs the tool.

The formats

• OpenAI Responses: { "type": "function", "name", "description", "parameters", "strict" }; Chat Completions puts the same fields under "function" (function calling). Names: letters, digits, underscores and dashes, up to 64 characters.
• Anthropic: { "name", "description", "input_schema" }, plus "strict": true for strict tool use (define tools); names up to 128 characters.
• Gemini: { "functionDeclarations": [ { "name", "description", "parameters" } ] }, where parameters is an OpenAPI 3.0 subset (types in capitals, nullable, no additionalProperties or $ref) — or parametersJsonSchema with plain JSON Schema (FunctionDeclaration).
• MCP: { "name", "title", "description", "inputSchema" } with optional behaviour annotations (MCP tools); names of 1 to 128 letters, digits, underscores, hyphens and dots.

Strict mode

With strict mode, the model’s arguments are guaranteed to match the schema — but only a subset of JSON Schema is allowed. OpenAI requires "additionalProperties": false on every object and every property in required (optional values become a type with null), and does not support allOf, not, if/then/else, dependentRequired or dependentSchemas; formats are limited to date-time, time, date, duration, email, hostname, ipv4, ipv6 and uuid, and fine-tuned models also refuse length, pattern, range and item limits (supported schemas). Anthropic’s strict tool use refuses recursive schemas, numeric and string-length limits, array limits other than minItems of 0 or 1, and some regular-expression features, and allows 24 optional and 16 union-typed parameters across a request’s strict tools (JSON Schema limitations).

Checking arguments

Paste the arguments object a model returned, the JSON string Chat Completions puts in arguments, or a whole tool call: the page validates it against the parameters (JSON Schema draft 2020-12) and lists every problem with its path — a missing required property, a value outside the allowed list, a property the schema does not allow. Use it to debug a tool before you trust a model’s output, and to test edge cases you put in a description.

Limitations

• The form edits the common part of JSON Schema (types, enums, formats, limits, defaults, nested objects and arrays); schemas with $ref, anyOf, oneOf or allOf are edited as JSON, and every output still supports them.
• Strict-mode checks follow the providers’ published lists; a provider may still refuse a schema that is very large or complex.
• Gemini’s OpenAPI subset cannot express everything JSON Schema can: the notes list what was dropped, and parametersJsonSchema keeps it all.
• The TypeScript, Zod and Pydantic code covers the same subset; review it before you depend on it.

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