Tool calling with closed-source models is seamless. You pass a list of functions to the API, the model calls them, you get structured JSON back.
Function calling | OpenAI API
Learn how function calling enables large language models to connect to external data and systems.
OpenAI Model Spec
The Model Spec specifies desired behavior for the models underlying OpenAI's products (including our APIs).
Introducing gpt-oss
We’re releasing gpt-oss-120b and gpt-oss-20b—two state-of-the-art open-weight language models that deliver strong real-world performance at low cost. Available under the flexible Apache 2.0 license, these models outperform similarly sized open models on reasoning tasks, demonstrate strong tool use capabilities, and are optimized for efficient deployment on consumer hardware.
Magnitude
The best way to code with open models. Magnitude has 9 repositories available. Follow their code on GitHub.
openbench: Provider-agnostic, open-source evaluation infrastructure for language models
Provider-agnostic, open-source evaluation infrastructure for language models
Ollama
Ollama is the easiest way to automate your work using open models, while keeping your data safe.
Open Responses
This is the standardization effort I've most wanted in the world of LLMs: a vendor-neutral specification for the JSON API that clients can use to talk to hosted LLMs. Open …
Google for Developers Blog - News about Web, Mobile, AI and Cloud
Google Cloud API Gateway now offers a model routing feature in Public Preview, allowing developers to dynamically route traffic to models like Gemini, Claude, or OpenAI OSS-GPT without hardcoding endpoints or managing open-source proxies. Developers can easily configure these routing rules directly within their OpenAPI 3.x specifications by mapping virtual model names to specific backend targets on a shared host. Once deployed, the Gateway acts as a serverless ingress layer that accepts standard OpenAI-compatible requests, automatically transcodes the payload to the native schema of the target model, and routes the traffic on the fly.
Open models by OpenAI
Advanced open-weight reasoning models to customize for any use case and run anywhere.
hey-api/openapi-ts
🌀 OpenAPI to TypeScript codegen. Production-ready SDKs, Zod schemas, TanStack Query hooks, and 20+ plugins. Used by Vercel, OpenCode, and PayPal.
Steve Krouse on Twitter / X
*gets up on soap box*With the announcement of this new "code mode" from Anthropic and Cloudflare, I've gotta rant about LLMs, MCP, and tool-calling for a second Let's all remember where this startedLLMs were bad at writing JSONSo OpenAI asked us to write good JSON schemas…— Steve Krouse (@stevekrouse) November 7, 2025
The control layer for AI
The industry spent two years teaching LLMs to speak JSON. That work mattered: free-form text was unusable in production. Today, every serious inference provider uses some implementation (often open-source) of structured output. Structured output has become essential infrastructure that the team at .txt is proud to have spearheaded. Even as the ecosystem evolves, and open-source alternatives emerged, our engine remains the state of the art.
The control layer for AI
The industry spent two years teaching LLMs to speak JSON. That work mattered: free-form text was unusable in production. Today, every serious inference provider uses some implementation (often open-source) of structured output. Structured output has become essential infrastructure that the team at .txt is proud to have spearheaded. Even as the ecosystem evolves, and open-source alternatives emerged, our engine remains the state of the art.