Feature

Function Calling & Tool Use

Structured tool definitions LLMs can invoke.

18 providers 249 APIs 17 declared variants

Capabilities are exposed as tool or function definitions an LLM can call with structured arguments. The mechanism behind most agentic integrations today.

18 API providers on the APIs.io network offer function calling & tool use. The highest-rated are NVIDIA NIM, Vapi, Anthropic, Exa, Claude.

Providers

Ranked by API Evangelist rating — Exemplar and Strong are expanded by default.

Exemplar 7 Complete, well-documented, and agent-ready
NVIDIA NIM NVIDIA NIM (NVIDIA Inference Microservices) is a catalog of GPU-accelerated, containerized AI inference mic... 11 APIs 75.7 Vapi Vapi is a San Francisco-based voice AI platform that lets developers build real-time, low-latency voice age... 15 APIs 75.7 Anthropic Anthropic is an AI safety company and creator of the Claude family of large language models (Opus, Sonnet, ... 24 APIs 75.0 Exa Exa is a web search API and AI research platform built specifically for LLMs and agents — semantic and keyw... 19 APIs 74.9 Claude Anthropic's Claude AI assistant API for natural language processing and conversation. 9 APIs 70.2 OpenAI APIs for accessing OpenAI's artificial intelligence models including GPT, DALL-E, Whisper, and Embeddings. 51 APIs 69.0 Mistral AI Mistral AI provides state-of-the-art large language models and AI APIs for developers and enterprises. 11 APIs 68.3
Strong 5 Solid coverage with minor gaps
OpenRouter OpenRouter is an API platform that provides unified access to multiple AI language models through a single ... 7 APIs 65.4 Hyperbolic Hyperbolic is an open-access AI cloud and decentralized GPU marketplace serving 200,000+ builders with affo... 6 APIs 65.0 Perplexity Perplexity AI is an answer engine that delivers accurate answers to complex questions using large language ... 12 APIs 64.3 Lakera Lakera is an AI-native security platform that protects GenAI applications, agents, and workforces from prom... 4 APIs 63.2 arcade-dev Arcade.dev (formerly Arcade AI) is the MCP runtime for production AI agents — a platform that lets agents s... 8 APIs 58.5
Developing 4 Usable, with meaningful gaps to close
Gemini Google's Gemini API provides access to state-of-the-art generative AI models for text generation, multimoda... 20 APIs 52.4 Lunar Energy Lunar Energy is a Mountain View, California-based residential clean-energy company founded in August 2020 b... 14 APIs 48.9 cohere Generates a text response to a user message and streams it down, token by token. 9 APIs 46.1 Docling Docling is an open-source toolkit for parsing diverse document formats — PDF, DOCX, PPTX, XLSX, HTML, image... 18 APIs 44.0
Thin 1 Limited public surface area
Agent Skills A collection of resources, APIs, and standards related to AI agent skills and capabilities. Agent skills re... 3 APIs 32.8
Minimal 1 Almost no public developer surface
Carnival Corporation Carnival Corporation & plc is the world's largest cruise company operating eight cruise line brands (Carniv... 8 APIs 6.9

What Providers Actually Declared

This feature is a canonical term. These are the free-text strings providers wrote in their own apis.yml that map onto it.

AI Agent Security — runtime protection for autonomous agents and tool useFunction CallingFunction calling and tool useFunction calling, JSON mode, tool use, and structured outputs across compatible LLMsMCP runtime engine for secure, multi-user AI agent tool callingMCP server (docling-mcp) exposes parsing as agent tools for Claude, Cursor, Gemini and other clientsMessages API with text, vision, tool use, extended thinking, streaming, structured outputsRemote MCP server exposes the Customer API as agent toolsStreaming, tool/function calling, structured JSON output, vision input on supported modelsStructured Outputs for JSON-schema-constrained extraction from callsStructured output (JSON schema)Structured outputs with web-grounded citationsTool UseTool use / function calling (model-dependent)Tool use and function callingTravel Agent ToolsVision, tool use, structured outputs, and function calling

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