AI Function Calling: 10 APIs Side-by-Side

Function calling / tool use comparison. Schema support, reliability, error handling.

The problem

Function calling is the foundation of AI agents. We tested 10 providers with 30+ tool definitions and 200 test queries.

Schema support

JSON Schema, OpenAPI 3, custom formats. Per-provider: what they accept.

Reliability

How often does the model call the right function with the right arguments?

Error handling

What happens when a function returns an error? How does the model recover?

Code

Per-provider function calling client in clients/. Includes a unified abstraction layer.

Read more

Multi-model gateway: apex.apiguider.com. API cost: codecost.apiguider.com.

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Author: apiguider.com editorial team (see apex.apiguider.com/about/)

Topics: ai-api, function-calling, tool-use, agents