Which LLM API providers support function calling and strict JSON mode for automation workflows?

Several major LLM API providers support function calling and some form of strict JSON / structured output for automation workflows. Here’s a practical list:

Providers with strong support

1) OpenAI

• Function calling / tool calling: Yes
• Strict JSON / structured outputs: Yes
• Best known for reliable structured outputs and agent/tool workflows.

2) Anthropic

• Function calling / tool use: Yes
• Strict JSON / structured outputs: Supported via tool use and prompting patterns; exact “strict JSON mode” is less native than OpenAI’s structured outputs, but usable for automation.

3) Google Gemini API

• Function calling: Yes
• JSON output / schema-based output: Yes, supports structured responses and tool use.
• Good option for workflows needing integration with Google ecosystem.

4) Mistral API

• Function calling / tool calling: Yes
• JSON mode / structured outputs: Supported in newer API features and model variants.
• Useful if you want a lightweight alternative.

5) Cohere

• Tool use / function calling: Yes
• Structured output: Available in their API offerings, depending on model and endpoint.

6) xAI Grok API

• Function calling/tool use: Available in supported API versions.
• Structured output: Limited compared with OpenAI/Gemini, but improving.

Also commonly used through compatible APIs

Some providers expose OpenAI-compatible endpoints, which often make it easier to use function calling patterns:

• Together AI
• Fireworks AI
• Groq
• DeepSeek
• OpenRouter (as a router/aggregator rather than a model provider)

Support for “strict JSON” varies by model and endpoint, so you need to verify per model.

What to look for

For automation workflows, the key features are:

• Tool/function calling
• Schema-constrained output
• JSON mode / response format
• Deterministic retries / validation
• Streaming support if you need low latency

Best practical choices

If you want the most dependable setup today:

1. OpenAI — strongest structured-output tooling
2. Google Gemini — good structured output + function calling
3. Anthropic — excellent tool use, though structured JSON is usually less “strict” natively
4. Mistral — solid if you want open-ish deployment options

If you want, I can also give you a comparison table with:

• provider
• function calling support
• strict JSON support
• schema enforcement
• open-source/open-weight options
• pricing/latency notes