support/docs for json mode

[jtoy]

does the gem support json mode as outlined here: https://ai.google.dev/gemini-api/docs/api-overview#curl_4

[weilandia]

The docs here work, but the confusing part is that json mode only works when using Vertex.

https://ai.google.dev/gemini-api/docs/api-overview#json

i.e.

client.generate_content(
  contents: messages,
  system_instructions: options[:system_message],
  generation_config: { response_mime_type: "application/json" }
)

[gbaptista]

@weilandia Thanks for testing and sharing! Yeah, I couldn't find examples/documentation to make it work outside of Vertex.

@jtoy Thanks for opening the issue! I added examples and documentation in the README:

• JSON Format Responses
  • JSON Schema

[weilandia]

Heads up: Using JSON schema in the generation config only works with Pro.

"While Gemini 1.5 Flash models only accept a text description of the JSON schema you want returned, the Gemini 1.5 Pro models let you pass a schema object (or a Python type equivalent), and the model output will strictly follow that schema. This is also known as controlled generation or constrained decoding."

[gbaptista]

@weilandia Here's my test results:

• ❌ Does not support JSON mode.
• 🟡 Supports JSON mode but not Schema.
• ✅ Supports JSON mode and Schema.
• 🔒 I don't have access to the model.

Model	Vertex AI	Generative Language
gemini-pro-vision	❌	🔒
gemini-pro	🟡	❌
gemini-1.5-pro-preview-0514	✅	🔒
gemini-1.5-pro-preview-0409	✅	🔒
gemini-1.5-pro	✅	❌
gemini-1.5-flash-preview-0514	🟡	🔒
gemini-1.5-flash	🟡	❌
gemini-1.0-pro-vision-latest	🔒	🔒
gemini-1.0-pro-vision-001	❌	🔒
gemini-1.0-pro-vision	❌	🔒
gemini-1.0-pro-latest	🔒	❌
gemini-1.0-pro-002	🟡	🔒
gemini-1.0-pro-001	❌	❌
gemini-1.0-pro	🟡	❌
gemini-ultra	🔒	🔒
gemini-1.0-ultra	🔒	🔒
gemini-1.0-ultra-001	🔒	🔒