Native json output support for agents

[rashadism]

Description

I've been playing around with fantasy recently and really loving it!

Is there currently a way to get structured output from agents similar to object.Generate[T] for models? I couldn't find one in the docs or code.

I got it working with a tool + HasToolCall stop condition, but couldn't figure out how to get json mode working with agents.

If it is not supported yet, I'd suggest something like follows:

Current interfaces

  type LanguageModel interface {
      Generate(context.Context, Call) (*Response, error)
      Stream(context.Context, Call) (StreamResponse, error)
      GenerateObject(context.Context, ObjectCall) (*ObjectResponse, error)
      StreamObject(context.Context, ObjectCall) (ObjectStreamResponse, error)
      Provider() string
      Model() string
  }

  type Agent interface {
      Generate(context.Context, AgentCall) (*AgentResult, error)
      Stream(context.Context, AgentStreamCall) (*AgentResult, error)
  }

Suggestion: Add ObjectGenerator interface

  type ObjectGenerator interface {
      GenerateObject(context.Context, ObjectCall) (*ObjectResponse, error)
      StreamObject(context.Context, ObjectCall) (ObjectStreamResponse, error)
  }

Have Agent implement this interface. The agent would resolve the mode from its underlying model's provider(Or should this be configurable for both agents and models?)

This would allow object.Generate[T] to work seamlessly with both agents and models

  // Works with model
  result, _ := object.Generate[Analysis](ctx, model, fantasy.ObjectCall{...})

  // Same API works with agent  
  result, _ := object.Generate[Analysis](ctx, agent, fantasy.ObjectCall{...})

Would love to hear on this, and will be happy to send a PR if this looks good and aligns with the project direction