tool

type Previewer interface {
	Preview(req message.ToolUseRequest) (variant, subject string)
}

type Result struct {
	// ModelOutput is the tool_result body returned to the model.
	ModelOutput string
	// Display is the text shown in the TUI history. When empty, the harness
	// falls back to ModelOutput. Set it only when the model-facing text
	// differs from what the user should see — e.g. a one-line preview of a
	// long body, or a colored diff.
	Display string
	// Variant labels the sub-action: the skill name, the editor command, the
	// LSP action, etc. May be empty when not meaningful.
	Variant string
	// Subject labels the target the action operates on: a file path, an
	// argument string, a shell command. May be empty.
	Subject string
	// IsError marks the invocation as failed. Surfaced to both the model
	// (is_error on the tool result) and the TUI. Tool errors are recoverable:
	// they never abort the agentic loop.
	IsError bool
	// Usage carries pre-costed usage (TotalCost already computed against
	// the subagent's own model schedule) for tools that spawn nested
	// subagent conversations, such as research_codebase and review_ticket.
	// It stays the zero value for ordinary tools that never spawn a
	// subagent, so the parent session has nothing extra to fold in.
	Usage usage.SessionUsage
}

type Tool interface {
	Definition() tooldef.Definition
	Execute(ctx context.Context, req message.ToolUseRequest) Result
}