type FrontMatter struct {
    Name        string      `yaml:"name"`
    Description string      `yaml:"description"`
    Context     ContextMode `yaml:"context"`
    Agent       string      `yaml:"agent"`
    Model       string      `yaml:"model"`
}

const (
    ContextModeFork            ContextMode = "fork"              // Isolated context
    ContextModeForkWithContext ContextMode = "fork_with_context" // Copy history messages
)

type Skill struct {
    FrontMatter
    Content       string
    BaseDirectory string
}

type Backend interface {
    List(ctx context.Context) ([]FrontMatter, error)
    Get(ctx context.Context, name string) (Skill, error)
}

type BackendFromFilesystemConfig struct {
    Backend filesystem.Backend
    BaseDir string
}

func NewBackendFromFilesystem(ctx context.Context, config *BackendFromFilesystemConfig) (Backend, error)

// AgentHubOptions passed to AgentHub.Get
type AgentHubOptions = TypedAgentHubOptions[*schema.Message]

type TypedAgentHubOptions[M adk.MessageType] struct {
    // Model is the model instance specified in the skill frontmatter (resolved via ModelHub).
    // nil means the skill did not specify a model override; implementations should use the default model.
    Model model.BaseModel[M]
}

// AgentHub provides Agent instances for Context mode
type AgentHub = TypedAgentHub[*schema.Message]

type TypedAgentHub[M adk.MessageType] interface {
    // Get returns an Agent by name. When name is empty, should return the default Agent.
    Get(ctx context.Context, name string, opts *TypedAgentHubOptions[M]) (adk.TypedAgent[M], error)
}

// ModelHub resolves model instances by name
type ModelHub = TypedModelHub[*schema.Message]

type TypedModelHub[M adk.MessageType] interface {
    Get(ctx context.Context, name string) (model.BaseModel[M], error)
}

type SubAgentInput = TypedSubAgentInput[*schema.Message]

type TypedSubAgentInput[M adk.MessageType] struct {
    Skill        Skill
    Mode         ContextMode
    RawArguments string   // Raw JSON arguments
    SkillContent string   // Constructed Skill content
    History      []M      // Conversation history (only in fork_with_context mode)
    ToolCallID   string   // Tool call ID (only in fork_with_context mode)
}

type SubAgentOutput = TypedSubAgentOutput[*schema.Message]

type TypedSubAgentOutput[M adk.MessageType] struct {
    Skill        Skill
    Mode         ContextMode
    RawArguments string
    Messages     []M      // All messages produced by the sub-Agent
    Results      []string // Extracted assistant message text content
}

type Config = TypedConfig[*schema.Message]

type TypedConfig[M adk.MessageType] struct {
    Backend           Backend
    SkillToolName     *string
    AgentHub          TypedAgentHub[M]
    ModelHub          TypedModelHub[M]

    CustomSystemPrompt    SystemPromptFunc
    CustomToolDescription ToolDescriptionFunc
    CustomToolParams      func(ctx context.Context, defaults map[string]*schema.ParameterInfo) (map[string]*schema.ParameterInfo, error)
    BuildContent          func(ctx context.Context, skill Skill, rawArgs string) (string, error)
    BuildForkMessages     func(ctx context.Context, in TypedSubAgentInput[M]) ([]M, error)
    FormatForkResult      func(ctx context.Context, in TypedSubAgentOutput[M]) (string, error)
}

func NewMiddleware(ctx context.Context, config *Config) (adk.ChatModelAgentMiddleware, error)

// 1. Create Backend
backend, err := skill.NewBackendFromFilesystem(ctx, &skill.BackendFromFilesystemConfig{
    Backend: fsBackend,
    BaseDir: "/path/to/skills",
})
if err != nil {
    return err
}

// 2. Create Middleware
handler, err := skill.NewMiddleware(ctx, &skill.Config{
    Backend:  backend,
    AgentHub: myAgentHub, // Optional, only needed for fork mode
    ModelHub: myModelHub, // Optional, only needed when using model field
})
if err != nil {
    return err
}

// 3. Pass to Agent's Handlers
agent, err := adk.NewChatModelAgent(ctx, &adk.ChatModelAgentConfig{
    // ... other config
    Handlers: []adk.ChatModelAgentMiddleware{handler},
})