Struct ToolSpec 

pub struct ToolSpec {
    pub name: &'static str,
    pub description: &'static str,
    pub input_schema: fn() -> Value,
    pub data_schema: DataSchema,
    pub mutates: bool,
    pub facet: Facet,
    pub handler: fn(&Ctx, &Value) -> Result<ToolOutcome, EngineFault>,
}

One tool, shared verbatim by the CLI and the MCP server (design §2.2).

Fields

name: &'static str

Wire name, e.g. "render". Also the CLI subcommand stem.

description: &'static str

Human/agent-facing description. This is ALSO the embedded agent prompt — it carries behavioral guidance, not just a label — and is rendered into MCP tools/list and scrybe <cmd> --help verbatim.

input_schema: fn() -> Value

JSON Schema for arguments (MCP inputSchema; also drives CLI arg parse).

data_schema: DataSchema

Versioned, typed schema for the tool’s stable data payload.

mutates: bool

Does this tool change editor/disk/app state? Gates read-only agents and dry-run mode.

facet: Facet

Tool group for progressive disclosure + feature gating.

handler: fn(&Ctx, &Value) -> Result<ToolOutcome, EngineFault>

The one implementation, shared by both front ends. Ok(outcome) even for business failures (a tool_error inside the outcome); Err is reserved for EngineFaults — e.g. the transport failing mid-request.