Function invoke_tool 

pub fn invoke_tool(tool_name: &str, argument: &str, ctx: &DispatchCtx) -> String

Invoke a tool with an argument. OSS default: resolves argument through ctx.let_bindings (literal if missing) + returns canonical "tool:<name>(<resolved_argument>)". Enterprise overrides hook the Fase 22 tool registry + per-provider dispatch (Anthropic / OpenAI / etc.). The D8 cross-cutting fix (33.y.k) extends pure_shape::run_pure_shape to plumb ChatRequest.tools so apply: <tool> on a Step activates real upstream tool-calling on the wire.