Sashiko is a small Ruby gem that adds a declarative span DSL on top of OpenTelemetry, plus helpers for keeping trace context attached as work crosses Thread, Fiber, queue, HTTP, and Ractor boundaries. Includes a span-replay mechanism for emitting spans from Ractor workers, optional Faraday and Anthropic adapters, RBS signatures, and a Ruby::Box-aware `tracer:` injection path for multi-tenant observability.