W‑1 Handshake Protocol: Minimal Reciprocity for Agency Detection

The W‑1 Handshake Protocol defines a minimal, substrate‑agnostic method for detecting intelligent agency using a one‑bit reciprocal challenge. Instead of relying on shared language, mathematics, or symbolic systems, W‑1 reduces first contact to the closure of a causal loop: an observer sends a binary probe, and agency is inferred if the response is coherent, non‑random, and conditionally dependent on the probe. The protocol formalises the physical and informational requirements for establishing intentional reciprocity across any medium capable of expressing two distinguishable states. This preprint includes the full protocol specification, detection criteria, failure modes, applications, and a mathematical appendix covering non‑randomness, conditionality, and causal timing. W‑1 reframes intelligence detection from message decoding to interaction design.