# 02 — Prefill/decode binary HTTP protocol

Status: ready-for-agent

## What to build

Split the activation protocol into explicit prefill and decode-step calls using the existing binary HTTP direction from ADR-0008. The completed slice should work against a stub backend so payload shape, route/session headers, relay preservation, and failure behavior are testable before real KV cache work begins.

## Acceptance criteria

- [ ] Prefill accepts chunked binary activations with route/session metadata and forwards them through the selected route.
- [ ] Decode-step accepts a one-step binary activation and forwards a one-step activation through the selected route.
- [ ] Decode-step payload size is independent of prompt length in protocol tests.
- [ ] Relay forwarding preserves route/session headers, shape, dtype, position, and wire version.
- [ ] Legacy `/forward` either remains as a compatibility wrapper or fails with a clear wire-version error.
- [ ] Tests cover prefill chunking, decode-step shape validation, relay preservation, and malformed header rejection.

## Blocked by

- 01 — Route Session lifecycle.
