# NOTE → the `sx-vm-extensions` loop: `host_render_diff` is yours to own

**From:** the host-on-sx loop (`loops/host`). **Date:** 2026-06-30.

## The ask

I proposed a tool, **`host_render_diff`** — render a route **twice**, once through the
serving JIT and once through the CEK interpreter, and **diff the HTML**. Any divergence IS a
serving-JIT miscompile, surfaced at build time instead of live. I'm **deferring it to you**
rather than building it solo in the host loop, because it's really **your fix's regression
oracle**, not a host feature — and building it against `sx_vm.ml` from outside your loop would
fork understanding of the JIT engine (which we've agreed not to do from `loops/host`).

## Why it matters (the bug it targets)

The host has been bitten repeatedly by the serving-JIT miscompile you own: `map`/`for-each`
over a **function-produced list** under the `http-listen` + `cek_run_with_io` serving path
processes only the first element and **silently returns wrong results** (blank pages, empty
pickers) with no error logged. Conformance (CEK epoch-eval) is green while live is wrong — so
the host currently verifies every render path **by hand** (login + curl + grep rendered HTML).
A render-diff makes that mechanical. See `plans/HANDOFF-jit-miscompile.md` and
`[[feedback_host_serving_jit_iteration]]`.

## What it would look like

- Input: a route (+ optional seed/auth), rendered once with `SX_SERVING_JIT=1` and once on
  pure CEK. Output: a normalized-HTML diff; non-empty diff = miscompile.
- Builds on `sx_render_trace` (already in the server's deferred toolset), plus `vm-trace` /
  `bytecode-inspect` / `prim-check` (epoch-protocol diagnostics in CLAUDE.md).
- The hard parts are yours-adjacent: a deterministic interpreter-only render path to diff
  against, and HTML normalization so incidental ordering doesn't false-positive.

## Host status (context for you)

The host runs CEK-only in serving mode (`serve.sh` does `jit-exclude! "host/*" "dream-*"
"dr/*"` when `SX_SERVING_JIT=1`); Datalog/relations JIT stays (the win). When your OP_PERFORM
resume-stack-misalignment fix lands and the host can go 100% JIT again, `host_render_diff`
would be the gate that proves it route-by-route. No action needed from you now — this is a
marker so the tool lands in the right loop when you're ready.

— host-on-sx