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+# Plan: SX-native engine tests (browser-independent)
+
+## Goal
+
+Move the host's *interactive* test coverage from Playwright (`.spec.js`, drives a real
+Chromium) into **SX harness tests** that drive the hypermedia engine against a **mock
+platform** — no browser. Reserve Playwright for the one irreducible real-browser fact:
+"the WASM kernel actually compiles, boots, and loads modules content-addressed."
+
+**Why (the principle):** the SX engine (`web/engine.sx` + `web/orchestration.sx`) has no
+hard browser dependency — it talks to a *platform* (fetch, DOM ops, timers) that is
+injected. The harness supplies a mock platform, so engine behaviour (fetch → swap →
+DOM mutation) is asserted with zero browser. The same engine could therefore drive
+*something else* (a server-side DOM, a native UI) — the SX tests prove that
+independence by running without one. This is consistent with
+`[[project_zero_dependencies]]` and `[[feedback_runtime_control]]` (build IN the runtime).
+
+## Current state (2026-06-29)
+
+- **Already SX:** the 272 host conformance tests (`lib/host/tests/*.sx`, `spec/harness.sx`
+  mock-IO). The picker's *server contract* is SX too (`lib/host/tests/blog.sx`:
+  `picker form declaratively wired`, `load-more sentinel`, `no-sentinel-on-short-page`).
+- **Still Playwright (`.spec.js`):** `lib/host/playwright/relate-picker.spec.js` (7 tests)
+  and `spa-check.spec.js` (4) — real-browser checks of populate / filter / paging /
+  relate-delete / remove-button / boosted-nav / error-retry / WASM boot.
+
+## Infrastructure that already exists (the enabler — verified)
+
+- `spec/harness.sx` — `make-harness`, `default-platform` with **`:fetch` overridable**
+  (`(fn (url &rest opts) {:status 200 :body "" :ok true})`), plus DOM ops, `:now`, etc.
+- `web/harness-web.sx` — `(define-library (sx harness-web))` exports: `mock-element`,
+  `mock-set-attr!`, `mock-append-child!`, `mock-get-attr`, `mock-add-listener!`,
+  **`simulate-click` / `simulate-input` / `simulate-event`**, `assert-text`, `assert-attr`,
+  `assert-class`, `assert-no-class`, `assert-child-count`, `assert-event-fired`,
+  `make-web-harness`, render-audit helpers.
+- `web/tests/` — existing SX engine tests: `test-orchestration.sx` (17 deftests),
+  `test-forms.sx` (25), `test-swap-integration.sx` (43, mock-response → swap → assert),
+  `test-engine.sx`, `test-handlers.sx`. **`test-swap-integration.sx` is the reference
+  pattern** (it sets `_mock-body`/`_mock-headers`/`_mock-content-type`, drives a swap,
+  asserts the result).
+- Runner: `hosts/ocaml/bin/run_tests.ml` scans `spec/tests/`, `lib/tests/`, `web/tests/`
+  and loads `harness-web.sx` + `harness-reactive.sx`. Run via the `sx_test host="ocaml"`
+  MCP tool (or `./scripts/sx-build-all.sh`). JS runner: `hosts/javascript/run_tests.js`
+  also loads the web harnesses.
+
+## Phases
+
+### Phase 0 — Proof of concept (small): one behavior, SX
+Port **relate → delete row** to an SX harness test (new `web/tests/test-relate-picker.sx`):
+1. Build a mock DOM: a `.rp-results` `<ul>` containing one candidate `<li id="cand-related-x">`
+   with the relate `<form sx-post=/x/relate sx-target=#cand-related-x sx-swap=delete>`.
+2. `process-elements` (or `bind-triggers`) the tree so the form's submit is bound.
+3. Mock `:fetch` to return `{:status 200 :ok true :body ""}`.
+4. `simulate-click` the button (or `simulate-event` "submit" on the form).
+5. Assert the `<li>` is gone (`assert-child-count` results = 0).
+This validates the **mock-DOM → execute-request → swap-dom-nodes** loop in SX end to end.
+**If it reads cleanly, the rest is mechanical.**
+
+### Phase 1 — Port the picker's interactive behaviors (medium)
+Same file, more deftests, each = mock fetch + simulate + assert:
+- **filter narrows**: `:fetch` returns N candidate rows for `q=...`; `simulate-input` the
+  filter; assert child-count == N.
+- **sentinel paging**: `:fetch` returns rows + a `<li class=rp-more sx-trigger=revealed>`;
+  fire the revealed/intersect path; assert more rows appended, sentinel replaced.
+- **load populate**: `load` trigger → fetch → assert results filled.
+- **error/retry visible state**: `:fetch` rejects → assert `.sx-error` class added
+  (`assert-class`), then succeeds → assert cleared.
+
+### Phase 2 — Trim Playwright to a boot smoke (small)
+Keep ONLY what needs a real browser in `relate-picker.spec.js` / `spa-check.spec.js`:
+- WASM kernel compiles + boots (`data-sx-ready`).
+- modules load **content-addressed** (`/sx/h/` fetches, 0 path `.sxbc`).
+- one boosted nav swaps `#content`.
+Delete the per-behavior browser tests now covered by SX. Net: ~2 browser tests + an
+SX suite.
+
+### Phase 3 — Stretch: the engine drives a non-browser target ("something else")
+The mock platform already *is* a non-browser target. This phase makes it a real
+alternative renderer (e.g. server-side DOM string-builder, or a native UI shim) and runs
+the same picker through it — concretely proving platform-independence. Optional / future.
+
+## Gaps & risks to resolve during Phase 0
+
+- **Mock-DOM completeness:** `swap-dom-nodes` uses `morph-children`, `dom-replace-child`,
+  `dom-insert-after/before/prepend/append`, `dom-remove-child`, `dom-parent`,
+  `dom-first-child`, `dom-clone`, `dom-is-fragment?`. Confirm `harness-web`'s mock DOM
+  implements (or can be extended for) these. `test-swap-integration.sx` already swaps, so
+  most exist; check `delete`/`outerHTML`/fragment paths specifically.
+- **fetch callback shape:** the engine's `fetch-request` calls back
+  `(resp-ok status get-header text)`; the platform `:fetch` returns `{:status :body :ok}`.
+  Confirm/adapt the bridge (see how `test-swap-integration.sx` feeds `_mock-body` etc.).
+- **trigger binding without a browser:** `simulate-click` fires bound listeners — the form
+  must be processed first (`process-elements` on the mock root, or bind directly).
+- **component expansion:** `~relate-picker` need not be expanded for these tests — assert
+  on the *rendered* candidate rows / form markup directly (build the mock DOM from the
+  expanded HTML the server produces, which is already SX-testable server-side).
+
+## Tracked loose ends (separate from this plan)
+- **unrelate "clever" in-place delete** (just-the-row, no `#content` re-render): now that
+  `bind-boost-form` is fixed the remove button works via a boosted POST→swap; the
+  minimal-mutation version (sx-post + `sx-swap=delete` on the current-row) is a further
+  refinement — earlier attempt didn't fire, revisit with the binding now understood.
+- **`hs-repeat-times`** bytecode test (architecture worktree): harness `host-new` stub bug
+  masks a pre-existing `beingTold` resume-env bug. See the diagnosis in this session.
+
+## Done-when
+- `web/tests/test-relate-picker.sx` covers populate / filter / paging / relate-delete /
+  error-retry in SX, green under `sx_test host="ocaml"`.
+- Playwright trimmed to the boot smoke; suite still green.
+- (Stretch) the picker runs through a non-browser platform.