diff --git a/docs/noise/vulcanus-cliffs-NOTES.md b/docs/noise/vulcanus-cliffs-NOTES.md
index 10783655..cd63c5a1 100644
--- a/docs/noise/vulcanus-cliffs-NOTES.md
+++ b/docs/noise/vulcanus-cliffs-NOTES.md
@@ -1715,3 +1715,45 @@ geyser's were in #100).
 this time on the mechanism's geometry rather than on the ceiling argument that
 died with the recall gap.** What remains unexplained: 25 surplus, 6 missing (all
 lava-rejection over-rejections), and the 33 wrong orientations.
+
+## The orientation residual is NOT a boundary tie - and the cliff channel still has no oracle (#84, 2026-08-02)
+
+`cliffOrientationResidual.spec.ts` pins the shape: every wrong cell differs in
+exactly ONE edge, always an OVER-detection (game finds no crossing, port finds
+one). That shape has an obvious cheap explanation, and it is worth writing down
+that it is **wrong**, because it eliminates a whole class of cause.
+
+`crossesCliff` decides on the SIGN of `elevation - boundary`. If an endpoint sat
+within float noise of a band boundary, the ~1e-6 our fields agree to would flip
+the crossing, the residual would be an irreducible precision limit, and there
+would be nothing to fix.
+
+**Measured: every crossing edge in a wrong cell sits at least 0.205 from its
+boundary**, median ~9.9 - four to seven orders of magnitude clear of float
+noise. For the game to disagree, its elevation there must differ from ours by
+more than 0.2. That is a real field or rule difference.
+
+Non-vacuity: the overall minimum across all 2,920 crossing edges is 6.4e-3,
+thirty times tighter, so "far from the boundary" is a property of the wrong
+cells rather than of the sample. `test/cliffOrientationMargin.spec.ts`.
+
+### The one input never checked corner-by-corner
+
+`oracle-vulcanus-cliff-corner-fields-entity-regions` holds the **tile** channel.
+That is stated in prose at the top of `vulcanusCliffCornerFields.spec.ts`, and
+is now asserted as a number: against our per-tile elevation the worst corner
+differs by **4.8e-2**; against the grid-4 cliff channel the cliff generator
+actually reads, by **96.09**.
+
+So **the grid-4 cliff-elevation channel has no per-corner oracle at all.** It is
+the only input to the placement rule never checked against the game corner by
+corner, and after the margin result it is also the only remaining candidate that
+could move an endpoint the required 0.2.
+
+**Capturing it is the next concrete step**, and it is not a plain
+`calculate_tile_properties` dump - that is the 1-tile program and is exactly what
+produced the wrong-channel fixture. It needs the cliff-channel value routed out
+of the 4-grid program, e.g. a mod that publishes
+`vulcanus_basalt_lakes_multisample` at grid 4 into a readable tile property.
+Until that exists, "the fields are exonerated" cannot be said of the channel that
+matters - and #93's exoneration rested on a substitution in the tile channel.
diff --git a/test/cliffOrientationMargin.spec.ts b/test/cliffOrientationMargin.spec.ts
new file mode 100644
index 00000000..cfb4d54f
--- /dev/null
+++ b/test/cliffOrientationMargin.spec.ts
@@ -0,0 +1,213 @@
+import { describe, expect, it } from "vite-plus/test";
+
+import corners from "./fixtures/oracle-vulcanus-cliff-corner-fields-entity-regions.seed123456.json";
+import entities from "./fixtures/oracle-vulcanus-cliff-entities.seed123456.json";
+import {
+  crossesCliff,
+  makeCliffPlacementFromFields,
+  smoothingKnots,
+} from "../src/noise/cliffs/cliffPlacement";
+import {
+  CLIFF_CELL_CENTER_X,
+  CLIFF_CELL_CENTER_Y,
+  CLIFF_CODE_TO_ORIENTATION,
+  CLIFF_GRID_SIZE,
+  CLIFF_ORIENTATION_NAMES,
+} from "../src/noise/cliffs/cliffCatalog";
+import {
+  VULCANUS_CLIFF_ELEVATION_0,
+  VULCANUS_CLIFF_ELEVATION_INTERVAL,
+  VULCANUS_CLIFF_SMOOTHING,
+  makeVulcanusCliffFields,
+} from "../src/noise/cliffs/vulcanusCliffFields";
+import { makeVulcanusStack } from "../src/noise/tiles/vulcanusCatalog";
+import { withCtxDefaults } from "../src/noise/eval/ctx";
+
+interface Ent {
+  x: number;
+  y: number;
+  name: string;
+  orientation?: string | null;
+}
+interface Case {
+  region: { x0: number; y0: number; x1: number; y1: number };
+  cliffs: Ent[];
+}
+
+const INPUT = { seed0: 123456, startingPositions: [{ x: 0, y: 0 }] };
+const ctx = withCtxDefaults(INPUT);
+const fields = makeVulcanusCliffFields(ctx);
+const E0 = VULCANUS_CLIFF_ELEVATION_0;
+const INTERVAL = VULCANUS_CLIFF_ELEVATION_INTERVAL;
+const S = VULCANUS_CLIFF_SMOOTHING;
+
+const rawE = new Map<string, number>();
+const rawElev = (i: number, j: number): number => {
+  const k = `${String(i)},${String(j)}`;
+  let v = rawE.get(k);
+  if (v === undefined) {
+    v = fields.cliffElevation(i * CLIFF_GRID_SIZE, j * CLIFF_GRID_SIZE);
+    rawE.set(k, v);
+  }
+  return v;
+};
+const elevAt = (i: number, j: number): number => {
+  const kx = smoothingKnots(i);
+  const ky = smoothingKnots(j);
+  const bil =
+    (1 - kx.t) * (1 - ky.t) * rawElev(kx.lo, ky.lo) +
+    kx.t * (1 - ky.t) * rawElev(kx.hi, ky.lo) +
+    (1 - kx.t) * ky.t * rawElev(kx.lo, ky.hi) +
+    kx.t * ky.t * rawElev(kx.hi, ky.hi);
+  return S === 1 ? bil : (1 - S) * rawElev(i, j) + S * bil;
+};
+interface Corner {
+  elev: number;
+  cliff: number;
+}
+const corner = (i: number, j: number): Corner => ({
+  elev: elevAt(i, j),
+  cliff: fields.cliffiness(i * CLIFF_GRID_SIZE, j * CLIFF_GRID_SIZE),
+});
+
+/** How far the two endpoints sit from the band boundary `crossesCliff` uses. */
+const margin = (a: number, b: number): number => {
+  const boundary = E0 + INTERVAL * Math.floor((Math.max(a, b) - E0) / INTERVAL);
+  return Math.min(Math.abs(a - boundary), Math.abs(b - boundary));
+};
+
+const edgesOf = (cx: number, cy: number): [Corner, Corner][] => {
+  const a = corner(cx, cy);
+  const b = corner(cx, cy + 1);
+  const d = corner(cx + 1, cy);
+  const f = corner(cx + 1, cy + 1);
+  return [
+    [a, b],
+    [d, f],
+    [a, d],
+    [b, f],
+  ];
+};
+
+const crossingMarginsIn = (cx: number, cy: number): number[] =>
+  edgesOf(cx, cy)
+    .filter(([u, v]) => crossesCliff(u.elev, v.elev, (u.cliff + v.cliff) / 2, E0, INTERVAL) !== 0)
+    .map(([u, v]) => margin(u.elev, v.elev));
+
+/**
+ * **The orientation residual is not a floating-point tie at the band boundary.**
+ *
+ * `test/cliffOrientationResidual.spec.ts` pins the residual's shape: every wrong
+ * cell differs from the game in exactly ONE edge, and it is always an
+ * OVER-detection - the game finds no crossing there and the port finds one.
+ *
+ * That shape has an obvious cheap explanation which turns out to be wrong, and
+ * ruling it out is worth a spec because it eliminates a whole class of cause.
+ * `crossesCliff` decides by the SIGN of `elevation - boundary` on each endpoint,
+ * so if an endpoint sat within float noise of a band boundary, a difference of
+ * 1e-6 between our field and the game's would flip the crossing - and the port's
+ * fields agree with the game's to about that order. Under that story the residual
+ * would be an irreducible precision limit and there would be nothing to fix.
+ *
+ * **It is not that.** Every crossing edge in a wrong cell sits at least **0.2**
+ * from its boundary, with a median near 10 - four to seven orders of magnitude
+ * clear of float noise. For the game to disagree, its elevation at that corner
+ * must differ from ours by more than 0.2, which is a real field difference, not
+ * a rounding one.
+ *
+ * So the residual is a genuine disagreement about a value or a rule, and it is
+ * worth continuing to hunt.
+ */
+describe("the orientation over-detections are not boundary ties", () => {
+  const wrongCellMargins: number[] = [];
+  const allCrossingMargins: number[] = [];
+
+  for (const c of entities.cases as unknown as Case[]) {
+    const r = c.region;
+    const byPos = new Map<string, string>();
+    for (const e of c.cliffs)
+      if (e.name === "cliff-vulcanus" && typeof e.orientation === "string")
+        byPos.set(`${String(e.x)},${String(e.y)}`, e.orientation);
+
+    for (const p of makeCliffPlacementFromFields(fields, {
+      elevation0: E0,
+      interval: INTERVAL,
+      smoothing: S,
+    }).placedCells(r.x0, r.y0, r.x1, r.y1)) {
+      const gameOrient = byPos.get(`${String(p.x)},${String(p.y)}`);
+      if (gameOrient === undefined) continue;
+      const cx = (p.x - CLIFF_CELL_CENTER_X) / CLIFF_GRID_SIZE;
+      const cy = (p.y - CLIFF_CELL_CENTER_Y) / CLIFF_GRID_SIZE;
+      const ms = crossingMarginsIn(cx, cy);
+      allCrossingMargins.push(...ms);
+      const id = CLIFF_CODE_TO_ORIENTATION[p.code];
+      const oursName = id === undefined ? "?" : CLIFF_ORIENTATION_NAMES[id];
+      if (oursName !== gameOrient) wrongCellMargins.push(...ms);
+    }
+  }
+
+  it("compares a real population, not a handful", () => {
+    // ~37 wrong cells carrying two crossing edges each, against every crossing
+    // edge of every matched cell.
+    expect(wrongCellMargins.length).toBeGreaterThan(50);
+    expect(allCrossingMargins.length).toBeGreaterThan(2000);
+  }, 120000);
+
+  it("puts every crossing edge in a wrong cell far from its band boundary", () => {
+    const min = Math.min(...wrongCellMargins);
+    // Measured 0.205. Asserted as a bound rather than the exact value so a field
+    // change that keeps the conclusion does not fail the spec spuriously.
+    expect(min).toBeGreaterThan(0.1);
+    // Four orders of magnitude clear of the ~1e-6 the fields agree to.
+    expect(min).toBeGreaterThan(1e-4 * 1000);
+  }, 120000);
+
+  /**
+   * Non-vacuity, and it matters here: the bound above would be unremarkable if
+   * NO edge anywhere sat near a boundary. Some do - the overall minimum is about
+   * 6e-3, thirty times tighter than the worst wrong cell - so "far from the
+   * boundary" is a property of the wrong cells rather than of the sample.
+   */
+  it("is a property of the wrong cells, not of every edge", () => {
+    expect(Math.min(...allCrossingMargins)).toBeLessThan(Math.min(...wrongCellMargins) / 10);
+  }, 120000);
+});
+
+/**
+ * **The corner fixture is the TILE channel, and this pins it so.**
+ *
+ * `test/vulcanusCliffCornerFields.spec.ts` says so in prose at the top, and its
+ * substitution deliberately feeds `vulcanus_elevation` into `cliffElevation` to
+ * preserve the history of how the wrong channel stayed hidden. Prose is not a
+ * guard, and this is the single most expensive mistake this subsystem has made
+ * (#83) - so the identification is asserted here as a number.
+ *
+ * The gap it leaves is the important part: **the grid-4 cliff-elevation channel
+ * has no per-corner oracle at all.** It is the one input to the placement rule
+ * that has never been checked against the game corner by corner, and after the
+ * measurement above it is also the only remaining candidate that could move an
+ * endpoint by the required 0.2. Capturing it is the next concrete step.
+ */
+describe("which elevation channel the corner fixture holds", () => {
+  it("matches the per-tile channel and NOT the grid-4 cliff channel", () => {
+    const stack = makeVulcanusStack(INPUT);
+    const cliffFields = makeVulcanusCliffFields(stack.ctx, stack);
+    const keys = corners.corners;
+    const elev = corners.elevation;
+
+    let maxVsTile = 0;
+    let maxVsCliff = 0;
+    for (let i = 0; i < keys.length; i++) {
+      const [is, js] = keys[i].split(",");
+      const x = Number(is) * CLIFF_GRID_SIZE;
+      const y = Number(js) * CLIFF_GRID_SIZE;
+      maxVsTile = Math.max(maxVsTile, Math.abs(stack.elevation.elevation(x, y) - elev[i]));
+      maxVsCliff = Math.max(maxVsCliff, Math.abs(cliffFields.cliffElevation(x, y) - elev[i]));
+    }
+
+    expect(keys.length).toBe(12675);
+    // Measured: 4.8e-2 against the tile channel, 96.09 against the cliff channel.
+    expect(maxVsTile).toBeLessThan(0.1);
+    expect(maxVsCliff).toBeGreaterThan(50);
+  }, 120000);
+});