diff --git a/src/globe/testing/contractSuite.mjs b/src/globe/testing/contractSuite.mjs
index f0e0488b8..2a2f705fc 100644
--- a/src/globe/testing/contractSuite.mjs
+++ b/src/globe/testing/contractSuite.mjs
@@ -1417,6 +1417,78 @@ export function createContractSuite(
         );
       },
     ],
+    [
+      'picker.pickPosition and projector.worldToWindow round-trip within 1.5 px',
+      async () => {
+        if (!engine.picker.pickPositionSupported) return;
+        // Straight down and tilted; screen points with no surface are skipped.
+        // Over New York, low enough for map detail.
+        const views = [
+          { height: 20000, heading: 0, pitch: -math.PI_OVER_TWO },
+          { height: 5000, heading: 0.7, pitch: -1.0 },
+        ];
+        let hits = 0;
+        const misses = [];
+        for (const { height: viewHeight, heading, pitch } of views) {
+          camera.setView({
+            destination: cartesian3.fromDegrees(-73.78, 40.64, viewHeight),
+            orientation: { heading, pitch, roll: 0 },
+          });
+          // Picks read the depth of the last drawn frame: draw this view.
+          await settle(3);
+          const { width, height } = engine.host.size();
+          const screens = [
+            [0.5, 0.5],
+            [0.3, 0.4],
+            [0.7, 0.6],
+            [0.5, 0.75],
+          ].map(([x, y]) =>
+            cartesian2.create(Math.round(width * x), Math.round(height * y)),
+          );
+          // The depth a pick reads needs drawn frames.
+          await until(
+            () => engine.picker.pickPosition(screens[0]) !== undefined,
+            'a surface under the centre for pickPosition',
+          );
+          for (const screen of screens) {
+            const position = engine.picker.pickPosition(screen);
+            if (position === undefined) continue;
+            const back = engine.projector.worldToWindow(position);
+            const error = back
+              ? Math.hypot(back.x - screen.x, back.y - screen.y)
+              : Infinity;
+            hits += 1;
+            if (!(error <= 1.5))
+              misses.push(
+                `(${screen.x}, ${screen.y}) → ${back ? `(${back.x.toFixed(2)}, ${back.y.toFixed(2)})` : 'undefined'}`,
+              );
+          }
+        }
+        ensure(hits > 0, 'pickPosition hit no surface');
+        ensure(
+          misses.length === 0,
+          `round-trip off by more than 1.5 px: ${misses.join('; ')}`,
+        );
+      },
+    ],
+    [
+      'maps.setTerrain: keyless terrain resolves; an unknown kind rejects',
+      async () => {
+        await engine.maps.setTerrain({ id: 'keyless', kind: 'keyless' });
+        let rejected = null;
+        try {
+          await engine.maps.setTerrain({
+            id: 'no-such-terrain',
+            kind: 'no-such-terrain',
+          });
+        } catch (error) {
+          rejected = error;
+        }
+        ensure(rejected !== null, 'an unknown terrain kind resolved');
+        // The keyless terrain still applies after the rejection.
+        await engine.maps.setTerrain({ id: 'keyless', kind: 'keyless' });
+      },
+    ],
     [
       'camera.commanded: fires for each camera command, never for reads',
       async () => {
diff --git a/src/globe/testing/fakeEngine.js b/src/globe/testing/fakeEngine.js
index a93d827e2..3ede75e44 100644
--- a/src/globe/testing/fakeEngine.js
+++ b/src/globe/testing/fakeEngine.js
@@ -24,6 +24,8 @@ const { cartesian2, cartesian3, cartographic, ellipsoid, math, matrix4 } = geo;
 const WGS84 = ellipsoid.WGS84;
 const RADII = WGS84.radii;
 const MAXIMUM_RADIUS = Math.max(RADII.x, RADII.y, RADII.z);
+// The terrain kinds the engines build themselves (src/maps/registry.js).
+const TERRAIN_KINDS = Object.freeze(['ellipsoid', 'keyless', 'world']);
 const DEFAULT_FLIGHT_SECONDS = 3;
 const DEFAULT_ZOOM_AMOUNT = 100000; // Cesium's camera.defaultZoomAmount
 const ZERO_RADIUS_SPHERE_RANGE = 100; // Cesium's MINIMUM_ZOOM
@@ -845,6 +847,13 @@ export function createFakeEngine(options = {}) {
       },
       async setTerrain(source) {
         record('maps.setTerrain', [source]);
+        // As the real engines: a prebuilt source or a known kind.
+        const kind = source?.kind ?? source?.id;
+        if (
+          typeof source?.create !== 'function' &&
+          !TERRAIN_KINDS.includes(kind)
+        )
+          throw new TypeError(`Unknown terrain source: ${kind}`);
         fake.maps.terrain = source;
       },
       async setPhotorealTiles(source) {