diff --git a/frontend/resources/wasm-playground/BENCHMARKS.md b/frontend/resources/wasm-playground/BENCHMARKS.md
index d209c3ede4..e8cc9abfa5 100644
--- a/frontend/resources/wasm-playground/BENCHMARKS.md
+++ b/frontend/resources/wasm-playground/BENCHMARKS.md
@@ -129,6 +129,26 @@ These runs used dirty source trees and are workflow evidence, not a comparison
 of renderer changes. Shadow visual validation and default sampling remain open.
 Keep the full workloads and provisional defaults until these limits are resolved.
 
+Isolated September 17 probes reproduce the shadow pattern without changing
+workloads or defaults. `verify.js --filter shadows/default/load` passes through
+`Full` on SwiftShader; `--filter shadows/default/pan` and
+`--filter shadows/default/zoom` fail with `WebGL context lost` in the warm
+interaction loop that calls `_render_from_cache` per frame. Load success records
+renderer submission only, not displayed pixels. The fixture keeps 1000 shapes
+with two drop shadows and one inner shadow, seven FFI arguments per shadow,
+style 0 for drop and 1 for inner, and a boolean hidden flag, matching
+`render-wasm/src/wasm/shadows.rs`. No fixture defect, workload reduction, or
+renderer change is claimed.
+
+Sampling stays provisional at 3 warm-ups, 10 measured attempts, and 30s timeout.
+The full pilot ran 240 attempts in 1543337 ms, about 6.4 s per attempt; the
+one-attempt workflow run took 156466 ms for 21 attempts, about 7.4 s per
+attempt. The 2-5 minute goal allows roughly 18-46 attempts in total, or one to
+two per case across 21 cases, below the 10 valid attempts needed for median
+intervals. Use zero-warm-up single attempts with `--filter` for workflow
+checks; use the full provisional counts for intervals. Final calibration needs
+a stable graphics environment with passing shadows.
+
 The implementation roadmap, pilot evidence, and remaining calibration work live
 in Serena memory `render-wasm/performance/core` and its numbered tickets. The
 2-5 minute full-suite goal excludes build time and is a target, not a guarantee
diff --git a/frontend/scripts/renderer-benchmarks/tests/scenes.test.js b/frontend/scripts/renderer-benchmarks/tests/scenes.test.js
index b5a8f40681..a9a47c1e0f 100644
--- a/frontend/scripts/renderer-benchmarks/tests/scenes.test.js
+++ b/frontend/scripts/renderer-benchmarks/tests/scenes.test.js
@@ -96,3 +96,28 @@ test("clip effects use valid enum values and actual booleans", () => {
     }
   }
 });
+
+test("shadow workload retains intended effects with complete valid FFI calls", () => {
+  const scenario = scenarios.find(({ id }) => id === "shadows");
+  const params = scenario.cases[0].params;
+  assert.equal(params.count, 1000);
+  assert.equal(params.shadows.length, 3);
+  assert.deepEqual(
+    params.shadows.map((shadow) => shadow[5]),
+    [0, 0, 1],
+  );
+  const scene = scenario.createScene(params, 42);
+  assert.equal(scene.shapes.length, 1000);
+  for (const shape of scene.shapes) {
+    assert.deepEqual(shape.shadows, params.shadows);
+    for (const shadow of shape.shadows) {
+      assert.equal(shadow.length, 7);
+      assert.ok(Number.isInteger(shadow[0]));
+      for (const value of shadow.slice(1, 5)) {
+        assert.ok(Number.isFinite(value));
+      }
+      assert.ok([0, 1].includes(shadow[5]));
+      assert.equal(shadow[6], false);
+    }
+  }
+});