diff --git a/harness/playwright.config.js b/harness/playwright.config.js
index 951838b0..c36d491c 100644
--- a/harness/playwright.config.js
+++ b/harness/playwright.config.js
@@ -20,13 +20,18 @@ export default defineConfig({
     trace: "retain-on-failure",
   },
   // Chromium + Firefox run the OPFS battery; WebKit has no OPFS at all and is
-  // kept for the day it does. Every other spec is Chromium-only: clients.md.
+  // kept for the day it does. Firefox ALSO runs the standalone half of the
+  // shell spec — the deployment improvise-bck's criterion (1) names, which
+  // mocks nothing; the `@mocked-peer` block it excludes is the one the
+  // routeWebSocket argument was actually made about (improvise-zkkw). Every
+  // other spec is Chromium-only: clients.md.
   projects: [
     { name: "chromium", use: { ...devices["Desktop Chrome"] } },
     {
       name: "firefox",
       use: { ...devices["Desktop Firefox"] },
-      testMatch: /opfs\.spec\.js/,
+      testMatch: /(opfs|shell)\.spec\.js/,
+      grepInvert: /@mocked-peer/,
     },
     {
       name: "webkit",
diff --git a/harness/tests/shell.spec.js b/harness/tests/shell.spec.js
index f494c340..0b51272f 100644
--- a/harness/tests/shell.spec.js
+++ b/harness/tests/shell.spec.js
@@ -53,7 +53,11 @@ async function outlastTheWatchdog(page) {
   ).toBeGreaterThanOrEqual(jump);
 }
 
-test.describe("the shell reports what its peer is doing", () => {
+// Tagged, and the tag is what the config's `grepInvert` reads: these four
+// mock their peer with `routeWebSocket`, which is slower and flakier across
+// three engines. That argument is about THESE, and was never made about the
+// standalone block below (improvise-zkkw).
+test.describe("the shell reports what its peer is doing", { tag: "@mocked-peer" }, () => {
   // improvise-hwpi. The fault is injected at the *peer*, not in the page, so
   // this exercises the real `on_frame` and the real catch.
   test("a frame the client cannot fold says so on screen", async ({ page }) => {
@@ -122,6 +126,10 @@ test.describe("the shell reports what its peer is doing", () => {
   });
 });
 
+// Chromium AND Firefox: this is the deployment improvise-bck's criterion (1)
+// is about, it mocks nothing, and the engine's OPFS is already exercised by
+// `opfs.spec.js` — so the one recorded reason for Chromium-only does not
+// reach it (improvise-zkkw).
 test.describe("the standalone deployment", () => {
   // No `?ws=`, so the page spawns the in-page worker over OPFS. If this draws
   // a grid, worker.js, bindings.rs, OpfsFs and the seed fetch all ran.