diff --git a/AGENTS.md b/AGENTS.md
index 6c47e2c1..d8f2b3f7 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -946,7 +946,7 @@ SMOKE_BROWSER=webkit npm run smoke:web:engine    # any engine, on demand
 SMOKE_BROWSER=firefox npm run smoke:web:app      # one smoke, one engine
 ```
 
-**Firefox runs in the local pre-push gate (`npm run ci`) and NOT in GitHub CI.** That split is the whole design — see the gate bullet below. `ENGINE_SMOKES` in `scripts/run-engine-smokes.mjs` is the single list of which smokes are engine-sensitive; add a fourth there and every engine picks it up. `smoke:web:firefox` passes the engine as an argument rather than relying on the ambient `SMOKE_BROWSER`, so the gate cannot be silently redirected to another engine by a stray variable — verified, an explicit argument beats it.
+**Firefox runs in the local pre-push gate (`npm run ci`) and NOT in GitHub CI.** That split is the whole design — see the gate bullet below. `ENGINE_SMOKES` in `scripts/run-engine-smokes.mjs` is the single list of which smokes are engine-sensitive, and **every tier reads it** — the default Chromium run inside `npm run smoke`, the Firefox pass in the gate, and the on-demand command. Add a fourth there and all three pick it up. A tier that enumerated the smokes itself would silently run fewer and still pass, so `run-engine-smokes.test.mjs` asserts that none of them does. `smoke:web:firefox` passes the engine as an argument rather than relying on the ambient `SMOKE_BROWSER`, so the gate cannot be silently redirected to another engine by a stray variable — verified, an explicit argument beats it.
 
 Everything about the selection lives in **`scripts/lib/headless-browser.mjs`** — `resolveBrowserName`, `loadBrowser`, and the `attachPageDiagnostics` / `FATAL_CONSOLE` split the smokes had each hand-rolled. Reach for it rather than launching Playwright in a new script.
 
diff --git a/package.json b/package.json
index eba1ca4c..82771596 100644
--- a/package.json
+++ b/package.json
@@ -66,7 +66,7 @@
     "coverage:tui": "cd clients/tui && npm run test:coverage",
     "coverage:web": "cd clients/web && npm run test:coverage",
     "coverage:launcher": "cd clients/launcher && npm run test:coverage",
-    "smoke": "npm run smoke:launcher && npm run smoke:cli && npm run smoke:tui && npm run smoke:web && npm run smoke:web:browser && npm run smoke:web:app && npm run smoke:web:elicit",
+    "smoke": "npm run smoke:launcher && npm run smoke:cli && npm run smoke:tui && npm run smoke:web && npm run smoke:web:chromium",
     "smoke:cli": "node scripts/smoke-cli.mjs",
     "smoke:tui": "node scripts/smoke-tui.mjs",
     "smoke:web": "node scripts/smoke-web.mjs",
@@ -74,6 +74,7 @@
     "smoke:web:app": "node scripts/install-smoke-browser.mjs && node scripts/smoke-web-app.mjs",
     "smoke:web:elicit": "node scripts/install-smoke-browser.mjs && node scripts/smoke-web-elicitation.mjs",
     "smoke:web:engine": "node scripts/run-engine-smokes.mjs",
+    "smoke:web:chromium": "node scripts/run-engine-smokes.mjs chromium",
     "smoke:web:firefox": "node scripts/run-engine-smokes.mjs firefox",
     "smoke:launcher": "node scripts/smoke-launcher.mjs",
     "pack:verify": "node scripts/install-smoke-browser.mjs chromium && node scripts/pack-and-verify.mjs",
diff --git a/scripts/run-engine-smokes.mjs b/scripts/run-engine-smokes.mjs
index 29c041a7..71883b54 100644
--- a/scripts/run-engine-smokes.mjs
+++ b/scripts/run-engine-smokes.mjs
@@ -8,10 +8,13 @@
  * from Node does, on every platform — the same reason `install-smoke-browser`
  * exists rather than an inline shell expansion.
  *
- * It also gives the smoke list ONE home. It used to be an `&&` chain in
- * package.json alongside a second chain in the workflow; adding a fourth smoke
- * meant remembering both. Now `ENGINE_SMOKES` is the list, and both the
- * on-demand command and the pre-push gate run through it.
+ * It also gives the smoke list ONE home. `ENGINE_SMOKES` is that list, and
+ * EVERY tier reads it — the default Chromium run inside `npm run smoke`
+ * (`smoke:web:chromium`), the Firefox pass in the pre-push gate
+ * (`smoke:web:firefox`), and the on-demand `smoke:web:engine`. Getting that
+ * wrong is silent: a tier that enumerated the smokes itself would simply run
+ * fewer of them and still pass, so `run-engine-smokes.test.mjs` asserts that no
+ * tier names an individual smoke.
  *
  * Usage: `node scripts/run-engine-smokes.mjs [engine]`. With no argument the
  * engine comes from `SMOKE_BROWSER` (default `chromium`).
diff --git a/scripts/run-engine-smokes.test.mjs b/scripts/run-engine-smokes.test.mjs
index ca8516c6..47197d17 100644
--- a/scripts/run-engine-smokes.test.mjs
+++ b/scripts/run-engine-smokes.test.mjs
@@ -12,12 +12,15 @@
  */
 
 import assert from "node:assert/strict";
-import { existsSync } from "node:fs";
+import { existsSync, readFileSync } from "node:fs";
 import { join } from "node:path";
 import { describe, it } from "node:test";
 import { ENGINE_SMOKES } from "./run-engine-smokes.mjs";
 
 const scriptDir = import.meta.dirname;
+const scripts = JSON.parse(
+  readFileSync(join(scriptDir, "..", "package.json"), "utf8"),
+).scripts;
 
 describe("ENGINE_SMOKES", () => {
   it("names every browser-driven smoke, and each one exists", () => {
@@ -43,3 +46,48 @@ describe("ENGINE_SMOKES", () => {
     assert.equal(ENGINE_SMOKES[0], "smoke-web-browser.mjs");
   });
 });
+
+describe("every engine tier consumes ENGINE_SMOKES", () => {
+  // The point of the list is defeated if any tier enumerates the smokes itself.
+  // `npm run smoke` used to do exactly that (Copilot, #2133): a fourth entry in
+  // ENGINE_SMOKES would have reached the Firefox and on-demand runs and silently
+  // skipped the DEFAULT Chromium run — which is the one GitHub CI executes. That
+  // is invisible at runtime, because a tier running fewer smokes still passes.
+
+  it("the Chromium tier goes through the runner, not its own list", () => {
+    assert.match(scripts["smoke:web:chromium"], /run-engine-smokes\.mjs/);
+    assert.match(scripts.smoke, /smoke:web:chromium/);
+    for (const individual of [
+      "smoke:web:browser",
+      "smoke:web:app",
+      "smoke:web:elicit",
+    ]) {
+      assert.ok(
+        !scripts.smoke.includes(individual),
+        `\`smoke\` names ${individual} directly — it must route through ` +
+          "`smoke:web:chromium` so every tier reads one list",
+      );
+    }
+  });
+
+  it("the Firefox tier goes through the runner and is in the pre-push gate", () => {
+    assert.match(scripts["smoke:web:firefox"], /run-engine-smokes\.mjs/);
+    assert.match(scripts.ci, /smoke:web:firefox/);
+  });
+
+  it("each gated tier names its engine explicitly rather than reading the env", () => {
+    // An ambient SMOKE_BROWSER must not be able to redirect a gate: without the
+    // literal engine, `SMOKE_BROWSER=firefox npm run ci` would run Firefox twice
+    // and never exercise Chromium at all.
+    assert.match(
+      scripts["smoke:web:chromium"],
+      /run-engine-smokes\.mjs chromium$/,
+    );
+    assert.match(
+      scripts["smoke:web:firefox"],
+      /run-engine-smokes\.mjs firefox$/,
+    );
+    // The on-demand entry point is the one that SHOULD follow the environment.
+    assert.match(scripts["smoke:web:engine"], /run-engine-smokes\.mjs$/);
+  });
+});