diff --git a/src/fetch/router.ts b/src/fetch/router.ts
index cf1f9aea..812e426f 100644
--- a/src/fetch/router.ts
+++ b/src/fetch/router.ts
@@ -792,6 +792,26 @@ export class SmartRouter {
       screenshot,
       signal,
       stealth: stealthForBrowser(getConfig(), { antiBotEscalation: false }),
+      // WHY A THUNK AND NOT A `fallback`. Every OTHER escalation reaches the browser
+      // holding an HTTP result it already paid for, so it hands that over as `fallback`
+      // for free. This path starts AT the browser because the host is MARKED, so there
+      // is no such result — and eagerly fetching one would put a full HTTP round-trip in
+      // front of every domain-marked fetch on a healthy machine, which is the cost the
+      // mark exists to avoid. Deferring it means the lower tier is paid for only on the
+      // hosts that actually cannot reach the browser rung.
+      //
+      // The mark is a PREFERENCE, not a requirement: it is set by an under-threshold
+      // body, a `__NEXT_DATA__` blob or a high script ratio, none of which claim HTTP
+      // cannot serve the page. Without this, a machine with no browser engine answered
+      // `browser_engine_unavailable` for a page plain HTTP had returned in full one call
+      // earlier — and on a fresh install with no engine that is every fetch of the host,
+      // forever.
+      fallbackFetch: async () => {
+        if (!this.httpClient) return null;
+        const lower = await this.httpClientFetch(url, { headers, conditionalHeaders, signal });
+        this.ensureStats(domain);
+        return this.toRawFetchResult(lower);
+      },
     });
   }
 
@@ -871,11 +891,20 @@ export class SmartRouter {
 
   private async browserFetch(
     url: string,
-    options: BrowserFetchArgs & { fallback?: RawFetchResult },
+    options: BrowserFetchArgs & {
+      fallback?: RawFetchResult;
+      /**
+       * A lower tier that has not been fetched yet, for the call sites that reach the
+       * browser without one in hand. Invoked ONLY when acquisition fails, so a healthy
+       * machine never pays for it. Resolving `null` (or throwing) means the lower tier
+       * had nothing to give, and the actionable error stands.
+       */
+      fallbackFetch?: () => Promise<RawFetchResult | null>;
+    },
   ): Promise<RawFetchResult | StageError> {
     if (!this.browserPool) throw new Error('SmartRouter: browserPool not configured');
 
-    const { fallback, ...browserOptions } = options;
+    const { fallback, fallbackFetch, ...browserOptions } = options;
     const acquired = await this.browserAcquirer.ensureBrowser();
     if (acquired !== 'ready') {
       const logger = createLogger('fetch');
@@ -897,14 +926,30 @@ export class SmartRouter {
       const companion = await this.companionRungFetch(url, browserOptions, acquired);
       if (companion) return this.guardChallengeShell(companion);
 
-      if (fallback) {
+      // Nothing in hand — ask the deferred lower tier whether it can serve the page.
+      // Tried AFTER the companion rung: a real browser is a better answer than HTTP.
+      let lowerTier = fallback;
+      if (!lowerTier && fallbackFetch) {
+        try {
+          lowerTier = (await fallbackFetch()) ?? undefined;
+        } catch (err) {
+          logger.debug('deferred lower-tier fetch failed; falling through to the actionable error', {
+            url,
+            error: err instanceof Error ? err.message : String(err),
+          });
+        }
+      }
+
+      if (lowerTier) {
         logger.info('browser engine not ready within budget — returning lower-tier content with note', { url });
-        // The browser was the escalation target because the lower tier returned
-        // a challenge shell (or an anti-bot-status challenge body). If we cannot
-        // acquire it, we must NOT fall back to returning that shell as content —
-        // guard it so a challenge fallback becomes blocked_by_challenge, while
-        // legit lower-tier content passes through unchanged with the note.
-        return this.guardChallengeShell({ ...fallback, warning: BROWSER_INSTALLING_NOTE });
+        // The lower tier is guarded whichever way it arrived. When the browser was the
+        // ESCALATION target, the thing that triggered the escalation may well be a
+        // challenge shell (or an anti-bot-status challenge body), and returning that as
+        // content would pass an interstitial off as the page. When it arrived from the
+        // deferred fetch above it is a fresh response that has never been classified at
+        // all. Both become blocked_by_challenge if they are challenges; legit lower-tier
+        // content passes through unchanged with the note.
+        return this.guardChallengeShell({ ...lowerTier, warning: BROWSER_INSTALLING_NOTE });
       }
       logger.info('browser engine not ready within budget and no lower-tier content — failing with actionable error', { url });
       return {
diff --git a/tests/integration/px2-rc/rc-exit-gate.test.ts b/tests/integration/px2-rc/rc-exit-gate.test.ts
index 1441b1b4..c25fc0fe 100644
--- a/tests/integration/px2-rc/rc-exit-gate.test.ts
+++ b/tests/integration/px2-rc/rc-exit-gate.test.ts
@@ -555,6 +555,23 @@ describe.skipIf(RC_GATE_DISABLED)('PX2 RC exit gate — fresh install, registrat
               `<main><h1>Changelog</h1><p>${CHANGELOG_V2}</p></main></body></html>`,
           );
 
+          // WHICH OF THE TWO IT WAS (#521). Not the fixture, and not the
+          // warmup/acquirer disagreement the issue suspected either — measured on a
+          // fresh packed install with a live network, `wigolo warmup --browser`
+          // reports `Browser: ok` and the NEXT process's acquirer reports the driver
+          // and the binary both present. The disagreement was never real; under this
+          // suite's egress fence the driver simply cannot be acquired at all, which is
+          // a correct answer for a machine with no network.
+          //
+          // The red was a PRODUCT defect one layer up. `/changelog`'s body is under the
+          // 200-character visible-text threshold, so the seeding fetch tripped SPA-shell
+          // detection and MARKED the fixture host `preferPlaywright`. That first fetch
+          // still succeeded — the escalation had the HTTP result in hand and degraded to
+          // it. This re-read then took the domain-marked path, which started AT the
+          // browser tier with nothing in hand and hard-failed `browser_engine_unavailable`
+          // for a page HTTP had just served in full. `router.ts` now defers a lower-tier
+          // fetch onto that branch, so the arm passes through HTTP and never depends on
+          // whether an engine could be acquired.
           const refreshed = await session.call('fetch', {
             url: `${site.url}/changelog`,
             force_refresh: true,
diff --git a/tests/unit/fetch/router-browser-acquire.test.ts b/tests/unit/fetch/router-browser-acquire.test.ts
index 2cf757ce..b1e35c6e 100644
--- a/tests/unit/fetch/router-browser-acquire.test.ts
+++ b/tests/unit/fetch/router-browser-acquire.test.ts
@@ -286,5 +286,66 @@ describe('SmartRouter — lazy browser acquisition threading (D3)', () => {
       expect('error' in result).toBe(true);
       expect((result as { hint?: string; error_reason: string }).hint ?? '').toMatch(/wigolo warmup --browser/);
     });
+
+    /**
+     * THE MARK IS A PREFERENCE, NOT A REQUIREMENT — and this is the one branch that
+     * used to read it as a requirement.
+     *
+     * `preferPlaywright` is set by heuristics that never claim HTTP cannot serve the
+     * page: a body under the 200-character visible-text threshold, a `__NEXT_DATA__`
+     * blob, a high script ratio. The escalation that SETS the mark hands `browserFetch`
+     * the HTTP result as `fallback`, so a machine with no browser engine degrades to
+     * that content with the actionable note. The very next fetch of the same host takes
+     * `browserOrHttpForBinary` instead, which had no fallback to hand over — so the same
+     * host, one call later, answered `browser_engine_unavailable` for a page plain HTTP
+     * had just returned in full.
+     *
+     * That is the shape PX2's RC exit gate met: fetch a short fixture page (marks the
+     * host), then re-read it with `force_refresh` (domain-marked path) and get a hard
+     * error on a fresh install that has no engine and — behind the gate's egress fence —
+     * can never acquire one.
+     *
+     * The two fetches share ONE router because the mark is in-memory per router
+     * (`ensureStats`); splitting them would drop the precondition the case is about.
+     */
+    it('the domain-marked path degrades to lower-tier content instead of hard-failing', async () => {
+      vi.mocked(httpClient.fetch).mockResolvedValue(makeHttpResult(SPA_SHELL_HTML));
+      const { acquirer } = makeAcquirer('unavailable');
+      const router = new SmartRouter({ httpClient, browserPool, pdfProbe: async () => false, browserAcquirer: acquirer, systemBrowserFetch: noInstalledBrowser });
+
+      // First fetch: SPA-shell detection marks the host and escalates. Already covered
+      // above; asserted here only to prove the precondition actually landed.
+      const first = await router.fetch('https://marked.example/page') as RawFetchResult;
+      expect(first.method).toBe('http');
+
+      // Second fetch: same host, now domain-marked, so it starts AT the browser tier.
+      const second = await router.fetch('https://marked.example/other') as RawFetchResult;
+
+      expect(browserPool.fetchWithBrowser).not.toHaveBeenCalled();
+      expect('error' in second, `domain-marked re-read hard-failed: ${JSON.stringify(second)}`).toBe(false);
+      expect(second.method).toBe('http');
+      expect(second.html).toBe(SPA_SHELL_HTML);
+      expect(second.warning).toMatch(/browser engine installing/);
+      expect(second.warning).toMatch(/wigolo warmup --browser/);
+    });
+
+    it('the domain-marked path still hard-fails when the lower tier has nothing to give', async () => {
+      const { acquirer } = makeAcquirer('unavailable');
+      const router = new SmartRouter({ httpClient, browserPool, pdfProbe: async () => false, browserAcquirer: acquirer, systemBrowserFetch: noInstalledBrowser });
+
+      vi.mocked(httpClient.fetch).mockResolvedValue(makeHttpResult(SPA_SHELL_HTML));
+      await router.fetch('https://marked2.example/page');
+
+      // The host is marked; now HTTP itself is down, so the degradation has no content
+      // to return and the actionable error is the correct answer. Without this the fix
+      // above could have swallowed a real failure into a silent empty success.
+      vi.mocked(httpClient.fetch).mockRejectedValue(new Error('refused'));
+      const result = await router.fetch('https://marked2.example/other');
+
+      expect('error' in result).toBe(true);
+      const err = result as { error: string; error_reason: string; hint?: string };
+      expect(err.error).toBe('browser_engine_unavailable');
+      expect(err.hint ?? err.error_reason).toMatch(/wigolo warmup --browser/);
+    });
   });
 });