Posthog Adblock Bypass

PostHog ad-blocker bypass for Next.js

When to use this

The user has posthog-js working mostly — events, feature flags, decide, config all reach PostHog fine — but session recording and/or dead-click capture aren't working, and DevTools shows:

• /static/dead-clicks-autocapture.js failing
• /static/recorder.js or /static/posthog-recorder.js failing
• ERR_BLOCKED_BY_CLIENT or a synthesized 404 (the file actually returns 200 via curl)

Root cause: ad blockers (uBlock Origin / Brave Shields / AdBlock Plus / EasyPrivacy) match on filename patterns (recorder, autocapture, dead-clicks) regardless of host. A reverse proxy alone doesn't help — renaming on a different host still trips the same filter. Confirmation step: ask the user to disable their ad blocker on the page; if the requests succeed, this skill applies.

This skill applies the surgical fix: only those three files divert through the Next.js app under bland filenames. All other PostHog traffic (events, decide, config, recording uploads) keeps using the existing api_host.

Prerequisites to confirm before implementing

1. The project is Next.js (App Router or Pages Router both fine) with posthog-js from npm.
2. Server-side: curl -I https://<api_host>/static/dead-clicks-autocapture.js?v=1.x returns 200. If it returns 404, the upstream proxy is misconfigured — fix that first, this skill won't help.
3. Identify the PostHog cloud region: US (us-assets.i.posthog.com) or EU (eu-assets.i.posthog.com). Default to US if unclear, but ask.
4. Locate the existing posthog.init(...) call (commonly src/lib/posthog.ts, a provider component, or app/layout.tsx). Note the file so you can hook the rewriter before init.
5. Session replay must be enabled in the PostHog project dashboard (Settings → Project → Session replay → "Record user sessions"). If it's off, posthog-js never even tries to load the recorder, so this skill produces no visible effect. Surface this check to the user.

Implementation — three changes

1. Add rewrites to next.config.{ts,js,mjs}

Insert a rewrites() block. Use /_p/ as the alias prefix (or any path that doesn't contain words like recorder, analytics, tracking, posthog).

tsCopy code
import type { NextConfig } from "next";

const PH_ASSETS = "https://us-assets.i.posthog.com"; // EU: "https://eu-assets.i.posthog.com"

const nextConfig: NextConfig = {
  // ...existing config...
  async rewrites() {
    return [
      { source: "/_p/dc.js", destination: `${PH_ASSETS}/static/dead-clicks-autocapture.js` },
      { source: "/_p/r.js",  destination: `${PH_ASSETS}/static/recorder.js` },
      { source: "/_p/r2.js", destination: `${PH_ASSETS}/static/posthog-recorder.js` },
    ];
  },
};

export default nextConfig;

Why all three: posthog-js renamed the recorder chunk across versions. Covering both recorder.js and posthog-recorder.js makes the skill version-resilient. Rewrites preserve the ?v=1.x.y query string automatically and incur no Node overhead.

If the project already has a rewrites() or redirects() block, merge rather than replace.

2. Create the script-tag rewriter

New file at src/lib/posthog-rewrite-loader.ts (adjust path to match the repo's lib convention):

tsCopy code
const REWRITES: Array<[RegExp, string]> = [
  [/\/static\/dead-clicks-autocapture\.js/, "/_p/dc.js"],
  [/\/static\/posthog-recorder\.js/,        "/_p/r2.js"],
  [/\/static\/recorder\.js/,                "/_p/r.js"],
];

export function installPostHogScriptRewriter() {
  if (typeof window === "undefined") return;
  const w = window as Window & { __phRewriteInstalled?: boolean };
  if (w.__phRewriteInstalled) return;
  w.__phRewriteInstalled = true;

  const desc = Object.getOwnPropertyDescriptor(HTMLScriptElement.prototype, "src");
  if (!desc?.set || !desc?.get) return;

  Object.defineProperty(HTMLScriptElement.prototype, "src", {
    configurable: true,
    enumerable: true,
    get() {
      return desc.get!.call(this);
    },
    set(value: string) {
      if (typeof value === "string") {
        for (const [pattern, replacement] of REWRITES) {
          if (pattern.test(value)) {
            const qIndex = value.indexOf("?");
            const query = qIndex >= 0 ? value.slice(qIndex) : "";
            value = replacement + query; // strip host -> same-origin
            break;
          }
        }
      }
      desc.set!.call(this, value);
    },
  });
}

Notes:

• The __phRewriteInstalled flag is required because Next.js dev HMR can re-execute modules and Object.defineProperty would otherwise throw.
• Stripping the host (replacing the entire URL with /_p/dc.js?v=...) makes the request same-origin, avoiding a CORS preflight on top of the blocker bypass.
• The patcher targets the src property setter on the prototype — posthog-js loads chunks by setting script.src, so this catches them. It does not catch fetch() or dynamic import() (not used by posthog-js for these chunks today, but worth noting if a future SDK version changes).

3. Wire the rewriter to run before posthog.init

The patcher must be installed before any posthog.init call. Module-level code in the same file as posthog.init works because module evaluation precedes useEffect:

tsCopy code
// src/lib/posthog.ts (or wherever posthog.init lives)
import posthog from "posthog-js";
import { installPostHogScriptRewriter } from "@/lib/posthog-rewrite-loader";

if (typeof window !== "undefined") {
  installPostHogScriptRewriter();
}

export const initPostHog = () => {
  if (typeof window !== "undefined") {
    posthog.init(process.env.NEXT_PUBLIC_POSTHOG_KEY!, {
      api_host: process.env.NEXT_PUBLIC_POSTHOG_HOST, // unchanged
      // ...existing config...
    });
  }
};

export { posthog };

If posthog.init is called from <Script strategy="beforeInteractive"> in app/layout.tsx instead of from a client module, put the patcher in its own <Script strategy="beforeInteractive"> block placed above the PostHog one. Don't use afterInteractive — by then posthog may have already started loading chunks.

Verification

1. Server side — after deploy:

   bashCopy code
   curl -I "https://<your-domain>/_p/dc.js?v=1.0"
   curl -I "https://<your-domain>/_p/r2.js?v=1.0"
   

   Both should return 200, content-type: text/javascript. If 404, the rewrite isn't picked up — Next.js needs a rebuild after next.config changes.

2. Browser with ad blocker enabled — in DevTools Network:

   • Requests to /_p/dc.js and /_p/r2.js succeed (200).
   • No /static/dead-clicks-autocapture.js or /static/posthog-recorder.js requests visible.
   • In uBlock Origin's Logger, /_p/* entries should show -- (allowed), not red.

3. PostHog dashboard:

   • Live Events shows dead-click events firing.
   • Session replay shows recordings (allow ~5 min for ingestion).
   • If recordings still don't appear, the issue is likely the dashboard toggle (Settings → Session replay), not this skill.

Common follow-ups and pitfalls

• "It still doesn't work after deploy" — rewrites only take effect after next build. Local dev needs npm run dev restart. Production needs a redeploy.
• "A different chunk is now blocked" — surveys, web-vitals, exception-autocapture. Add a fourth rewrite + a fourth regex entry. Pattern is identical.
• "Capture endpoints are also blocked" — /e/, /decide/, /array/*/config.js getting blocked is a different problem (host-level rather than filename-level). Solve with a PostHog reverse proxy on the user's own domain, not this skill. PostHog's docs cover that.
• "Want to proxy everything through Next.js instead" — possible but discouraged. Session recording is high-volume (kilobytes per second per active user); routing through Next.js multiplies function invocations 10–100×. The PostHog-managed proxy is purpose-built for this load. Use full-proxy only if you have a hard CSP/compliance requirement.
• SDK upgrades — if posthog-js renames a chunk in a future version, the rewriter silently misses it. After upgrades, watch DevTools Network for any /static/*.js request leaking through and add a regex entry.
• Network-level blockers (Pi-hole / NextDNS) blocking by domain are not bypassed by this. They're a much smaller cohort than browser extensions and your own domain isn't on their lists.

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