diff --git a/src/background/contentScript.ts b/src/background/contentScript.ts
index 22628da316..d31535fd94 100644
--- a/src/background/contentScript.ts
+++ b/src/background/contentScript.ts
@@ -46,19 +46,19 @@ function makeTargetKey(target: Target): string {
   return JSON.stringify({ tabId: target.tabId, frameId: target.frameId });
 }
 
-/**
- * Runtime message handler to handle ENSURE_CONTENT_SCRIPT_READY messages sent from the contentScript
- */
 // eslint-disable-next-line @typescript-eslint/promise-function-async -- Message handlers must return undefined to "pass through", not Promise<undefined>
-function onContentScriptReadyMessage(
+function onMessage(
   message: unknown,
   sender: Runtime.MessageSender,
-): Promise<void> | undefined {
+): Promise<unknown> | undefined {
   if (
-    isRemoteProcedureCallRequest(message) &&
-    message.type === ENSURE_CONTENT_SCRIPT_READY &&
-    sender.id === browser.runtime.id
+    !isRemoteProcedureCallRequest(message) ||
+    sender.id !== browser.runtime.id
   ) {
+    return; // Don't handle message
+  }
+
+  if (message.type === ENSURE_CONTENT_SCRIPT_READY) {
     const key = makeSenderKey(sender);
 
     try {
@@ -73,6 +73,10 @@ function onContentScriptReadyMessage(
     return Promise.resolve();
   }
 
+  if (message.type === "WHO_AM_I") {
+    return Promise.resolve(sender);
+  }
+
   // Don't return anything to indicate this didn't handle the message
 }
 
@@ -162,5 +166,5 @@ async function ensureContentScriptWithoutTimeout(
 }
 
 export function initContentScriptReadyListener() {
-  browser.runtime.onMessage.addListener(onContentScriptReadyMessage);
+  browser.runtime.onMessage.addListener(onMessage);
 }
diff --git a/src/contentScript/contentScript.ts b/src/contentScript/contentScript.ts
index fc3ddbce44..349453f9b2 100644
--- a/src/contentScript/contentScript.ts
+++ b/src/contentScript/contentScript.ts
@@ -31,6 +31,7 @@ import {
 import { onContextInvalidated } from "webext-events";
 import { logPromiseDuration } from "@/utils/promiseUtils";
 import { initRuntimeLogging } from "@/development/runtimeLogging";
+import { type Runtime } from "webextension-polyfill";
 
 // eslint-disable-next-line prefer-destructuring -- process.env substitution
 const DEBUG = process.env.DEBUG;
@@ -53,7 +54,7 @@ void initRuntimeLogging();
 
 // See note in `@/contentScript/ready.ts` for further details about the lifecycle of content scripts
 async function initContentScript() {
-  const context = top === self ? "" : `in frame ${location.href}`;
+  const urlInfo = top === self ? "" : `in frame ${location.href}`;
   const uuid = uuidv4();
 
   if (isInstalledInThisSession()) {
@@ -62,12 +63,26 @@ async function initContentScript() {
     // Prior to 1.7.31 we had been using `webext-dynamic-content-scripts` which can inject the same content script
     // multiple times: https://github.com/pixiebrix/pixiebrix-extension/pull/5743
     console.warn(
-      `contentScript: was requested twice in the same context, skipping content script initialization ${context}`,
+      `contentScript: was requested twice in the same context, skipping content script initialization ${urlInfo}`,
     );
     return;
   }
 
-  console.debug(`contentScript: importing ${uuid} ${context}`);
+  // Do not use the Messenger, it cannot appear in this bundle
+  const context: Runtime.MessageSender | undefined =
+    await browser.runtime.sendMessage({ type: "WHO_AM_I" });
+  if (!context) {
+    console.error(
+      "contentScript: nobody answered the WHO_AM_I context check. Loading might fail later.",
+    );
+  } else if (!("tab" in context)) {
+    console.warn("contentScript: not available in tabless iframes", {
+      context,
+    });
+    return;
+  }
+
+  console.debug(`contentScript: importing ${uuid} ${urlInfo}`);
 
   setInstalledInThisSession();