diff --git a/CHANGELOG.md b/CHANGELOG.md
index 95a4d64a9..c0d084495 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -32,6 +32,7 @@ All notable changes to OriginWeave are documented in this file. The format follo
 - Proposed product-wide target-architecture ADRs for the Rust control plane, isolated execution modes, typed actions, semantic observation/stale-node authority, prompt-injection and secret separation, resource-governor priority, provenance evidence, browser/protocol adapters, crawler policy, and hourly automation operational closure; these remain Proposed rather than shipped claims until protected review and merge.
 - Pinned real-Chromium Manifest V3 compatibility evidence for a controlled history add/read/delete lifecycle confined to the ephemeral loopback fixture profile, with cleanup verification and no OriginWeave Agent history-authority claim.
 - Pinned real-Chromium Manifest V3 update-migration evidence using a trial-local unpacked-extension copy, a controlled `1.0.0` to `1.0.1` transition, persisted profile state, and explicit schema migration without modifying the checked-in fixture or granting Agent authority.
+- Pinned real-Chromium Manifest V3 content-script isolated-world evidence that requires the page main world and extension isolated world to retain distinct JavaScript globals while sharing only a controlled DOM sentinel.
 
 ### Changed
 
diff --git a/tests/fixtures/mv3_basic/content_script.js b/tests/fixtures/mv3_basic/content_script.js
index 1bde75ed9..161d5db51 100644
--- a/tests/fixtures/mv3_basic/content_script.js
+++ b/tests/fixtures/mv3_basic/content_script.js
@@ -1,7 +1,13 @@
 "use strict";
 
 (async () => {
-  document.documentElement.dataset.originweaveContentScript = "ready";
+  window.originweaveWorldSentinel = "extension";
+  await new Promise((resolve) => setTimeout(resolve, 75));
+  const pageWorldSentinel = document.documentElement.dataset.originweavePageWorld;
+  document.documentElement.dataset.originweaveContentScript =
+    window.originweaveWorldSentinel === "extension" && pageWorldSentinel === "page"
+      ? "ready"
+      : "missing";
 
   const previous = await chrome.storage.local.get("originweave_content");
   const storagePersisted = previous.originweave_content === "ready";
diff --git a/tests/fixtures/mv3_basic/page.html b/tests/fixtures/mv3_basic/page.html
index cc224cff0..ef4f2bb7a 100644
--- a/tests/fixtures/mv3_basic/page.html
+++ b/tests/fixtures/mv3_basic/page.html
@@ -12,6 +12,14 @@ <h1>OriginWeave MV3 compatibility fixture</h1>
   </main>
   <script>
     "use strict";
+    window.originweaveWorldSentinel = "page";
+    const publishPageWorld = () => {
+      document.documentElement.dataset.originweavePageWorld =
+        window.originweaveWorldSentinel;
+    };
+    publishPageWorld();
+    setInterval(publishPageWorld, 25);
+
     document.querySelector("#fixture-button").addEventListener("click", () => {
       document.querySelector("#fixture-output").textContent = "clicked";
     });
diff --git a/tests/test_mv3_isolated_world_contract.py b/tests/test_mv3_isolated_world_contract.py
new file mode 100644
index 000000000..15a36bece
--- /dev/null
+++ b/tests/test_mv3_isolated_world_contract.py
@@ -0,0 +1,44 @@
+"""Fail-first contract for real Manifest V3 content-script isolated-world evidence."""
+
+from __future__ import annotations
+
+import pathlib
+import unittest
+
+ROOT = pathlib.Path(__file__).resolve().parents[1]
+FIXTURE = ROOT / "tests" / "fixtures" / "mv3_basic"
+
+
+class ManifestV3IsolatedWorldContractTests(unittest.TestCase):
+    """Require content-script compatibility to prove main/isolated world separation."""
+
+    def test_page_continuously_exposes_its_main_world_sentinel(self) -> None:
+        """The page must re-read its own window after the content script runs."""
+
+        page = (FIXTURE / "page.html").read_text(encoding="utf-8")
+        for expected in (
+            'window.originweaveWorldSentinel = "page"',
+            "originweavePageWorld",
+            "setInterval",
+        ):
+            with self.subTest(expected=expected):
+                self.assertIn(expected, page)
+
+    def test_content_ready_depends_on_isolated_world_separation(self) -> None:
+        """The existing content-script gate must fail if page and extension worlds collapse."""
+
+        content = (FIXTURE / "content_script.js").read_text(encoding="utf-8")
+        for expected in (
+            'window.originweaveWorldSentinel = "extension"',
+            "originweavePageWorld",
+            'window.originweaveWorldSentinel === "extension"',
+            'pageWorldSentinel === "page"',
+        ):
+            with self.subTest(expected=expected):
+                self.assertIn(expected, content)
+        self.assertIn("setTimeout", content)
+        self.assertIn("originweaveContentScript", content)
+
+
+if __name__ == "__main__":
+    unittest.main()