diff --git a/Packages/macOS/CmuxNext/CEFShim/src/artifact_page.h b/Packages/macOS/CmuxNext/CEFShim/src/artifact_page.h
new file mode 100644
index 000000000000..11ebe2c6ecfc
--- /dev/null
+++ b/Packages/macOS/CmuxNext/CEFShim/src/artifact_page.h
@@ -0,0 +1,79 @@
+// Offline artifact documents for the shared CEF agent page. No CEF dependency;
+// exercised by test-cmux-page-path-cpp.sh alongside the scheme's path policy.
+#ifndef CMUX_SHIM_ARTIFACT_PAGE_H_
+#define CMUX_SHIM_ARTIFACT_PAGE_H_
+
+#include <string>
+
+namespace cmux_shim {
+inline constexpr char kArtifactFrameURL[] = "cmux-page://cmux.agent/__artifact";
+inline constexpr char kArtifactCSP[] =
+    "sandbox allow-scripts; default-src 'none'; script-src 'unsafe-inline'; style-src 'unsafe-inline'; "
+    "img-src data: blob:; media-src data: blob:; font-src data:; connect-src 'none'; "
+    "frame-src 'none'; worker-src 'none'; form-action 'none'; base-uri 'none'; "
+    "frame-ancestors cmux-page://cmux.agent";
+
+inline bool IsArtifactFrameURL(const std::string& url) {
+  return url.substr(0, url.find_first_of("?#")) == kArtifactFrameURL;
+}
+
+inline bool IsAgentPageURL(const std::string& url) {
+  const std::string origin = "cmux-page://cmux.agent";
+  return url.compare(0, origin.size(), origin) == 0 &&
+         (url.size() == origin.size() || url[origin.size()] == '/' || url[origin.size()] == '?' ||
+          url[origin.size()] == '#');
+}
+
+inline bool ArtifactContextRestricted(const std::string& frame, const std::string& parent) {
+  return IsArtifactFrameURL(frame) || IsArtifactFrameURL(parent) ||
+         (frame == "about:srcdoc" && IsAgentPageURL(parent));
+}
+
+// Only an empty, direct child of the agent page may load the bootstrap once.
+// After commit, self navigation (including refresh), children and srcdoc
+// replacement are denied, regardless of the destination's scheme.
+inline bool ArtifactNavigationRefused(bool main, const std::string& frame, const std::string& parent,
+                                      const std::string& target) {
+  if (ArtifactContextRestricted(frame, parent)) return true;
+  if (IsArtifactFrameURL(target)) {
+    return main || target != kArtifactFrameURL || !IsAgentPageURL(parent) ||
+           (!frame.empty() && frame != "about:blank");
+  }
+  return false;
+}
+
+inline bool ArtifactResourceAllowed(const std::string& url, bool image_media_or_font) {
+  return image_media_or_font && (url.rfind("data:", 0) == 0 || url.rfind("blob:", 0) == 0);
+}
+
+// Same one-shot protocol as AgentPaneRenderFrame. The response's sandbox
+// makes the origin opaque even if an embedder omits its iframe sandbox. A
+// passive preview remains srcdoc with sandbox=""; only Run grants scripts.
+inline char kArtifactDocument[] = R"HTML(<!doctype html>
+<html><head><meta charset="utf-8"><script>
+"use strict";
+(function () {
+  var host = parent;
+  function size() {
+    var body = document.body;
+    host.postMessage({type:"cmux-render-size",height:Math.ceil(Math.max(
+      document.documentElement.scrollHeight,body ? body.scrollHeight : 0))},"*");
+  }
+  function show(event) {
+    var data = event.data;
+    if (event.source !== host || !data || data.type !== "cmux-render" || typeof data.html !== "string") return;
+    removeEventListener("message",show);
+    document.open(); document.write(data.html); document.close();
+    var theme = document.createElement("style");
+    theme.textContent = typeof data.css === "string" ? data.css : "";
+    var head = document.head || document.documentElement;
+    head.insertBefore(theme,head.firstChild);
+    new ResizeObserver(size).observe(document.documentElement);
+    addEventListener("load",size); size();
+  }
+  addEventListener("message",show);
+  host.postMessage({type:"cmux-render-ready"},"*");
+})();
+</script></head></html>)HTML";
+}  // namespace cmux_shim
+#endif  // CMUX_SHIM_ARTIFACT_PAGE_H_
diff --git a/Packages/macOS/CmuxNext/CEFShim/src/cmux_page_path.h b/Packages/macOS/CmuxNext/CEFShim/src/cmux_page_path.h
index c7647123d201..6fbd5d143831 100644
--- a/Packages/macOS/CmuxNext/CEFShim/src/cmux_page_path.h
+++ b/Packages/macOS/CmuxNext/CEFShim/src/cmux_page_path.h
@@ -13,6 +13,8 @@
 #include <utility>
 #include <vector>
 
+#include "artifact_page.h"
+
 namespace cmux_shim {
 
 inline constexpr char kCmuxPageScheme[] = "cmux-page";
@@ -201,6 +203,7 @@ inline bool GrantsCmuxPageLocalNetwork(const std::string& requesting_origin, uin
 inline bool PageRequestAllowed(bool main_frame, const std::string& frame_url, const std::string& parent_url,
                                const std::string& domain) {
   if (main_frame) return true;
+  if (ArtifactContextRestricted(frame_url, parent_url)) return false;
   const std::string origin = std::string(kCmuxPageScheme) + "://" + domain;
   auto same = [&](const std::string& url) {
     return url.size() >= origin.size() && url.compare(0, origin.size(), origin) == 0 &&
diff --git a/Packages/macOS/CmuxNext/CEFShim/src/shim_client.mm b/Packages/macOS/CmuxNext/CEFShim/src/shim_client.mm
index 35f3beb21875..095ac9940087 100644
--- a/Packages/macOS/CmuxNext/CEFShim/src/shim_client.mm
+++ b/Packages/macOS/CmuxNext/CEFShim/src/shim_client.mm
@@ -9,6 +9,7 @@
 #include "include/cef_parser.h"
 #include "include/cef_resource_request_handler.h"
 #include "command_line_switches.h"
+#include "artifact_page.h"
 #include "external_url.h"
 #include "local_file_handoff.h"
 #include "local_file_scope.h"
@@ -165,6 +166,17 @@ cef_return_value_t OnBeforeResourceLoad(CefRefPtr<CefBrowser>, CefRefPtr<CefFram
   return std::string();
 }
 
+// Inspect the whole ancestry: blank/srcdoc children cannot shed the offline
+// document's restrictions by inheriting an ancestor's ordinary page URL.
+bool RestrictedArtifactFrame(CefRefPtr<CefFrame> frame) {
+  while (frame) {
+    CefRefPtr<CefFrame> parent = frame->GetParent();
+    if (ArtifactContextRestricted(frame->GetURL().ToString(), parent ? parent->GetURL().ToString() : "")) return true;
+    frame = parent;
+  }
+  return false;
+}
+
 class Client : public CefClient,
                public CefDisplayHandler,
                public CefLoadHandler,
@@ -282,6 +294,10 @@ bool OnChromeCommand(CefRefPtr<CefBrowser> browser, int command_id, cef_window_o
   // tab in the other store with the same URL.
   bool OnBeforeBrowse(CefRefPtr<CefBrowser> browser, CefRefPtr<CefFrame> frame, CefRefPtr<CefRequest> request,
                       bool user_gesture, bool is_redirect) override {
+    CefRefPtr<CefFrame> parent = frame->GetParent();
+    if (RestrictedArtifactFrame(frame) ||
+        ArtifactNavigationRefused(frame->IsMain(), frame->GetURL().ToString(),
+                                  parent ? parent->GetURL().ToString() : "", request->GetURL().ToString())) return true;
     if (!frame->IsMain()) return false;
     int id = browser->GetIdentifier();
     std::string url = request->GetURL().ToString();
@@ -311,6 +327,12 @@ bool OnBeforeBrowse(CefRefPtr<CefBrowser> browser, CefRefPtr<CefFrame> frame, Ce
                                                                  CefRefPtr<CefRequest> request, bool, bool,
                                                                  const CefString&, bool&) override {
     const std::string url = request->GetURL().ToString();
+    const auto type = request->GetResourceType();
+    if (RestrictedArtifactFrame(frame) &&
+        !ArtifactResourceAllowed(url, type == RT_IMAGE || type == RT_MEDIA || type == RT_FONT_RESOURCE)) {
+      static CefRefPtr<CefResourceRequestHandler> refusal = new LocalFileRefusal();
+      return refusal;
+    }
     // A request with no frame (a worker, a CefURLRequest) has no folder to
     // judge it by: its file: loads are refused.
     if (LocalFileSchemeURL(url) && request->GetResourceType() != RT_MAIN_FRAME &&
diff --git a/Packages/macOS/CmuxNext/CEFShim/src/shim_page_scheme.mm b/Packages/macOS/CmuxNext/CEFShim/src/shim_page_scheme.mm
index 2061ce638b41..73e2449baa7e 100644
--- a/Packages/macOS/CmuxNext/CEFShim/src/shim_page_scheme.mm
+++ b/Packages/macOS/CmuxNext/CEFShim/src/shim_page_scheme.mm
@@ -167,6 +167,21 @@ int NextToken() {
       frame_url = frame->GetURL().ToString();
       if (CefRefPtr<CefFrame> parent = frame->GetParent()) parent_url = parent->GetURL().ToString();
     }
+    const std::string url = request->GetURL().ToString();
+    if (IsArtifactFrameURL(url)) {
+      if (domain_ != "cmux.agent" || request->GetMethod().ToString() != "GET" ||
+          request->GetResourceType() != RT_SUB_FRAME ||
+          ArtifactNavigationRefused(main_frame, frame_url, parent_url, url)) {
+        static char kBody[] = "Forbidden";
+        return Respond(403, "Forbidden", "text/plain", Headers(), kBody, sizeof(kBody) - 1);
+      }
+      CefResponse::HeaderMap headers;
+      headers.emplace("Content-Security-Policy", kArtifactCSP);
+      headers.emplace("Cache-Control", "no-store");
+      headers.emplace("X-Content-Type-Options", "nosniff");
+      headers.emplace("Referrer-Policy", "no-referrer");
+      return Respond(200, "OK", "text/html", headers, kArtifactDocument, sizeof(kArtifactDocument) - 1);
+    }
     if (!PageRequestAllowed(main_frame, frame_url, parent_url, domain_)) {
       static char kBody[] = "Forbidden";
       return Respond(403, "Forbidden", "text/plain", Headers(), kBody, sizeof(kBody) - 1);
diff --git a/Packages/macOS/CmuxNext/CEFShim/tests/cmux_page_path_test.cpp b/Packages/macOS/CmuxNext/CEFShim/tests/cmux_page_path_test.cpp
index 6acfcc3a3cd9..311e49171412 100644
--- a/Packages/macOS/CmuxNext/CEFShim/tests/cmux_page_path_test.cpp
+++ b/Packages/macOS/CmuxNext/CEFShim/tests/cmux_page_path_test.cpp
@@ -84,6 +84,51 @@ int main() {
   // A child frame being navigated: its parent decides.
   Allowed(false, "about:blank", "cmux-page://cmux.agent/", "cmux.agent", true);
   Allowed(false, "about:blank", "https://evil.test/", "cmux.agent", false);
+  // Artifact documents must never inherit the page host's file authority.
+  Allowed(false, "cmux-page://cmux.agent/__artifact", "cmux-page://cmux.agent/", "cmux.agent", false);
+  Allowed(false, "about:blank", "cmux-page://cmux.agent/__artifact", "cmux.agent", false);
+  Allowed(false, "about:srcdoc", "cmux-page://cmux.agent/", "cmux.agent", false);
+  {
+    const std::string page = "cmux-page://cmux.agent/";
+    const std::string artifact = cmux_shim::kArtifactFrameURL;
+    auto navigation = [&](bool main, const std::string& current, const std::string& parent,
+                          const std::string& target, bool refused) {
+      Expect(cmux_shim::ArtifactNavigationRefused(main, current, parent, target) == refused,
+             "artifact navigation " + current + " -> " + target + " parent " + parent);
+    };
+    navigation(false, "about:blank", page, artifact, false);
+    navigation(false, "", page, artifact, false);
+    navigation(true, "about:blank", "", artifact, true);
+    for (const auto& parent : {"https://evil.test/", "cmux-page://cmux.agent.evil/",
+                               "cmux-page://cmux.history/", "cmux-page://evil@cmux.agent/"}) {
+      navigation(false, "about:blank", parent, artifact, true);
+    }
+    for (const auto& target : {artifact, artifact + "?copy=1", page, std::string("about:srcdoc"),
+                               std::string("https://example.test/"), std::string("file:///tmp/a.html"),
+                               std::string("data:text/html,replace"), std::string("javascript:alert(1)")}) {
+      navigation(false, artifact, page, target, true);
+      navigation(false, "about:srcdoc", page, target, true);
+      navigation(false, "about:blank", artifact, target, true);
+    }
+    navigation(false, "about:blank", page, artifact + "?copy=1", true);
+    navigation(false, page, "", "https://example.test/", false);
+    navigation(false, "about:blank", page, "http://localhost:3000/", false);
+    for (const auto& url : {"https://example.test/a.png", "http://127.0.0.1:4000/api", "file:///tmp/a.png",
+                            "cmux-page://cmux.agent/index.html", "cmux-agent://pane/index.html"}) {
+      Expect(!cmux_shim::ArtifactResourceAllowed(url, true), "offline artifact refuses " + std::string(url));
+    }
+    Expect(cmux_shim::ArtifactResourceAllowed("data:image/png;base64,AA", true), "inline image allowed");
+    Expect(cmux_shim::ArtifactResourceAllowed("blob:null/test", true), "inline media allowed");
+    Expect(!cmux_shim::ArtifactResourceAllowed("data:text/html,nested", false), "inline document refused");
+    Expect(!cmux_shim::ArtifactResourceAllowed("blob:null/script", false), "blob script refused");
+    const std::string policy = cmux_shim::kArtifactCSP;
+    Expect(policy.find("sandbox allow-scripts;") == 0 && policy.find("allow-same-origin") == std::string::npos,
+           "artifact response enforces opaque origin");
+    Expect(policy.find("connect-src 'none'") != std::string::npos &&
+               policy.find("frame-src 'none'") != std::string::npos &&
+               policy.find("form-action 'none'") != std::string::npos,
+           "artifact response refuses connections, children and forms");
+  }
   {
     auto headers = cmux_shim::PageResponseHeaders("default-src 'self'");
     bool xfo = false, corp = false, nosniff = false, csp = false;