diff --git a/packages/op-vscode/src/vscode/pen-editor-provider.ts b/packages/op-vscode/src/vscode/pen-editor-provider.ts
index 8b7e2f777..172fa19dd 100644
--- a/packages/op-vscode/src/vscode/pen-editor-provider.ts
+++ b/packages/op-vscode/src/vscode/pen-editor-provider.ts
@@ -13,7 +13,7 @@ import { PenSession, type SessionHost } from "../session/pen-session";
 import type { SessionRegistry } from "../session/session-registry";
 import type { BridgeOutboundToPage } from "../protocol/bridge";
 import { encodeOutbound } from "../protocol/bridge";
-import { buildBootHtml, buildWebviewHtml } from "./webview-shell";
+import { appendEmbedQuery, buildBootHtml, buildWebviewHtml } from "./webview-shell";
 import { pickRestartSource, resolveDaemonBinary, type LatestDurable } from "./restart-source";
 import { isShellControl, parseShellReadyOrigin } from "./shell-messages";
 import { encodeFigBytes, figSaveTargetPath, isFigPath } from "./fig-source";
@@ -249,8 +249,13 @@ export class PenEditorProvider implements vscode.CustomEditorProvider<PenDocumen
   ): Promise<void> {
     const key = document.uri.fsPath;
     const nonce = makeNonce();
-    const external = await vscode.env.asExternalUri(vscode.Uri.parse(`${client.baseUrl}/?embed=vscode`));
-    panel.webview.html = buildWebviewHtml({ iframeSrc: external.toString(), nonce });
+    // The embed flag is appended AFTER asExternalUri: putting it in the Uri
+    // makes toString() percent-encode the "=" ("?embed%3Dvscode"), which the
+    // wasm-side EmbedHost::from_query correctly refuses — the editor then
+    // silently renders full chrome inside the plugin.
+    const external = await vscode.env.asExternalUri(vscode.Uri.parse(`${client.baseUrl}/`));
+    const iframeSrc = appendEmbedQuery(external.toString(true));
+    panel.webview.html = buildWebviewHtml({ iframeSrc, nonce });
 
     const host = this.makeHost(document, panel, watcherState);
     const session = new PenSession(host, client.handshake.token, document.bootJson);
diff --git a/packages/op-vscode/src/vscode/webview-shell.test.ts b/packages/op-vscode/src/vscode/webview-shell.test.ts
index 9f28041f9..1a320c47d 100644
--- a/packages/op-vscode/src/vscode/webview-shell.test.ts
+++ b/packages/op-vscode/src/vscode/webview-shell.test.ts
@@ -1,5 +1,5 @@
-import { test, expect } from "bun:test";
-import { buildBootHtml, buildWebviewHtml, originOf } from "./webview-shell";
+import { describe, expect, test } from "bun:test";
+import { appendEmbedQuery, buildBootHtml, buildWebviewHtml, originOf } from "./webview-shell";
 
 test("originOf extracts scheme://host:port", () => {
   expect(originOf("http://127.0.0.1:45001/")).toBe("http://127.0.0.1:45001");
@@ -56,3 +56,17 @@ test("nonce is not reused across boot and full unless the caller reuses it", ()
   expect(buildBootHtml("A")).toContain('nonce="A"');
   expect(buildWebviewHtml({ iframeSrc: "http://x.y:1/", nonce: "B" })).toContain('nonce="B"');
 });
+
+describe("appendEmbedQuery", () => {
+  test("appends ?embed=vscode to a bare externalized URL", () => {
+    expect(appendEmbedQuery("http://127.0.0.1:49793/")).toBe("http://127.0.0.1:49793/?embed=vscode");
+  });
+  test("appends with & when the base already has a query", () => {
+    expect(appendEmbedQuery("https://tunnel.example/x/?tkn=abc")).toBe(
+      "https://tunnel.example/x/?tkn=abc&embed=vscode",
+    );
+  });
+  test("never percent-encodes the equals sign", () => {
+    expect(appendEmbedQuery("http://127.0.0.1:1/")).not.toContain("%3D");
+  });
+});
diff --git a/packages/op-vscode/src/vscode/webview-shell.ts b/packages/op-vscode/src/vscode/webview-shell.ts
index 854406d36..9c99881d2 100644
--- a/packages/op-vscode/src/vscode/webview-shell.ts
+++ b/packages/op-vscode/src/vscode/webview-shell.ts
@@ -11,6 +11,15 @@ export function originOf(url: string): string {
   return new URL(url).origin;
 }
 
+/** Append the `embed=vscode` flag to an already-externalized editor URL.
+ *  Kept as plain string concatenation (never `vscode.Uri`): Uri.toString()
+ *  percent-encodes the query's "=" into "%3D", which the wasm-side
+ *  `EmbedHost::from_query` refuses by design. Tolerates a base that already
+ *  carries a query (e.g. tunnel tokens in remote scenarios). */
+export function appendEmbedQuery(base: string): string {
+  return `${base}${base.includes("?") ? "&" : "?"}embed=vscode`;
+}
+
 /** Phase 1: no iframe. On load it reports window.origin so the extension can
  *  spawn the daemon with the correct --allow-origin, then waits for the
  *  extension to replace the HTML with the full shell. */