feat(mcp-server): the canvas shell is the default preview, the classic page behind DRAWIO_PREVIEW_UI=classic

packages/mcp-server/src/drawing-guide.ts

@@ -29,7 +29,7 @@ export const DRAWING_GUIDE = `# Draw.io drawing guide
 - After a successful create_new_diagram or edit_diagram call, do not describe the diagram; the user sees it in the preview. One short sentence at most.
 - For artistic requests (a cat, a logo, a scene), compose the picture from standard shapes and connectors while keeping it clear.
 - To clear the canvas to one blank page, call create_new_diagram with only the two root cells <mxCell id="0"/><mxCell id="1" parent="0"/>; the previous diagram stays in History.
-- The preview page has History (it saves a snapshot before every AI change and can restore any of the last 20 versions) and Download. You can make changes freely; nothing is lost.
+- The preview keeps the last 20 versions (a snapshot before every AI change) and can restore any of them. You can make changes freely; nothing is lost.
 - If the user asks to undo or go back, call restore_version instead of re-sending the earlier XML from memory.
 
 ## The XML you send

packages/mcp-server/src/http-server.ts

@@ -110,15 +110,21 @@ const PACKAGE_VERSION: string = JSON.parse(
 ).version
 
 /**
- * The preview page start_session opens: the classic page (src/preview), or
- * the canvas shell with DRAWIO_PREVIEW_UI=shell. The shell becomes the
- * default once it does everything the classic page does.
+ * The preview page start_session opens: the canvas shell (shell/, the web
+ * app's canvas), or with DRAWIO_PREVIEW_UI=classic the classic page
+ * (src/preview), kept for one more release.
  */
 export type PreviewUi = "classic" | "shell"
-export const PREVIEW_UI: PreviewUi =
-    (process.env.DRAWIO_PREVIEW_UI ?? "").toLowerCase() === "shell"
-        ? "shell"
-        : "classic"
+
+export function previewUiFromEnv(
+    env: NodeJS.ProcessEnv = process.env,
+): PreviewUi {
+    return (env.DRAWIO_PREVIEW_UI ?? "").toLowerCase() === "classic"
+        ? "classic"
+        : "shell"
+}
+
+export const PREVIEW_UI: PreviewUi = previewUiFromEnv()
 
 export function previewUrl(
     port: number,

packages/mcp-server/src/index.ts

@@ -1268,7 +1268,7 @@ server.registerTool(
                     content: [
                         {
                             type: "text",
-                            text: "The classic preview page cannot read the selection; start the server with DRAWIO_PREVIEW_UI=shell. Ask the user which shapes they mean, or call get_diagram.",
+                            text: "The classic preview page (DRAWIO_PREVIEW_UI=classic) cannot read the selection. Ask the user which shapes they mean, or call get_diagram.",
                         },
                     ],
                 }

packages/mcp-server/tests/e2e/shell.spec.ts

@@ -1,8 +1,8 @@
 /**
- * The canvas shell as start_session opens it with DRAWIO_PREVIEW_UI=shell:
- * the built server (dist/index.js, or the copy MCP_SERVER_ENTRY names: CI
- * installs the packed tarball in an empty directory) runs over stdio as an
- * MCP client would start it, and headless Chromium opens the URL it returns.
+ * The canvas shell as start_session opens it by default: the built server
+ * (dist/index.js, or the copy MCP_SERVER_ENTRY names: CI installs the packed
+ * tarball in an empty directory) runs over stdio as an MCP client would
+ * start it, and headless Chromium opens the URL it returns.
  */
 import { mkdtempSync, readFileSync, rmSync } from "node:fs"
 import { tmpdir } from "node:os"
@@ -26,7 +26,6 @@ test.beforeAll(async () => {
         env: {
             ...process.env,
             DRAWIO_DATA_DIR: dataDir,
-            DRAWIO_PREVIEW_UI: "shell",
             // Away from the web app's dev server and other test runs
             PORT: String(6100 + Math.floor(Math.random() * 400)),
             BROWSER: "none",

packages/mcp-server/tests/http-server.test.ts

@@ -27,6 +27,7 @@ import {
     keepInHistory,
     onSessionRecreate,
     onStateChange,
+    previewUiFromEnv,
     previewUrl,
     requestExport,
     requestSelection,
@@ -928,7 +929,15 @@ describe("canvas shell page", () => {
         }
     })
 
-    it("is what start_session opens with DRAWIO_PREVIEW_UI=shell", () => {
+    it("is what start_session opens, unless DRAWIO_PREVIEW_UI=classic", () => {
+        expect(previewUiFromEnv({})).toBe("shell")
+        expect(previewUiFromEnv({ DRAWIO_PREVIEW_UI: "shell" })).toBe("shell")
+        expect(previewUiFromEnv({ DRAWIO_PREVIEW_UI: "classic" })).toBe(
+            "classic",
+        )
+        expect(previewUiFromEnv({ DRAWIO_PREVIEW_UI: "Classic" })).toBe(
+            "classic",
+        )
         expect(previewUrl(6002, "mcp-x", "shell")).toBe(
             "http://localhost:6002/shell/?mcp=mcp-x",
         )

packages/mcp-server/tests/server-wiring.test.ts

@@ -95,9 +95,9 @@ beforeAll(async () => {
             ...process.env,
             DRAWIO_DATA_DIR: dataDir,
             // For the session tests at the end: no browser window, the
-            // canvas shell as the preview, a port away from other servers
+            // default preview (the canvas shell), a port away from other
+            // servers
             BROWSER: "none",
-            DRAWIO_PREVIEW_UI: "shell",
             PORT: String(6500 + Math.floor(Math.random() * 400)),
         },
     }) as ChildProcessWithoutNullStreams