fix(pdf-server): resolve host-qualified tool names for the viewer's own tool calls

examples/pdf-server/src/mcp-app.ts

@@ -76,6 +76,7 @@ import {
   getAnnotationPreview,
   getAnnotationColor,
 } from "./annotation-panel.js";
+import { resolveToolName } from "./tool-names.js";
 import "./global.css";
 import "./mcp-app.css";
 
@@ -706,6 +707,16 @@ const app = new App(
   { autoResize: false },
 );
 
+/**
+ * Name to use for `app.callServerTool` when calling one of this server's own
+ * tools. Hosts that aggregate servers may qualify tool names (for example
+ * `pdf-server-save_pdf`); the prefix is recovered from the name the host
+ * invoked `display_pdf` under. See tool-names.ts and ext-apps issue #753.
+ */
+function toolName(baseName: string): string {
+  return resolveToolName(baseName, app.getHostContext()?.toolInfo?.tool?.name);
+}
+
 // UI State functions
 function showLoading(text: string) {
   loadingTextEl.textContent = text;
@@ -2575,7 +2586,7 @@ async function handleGetViewerState(requestId: string): Promise<void> {
   };
 
   await app.callServerTool({
-    name: "submit_viewer_state",
+    name: toolName("submit_viewer_state"),
     arguments: { requestId, state: JSON.stringify(state, null, 2) },
   });
 }
@@ -2662,7 +2673,7 @@ async function handleGetPages(cmd: {
   // Submit results back to server
   try {
     await app.callServerTool({
-      name: "submit_page_data",
+      name: toolName("submit_page_data"),
       arguments: { requestId: cmd.requestId, pages: results },
     });
     log.info(
@@ -3182,7 +3193,7 @@ async function savePdf(): Promise<void> {
     const base64 = uint8ArrayToBase64(pdfBytes);
 
     const result = await app.callServerTool({
-      name: "save_pdf",
+      name: toolName("save_pdf"),
       arguments: { url: pdfUrl, data: base64 },
     });
 
@@ -4339,7 +4350,7 @@ async function fetchChunk(
   const request = (async (): Promise<RangeResult> => {
     try {
       const result = await app.callServerTool({
-        name: "read_pdf_bytes",
+        name: toolName("read_pdf_bytes"),
         arguments: { url, offset: begin, byteCount: end - begin },
       });
 
@@ -4946,7 +4957,7 @@ async function processCommands(commands: PdfCommand[]): Promise<void> {
           log.error("get_pages failed — submitting empty result:", err);
           await app
             .callServerTool({
-              name: "submit_page_data",
+              name: toolName("submit_page_data"),
               arguments: { requestId: cmd.requestId, pages: [] },
             })
             .catch(() => {});
@@ -4959,15 +4970,15 @@ async function processCommands(commands: PdfCommand[]): Promise<void> {
           const pdfBytes = await getAnnotatedPdfBytes();
           const base64 = uint8ArrayToBase64(pdfBytes);
           await app.callServerTool({
-            name: "submit_save_data",
+            name: toolName("submit_save_data"),
             arguments: { requestId: cmd.requestId, data: base64 },
           });
           log.info(`save_as: submitted ${pdfBytes.length} bytes`);
         } catch (err) {
           log.error("save_as: failed to build bytes — submitting error:", err);
           await app
             .callServerTool({
-              name: "submit_save_data",
+              name: toolName("submit_save_data"),
               arguments: {
                 requestId: cmd.requestId,
                 error: err instanceof Error ? err.message : String(err),
@@ -4984,7 +4995,7 @@ async function processCommands(commands: PdfCommand[]): Promise<void> {
           log.error("get_viewer_state failed — submitting error:", err);
           await app
             .callServerTool({
-              name: "submit_viewer_state",
+              name: toolName("submit_viewer_state"),
               arguments: {
                 requestId: cmd.requestId,
                 error: err instanceof Error ? err.message : String(err),
@@ -5058,7 +5069,7 @@ async function pollLoop(): Promise<void> {
   while (polling && viewUUID) {
     try {
       const result = await app.callServerTool({
-        name: "poll_pdf_commands",
+        name: toolName("poll_pdf_commands"),
         arguments: { viewUUID },
       });
       if (result.isError) {

examples/pdf-server/src/tool-names.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, test } from "bun:test";
+import { resolveToolName } from "./tool-names";
+
+describe("resolveToolName", () => {
+  test("applies the host's prefix to other tools", () => {
+    expect(resolveToolName("save_pdf", "pdf-server-display_pdf")).toBe(
+      "pdf-server-save_pdf",
+    );
+    expect(resolveToolName("poll_pdf_commands", "a__b__display_pdf")).toBe(
+      "a__b__poll_pdf_commands",
+    );
+  });
+
+  test("keeps the bare name when the host does not rewrite names", () => {
+    expect(resolveToolName("save_pdf", "display_pdf")).toBe("save_pdf");
+  });
+
+  test("keeps the bare name when there is no host context", () => {
+    expect(resolveToolName("save_pdf", undefined)).toBe("save_pdf");
+    expect(resolveToolName("save_pdf", null)).toBe("save_pdf");
+    expect(resolveToolName("save_pdf", "")).toBe("save_pdf");
+  });
+
+  test("keeps the bare name when the invoked tool is not the entry tool", () => {
+    expect(resolveToolName("save_pdf", "other-server-get-time")).toBe(
+      "save_pdf",
+    );
+  });
+});

examples/pdf-server/src/tool-names.ts

@@ -0,0 +1,27 @@
+/**
+ * Resolve the name a widget must use to call one of this server's own tools.
+ *
+ * A host that aggregates several MCP servers behind one session (mcphub, for
+ * example) can qualify tool names to avoid collisions, so `save_pdf` becomes
+ * `pdf-server-save_pdf`. `callServerTool` must then use the qualified name, but
+ * the widget only knows the bare one. The host does tell us the name it
+ * invoked the entry tool under (`toolInfo.tool.name`), so the prefix it added
+ * to the entry tool can be recovered and applied to every other tool.
+ *
+ * Falls back to the bare name when the host does not rewrite names, or when
+ * the invoked name is not this server's entry tool.
+ *
+ * See https://github.com/modelcontextprotocol/ext-apps/issues/753
+ */
+
+/** The only tool that mounts the viewer (the one with `_meta.ui.resourceUri`). */
+export const ENTRY_TOOL = "display_pdf";
+
+export function resolveToolName(
+  baseName: string,
+  invokedName: string | undefined | null,
+  entryTool: string = ENTRY_TOOL,
+): string {
+  if (!invokedName || !invokedName.endsWith(entryTool)) return baseName;
+  return invokedName.slice(0, invokedName.length - entryTool.length) + baseName;
+}