feat(app): throw helpful error when callServerTool is called with a string (modelcontextprotocol#449)

Users frequently call callServerTool("tool_name", args) instead of the
correct callServerTool({ name: "tool_name", arguments: args }), resulting
in a silent/confusing failure. Detect this and throw immediately with
a message that shows the correct call shape.

Also fixes the incorrect example in examples/pdf-server/README.md that
demonstrated this same wrong usage pattern.

Fixes modelcontextprotocol#386

https://claude.ai/code/session_01GqAyN4Ux7svWoU2HqsSLZF

Co-authored-by: Claude <noreply@anthropic.com>

Author: ochafik

examples/pdf-server/README.md

@@ -63,7 +63,10 @@ On some host platforms, tool calls have size limits, so large PDFs cannot be sen
 ```typescript
 // Load in chunks with progress
 while (hasMore) {
-  const chunk = await app.callServerTool("read_pdf_bytes", { url, offset });
+  const chunk = await app.callServerTool({
+    name: "read_pdf_bytes",
+    arguments: { url, offset },
+  });
   chunks.push(base64ToBytes(chunk.bytes));
   offset += chunk.byteCount;
   hasMore = chunk.hasMore;

src/app-bridge.test.ts

@@ -678,6 +678,19 @@ describe("App <-> AppBridge integration", () => {
       expect(result.content).toEqual(resultContent);
     });
 
+    it("callServerTool throws a helpful error when called with a string instead of params object", async () => {
+      await bridge.connect(bridgeTransport);
+      await app.connect(appTransport);
+
+      await expect(
+        // @ts-expect-error intentionally testing wrong usage
+        app.callServerTool("my_tool"),
+      ).rejects.toThrow(
+        'callServerTool() expects an object as its first argument, but received a string ("my_tool"). ' +
+          'Did you mean: callServerTool({ name: "my_tool", arguments: { ... } })?',
+      );
+    });
+
     it("onlistresources setter registers handler for resources/list requests", async () => {
       const requestParams = {};
       const resources = [{ uri: "test://resource", name: "Test" }];

src/app.ts

@@ -724,6 +724,12 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
     params: CallToolRequest["params"],
     options?: RequestOptions,
   ): Promise<CallToolResult> {
+    if (typeof params === "string") {
+      throw new Error(
+        `callServerTool() expects an object as its first argument, but received a string ("${params}"). ` +
+          `Did you mean: callServerTool({ name: "${params}", arguments: { ... } })?`,
+      );
+    }
     return await this.request(
       { method: "tools/call", params },
       CallToolResultSchema,