fix: workaround models not providing arguments when everything is optional

src/server.ts

@@ -8,6 +8,8 @@ import { mongoLogId } from "mongodb-log-writer";
 import { ObjectId } from "mongodb";
 import { Telemetry } from "./telemetry/telemetry.js";
 import { UserConfig } from "./config.js";
+import { CallToolRequestSchema, CallToolResult } from "@modelcontextprotocol/sdk/types.js";
+import assert from "assert";
 
 export interface ServerOptions {
     session: Session;
@@ -33,6 +35,29 @@ export class Server {
         this.registerTools();
         this.registerResources();
 
+        // This is a workaround for an issue we've seen with some models, where they'll see that everything in the `arguments`
+        // object is optional, and then not pass it at all. However, the MCP server expects the `arguments` object to be if
+        // the tool accepts any arguments, even if they're all optional.
+        //
+        // see: https://github.com/modelcontextprotocol/typescript-sdk/blob/131776764536b5fdca642df51230a3746fb4ade0/src/server/mcp.ts#L705
+        // Since paramsSchema here is not undefined, the server will create a non-optional z.object from it.
+        const existingHandler = (
+            this.mcpServer.server["_requestHandlers"] as Map<
+                string,
+                (request: unknown, extra: unknown) => Promise<CallToolResult>
+            >
+        ).get(CallToolRequestSchema.shape.method.value);
+
+        assert(existingHandler, "No existing handler found for CallToolRequestSchema");
+
+        this.mcpServer.server.setRequestHandler(CallToolRequestSchema, (request, extra): Promise<CallToolResult> => {
+            if (!request.params.arguments) {
+                request.params.arguments = {};
+            }
+
+            return existingHandler(request, extra);
+        });
+
         await initializeLogger(this.mcpServer, this.userConfig.logPath);
 
         await this.mcpServer.connect(transport);

tests/integration/tools/mongodb/metadata/connect.test.ts

@@ -15,6 +15,23 @@ describeWithMongoDB("Connect tool", (integration) => {
         },
     ]);
 
+    describe("without arguments", () => {
+        it("prompts for connection string if not set", async () => {
+            const response = await integration.mcpClient().callTool({ name: "connect" });
+            const content = getResponseContent(response.content);
+            expect(content).toContain("No connection details provided");
+        });
+
+        it("connects to the database if connection string is set", async () => {
+            config.connectionString = integration.connectionString();
+
+            const response = await integration.mcpClient().callTool({ name: "connect" });
+            const content = getResponseContent(response.content);
+            expect(content).toContain("Successfully connected");
+            expect(content).toContain(integration.connectionString());
+        });
+    });
+
     describe("with default config", () => {
         describe("without connection string", () => {
             it("prompts for connection string", async () => {