Fix request-metadata handling of empty HTTP probes

src/scenarios/client/request-metadata.test.ts

@@ -6,7 +6,10 @@ import {
 } from './auth/test_helpers/testClient';
 import { getHandler } from '../../../examples/clients/typescript/everything-client';
 import { getScenario } from '../index';
-import { DECLARED_CHECK_IDS } from './request-metadata';
+import {
+  DECLARED_CHECK_IDS,
+  RequestMetadataScenario
+} from './request-metadata';
 
 // A bad client that does not send _meta
 async function badClient(serverUrl: string) {
@@ -246,6 +249,48 @@ describe('request-metadata client scenario — client never connects', () => {
   });
 });
 
+describe('request-metadata client scenario — HTTP handling', () => {
+  test.each(['GET', 'DELETE'])(
+    'rejects an empty-body %s request without parsing it as JSON',
+    async (method) => {
+      const scenario = new RequestMetadataScenario();
+      const { serverUrl } = await scenario.start(testScenarioContext());
+
+      try {
+        const response = await fetch(serverUrl, { method });
+
+        expect(response.status).toBe(405);
+        expect(response.headers.get('allow')).toBe('POST');
+        expect(await response.text()).toBe('Method Not Allowed');
+      } finally {
+        await scenario.stop();
+      }
+    }
+  );
+
+  test('returns a JSON-RPC parse error for malformed POST bodies', async () => {
+    const scenario = new RequestMetadataScenario();
+    const { serverUrl } = await scenario.start(testScenarioContext());
+
+    try {
+      const response = await fetch(serverUrl, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: '{'
+      });
+
+      expect(response.status).toBe(400);
+      expect(await response.json()).toMatchObject({
+        jsonrpc: '2.0',
+        id: null,
+        error: { code: -32700 }
+      });
+    } finally {
+      await scenario.stop();
+    }
+  });
+});
+
 describe('request-metadata client scenario — negative tests', () => {
   test('client fails when omitting _meta', async () => {
     const runner = new InlineClientRunner(badClient);

src/scenarios/client/request-metadata.ts

@@ -129,12 +129,39 @@ export class RequestMetadataScenario implements Scenario {
     req: http.IncomingMessage,
     res: http.ServerResponse
   ): void {
+    // This scenario only evaluates the metadata attached to POST requests.
+    // Clients may probe a Streamable HTTP endpoint with an empty-body GET
+    // before sending MCP requests, so reject unsupported methods before
+    // attempting to parse a JSON-RPC body.
+    if (req.method !== 'POST') {
+      res.writeHead(405, { Allow: 'POST' });
+      res.end('Method Not Allowed');
+      return;
+    }
+
     let body = '';
     req.on('data', (chunk) => {
       body += chunk.toString();
     });
     req.on('end', () => {
-      const request = JSON.parse(body);
+      let request;
+      try {
+        request = JSON.parse(body);
+      } catch (error) {
+        res.writeHead(400, { 'Content-Type': 'application/json' });
+        res.end(
+          JSON.stringify({
+            jsonrpc: '2.0',
+            id: null,
+            error: {
+              code: -32700,
+              message: `Parse error: ${String(error)}`
+            }
+          })
+        );
+        return;
+      }
+
       this.requestsObserved++;
 
       // Extract version and headers