fix(server): accept metadata on stateful initialize

examples/servers/typescript/everything-server.ts

@@ -1284,13 +1284,21 @@ app.post('/mcp', async (req, res) => {
   const meta = params._meta;
   const metaVersion = meta?.['io.modelcontextprotocol/protocolVersion'];
 
-  // A request that carries no `_meta` and names a legacy session-era revision
-  // in the header is legacy traffic; it is served by the session path below
-  // instead of being rejected for missing per-request metadata.
+  // An initialize request negotiates its version in params and may carry the
+  // optional RequestParams `_meta` field. Route session-era versions through
+  // the stateful transport even when that metadata is present.
+  const isLegacyInitializeRequest =
+    isInitializeRequest(body) &&
+    typeof params.protocolVersion === 'string' &&
+    LEGACY_SESSION_PROTOCOL_VERSIONS.includes(params.protocolVersion);
+
+  // A non-initialize request that carries no `_meta` and names a legacy
+  // session-era revision in the header is also legacy traffic.
   const isLegacySessionEraRequest =
-    meta === undefined &&
-    reqVersion !== undefined &&
-    LEGACY_SESSION_PROTOCOL_VERSIONS.includes(reqVersion);
+    isLegacyInitializeRequest ||
+    (meta === undefined &&
+      reqVersion !== undefined &&
+      LEGACY_SESSION_PROTOCOL_VERSIONS.includes(reqVersion));
 
   if (!sessionId && (reqVersion || meta) && !isLegacySessionEraRequest) {
     // Missing Transport Header Validation Check

src/scenarios/server/all-scenarios.test.ts

@@ -177,6 +177,37 @@ describe('Server Scenarios', () => {
     expect(nonFailures.length).toBe(checks.length);
   }
 
+  it('accepts request metadata on a stateful initialize', async () => {
+    const response = await fetch(serverUrl, {
+      method: 'POST',
+      headers: {
+        Accept: 'application/json, text/event-stream',
+        'Content-Type': 'application/json'
+      },
+      body: JSON.stringify({
+        jsonrpc: '2.0',
+        id: 1,
+        method: 'initialize',
+        params: {
+          protocolVersion: '2025-11-25',
+          capabilities: {},
+          clientInfo: { name: 'metadata-test', version: '1.0.0' },
+          _meta: {}
+        }
+      })
+    });
+
+    const sessionId = response.headers.get('mcp-session-id');
+    expect(response.status).toBe(200);
+    expect(sessionId).toBeTruthy();
+    expect(await response.text()).toContain('"protocolVersion":"2025-11-25"');
+
+    await fetch(serverUrl, {
+      method: 'DELETE',
+      headers: { 'mcp-session-id': sessionId! }
+    });
+  });
+
   for (const scenarioName of scenarios) {
     it(`${scenarioName}`, async () => {
       await expectScenarioToPass(scenarioName);