diff --git a/.changeset/green-dryers-film.md b/.changeset/green-dryers-film.md
new file mode 100644
index 0000000000..dbad2e856f
--- /dev/null
+++ b/.changeset/green-dryers-film.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/server": patch
+---
+
+fix(server): reject requests before initialization
diff --git a/packages/server/src/server/server.ts b/packages/server/src/server/server.ts
index 1582f8c6eb..70a6aaab92 100644
--- a/packages/server/src/server/server.ts
+++ b/packages/server/src/server/server.ts
@@ -285,6 +285,7 @@ export class Server extends Protocol<ServerContext> {
     private _capabilities: ServerCapabilities;
     private _instructions?: string;
     private _jsonSchemaValidator: jsonSchemaValidator;
+    private _receivedInitialize = false;
     private _cacheHints?: ServerOptions['cacheHints'];
     private _requestStateVerify?: (state: string, ctx: ServerContext) => unknown | Promise<unknown>;
     private _inputRequiredServing: { maxRounds: number; roundTimeoutMs: number; legacyShim: boolean };
@@ -473,6 +474,17 @@ export class Server extends Protocol<ServerContext> {
         method: string,
         handler: (request: JSONRPCRequest, ctx: ServerContext) => Promise<Result>
     ): (request: JSONRPCRequest, ctx: ServerContext) => Promise<Result> {
+        const lifecycleHandler: (request: JSONRPCRequest, ctx: ServerContext) => Promise<Result> = async (request, ctx) => {
+            if (!ctx.http && ctx.sessionId === undefined && !this._receivedInitialize && method !== 'initialize' && method !== 'ping') {
+                throw new ProtocolError(ProtocolErrorCode.InvalidRequest, 'Server not initialized');
+            }
+            const result = await handler(request, ctx);
+            if (method === 'initialize') {
+                this._receivedInitialize = true;
+            }
+            return result;
+        };
+
         if (method !== 'tools/call') {
             const cacheHint = (this._cacheHints as Record<string, CacheHint | undefined> | undefined)?.[method];
             const isInputRequiredCapable = INPUT_REQUIRED_CAPABLE_METHODS.has(method);
@@ -481,7 +493,7 @@ export class Server extends Protocol<ServerContext> {
                 // whose result vocabulary does not include it is never
                 // mis-typed onto the wire.
                 return async (request, ctx) => {
-                    const result = await handler(request, ctx);
+                    const result = await lifecycleHandler(request, ctx);
                     if (isInputRequiredResult(result)) {
                         throw new ProtocolError(
                             ProtocolErrorCode.InternalError,
@@ -494,8 +506,8 @@ export class Server extends Protocol<ServerContext> {
             }
             return async (request, ctx) => {
                 const result = isInputRequiredCapable
-                    ? await this._invokeInputRequiredCapableHandler(method, handler, request, ctx)
-                    : await handler(request, ctx);
+                    ? await this._invokeInputRequiredCapableHandler(method, lifecycleHandler, request, ctx)
+                    : await lifecycleHandler(request, ctx);
                 if (isInputRequiredResult(result)) {
                     if (!isInputRequiredCapable) {
                         throw new ProtocolError(
@@ -530,7 +542,7 @@ export class Server extends Protocol<ServerContext> {
                 );
             }
 
-            const result = await this._invokeInputRequiredCapableHandler('tools/call', handler, request, ctx);
+            const result = await this._invokeInputRequiredCapableHandler('tools/call', lifecycleHandler, request, ctx);
             if (isInputRequiredResult(result)) {
                 // Already checked by the seam; the CallToolResult schema does
                 // not apply to it (no widening — InputRequiredResult travels
diff --git a/packages/server/test/server/server.test.ts b/packages/server/test/server/server.test.ts
index 0a96a0bb66..437a86e49b 100644
--- a/packages/server/test/server/server.test.ts
+++ b/packages/server/test/server/server.test.ts
@@ -4,6 +4,7 @@ import {
     InMemoryTransport,
     isJSONRPCResultResponse,
     LATEST_PROTOCOL_VERSION,
+    ProtocolErrorCode,
     SUPPORTED_PROTOCOL_VERSIONS
 } from '@modelcontextprotocol/core-internal';
 import { Server } from '../../src/server/server';
@@ -84,6 +85,73 @@ describe('Server', () => {
 
             await server.close();
         });
+
+        it('rejects requests before initialize', async () => {
+            const server = new Server({ name: 'test', version: '1.0.0' }, { capabilities: { tools: {} } });
+
+            server.setRequestHandler('tools/list', async () => ({ tools: [] }));
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await server.connect(serverTransport);
+
+            const responses: JSONRPCMessage[] = [];
+            clientTransport.onmessage = message => responses.push(message);
+            await clientTransport.start();
+
+            await clientTransport.send({
+                jsonrpc: '2.0',
+                method: 'notifications/initialized'
+            } as JSONRPCMessage);
+
+            await clientTransport.send({
+                jsonrpc: '2.0',
+                id: 1,
+                method: 'tools/list',
+                params: {}
+            } as JSONRPCMessage);
+
+            await vi.waitFor(() => expect(responses.some(message => 'id' in message && message.id === 1)).toBe(true));
+
+            const rejected = responses.find(message => 'id' in message && message.id === 1);
+            expect(rejected).toMatchObject({
+                error: {
+                    code: ProtocolErrorCode.InvalidRequest,
+                    message: 'Server not initialized'
+                }
+            });
+
+            await clientTransport.send({
+                jsonrpc: '2.0',
+                id: 2,
+                method: 'initialize',
+                params: {
+                    protocolVersion: LATEST_PROTOCOL_VERSION,
+                    capabilities: {},
+                    clientInfo: { name: 'test-client', version: '1.0.0' }
+                }
+            } as JSONRPCMessage);
+            await vi.waitFor(() => expect(responses.some(message => 'id' in message && message.id === 2)).toBe(true));
+
+            await clientTransport.send({
+                jsonrpc: '2.0',
+                method: 'notifications/initialized'
+            } as JSONRPCMessage);
+
+            await clientTransport.send({
+                jsonrpc: '2.0',
+                id: 3,
+                method: 'tools/list',
+                params: {}
+            } as JSONRPCMessage);
+
+            await vi.waitFor(() => expect(responses.some(message => 'id' in message && message.id === 3)).toBe(true));
+
+            expect(responses.find(message => 'id' in message && message.id === 3)).toMatchObject({
+                result: { tools: [] }
+            });
+
+            await server.close();
+        });
     });
 
     describe('getNegotiatedProtocolVersion', () => {