v2 backcompat: server legacy package (#2206)

Co-authored-by: Felix Weinberger <3823880+felixweinberger@users.noreply.github.com>

packages/server-legacy/README.md

@@ -0,0 +1,39 @@
+# @modelcontextprotocol/server-legacy
+
+> **Deprecated** — This package is a frozen copy of v1 code for migration purposes only. It will not receive new features and is planned for removal in v3.
+
+Provides two pieces of v1 server functionality removed in v2:
+
+- **SSE Transport** (`./sse`) — The `SSEServerTransport` class, replaced in v2 by `NodeStreamableHTTPServerTransport` (from `@modelcontextprotocol/node`) or `WebStandardStreamableHTTPServerTransport` (from `@modelcontextprotocol/server`)
+- **OAuth Authorization Server** (`./auth`) — The `mcpAuthRouter` and related helpers, removed in v2 because MCP servers should use dedicated OAuth providers
+
+## Installation
+
+```bash
+npm install @modelcontextprotocol/server-legacy
+```
+
+## Usage
+
+### SSE Transport (no Express dependency required)
+
+```ts
+import { SSEServerTransport } from '@modelcontextprotocol/server-legacy/sse';
+```
+
+### OAuth Auth Router (requires Express)
+
+```ts
+import { mcpAuthRouter } from '@modelcontextprotocol/server-legacy/auth';
+```
+
+### Everything (requires Express)
+
+```ts
+import { SSEServerTransport, mcpAuthRouter } from '@modelcontextprotocol/server-legacy';
+```
+
+## Migration
+
+- **SSE → StreamableHTTP**: Use `NodeStreamableHTTPServerTransport` from `@modelcontextprotocol/node` (Node.js) or `WebStandardStreamableHTTPServerTransport` from `@modelcontextprotocol/server` (Web Standard / Cloudflare Workers)
+- **Auth router → Dedicated IdP**: Use a dedicated OAuth provider (Auth0, Keycloak, etc.) instead of the built-in OAuth Authorization Server