McpStatelessSyncServer not initialized under spring.main.lazy-initialization=true, causing "MCP handler not configured" on every request

Summary

When spring.main.lazy-initialization=true is active, McpStatelessSyncServer is never eagerly constructed. Because setMcpHandler is only called from the server's constructor/build phase, WebMvcStatelessServerTransport.mcpHandler stays null permanently. Every incoming MCP request then hits the null-check at WebMvcStatelessServerTransport.handlePost (line ~146) and returns HTTP 500 with JSON-RPC error -32603:

{
  "jsonRpcError": {
    "code": -32603,
    "message": "MCP handler not configured"
  }
}

The transport bean and its RouterFunction are created eagerly (because Spring MVC's handler mapping requires them at context refresh), so the endpoint is reachable — but the handler wire is never made.

Related issue

#3868 describes the same root cause on the SSE transport (WebMvcSseServerTransportProvider). This issue covers the stateless transport path, which is a separate code path introduced later.

Environment

• Spring AI: 2.0.0-M5
• spring-ai-starter-mcp-server-webmvc
• spring.ai.mcp.server.protocol=STATELESS, spring.ai.mcp.server.type=SYNC
• spring.main.lazy-initialization=true
• Spring Boot 4.1.0-RC1 / Tomcat (WebMVC)

Stack trace (trimmed)

io.modelcontextprotocol.spec.McpError$Builder.build(McpError.java:72)
org.springframework.ai.mcp.server.webmvc.transport.WebMvcStatelessServerTransport.handlePost(WebMvcStatelessServerTransport.java:150)
org.springframework.web.servlet.function.support.HandlerFunctionAdapter.handle(HandlerFunctionAdapter.java:106)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
...

Root cause

McpServerStatelessAutoConfiguration.mcpStatelessSyncServer() has no @Lazy(false) guard. Under global lazy init, the bean is deferred until first programmatic access, which never happens via the normal request path. The transport's setMcpHandler is therefore never called.

Suggested fix

Add @Lazy(false) to the mcpStatelessSyncServer bean definition in McpServerStatelessAutoConfiguration:

@Bean
@Lazy(false)
@ConditionalOnProperty(prefix = McpServerProperties.CONFIG_PREFIX, name = "type", havingValue = "SYNC",
        matchIfMissing = true)
public McpStatelessSyncServer mcpStatelessSyncServer(McpStatelessServerTransport statelessTransport, ...) {
    ...
}

This mirrors the fix that should also apply to the SSE transport variant in #3868.

Start in McpServerStatelessAutoConfiguration.mcpStatelessSyncServer() and trace how it initializes WebMvcStatelessServerTransport; inspect WebMvcStatelessServerTransport.handlePost() for the null-handler path. Reproduce with the listed lazy-initialization and stateless SYNC settings, then verify MCP requests no longer return HTTP 500 with “MCP handler not configured.”