fix: lazy-load MCP SDK to avoid global state pollution in Next.js#141
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mr-kelly wants to merge 1 commit intovercel:mainfrom
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fix: lazy-load MCP SDK to avoid global state pollution in Next.js#141mr-kelly wants to merge 1 commit intovercel:mainfrom
mr-kelly wants to merge 1 commit intovercel:mainfrom
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The @modelcontextprotocol/sdk has dependencies (via undici) that modify the global Response object at import time. This breaks Next.js App Routes which rely on instanceof Response checks, causing "No response is returned" errors. This fix: - Converts eager SDK imports to type-only imports - Adds a lazy loadSdk() function that dynamically imports SDK modules - Defers StreamableHTTPServerTransport creation to first request - Calls loadSdk() before SDK usage in both HTTP and SSE endpoints This ensures the SDK is only loaded after Next.js has fully initialized. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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Hello all, this issue is impacting my project as well. Using Mr Kelly's patch is working. Is there a timeline for incorporating a fix for this issue into the vercel mcp-handler project? |
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The @modelcontextprotocol/sdk has transitive dependencies (via undici) that replace globalThis.Response when imported. This breaks Next.js App Router route handlers which validate responses with instanceof Response, causing "No response is returned from route handler" errors for ALL routes in the same process — not just the MCP endpoint. Lazy-loading alone (as proposed in vercel#141) only delays the problem. Once the MCP endpoint is hit, the global Response is replaced and all subsequent requests to any route handler fail. This fix: - Converts eager SDK imports to type-only imports - Adds a loadSdk() function that dynamically imports SDK modules - Saves globalThis.Response before import, restores it after - Defers StreamableHTTPServerTransport creation to first request - Calls loadSdk() at the start of mcpApiHandler Fixes vercel#140
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The @modelcontextprotocol/sdk has dependencies (via undici) that modify the global Response object at import time. This breaks Next.js App Routes which rely on instanceof Response checks, causing "No response is returned" errors.
This fix:
This ensures the SDK is only loaded after Next.js has fully initialized.