Securing Remote MCP Servers with OAuth 2.1 & Better-Auth

Why Static API Keys Fail for Multi-User AI Agents

Passing a shared root API key into an MCP server gives every prompt full database access. Instead, bind every MCP tool call to a scoped user token verified at the edge.

export async function verifyMcpBearer(request: Request, db: D1Database) {
  const authHeader = request.headers.get("authorization") || "";
  if (!authHeader.startsWith("Bearer ")) {
    throw new Response("Unauthorized MCP client", {
      status: 401,
      headers: { "WWW-Authenticate": 'Bearer realm="mcp"' },
    });
  }
  return { token: authHeader.slice(7).trim() };
}

Production Architecture Takeaway

Always return a standard WWW-Authenticate header on 401 responses so spec-compliant MCP clients automatically trigger the OAuth 2.1 authorization flow.