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Connect upstream MCP servers with high-level transport helpers
This page is kept for older links. The current concepts entrypoint is Servers and transports.

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Quick Start

Use stdio(...) for local upstream MCP servers: 
import { fentaris, mcp, stdio } from "@fentaris/core";

const proxy = fentaris({
  servers: [
    mcp("filesystem", {
      transport: stdio({
        command: "npx",
        args: ["-y", "@modelcontextprotocol/server-filesystem", "./demo-files"],
        stderr: "inherit",
      }),
    }),
  ],
});
Use streamableHttp(...) for native remote MCP HTTP endpoints: 
import { fentaris, mcp, streamableHttp } from "@fentaris/core";

const proxy = fentaris({
  servers: [
    mcp("remote", {
      transport: streamableHttp({
        url: "https://mcp.example.com/mcp",
        auth: {
          headers: { "x-tenant": "acme" },
          bearerToken: "service-token",
        },
      }),
    }),
  ],
});

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Transport Matrix

DirectionHelperUse case
Upstreamstdio(...)Local MCP process over stdin/stdout.
UpstreamstreamableHttp(...)Remote native MCP Streamable HTTP endpoint.
UpstreamSseMcpTransportLegacy SSE-capable MCP servers.
UpstreamHttpTransportCompatibility adapter for REST-like /listTools and /callTool endpoints.
Downstreamproxy.start()Default Streamable HTTP exposure for MCP clients.

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Stdio Options

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command

Executable to spawn.

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args

Arguments passed to the upstream process.

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env

Environment variables passed to the upstream process.

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stderr

How to handle stderr from the upstream process. Use inherit during local development to see upstream logs.

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HTTP-Family Auth

streamableHttp(...) supports static headers, bearer tokens, API keys, and per-user header resolvers. 
streamableHttp({
  url: "https://mcp.example.com/mcp",
  auth: {
    resolveHeaders: ({ user }) => ({
      "x-user-id": user.id ?? "anonymous",
      authorization: `Bearer ${user.tokens?.remote ?? ""}`,
    }),
  },
})
Fentaris does not acquire or refresh OAuth tokens. Pass an already-valid access token through a resolver when the upstream needs one.

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Close Cleanly

Close the proxy on shutdown so upstream transports can release processes, sockets, and sessions. 
process.on("SIGTERM", async () => {
  await proxy.close();
  process.exit(0);
});

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Low-Level API

Transport classes such as StdioTransport, StreamableHttpMcpTransport, SseMcpTransport, and exposure transports remain available for advanced integrations. New application docs should prefer stdio(...), streamableHttp(...), and proxy.start().