A composite Model Context Protocol (MCP) gateway is an MCP server to its upstream host and an MCP client to one or more downstream MCP servers. In TypeScript, the official SDK provides building blocks for both roles; your gateway supplies the policy and orchestration that decide what to expose, how to route calls, and which identities may use them. The mediator pattern is an architectural approach, not a requirement of the MCP specification.
How a composite MCP gateway works
Think of the gateway as three parts that meet at a policy boundary:
- Inbound server: presents a deliberate set of tools, resources, or prompts to the connected MCP host.
- Downstream clients: connect to servers that provide capabilities the gateway may use.
- Policy and orchestration: selects which capabilities to expose, maps names and schemas, applies authorization, and handles results and errors.
The official TypeScript SDK’s v2 line separates server and client packages: @modelcontextprotocol/server builds servers, while @modelcontextprotocol/client connects to them. The SDK documentation says v2 implements the 2026-07-28 MCP specification; check the current documentation and package versions when starting a project because both package details and protocol compatibility can change.
The SDK’s client connection guide describes a Client as holding one connection to one server. A gateway that integrates several downstream servers therefore needs to manage a client connection for each one, or place those connections behind its own routing layer. This is an architectural consequence of the client model, not a protocol-mandated gateway design.
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How to plan the implementation
1. Define the gateway’s public surface
Decide which downstream tools, resources, and prompts should be visible upstream. Avoid treating discovery as automatic permission to expose everything: choose what the gateway supports and what its callers are allowed to invoke. If downstream names or schemas need mapping, keep that mapping explicit so the gateway’s public contract remains understandable.
2. Manage a client for each downstream server
For each server, choose its transport, establish the connection, and complete initialization before relying on its capabilities. Initialization provides the negotiated protocol version, server capabilities, and instructions. Request only operations the connected server declares it supports. The SDK’s client guide documents the connection and initialization flow: v2 client connections.
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3. Route calls through policy
When an upstream request arrives, the gateway should identify the corresponding downstream connection and operation, check authorization, then translate or forward the request as its design requires. Decide how to present downstream errors and results to the host, and what to record for audit purposes. These are gateway responsibilities; the MCP mediator pattern does not prescribe one universal routing or policy layer.
4. Expose the server face
Separately configure the gateway as an MCP server for its upstream host. The TypeScript SDK repository describes optional thin adapters for Node HTTP, Express, Fastify, and Hono; these help wire a server into a web framework rather than supply MCP features or business logic. Confirm the current adapter packages in the official TypeScript SDK repository.
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Choose transport and session behavior
For new remote integrations, the SDK documentation presents Streamable HTTP as the modern transport. The appropriate choice depends on whether a downstream server is remote or process-local, whether the gateway needs session features, and whether it must accommodate older servers.
| Option | When it fits | Trade-off or caveat |
|---|---|---|
| Streamable HTTP | Connecting to a remote MCP server; the documented modern remote-server transport. | Supports HTTP POST request/response, optional SSE notifications, JSON-only response mode, and session management/resumability. |
| Stateless Streamable HTTP | Simple API-style server behavior without session tracking. | Does not provide session tracking. |
| Stateful Streamable HTTP | When session features and resumability are needed. | Session transports are held in memory; close idle sessions and set concurrent-session limits based on available memory. |
| stdio | Local integrations where the client spawns the server process. | The SDK communicates over that process’s standard input and output using JSON-RPC. |
| Legacy HTTP + SSE | Compatibility with older SSE-only servers. | Retained for backwards compatibility, not the default for a new deployment. The v2 client guide recommends trying Streamable HTTP first and using a fresh Client for SSE fallback. |
These transport and session details are documented in the SDK server guide, which is v1 documentation, so verify exact API parity before copying its implementation details into a v2 project. The v2 client guide covers connection selection and fallback for older servers: client connections.
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Design authentication across both boundaries
A gateway has at least two trust boundaries: the upstream host-to-gateway connection and each gateway-to-downstream connection. Decide separately how each boundary authenticates callers and what identity downstream requests represent. A downstream credential might represent the end user, a gateway service account, or a delegated identity; the choice affects authorization and audit attribution.
- Caller persona: distinguish interactive users from automated, non-user identities.
- Credential model: document how credentials are provisioned and whether the flow uses API keys, OAuth, or another mechanism.
- Delegation: state whether the gateway forwards user credentials, uses service credentials, or exchanges tokens.
- Authorization and audit: align advertised capabilities and invocation checks with policy, and retain records that identify the relevant caller and credential context.
An August 2026 enterprise gateway preprint discusses heterogeneous authentication, user and non-user personas, centralized governance, identity delegation, and OAuth token exchange as gateway concerns. Those are architectural proposals and claims in the paper, not requirements imposed by MCP: Kumar, Wang, and Manoharan, “A Gateway Architecture for Enterprise MCP Authentication”.
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Authentication of an upstream caller should not be treated as automatic permission to use every downstream capability. Scope the gateway’s exposed tools and its invocation checks to the authorization policy you intend to enforce. The SDK provides connection and server mechanisms, but the cited materials do not establish a universal authorization policy for gateways.
Local HTTP safeguards
The SDK’s v1 server guide describes a bearer-token pattern that verifies a presented token and checks that its resource or audience matches the expected server resource. It also warns that localhost HTTP servers need protection against DNS rebinding and describes host-header validation. These are useful security considerations, but the APIs cited are from v1 documentation; verify the matching v2 interfaces before using them.
What the mediator pattern and its reported results establish
A March 2026 preprint describes an MCP server that simultaneously acts as a client to downstream MCP servers and implements that mediator pattern in TypeScript. It is a worked architectural example, not normative MCP guidance: Abhinav Singh Parmar, “Separating Intelligence from Execution: A Workflow Engine for the Model Context Protocol”.
That paper reports more than a 99% reduction in per-execution token cost for its MCP Workflow Engine evaluation, comparing declarative workflow execution with repeated agent reasoning across 67 orchestrated steps and two MCP servers. It also reports completing a cluster graph with more than 1,200 nodes and 2,800 relationships in under 45 seconds for its described Kubernetes CMDB synchronization task. Both are author-reported results from the paper’s evaluations—not independent replications, general gateway benchmarks, or performance guarantees for another workload.
Practical decisions before deployment
- Remote or local downstreams: use Streamable HTTP for remote connections; use stdio when the client starts a local server process.
- Sessions or stateless requests: choose stateless behavior for an API-style endpoint that does not need sessions, or account for in-memory lifecycle and capacity if using stateful sessions.
- Compatibility: prefer Streamable HTTP for new remote integrations and retain legacy SSE fallback only where older downstream servers require it.
- Identity and delegation: decide whose identity downstream calls carry and how automated callers differ from interactive users.
- Authorization and audit: ensure the gateway’s public capabilities, invocation policy, and audit records agree with the chosen credential model.
- Version alignment: verify the SDK package versions, transport APIs, and protocol versions used by both gateway and downstream servers.
The official SDK repository frames MCP as a way for applications to provide context for language models while separating context provision from the LLM interaction itself. A composite gateway applies that separation across multiple downstream services, while leaving aggregation policy and trust decisions to the implementation.
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