What’s the Difference Between an MCP Server and MCP Client?

TL;DR:

The MCP Client is the agent’s access layer — it requests credentials and presents tokens.
The MCP Server is the platform’s control layer — it issues tokens, enforces delegation rules, and logs activity.

If you think in OAuth terms:

Client = the agent (like a bot, LLM, or automation)
Server = the SaaS platform it’s trying to access.

🧠 Why This Matters

As you implement Model Context Protocol (MCP) in your app or platform, you'll need to decide:

Who’s hosting the server?
Who’s building the client integration?
Where do delegation, token issuance, and auditing happen?

Let’s break down each side.

🤖 What Is an MCP Client?

An MCP Client is the agent-side component. It’s the logic that lives inside the agent (or is bundled with it) that:

Requests credentials (either directly or via delegated setup)
Handles token acquisition from an MCP server
Presents tokens when calling downstream APIs
Refreshes or revokes tokens as needed

Think of it as the identity and access interface for the agent.

Common Examples:

A GPT plugin needing access to a user’s CRM
A customer-side automation tool acting on behalf of a user
An LLM orchestrator calling a third-party SaaS API

The client might be:

A library
A lightweight SDK
A runtime wrapper around the agent

🔐 What Is an MCP Server?

An MCP Server is the provider-side infrastructure. It’s the component your platform exposes to:

Receive client identity requests
Verify agent identity and delegation
Issue scoped, signed access tokens
Log all access attempts and actions
Provide auditability and revocation

Think of the MCP server as the access gatekeeper for your platform’s APIs — it decides:

Who gets in
What they can do
How long they can do it
Whether it’s safe to allow it

🛠️ How MCP Server and Client Work Together

1. Client Requests Access

The agent’s MCP client requests a token for access, often including:

Agent ID
Requested scopes
Delegation proof (if acting on behalf of a user or org)

2. Server Verifies and Issues Token

The MCP server:

Authenticates the agent
Validates the delegation chain
Issues a signed, scoped token

3. Client Calls API

The agent uses that token to access your API, with:

TTL (time-to-live)
Scope restrictions
Tracing metadata

4. Server Logs Everything

The MCP server logs:

Who accessed what
When, and for how long
Whether access was revoked or refreshed
Audit metadata

📊 Quick Comparison Table

🧱 Analogy: Driver vs. Toll Booth

MCP Client = the driver (agent) who wants to access the highway (API)
MCP Server = the toll booth that checks identity, validates the ticket, and logs every crossing

🚀 How Prefactor Helps

At Prefactor, we provide a drop-in MCP server — so you don’t have to build it from scratch.

You focus on:

Enabling secure agent access
Defining scopes and permissions
Delivering your core product

We handle:

Token issuance
Delegation validation
Audit trail
Revocation logic

And your customers can use our MCP client SDKs or build their own — securely.

✅ Summary

MCP Clients are for agents who need access
MCP Servers are for platforms granting access
Both are essential parts of a secure, scalable agent access flow

If your app is being accessed by bots, LLMs, or customer-side automations — you’ll need both sides of MCP.