# What is an 'agent'? A class/instance definition, stress-tested against the 2026-07-28 MCP spec

> Source: <https://dev.to/langensjonathan/what-is-an-agent-a-classinstance-definition-stress-tested-against-the-2026-07-28-mcp-spec-5akp>
> Published: 2026-07-28 07:28:30+00:00

In the Gen AI / LLM space, two standards have emerged as the de facto choice in their domains:

One nuance to be fair to both: MCP can also act as an agent's public interface, when an agent is exposed as an MCP server inside a single trust domain. So A2A covers the cross-organization contract, and MCP covers everything inside it. Either way, the conclusion stands: neither protocol defines what an agent *is*.

To me it seems the terms 'agent' and 'agentic' are still not well defined, and this article is my attempt at an operational definition: one precise enough that you could build a host around it. That claim is not hypothetical — I am building an MCP host, and this definition is what fell out of that work.

"An agent is an LLM using tools in a loop" (Anthropic's framing) is directionally right, but it is not operational: it does not tell you what belongs in your `agents.json`

. The academic definitions (rational agents, BDI, FIPA) predate LLMs and do not map to context windows, token budgets or MCP servers. I want something in between: a definition you can serialize.

Scope: this is a definition for agents that live in an MCP host. Frameworks like LangGraph organize things differently, but I believe the same components show up under different names.

I split the concept in two parts, like a class and its instances. Both live in the MCP host. From here on: an **agent definition** is the class, an **agent** is the instance.

The agent definition consists of:

An agent is an instance of that class. In C# terms:

```
record AgentDefinition(
    HostLoop Loop,
    SystemContext Context,
    McpServerSet Servers,
    CapabilityBoundary Capabilities,
    Model Llm,
    TerminationLimits Limits,
    TaskContract Contract,
    ContextStrategy Strategy);

record Agent(
    AgentDefinition Definition,
    AgentTask Goal,
    ContextWindow WorkingContext,
    Principal Credentials,
    Budget Consumed);
```

Note the symmetry: the definition holds limits, contracts and capabilities; the instance holds counters, goals and credentials. Static in the class, dynamic in the instance.

The new MCP spec removes protocol sessions entirely. A server no longer remembers you between calls. All state now travels in explicit handles that the model itself can see: task handles for long-running work, workflow ids, and the `requestState`

blob a paused call hands back.

This breaks my 'working context window' as defined above. Those handles land in the context window, which means compacting — or worse, top-x-ing — becomes a *correctness* concern instead of a cost concern. Summarize away a task handle and the agent has orphaned remote work it can never resume, because the stateless server has no session through which to remind it.

So the instance needs a split:

The host loop also stops being free-form. It must now support:

`tasks/get`

)Here is where the stateless core points. If instance state is exactly {goal, context, handles, credentials, consumed budget}, and the protocol holds no hidden session on your behalf, then an agent is a *serializable value*. You can suspend it, persist it, and resume it in a different host process. Not a running process you must keep alive — a record you can store.

Production reality will take a while to catch up (connection reuse, servers caches, ...), but the direction is set by the spec itself.

I'm looking for feedback on this — especially from people running MCP hosts in production: what is in your agent config that this definition misses?