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

A developer proposes an operational definition of an 'agent' as a serializable class/instance split, stress-tested against the 2026-07-28 MCP spec. The definition separates static agent definitions from dynamic agent instances, and the stateless MCP spec forces instance state to be a serializable value, enabling suspension and resumption across hosts.

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?