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An agent host talks to a model API and, through an MCP client, to files, tools, and other agents.An agent host talks to a model API and, through an MCP client, to files, tools, and other agents.

Builder

Tools and MCP

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The model decides. A tool does. A protocol is the agreement that lets many tools plug into many hosts without a custom integration each time.

Why it matters. Without a tool, the agent can only talk. MCP matters when that tool should plug into more than one host. The smallest version is one function in the Builder guide: the model asks, your code runs, the text comes back. This page is the contract around that function. One action per tool. A tool that both reads and sends hides the send.
Path Builder
Time About 20 minutes
You need Memory, tools, and planning

What you will be able to do

  • Write a tool contract a teammate can review.
  • Explain MCP as the connection between a host and tool servers.
  • Place agent-to-agent handoff next to tool calls, and mark what is only a post's claim.

The contract, again, with teeth

From foundations, a tool has a name, a schema, a side effect, a permission, and a result. A few rules make that contract survivable:

Rule Why it is here
One side effect per tool "Search and also email the result" hides the email inside a read
The result includes failure The observe step needs the error, not a cheerful summary
Arguments are data File paths and URLs are inputs. Instructions hiding in those inputs are a threat, covered in security
The smallest permission A docs agent that can rewrite production is a bug you have not met yet
A person sees writes Reads can be wide. Writes, sends, and deletes get a review or a very small allow-list

MCP

MCP (Model Context Protocol) is an open protocol for connecting an agent host to external tools and data. The host embeds an MCP client. Each external capability is an MCP server: a file tree, a browser, a ticket system, a database. The model sees a list of tools with schemas. Calling one is an ordinary tool call. The protocol is what makes that list portable across hosts.

In the harness survey, a tool schema is the action interface, and the description that lands in the window is also context. MCP is one way to draw that boundary. The six-part frame is harness engineering.

flowchart TD
  Host["Agent host"] --> Model["Model API"]
  Host --> Client["MCP client"]
  Client --> Files["Server: files and docs"]
  Client --> Tools["Server: browser, shell, APIs"]
  Client --> Other["Another agent"]

The SVG at the top is the same picture, with the other-agent box marked as the piece a post claims rather than the piece the protocol itself requires.

Tip

Established. MCP is the protocol. It is not a particular vendor's agent, and it is not a model. A host can speak MCP and still be a poor agent if the loop, the memory, or the stop rule is weak.

Note

Per post. @Bober_smart says the OmniRoute gateway has "full support" for MCP and for an agent-to-agent architecture, used for automated model-pool management. That is a product claim. It is not a statement that every MCP host routes models, and it does not name a protocol version. The gateway's other claims are on the next page.

Agent-to-agent, beside tools

A tool returns data or a side effect. An agent-to-agent handoff gives another agent a goal and a boundary, and takes back a result. Use a handoff when the subtask needs its own loop. Use a tool when you want one action and one observation.

The orchestration page is what a handoff looks like when a lead agent plans the phases: orchestration.

A gateway is not the protocol

MCP attaches tools and data to the host. A gateway attaches the host's model calls to providers, often through one OpenAI-compatible endpoint. They sit on different arrows in the diagram. Products may bundle them. The bundle is a product choice. OmniRoute's post claims both.

Check yourself

  1. Which five clauses belong on a tool you would let an agent call unattended?
  2. In the diagram, what sits between the host and a file server?
  3. Which OmniRoute capability is per post, and which idea (MCP as a protocol) is established?

Sources

  • Established account of tool contracts and of MCP as an open protocol for tools and data.
  • @Bober_smart — 2026-10-08. MCP and agent-to-agent claims, per post.

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