MCP, the sockets of the harness

MCP — the Model Context Protocol — is how an agent reaches a real system without you writing an integration. A server exposes tools, resources, and prompts; the agent calls them. If instructions are the harness’s rules and agents are its roles, MCP servers are its sockets: the places where the editor plugs into the outside world.

The anatomy

The starter uses VS Code’s .vscode/mcp.json format with a servers key. Portable .mcp.json files instead use mcpServers. Agent Host does not read .vscode/mcp.json directly; VS Code forwards compatible configurations, but not servers with interactive input variables. Use the Local session target for this starter, or migrate the configuration before switching hosts.^⊙ Here is the harness config:

// The Demo Harness — drewbreyer.com/harness — starter file, as of 2026-07-06
{
  "servers": {
    "microsoft-learn": { "type": "http", "url": "https://learn.microsoft.com/api/mcp" },
    "github":          { "type": "http", "url": "https://api.githubcopilot.com/mcp/" },
    "playwright":      { "type": "stdio", "command": "npx", "args": ["@playwright/mcp@latest"] },
    "dataverse":       { "type": "http", "url": "https://yourorg.crm.dynamics.com/api/mcp" }
  }
}

Three of these you can wire in minutes. The Microsoft Learn server is remote and needs no authentication at all^⊙ . The GitHub server is remote and uses a one-time OAuth sign-in in the editor — no personal access token to mint or leak.^⊙ The Playwright server runs locally over stdio and downloads its browser on first use.^⊙ The fourth, Dataverse, waits until Part III, and its placeholder org URL must be replaced before it does anything.

Keep the menus small

An agent with forty tools chooses badly. Tool sets let you group tools so an agent is pointed at a small, relevant menu^⊙ — the librarian sees documentation tools, the operator sees the environment, and neither is tempted by the other’s. Scoping tools is not bureaucracy; it is how you keep an agent’s behavior legible.

Hooks, with caveats

The VS Code hooks experience is in preview. The Local harness has lifecycle events including PreToolUse and PostToolUse; other harnesses have different schemas and payloads. Select Local for this example and recheck the schema before changing the session target.^⊙ A customer-facing harness wants an audit trail, so the pack ships a hook that logs every tool invocation to a file:

{
  "hooks": {
    "PostToolUse": [
      {
        "run": "jq -c '{ts: now, tool: .tool_name, input: .tool_input}' >> .harness-log.jsonl"
      }
    ]
  }
}

It degrades gracefully: delete it and nothing else breaks. That is the right posture for a preview feature you are leaning on — useful, labeled, and optional.

In the field

Create .vscode/mcp.json with the three no-Dataverse servers and let VS Code start them. Ask an agent to look something up through the Learn server. If your laptop is org-managed and a server will not start, that is its own lesson: note which governance setting is blocking it, and move on.

Part I is done

You have a workbench: a repo, an instruction stack, prompt files and a skill, the first agent, and the sockets. The customer does not exist yet. In Part II, the engagement begins, and the harness goes to work.