# MCP

URL: https://tailrace.dev/docs/reference/mcp

> Reference for @tailrace/mcp - wrapTransport, withMcp, and McpWrapOptions.



`@tailrace/mcp` wraps an MCP client `Transport` so outbound `tools/call` arguments and inbound tool / `resources/read` results pass through Tailrace.

## Install [#install]

```bash
pnpm add @tailrace/core @tailrace/mcp @modelcontextprotocol/sdk
```

Peer dependency: `@modelcontextprotocol/sdk` `>=1` (bound against `1.29.0` `Transport`).

## Exports [#exports]

| Export            | Description                                           |
| ----------------- | ----------------------------------------------------- |
| `wrapTransport`   | Standalone transport wrapper                          |
| `withMcp`         | Attach fluent `transport(...)` to a Tailrace instance |
| `McpWrapOptions`  | Options type                                          |
| `TailraceWithMcp` | Fluent instance type                                  |

## Options [#options]

| Option       | Required | Notes                                   |
| ------------ | -------- | --------------------------------------- |
| `server`     | yes      | Policy key prefix `mcp:{server}/{tool}` |
| `agent`      | no       | Default `"default"`                     |
| `workflowId` | no       | Default `"default"`                     |
| `onDecision` | no       | Audit callback                          |

## Block shape [#block-shape]

JSON-RPC 2.0 error on the pending request id: `code: -32001`, message names entity + rule, `data: { type: "policy_violation", entity, rule }`. Transport stays open.

## Guides [#guides]

* [Govern MCP tool calls](/docs/guides/govern-mcp-tool-calls)
* [MCP integration](/docs/integrations/mcp)