How to Add Custom Tools to Crush With MCP Servers
Crush can talk to MCP servers, giving it tools beyond file edits. Here is how to register a server, scope its permissions, and use it…
Open protocol that lets AI models call tools, read files and query databases through a standard interface
MCP — the Model Context Protocol — is an open standard for connecting AI applications to external tools and data. Instead of each AI client writing custom integration code, a server exposes its tools once, and any MCP-compatible client can use them. Released by Anthropic in November 2024.
The Model Context Protocol is a specification published by Anthropic in November 2024 and since adopted by OpenAI, Google, Microsoft and most agent frameworks. It is not a model and not a product. It is a contract: how a client (an AI app) discovers and calls the capabilities a server exposes.
The problem it solves is combinatorial. Before MCP, five AI apps times twenty data sources meant a hundred bespoke integrations. With MCP, each data source ships one server, each app implements one client, and the grid collapses into a list.
Servers expose three primitives. Resources are readable data — a file, a database row, a page. Tools are callable functions with a JSON schema describing their arguments. Prompts are reusable templates the server can offer.
Transport is JSON-RPC 2.0. Local servers typically run over stdio (the client launches a subprocess); remote servers use streamable HTTP. SDKs exist for Python and TypeScript, plus community ports for Go, Java, Rust and C#.
A minimal server is about 30 lines of Python. That low floor is why the ecosystem exploded — there are reference servers for filesystem, Git, Postgres, Slack, Google Drive and dozens of SaaS tools.
| Client | MCP support | Notes |
|---|---|---|
| Claude Desktop / Claude Code | Native | Config file for local servers |
| Cursor | Native | Settings panel, per-project servers |
| ChatGPT desktop | Connectors | Rolled out through 2025 |
| VS Code / Copilot | Native | Agent mode consumes MCP servers |
| n8n, LangChain, OpenAI Agents SDK | Via adapters | Treat MCP as a tool source |
Security is the big one, and it is unresolved. An MCP server runs with whatever permissions you grant it. A malicious or compromised server can exfiltrate data, and prompt injection through tool output is a live attack path — a fetched web page can contain instructions the model follows.
There is no standard for authentication in the first revisions; OAuth support landed later and is still unevenly implemented across servers. Tool discovery is also expensive: a server with 40 tools eats context window before the model does any work, and clients handle that pruning differently.
Version churn. The spec moved through several revisions in under two years, and servers written for one revision sometimes break on newer clients with confusing errors.
Developers building AI features that need to touch real systems — a coding agent reading a repo, a support bot querying orders, an analyst bot hitting a warehouse. If you are wiring more than two AI apps to the same data, MCP saves you from writing the integration twice.
End users mostly consume it indirectly: the “connectors” panel in Claude or Cursor is MCP underneath. If you are evaluating agent platforms like Flowise or Composio, ask whether they speak MCP — that answer predicts how much custom glue you will write.