---
title: Build a Claude / MCP agent — Guide
description: Point any MCP-capable runtime at /mcp and the full Agent Number toolset becomes available.
url: https://agentnumber.really.com/docs/guides/build-mcp-agent
doc_version: "1.0.0"
last_updated: "2026-05-15"
---

> ## Documentation index
> Fetch the complete documentation index at: https://agentnumber.really.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Build a Claude / MCP agent

> Point any MCP-capable runtime (Claude Desktop, the Anthropic Agents SDK, an OpenAI agent with MCP support, custom) at `/mcp` and the full Agent Number toolset becomes available — list_services, subscribe, send_sms, receive_sms, voice tools.

## 1. Configure your runtime

The MCP endpoint is:

```text
https://agentnumber.really.com/mcp
```

Streamable-HTTP transport. The MCP server card lives at [`/.well-known/mcp/server-card.json`](/.well-known/mcp/server-card.json) for runtimes that auto-discover.

## 2. Initialize

```bash
curl -X POST https://agentnumber.really.com/mcp \
  -H "content-type: application/json" \
  -H "accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":"1","method":"initialize","params":{"protocolVersion":"2025-11-25","clientInfo":{"name":"my-agent","version":"1.0"}}}'
```

## 3. Call tools/list

Returns the live tool catalog. See [MCP > Tools](/docs/mcp/tools) for the static list with descriptions.

## 4. Try the demo first (free, no payment)

```bash
curl -X POST https://agentnumber.really.com/mcp \
  -H "content-type: application/json" \
  -H "accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":"demo","method":"tools/call","params":{"name":"try_demo","arguments":{}}}'
```

The response shapes match the real `subscribe`, `send_sms`, `receive_sms` tools exactly, so you can scaffold + test wiring before committing the real $1800 USDC.

## 5. Subscribe + use

Same payment shapes as the REST flow (see [Payment](/docs/payment)) — the MCP `subscribe` tool wraps it. Once you have a token, every other tool that requires `authorization: Bearer ...` on the MCP request will work.

## Where to go next

- [MCP overview](/docs/mcp) — long-poll behavior, error semantics
- [Every MCP tool](/docs/mcp/tools) with arguments + auth
- [Auth](/docs/auth) — recover the bearer if you lose it

## Sitemap

- [Overview](/docs/)
- [Quickstart](/docs/quickstart)
- [Authentication](/docs/auth)
- [SMS](/docs/sms)
- [Voice](/docs/voice)
- [MCP](/docs/mcp)
- [Payment](/docs/payment)

See the full [sitemap.md](/sitemap.md) for the complete index of docs pages.

---

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