# Secure a custom MCP client

Onboard any MCP client with SMA over HTTP, and the client-side rules that stop tool poisoning.

## Register

Your client has a shell or an HTTP library. Use either path.

```bash
npx -y securemyagent register --name my-client
npx -y securemyagent checkup --upload
```

Without Node, follow the raw HTTP contract in [skill.md](https://securemyagent.ai/skill.md):

1. `POST /v1/agents/challenge` with `{did}`.
2. Find a nonce so `sha256(challenge + ":" + nonce)` has `difficulty` leading zero bits.
3. Sign the challenge with Ed25519. Encode as base64url.
4. `POST /v1/agents/register`.

A hosted MCP endpoint, `mcp.securemyagent.ai`, is coming soon.

## Client rules

1. **Show tool descriptions to the user** on first connect and when they change. Hash each description and store the hash. A changed hash is a rug pull signal.
2. **Pin servers.** Store the package name, exact version and hash. Do not resolve `latest` at runtime.
3. **Keep servers apart.** One server must not see another server's tool output or credentials.
4. **Require approval** for tools with destructive verbs: `delete`, `send`, `transfer`, `exec`.
5. **Treat tool output as data.** Never let it change the system prompt or the tool list.
6. **Send secrets only to the host they belong to.** Never forward a token received from one server to another.
7. **Log every call**: time, server, tool, argument summary, result status.

## Description hash in shell

```bash
curl -sS https://your-server.example/mcp -H 'content-type: application/json' -H 'accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | sha256sum
```

Compare the hash on each start. A mismatch means the server changed.

## Heartbeat

```bash
curl -sS -X POST https://api.securemyagent.ai/v1/heartbeat -H "Authorization: Bearer $SMA_API_KEY"
```

Every 4 to 24 hours.

## Sources

- [OWASP MCP Top 10](https://owasp.org/projects/mcp-top-10)
- [Snyk on postmark-mcp](https://snyk.io/blog/malicious-mcp-server-on-npm-postmark-mcp-harvests-emails/)
- [Cycode on the OWASP MCP Top 10](https://cycode.com/blog/owasp-mcp-top-10/)