> ## Documentation Index
> Fetch the complete documentation index at: https://docs.varianz.io/llms.txt
> Use this file to discover all available pages before exploring further.

# AI-assisted test generation

> Use the Varianz MCP server to let coding agents discover VPoints, inspect schemas, and generate validated CEL expressions and complete tests.

The registry's VPoint catalog — every name, schema, and type — is machine-readable, which makes Varianz tests a natural fit for coding agents. The **Varianz MCP server** (`varianz-mcp`) exposes the catalog and generation tools over the Model Context Protocol, so agents like Claude Code can discover what's instrumented and produce validated stages and tests.

<Note>
  `varianz-mcp` is distributed alongside the other Varianz components (like the registry server) rather than through a public package index — it's included with your team's Varianz distribution.
</Note>

## Setup

`varianz-mcp` is a stdio adapter that connects to the Varianz services endpoint (`--services-addr`, default `http://[::1]:50052`), which runs alongside the registry in the full Varianz stack. Register it with your agent:

```json theme={null}
{
  "mcpServers": {
    "varianz": {
      "command": "varianz-mcp",
      "args": ["--services-addr", "http://localhost:50052"]
    }
  }
}
```

## The tools

Discovery and inspection:

| Tool                | Purpose                                                                |
| ------------------- | ---------------------------------------------------------------------- |
| `list_vpoints`      | All registered VPoints                                                 |
| `search_vpoints`    | Semantic search over the catalog ("payment", "things that send email") |
| `get_vpoint_detail` | A VPoint's full schema: types, fields, examples                        |

Generation and validation:

| Tool                | Purpose                                                                              |
| ------------------- | ------------------------------------------------------------------------------------ |
| `generate_cel`      | Produce a CEL expression for a VPoint + scenario description                         |
| `validate_cel`      | Compile-check an expression against the schema before it ever runs                   |
| `suggest_overrides` | Propose useful override scenarios for a VPoint                                       |
| `generate_test`     | Produce a complete test (pytest, JUnit, …) for a scenario across one or more VPoints |
| `ask_varianz`       | Free-form agent queries with session support, for multi-service reasoning            |

## Rules for agents

If you're pointing a coding agent at Varianz (or writing prompts for one), four rules prevent almost all failures:

1. **Call `get_vpoint_detail` before writing any CEL.** It shows the exact type and field names. Never guess — `ChargeResult` is not `ChargeResponse`, and struct fields are case-sensitive.
2. **Run every expression through `validate_cel`** before putting it in test code. Fix validation errors first; they're cheaper than runtime failures.
3. **Use the schema's exact types.** Missing required fields in struct literals are runtime errors.
4. **Always include `await_sync_or_fail(timeout)` between stage insertion and the trigger.** Without it, stages may not have reached the service.

These same rules apply to humans — the tools just make following them cheap.

## What good agent output looks like

A generated test should follow the standard lifecycle — insert, sync, trigger with the session header, assert:

```python theme={null}
def test_payment_decline_blocks_email(varianz):
    varianz.insert(cel("payment/charge",
        'ChargeResult { txnId: "", declined: true }'))     # validated via validate_cel
    varianz.insert(cel("email/send-confirmation",
        'sample("email-sent", invoke())'))

    varianz.await_sync_or_fail(timeout_ms=5000)

    http = requests.Session()
    http.headers["x-varianz-id"] = varianz.session_id
    resp = http.post("http://localhost:8080/checkout", json=payload)

    assert resp.status_code == 200
    with pytest.raises(AssertionError):
        varianz.assert_sample("email-sent").within(timedelta(seconds=3)).exists()
```

For the underlying concepts an agent (or you) needs, the canonical pages are [Stages](/concepts/stages), [How tests work](/testing/overview), and the [CEL reference](/reference/cel).