# `mcp/missing-argument` — Required argument is missing
- **type**: `https://github.com/zircote/nsip/blob/main/docs/reference/errors/mcp/missing-argument.md`
- **status**: 400 · **exit_code**: 1 · **class**: caller
- **`suggested_fix` applicability**: `machine_applicable`
- **`retry_after`**: never
## When it occurs
An MCP prompt or tool was invoked without a required argument. The envelope `detail` names the argument.
## Recovery
Provide the named argument and retry. (JSON-RPC code: `invalid_params` / -32602.)
## Example
```json
{
"type": "https://github.com/zircote/nsip/blob/main/docs/reference/errors/mcp/missing-argument.md",
"title": "Required argument is missing",
"status": 400,
"detail": "validation error: lpn_id",
"instance": "urn:nsip:details:6a7b8c9d-0e1f-2a3b-4c5d-6e7f8a9b0c1d",
"exit_code": 1,
"suggested_fix": "provide the required argument: lpn_id",
"docs_url": "https://github.com/zircote/nsip/blob/main/docs/reference/errors/mcp/missing-argument.md"
}
```