TL;DR: if the DataForSEO MCP server's Labs tools (`dataforseo_labs_google_ranked_keywords`, `dataforseo_labs_google_domain_rank_overview`) fail with `Invalid Field: 'language_code' (Code: 40501)` while other SERP tools work, a cached old npx build is passing `language_code` to endpoints that do not accept it. Run `npx -y dataforseo-mcp-server@latest` so npx fetches the current build instead of reusing its cache. The maintainer verified the current build handles the payload correctly, and the reporter confirmed the error is gone after updating.

```text
Error: API Error: Invalid Field: 'language_code'. (Code: 40501)
```

## Steps

1. Force the latest build:

```sh
npx -y dataforseo-mcp-server@latest
```

2. Re-run the Labs tool.

   Expected: no more 40501. Success check: the Labs endpoint returns data.

## When to use this

- dataforseo-mcp-server v2.8.10 via npx in Claude Desktop, only Labs tools fail.

## When NOT to use this

- All tools fail; check the API key and setup.
- You already run the latest build; report it fresh.

## Compatibility

- dataforseo-mcp-server latest.

## Variant phrasings

- "dataforseo labs language_code 40501"
- "dataforseo mcp invalid field language_code"

## Root cause

The stale cached build sent a field the Labs endpoints do not accept; the current build does not.

## Edge cases

- npx caching is the whole story here; the `-y` plus `@latest` bypasses it.

Source: https://github.com/dataforseo/mcp-server-typescript/issues/45