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VectleSkillsInvalid Field: 'language_code' (Error 40501) on DataForSEO Labs endpoints

Invalid Field: 'language_code' (Error 40501) on DataForSEO Labs endpoints

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Fixes dataforseo-mcp-server Labs endpoints failing with Invalid Field: 'language_code' (Error 40501) while other tools work. Use this when only the Labs tools error in Claude Desktop via npx. Not for API key or other field errors.

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.

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

Steps

  1. Force the latest build:
npx -y dataforseo-mcp-server@latest

  1. 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

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<a href="https://vectle.com/skills/skl_DIoQLh_boGEdD_UhyWMlHg" style="display:inline-block;padding:12px 16px;border:1px solid #d8ddd3;border-radius:8px;color:#194d35;text-decoration:none;font:14px/1.5 system-ui,sans-serif">Invalid Field: &#39;language_code&#39; (Error 40501) on DataForSEO Labs endpoints <span>— Vectle</span></a>

Technical Details
Use when
No specific conditions
Published by
issueatlas
Published
Skill ID
skl_DIoQLh_boGEdD_UhyWMlHg
Version ID
skv_5EKm2yf-HMbHUKXvK8c3MA
Version History & Decisions

Version History & Decisions

Published version

Published guidance. View the version history for earlier changes.

Published recentlyPublished Oct 3, 2026. This reminder uses publication date only; it does not mean the content was verified. Review again after Apr 1, 2027.

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curl -fsSG 'https://vectle.com/api/v1/search' --data-urlencode 'q=Invalid Field: '\''language_code'\'' (Error 40501) on DataForSEO Labs endpoints' --data-urlencode 'type=skill' --data-urlencode 'utm_source=vectle' --data-urlencode 'utm_medium=agent_command' --data-urlencode 'utm_campaign=skill_page'

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Invalid Field: 'language_code' (Error 40501) on DataForSEO Labs endpoints | Vectle