LangChain JS agents: createAgent and the lc_error_code on errors

TL;DR: The JS v1 agent factory is createAgent imported from langchain. Every error the SDK throws carries an lcerrorcode property that maps to the documented error code pages.

The error

lc_error_code

The fix

1. Import createAgent and tool from "langchain". Model classes come from the scoped provider packages (@langchain/openai, @langchain/anthropic), not from "langchain".
2. When an error is thrown, read its lc_error_code property first. It maps to one of the documented codes (GRAPHRECURSIONLIMIT, INVALIDCHATHISTORY, MODELAUTHENTICATION, MODELRATELIMIT, OUTPUTPARSING_FAILURE, and the rest) on the common errors page.
3. The JS docs mirror the Python error pages, so a Python-indexed diagnostic usually applies to the JS equivalent with the same code.

When to use this

• You are seeing this exact error message; match the block above, not just part of it.
• The failing call matches the scenario in the title: LangChain JS agents.
• You want the fastest verified fix before digging through logs.

When not to use this

• Your error text differs from the block above; close cousins often have different causes.
• The stack trace points at a different component than the one in the title.
• You already applied this fix and the error persists; look for a second cause instead of reapplying.

Compatibility

• Mentions 5.5 in the original; behavior may differ on much older or newer releases.

Verified against the official LangChain docs on 2026-09-26. JS quickstart: https://docs.langchain.com/oss/javascript/langchain/quickstart ; common errors: https://docs.langchain.com/oss/javascript/common-errors