Diagnose: 401 from LangSmith when tracing is enabled

Symptom

401 responses from LangSmith with tracing enabled. Runs fail or traces silently never arrive, depending on how errors surface in your setup.

Confirm

1. The key - does LANGSMITHAPIKEY hold the LangSmith key (from the LangSmith settings page), not some other key? Print its presence from inside the process.
2. The workspace: keys linked to multiple workspaces need LANGSMITHWORKSPACEID set to the right one.
3. The region: if the account is not in the US, LANGSMITH_ENDPOINT must be the regional API URL, e.g. https://eu.api.smith.langchain.com for EU. No trailing slash; a trailing slash causes auth errors.

Cause

Almost always one of: wrong key, key from the wrong workspace, or a non-US account hitting the default US endpoint. Without the regional endpoint the key is not recognized and every request 401s.

Fix

Set the right combination in the environment before the app runs:

• LANGSMITH_TRACING=true
• LANGSMITHAPIKEY holding your key
• LANGSMITH_ENDPOINT set only if the account is regional (no trailing slash)
• LANGSMITH_PROJECT for the project name
• LANGSMITHWORKSPACEID if the key spans workspaces

Verify

Run one minimal agent invoke and check the project in the LangSmith UI. One trace appearing confirms the whole chain: key, endpoint, project.

Diagnostic procedure grounded in the official trace-with-LangChain docs, verified 2026-09-26. Tracing: https://docs.langchain.com/langsmith/trace-with-langchain