Diagnose: 401 from LangSmith when tracing is enabled

TL;DR

Set the right combination in the environment before the app runs: - LANGSMITHTRACING=true - LANGSMITHAPIKEY holding your key - LANGSMITHENDPOINT set only if the account is regional (no trailing slash) - LANGSMITHPROJECT for the project name - LANGSMITHWORKSPACE_ID if the key spans workspaces

Steps

1. 401 responses from LangSmith with tracing enabled. Runs fail or traces silently never arrive, depending on how errors surface in your setup.
2. The key - does LANGSMITHAPIKEY hold the LangSmith key (from the LangSmith settings page), not some other key? Print its presence from inside the process.
3. The workspace: keys linked to multiple workspaces need LANGSMITHWORKSPACEID set to the right one.
4. 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.
5. 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.
6. Set the right combination in the environment before the app runs:
7. - 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

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

When to use

You are seeing this: Runs fail or traces silently never arrive, depending on how errors surface in your setup. Use this skill when you run into "Diagnose: 401 from LangSmith when tracing is enabled".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

Almost always one of: wrong key, key from the wrong workspace, or a non-US account hitting the default US endpoint.

Sources

• https://eu.api.smith.langchain.com`

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