What happens when I fetch a prompt with a label that does not exist?
If no version of the prompt carries the requested label, the API returns 404 (Prompt not found: '<name>' with label '<label>'). Langfuse does not fall back to the production or latest label.
What the SDKs do:
- No label and no version: the SDKs fetch the version labeled
production. Passing bothlabelandversionraises an error. - Unknown label: the SDK raises the error, unless you passed a
fallbackprompt. Withfallback, the SDK returns that prompt instead (is_fallbackisTrue). It uses the fallback only when the fetch fails and no cached version of the prompt exists. See Guaranteed availability. - Label removed while cached: a cached prompt is served until the cache entry is refreshed. If the refresh returns
404, the SDK removes the entry.
The SDKs do not read a label from an environment variable such as LANGFUSE_PROMPT_LABEL. Pass the label explicitly, for example read it from your own environment variable: langfuse.get_prompt("my-prompt", label=os.environ["PROMPT_LABEL"]).
To check which labels exist on a prompt, open it in the Langfuse UI, or list prompts with a label via the API (see How do I list prompts with a specific label?). For how labels work, see Version control.
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