Flux 2 Klein 9B Preview (BFL)

Flux 2 Klein 9B Preview (BFL) is the preview tier of the Klein 9B node in the ComfyUI-FLUX-BFL-API pack - same model family, same inputs, same one-click feel, but pointed at BFL's flux-2-klein-9b-preview endpoint instead of the stable one. It's how you ride along with BFL's latest Klein improvements before they're finalized. If you follow the Flux 2 line at all, this is the "try the new thing first" node.

And that's genuinely all it is. Mechanically, this is a byte-for-byte clone of Flux2Klein9b_BFL with a different URL. The entire personality of the node lives in that one word: preview. The weights behind the endpoint change when BFL updates them, so the model you hit today may not be the model you hit next month - that's the feature, and also the trap.

How it works

Standard pack flow: prompt, up to four base64 reference images, POST to flux-2-klein-9b-preview, poll get_result?id=... every 5 seconds, return an IMAGE. Everything about the request and response is identical to the stable Klein 9B node.

The inputs

• prompt - the description.
• input_image … input_image_4 - up to four references for consistency/editing.
• width / height - optional (≥64), default lets BFL decide.
• safety_tolerance - 0–5, default 2.
• output_format - jpeg (default) or png.
• Optional seed, webhook_url/secret, config.

Installing it

Same single-pack install as the rest:

cd ComfyUI/custom_nodes
git clone https://github.com/gelasdev/ComfyUI-FLUX-BFL-API.git

Restart, key in config.ini or via Flux Config (BFL). torch is the only declared dependency - nothing else to install, no models to fetch.

Where people get burned

• Building anything repeatable on it. Preview endpoints change under you. Batch jobs, client work, and saved workflows should point at the stable Klein 9B node; the preview is for seeing what's new.
• Expecting the preview to be better. Sometimes BFL's preview is a regression they're testing, sometimes it's the next big jump. Judge it per-generation and keep the stable node around as your baseline.
• Black output. The pack's standard failure tell - moderation, API error, or poll timeout. [BFL] logs print a curl equivalent for every call, so you can replay and see exactly what the preview endpoint returned.

The honest read: Flux 2 Klein 9B Preview is a curiosity node with a real purpose - being the first to know where Klein is heading. Use it to explore, then do the actual work on the stable node.