FLUX.3 [klein] on a 12 GB RTX 5070: four tries, one clean run

Partly worked2026-10-24 · FLUX.3 [klein] (bf16) · RTX 5070 12GB

Two portraits of an elderly man generated from the same prompt and seed, FLUX.2 klein on the left and FLUX.3 klein on the right — Same prompt, same seed 100. Left: FLUX.2 [klein] 4B (3.3 s). Right: FLUX.3 [klein] (11.8 s). Different models give different faces; this is a look at style, not a quality ranking.

FLUX.3 [klein] came out in October 2026. It needs a newer ComfyUI than the container image we use, so each rented node updates ComfyUI to v0.42.1 at start-up.

What happened

Try 1 — failed. The model files are gated on Hugging Face. The start-up downloader in our container did not send the access token, so the download stopped with HTTP 401. Fix: download the two gated files ourselves, with the token, before ComfyUI starts.
Try 2 — failed. Downloads worked (11.1 GB model, 8.9 GB text encoder). ComfyUI then exited about one second into the first image. Cause not found yet.
Try 3 — worked. Same settings on another node. 1024×1024, 15 steps, er_sde, cfg 1: 11.8 s per image (server-side median of 5 runs), 13.5 s measured from the client. Peak VRAM was about 11.4 GB of 12 GB.
Try 4 — failed. The comparison gallery (10 images) failed on every image. The log showed the 11 GB model download had failed, so ComfyUI ran without it. Fix: resumable retries and a size check against the server's file size; the node now stops instead of starting without the model.

Portrait of an elderly man made with FLUX.3 klein on the RTX 5070 — The clean run (try 3): FLUX.3 [klein], RTX 5070 12 GB, 1024×1024, 15 steps.

Observations (drafted with Astra, checked by the author)

In the side-by-side image, both portraits face the viewer beside a window. The left figure wears a cap and a zipped jacket; the right has uncovered white hair and an open checked shirt. Because the faces and clothes differ, it is hard to compare matching details.
Both portraits are lit from the left, with softer light on the far side and blurred interiors behind. The right portrait (FLUX.3) has warmer skin tones and deeper forehead creases.
In the single FLUX.3 image, fine beard strands, stray eyebrow hairs and skin highlights are clearly visible. Several chest hairs form conspicuous pale loops, which looks like an artefact.
No hands or readable text appear in these images, so we cannot judge those from this note.

What we learned

On a 12 GB card FLUX.3 [klein] fits, but only just. An 8 GB card is unlikely to work (not tested).
It is much slower than FLUX.2 [klein] 4B (about 3.3 s on the same GPU) at its default 15 steps.
One node is a reference value only. We want at least five nodes before calling it a result.