Fal API Flux Pro Fill

Flux Fill is Black Forest Labs' official inpainting and outpainting model - one of the "Flux Tools" they shipped alongside Depth, Canny, and Redux in late 2024, and it landed to real enthusiasm at the time (the announcement thread cleared 1,300 upvotes on r/StableDiffusion). This node is fal.ai's hosted version of the Pro tier of that tool, called from ComfyUI over the network rather than run locally.

The inputs are simple: image (required), mask_image (required), and the usual generation set - prompt (write it as a sentence, not tags), width/height (1024 default, step 8), num_inference_steps (28 default, 1–100), guidance_scale (3.5 default, 0.1–40), num_images (1–4), enable_safety_checker, and safety_tolerance, a 1–6 enum fal documents as running from strictest to most permissive content filtering. seed is optional.

On the mask: the schema doesn't spell out polarity, but the standard convention across ComfyUI inpaint tools is white marks the region to regenerate and black protects the rest - a fine first assumption, and easy to invert if your first test comes back backwards.

One thing worth knowing about how native Flux Fill behaves, even though this node abstracts most of it away: people running the local, native Flux Fill workflow in ComfyUI have repeatedly flagged that every VAE encode/decode pass is lossy, and that skipping a final composite step - pasting the generated region back over the untouched original - quietly degrades the whole image, not just the masked area. That's a local-pipeline concern, not something you build by hand here; this node sends image and mask_image to fal and gets one finished image back, so you don't control the encode/decode steps directly. But the same instinct is worth keeping: if you notice areas outside your mask drifting slightly from the source across a few generations, compositing the returned result back over your original outside the masked region yourself is a safer habit than assuming fal's pipeline never touches anything it shouldn't.

Output is IMAGE.

What's absent, deliberately: no LoRA input. Pro-tier Flux weights aren't published, so there's nothing to fine-tune - if you need a LoRA-driven inpaint, that's the Dev-with-LoRA-and-ControlNet-Inpaint node in this pack, not this one.

Installing it. ComfyUI Manager, search "ComfyUI-Fal-API-Flux," is the straightforward route. Manual installers clone the real repo - https://github.com/yhayano-ponotech/ComfyUI-Fal-API-Flux.git, not the placeholder URL that appears in the README's own example command - into custom_nodes, then pip install -r requirements.txt. After either path, create a config.ini in the pack's folder with a [falai] section and your fal.ai api_key, and restart ComfyUI. Nothing in this pack has any other way to authenticate.

Troubleshooting. Beyond the pack-wide basics (bad config.ini, an account without Pro-tier Flux access, reading the ComfyUI console for fal's actual error text), the most likely inpaint-specific miss is a mask that doesn't line up with your source image's dimensions - mismatched sizes are a common source of "it ignored my mask" reports across every hosted inpaint API, this one included.