debug-render

Debugging renders with run-to-node

When a final asset looks wrong (not when it errors), don't re-roll the whole graph and hope. Localize the fault. Render only as far as one stage and look at what that stage actually produces. The wrong-looking output is downstream of a first bad step. Find that step, fix it there, and everything after it follows.

The tool for this is panel_run with to_node_id ("run to node"), which uses ComfyUI's native partial execution. Only the target output node plus everything upstream of it renders; every other branch is skipped. That makes each probe fast and cheap, and the output is delivered to you automatically, same as any run. Don't poll.

When to reach for this

• A final image/video is wrong: artifacts, wrong subject/pose/composition, off color, soft/blurry, melted hands, wrong style.
• A conditioner isn't "taking" (ControlNet, IPAdapter, a mask, an inpaint, a LoRA) and you can't tell if the input to it is wrong or the node is wrong.
• A multi-stage pipeline (base → refiner, txt2img → upscale, decode → post) degrades, and you need to see which stage introduces the problem.
• You only want to test one branch of a multi-output graph without paying for the others.

If instead the run fails (red error, OOM, missing node, black image from a crash), use the troubleshooting skill. This skill is for outputs that complete but look wrong.

The one hard rule: to_node_id must be an OUTPUT node

ComfyUI can only run to an output node: SaveImage, PreviewImage, SaveVideo, SaveAudio, etc. (these show is_output: true in panel_query_graph detail rows). A bare KSampler / VAEDecode / preprocessor is not an output node, so you can't target it directly. To inspect a point that isn't already an output, add a preview tap there:

You want to see…	Wire type	Tap to add	Wiring
An intermediate image (post-process, refiner stage, composite)	IMAGE	PreviewImage	image → PreviewImage.images
A latent (after a sampler, before/after upscale)	LATENT	VAEDecode → PreviewImage	latent → VAEDecode.samples; reuse the graph's VAE → VAEDecode.vae; then → PreviewImage
A mask (inpaint, segmentation, attention)	MASK	MaskToImage → PreviewImage	mask → MaskToImage.mask → PreviewImage
A ControlNet/preprocessor map (depth/pose/canny)	usually IMAGE already	PreviewImage	the preprocessor's IMAGE output → PreviewImage.images
The actual prompt/conditioning	CONDITIONING	(can't preview as an image)	inspect widget values / the text node feeding it instead