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

Build a tap with panel_add_node + panel_connect, run to it, inspect, then panel_remove_node the tap when you're done (unless the user wants to keep it).

The loop

1. Read the graph. panel_graph_outline (then panel_query_graph for specifics). Note node ids, the output nodes (is_output: true), and every node's mode. A node in bypass/mute on the path is OFF and is a top cause of wrong renders. Fix modes first with panel_set_node_mode before you blame anything else.
2. Pick a probe point roughly in the middle of the suspect chain (bisect).
3. Get an output there. Target an existing output node, or add a preview tap (table above).
4. panel_run(to_node_id = that output node). Only that branch renders.
5. Look at what's delivered. Is this stage already wrong, or still fine?
   • Still fine: move the probe downstream; the fault is later.
   • Already wrong: move the probe upstream; the fault is earlier.
6. Repeat until you find the first stage whose output is bad. That node (or its inputs/widgets) is what to fix, via panel_set_widget, panel_set_node_mode, a rewire, or a different model. Then run-to-node there again to confirm the fix before running the full graph.
7. Clean up temporary preview taps (panel_remove_node) and do a final full panel_run to produce the real saved asset.

Symptom → where to probe first

• Whole image wrong subject/style. Preview the conditioning's source (the text/prompt node, the sampler's positive input via a decode of its latent).
• ControlNet ignored / wrong structure. Preview the preprocessor map (the depth/pose/canny IMAGE going into the ControlNet). A blank or wrong map means the problem is the preprocessor or its input, not the sampler.
• IPAdapter/reference not taking. Preview the reference image after any resize/crop the IPAdapter chain applies.
• Inpaint/outpaint bleeding. MaskToImage-preview the mask actually reaching the sampler (mask misalignment is the usual culprit).
• Refiner/upscale degrades it. Preview the base latent (VAEDecode tap) before the refiner vs after. If the base is good and the result is bad, the refiner/upscale stage is at fault.
• Right content, bad quality only. The fault is late (sampler steps/cfg, VAE, post), so probe near the end first.

Gotchas

• Output-node only. If you target a non-output node, the run is rejected with a message saying so. Add a preview tap instead.
• Modes matter. A bypassed/muted node on the probed path silently changes what renders. Check modes in step 1.
• Auto-delivery. The probe's output node fires an executed event and the image is delivered to you inline, including PreviewImage taps, which arrive labeled as a preview (temporary, not a saved file). Don't poll; just end the turn and read the delivered image. A preview tap with no SaveImage means "no saved output ran", which is expected for a probe.
• Shared queue. Each run-to-node queues a real (small) job. Don't stack probes. Run one, read it, then the next.
• Subgraphs. Output nodes nested inside a subgraph can't be targeted yet. Probe at the root graph (add the tap outside the subgraph, fed by an exposed output rail).

Sources

• Official: comfyui-mcp panel_run to_node_id product behavior (this repo).
• Empirical: probe-point recipes from observed wrong-render cases.