ComfyUI Troubleshooting Guide

Render completes but looks WRONG (artifacts, wrong subject/pose/color, a ControlNet/mask/LoRA not taking, a refiner degrading it)? That's not an error. Use the debug-render skill (list_packs with action: "skill_read", name: "debug-render") to localize the bad stage with run-to-node (panel_run to_node_id) by previewing intermediate steps. This guide is for runs that fail with an error, OOM, or missing node.

Error Diagnosis Strategy

When a workflow fails, follow this approach:

1. Get the error. Use get_history(action="diagnose") to retrieve the execution result with the full traceback, plus any missing models/nodes
2. Check logs. Use get_system_stats (action:"logs") with keyword filters like "error", "warning", "traceback"
3. Identify the failing node. The history response includes the node_id and node_type that failed
4. Cross-reference inputs. Use create_workflow (action:"node_info") to verify the failing node's expected input schema
5. Check models. Use list_local_models to verify all referenced model files exist

Out of Memory (OOM)

Error Pattern

torch.cuda.OutOfMemoryError: CUDA out of memory. Tried to allocate X MiB.
GPU 0 has a total capacity of 24.00 GiB of which X MiB is free.

Or:

RuntimeError: CUDA error: out of memory

Root Cause

The GPU does not have enough VRAM to hold the model weights, intermediate tensors, and latent images at the same time. Common triggers:

• High resolution images (2048x2048+)
• Multiple models loaded at the same time
• FP32 precision models on limited VRAM
• Video generation (LTXV, AnimateDiff) with many frames
• Large batch sizes

Fixes (in order of preference)

1. Reduce resolution. Drop to the model's native resolution (512 for SD 1.5, 1024 for SDXL/Flux)
2. Use FP8/FP16 quantized models. FP8 Flux models use ~8GB vs ~24GB for FP16
   • Search for FP8 variants: download_model({ action: "search", query: "flux fp8" }) or the same with "sdxl fp8"
3. Launch flags (the VRAM ladder). Offload via ComfyUI CLI flags:
   • --lowvram offloads text encoders / model parts to CPU
   • --novram is extreme offload, the go-to for long video (LTX 2 / WAN) OOM
   • --cache-none caches nothing (lowest RAM/VRAM); combine with --novram
   • --reserve-vram N reserves N GB so the GPU stops spilling into slow shared VRAM (Windows); typical 2 to 4
   • --disable-smart-memory forces offload to RAM when a run gets stuck or OOMs intermittently
   • Full matrix and recipes: comfyui-launch-flags
4. Free VRAM between generations. ComfyUI should auto-manage, but restarting clears leaked memory
5. Use tiled VAE decoding. For high-resolution images, tile the VAE decode step
   • Node: VAEDecodeTiled instead of VAEDecode
   • Breaks the image into tiles, decodes each separately, and stitches them together
6. Reduce batch size. Set batch_size to 1 in EmptyLatentImage
7. Avoid multiple models. Don't load two full checkpoints at the same time; use one checkpoint and LoRAs instead
8. For LTXV/video: always use FP8 quantized video models on 24GB cards

VRAM Estimates

Model	FP32	FP16	FP8
SD 1.5	~4GB	~2GB	~1GB
SDXL	~12GB	~6GB	~3GB
Flux Dev	~48GB	~24GB	~12GB
Flux Schnell	~48GB	~24GB	~12GB
LTXV	~20GB+	~10GB+	~6GB

Launch Flags — VRAM / Cache / Attention / Precision

ComfyUI's startup flags tune the speed↔VRAM tradeoff. Match them to the detected GPU (the panel orchestrator reports VRAM/GPU/torch/sage in its env block; pick the tier from there). Set them on the process that launches ComfyUI (or the --panel-orchestrator / connect command's ComfyUI, not the agent).

VRAM mode (pick ONE by card size)

Flag	Card	Behavior
--gpu-only	16GB+	Everything (CLIP/VAE/UNet) stays on GPU — fastest, max VRAM
--highvram	12–16GB	Models stay resident in GPU after use, no CPU offload
--normalvram	8–12GB	Default balance — unload to CPU RAM when idle
--lowvram	6–8GB	Split the UNet, aggressive CPU offload — slower
--novram	4–6GB	Extreme split/offload — for OOM even on lowvram, or long videos
--cpu	<4GB / no GPU	CPU only (very slow)

--reserve-vram N (GB) leaves headroom for the OS and other apps. Bump it if you OOM intermittently mid-run (VAE decode / audio round-trips spike).

Cache (RAM vs re-run speed)

Flag	Effect
--cache-classic	Default aggressive caching (fastest re-runs, most RAM)
--cache-lru N	Keep the last N node results (bounded RAM)
--cache-ram N	Cap cache to N GB of headroom
--cache-none	No caching — minimal RAM, re-runs every node

Attention (speed vs compatibility)

Flag	Notes
--use-sage-attention	Recommended — fast + efficient (needs SageAttention + Triton; see triton-sageattention)
--use-flash-attention	Very fast on supported GPUs
--use-pytorch-cross-attention	PyTorch 2.x native — best compatibility
--use-split-cross-attention	Lower VRAM, slower
--use-quad-cross-attention	Sub-quadratic optimization
(omit)	Auto-selects xFormers if available

Precision (UNet)

Flag	Effect
--fp16-unet	Half precision, ~50% VRAM
--bf16-unet	BFloat16, good balance (newer GPUs)
--fp8_e4m3fn-unet	8-bit float, max savings (newest GPUs)

Typical recipes:

• RTX 4090/5090 (24 to 32GB): --gpu-only --use-sage-attention --cache-classic
• 12 to 16GB: --highvram --use-sage-attention (or --fp8_e4m3fn-unet for big models)
• 8GB: --normalvram --use-sage-attention --cache-lru 20
• 6GB: --lowvram --use-split-cross-attention --cache-none
• OOM on long video: --novram --reserve-vram 2

Device Mismatch

Error Pattern

RuntimeError: Expected all tensors to be on the same device, but found at least
two devices, cuda:0 and cpu!

Root Cause

A tensor on the CPU is combined with a tensor on the GPU. This usually happens when:

• A custom node doesn't move tensors to the correct device
• Model offloading placed parts of the model on CPU
• A node produces CPU tensors while downstream expects GPU tensors

Fixes

1. Check if the error occurs with a specific custom node. Update or replace that node
2. If using --lowvram or --cpu, some nodes may not support CPU offloading
3. Restart ComfyUI to reset device state
4. Check if a custom node has a newer version that fixes device handling

Missing Nodes

Error Pattern

Cannot find node class 'NodeClassName'

Or in the execution response:

"error": {"type": "node_not_found", "message": "Cannot find node class 'X'"}

Root Cause

The workflow references a node type that is not installed. This happens when:

• A custom node pack is not installed
• A custom node pack is installed but failed to load (import error)
• The node was renamed or removed in a pack update

Fixes

1. Search for the node pack:

   search_custom_nodes(action="search", query="NodeClassName")
   

2. Install via ComfyUI Manager or the registry
3. Check logs for import errors:

   get_system_stats (action:"logs")(keyword="import")
   get_system_stats (action:"logs")(keyword="error")
   

   Import errors often reveal missing Python dependencies
4. Install missing Python dependencies. If the custom node requires a pip package:

   pip install missing-package
   

5. Restart ComfyUI after installing any custom node. Nodes are loaded at startup

NaN Tensor Errors

Error Pattern

RuntimeError: Input contains NaN

Or images come out as solid gray/noise with NaN warnings in logs.

Root Cause

Numerical instability during the diffusion process. Common triggers:

• CFG scale too high. Values above 15-20 can cause numerical overflow
• Corrupted model weights. Damaged download or incompatible merge
• FP16 overflow. Some operations overflow at half precision
• Incompatible LoRA. A LoRA trained for a different base model

Fixes

1. Lower CFG. Try CFG 7.0 for SD 1.5/SDXL, 1.0 for Flux
2. Use FP32 VAE. Some VAEs produce NaN in FP16. Switch to vae-ft-mse-840000-ema-pruned.safetensors (FP32)
3. Remove LoRAs. Test without LoRAs to isolate the cause
4. Re-download the model. Hash verification can detect corrupted files
5. Check LoRA compatibility. The LoRA must match the base model family

Dtype Mismatches

Error Pattern

RuntimeError: expected scalar type Float but found Half

Or:

RuntimeError: expected scalar type Half but found Float

Or:

RuntimeError: Input type (float) and bias type (c10::Half) should be the same

Root Cause

A model component expects one precision (FP32/FP16) but receives another. Most common with:

• VAE precision mismatch (FP16 model + FP32 VAE or vice versa)
• Mixed-precision LoRAs
• Custom nodes that force a specific dtype

Fixes

1. Use a separate VAE. Load an explicit FP32 VAE instead of the checkpoint's built-in VAE
   • Node: VAELoader with vae-ft-mse-840000-ema-pruned.safetensors
2. Match precision. If the model is FP16, use FP16-compatible nodes throughout
3. Force FP32 VAE decode. Some node packs offer VAEDecodeFP32 nodes
4. Check ComfyUI settings. The --force-fp32 flag forces everything to FP32 (uses more VRAM)

CLIP Token Overflow

Error Pattern

No explicit error. The prompt is truncated at 77 tokens without warning, and details mentioned late in the prompt are ignored.

Symptoms

• Later parts of long prompts have no effect on the image
• Adding more descriptive text doesn't change the output
• Removing early tokens suddenly makes later tokens work

Fixes

1. Use a BREAK token. Split the prompt at natural boundaries:

   subject description, pose, clothing, setting
   BREAK
   lighting, style, quality, camera angle
   

2. Use CLIPTextEncodeSDXL. SDXL's dual-CLIP processes two 77-token chunks
3. Prioritize important tokens. Put the most important descriptors first
4. Use fewer filler words. Remove articles and prepositions where possible
5. Use embeddings. Condense complex concepts into single tokens with textual inversions

Black Images

Error Pattern

No error in the execution. The workflow "succeeds" but produces completely black or near-black images.

Root Causes and Fixes

Cause	Diagnosis	Fix
denoise = 0	Check KSampler inputs	Set denoise to 1.0 for txt2img, 0.5-0.8 for img2img
cfg = 0	Check KSampler inputs	Set CFG to 7.0 (SD 1.5), 1.0 (Flux)
steps = 0	Check KSampler inputs	Set steps to 20+ (standard) or 4+ (turbo)
Wrong VAE	VAE doesn't match model	Use the correct VAE for the model family
Empty prompt	CLIPTextEncode has empty text	Add a text prompt
Wrong scheduler	Incompatible scheduler/sampler combo	Try "normal" scheduler with "euler" sampler
Seed collision	Extremely rare	Change the seed value
FP16 VAE overflow	VAE decode produces black	Use FP32 VAE or VAEDecodeTiled

Quick Diagnostic Checklist

1. Check denoise > 0 (should be 1.0 for txt2img)
2. Check cfg > 0 (should be 7.0 for SD 1.5, 1.0 for Flux)
3. Check steps > 0 (should be 20 for standard, 4 for turbo)
4. Verify the positive prompt is not empty
5. Try a different seed
6. Try a known-working sampler/scheduler combo: euler + normal

Connection Type Errors

Error Pattern

Output type 'IMAGE' doesn't match input type 'LATENT'

Or:

Required input 'model' of type 'MODEL' but got connection of type 'CLIP'

Root Cause

Connecting the wrong output slot of a node to an incompatible input. Often caused by using the wrong output index.

Fixes

1. Check output indices. Use create_workflow (action:"node_info") to verify the exact output order
   • CheckpointLoaderSimple outputs: 0=MODEL, 1=CLIP, 2=VAE
   • Getting index wrong: ["1", 0] gives MODEL, ["1", 1] gives CLIP
2. Verify connection format. ["nodeId", outputIndex], where node ID is a string and index is an integer
3. Check data type flow. The pipeline must follow the correct type chain:

   MODEL → KSampler
   CLIP → CLIPTextEncode → CONDITIONING → KSampler
   LATENT → KSampler → LATENT → VAEDecode → IMAGE
   VAE → VAEDecode, VAEEncode
   

Model Loading Errors

Error Pattern

FileNotFoundError: [Errno 2] No such file or directory: 'models/checkpoints/model.safetensors'

Or:

SafetensorError: Error reading file: invalid header

Or:

RuntimeError: PytorchStreamReader failed reading zip archive

Root Causes

• File not found. Model file doesn't exist at the referenced path
• Corrupted download. Incomplete or damaged file
• Wrong format. File is not a valid safetensors/pickle/checkpoint format

Fixes

1. Verify the model exists: list_local_models({ action: "list", model_type: "checkpoints" })
2. Check the exact filename. Model names in workflows must match the filename exactly (case-sensitive)
3. Re-download. If hash mismatch or corruption:

   download_model({ action: "download", url: "...", target_subfolder: "checkpoints" })
   

4. Check file size. A 1KB safetensors file is corrupted; re-download
5. Verify subfolder. Models must be in the correct subfolder (checkpoints/, loras/, vae/, etc.)

Torch / CUDA Version Errors

Error Pattern

RuntimeError: CUDA error: no kernel image is available for execution on the device

Or:

ImportError: cannot import name 'xxx' from 'torch'

Or:

AssertionError: Torch not compiled with CUDA enabled

Root Cause

PyTorch and CUDA version incompatibility, usually after:

• Updating PyTorch without matching CUDA toolkit
• Installing a custom node that downgrades/changes PyTorch
• Using pip install that pulls a CPU-only PyTorch

Fixes

1. Check current versions:

   get_system_stats()  # Shows PyTorch version and CUDA version
   

2. Verify CUDA availability. In Python: torch.cuda.is_available()
3. Reinstall PyTorch with CUDA. Visit pytorch.org for the correct install command matching your CUDA version
4. Pin PyTorch version. After fixing, avoid running pip install commands that might change PyTorch
5. Use ComfyUI's bundled venv. ComfyUI Desktop ships with a pre-configured Python environment

ComfyUI Desktop vs CLI Differences

Key Differences

Aspect	ComfyUI Desktop	ComfyUI CLI
Default port	8000	8188
Python	Embedded (bundled)	System/venv Python
Install location	AppData/Local/Programs/ComfyUI/	Wherever you cloned it
Custom nodes	Documents/ComfyUI/custom_nodes/	./custom_nodes/ in repo
Models	Documents/ComfyUI/models/	./models/ in repo
Config	extra_model_paths.yaml for shared paths	Same
Updates	Auto-updater in the app	git pull

Common Issues

• Wrong port. MCP tools default to 8188; if using Desktop, configure for port 8000
• Path confusion. Desktop separates user data from application files
• Custom node pip installs. Desktop's embedded Python may not be on PATH; install within the venv

Error-Specific Debugging Commands

Workflow Failed — Get Details

get_history(action="list")                       # Most recent execution
get_history(action="list", prompt_id="abc-123")  # Specific execution
get_history(action="diagnose")                   # Why the last run failed

The response includes:

• status.status_str: "success" or "error"
• status.messages: Timestamped execution messages
• outputs: Node outputs (images, etc.)
• Error traceback for failed nodes

Check Server Health

get_system_stats()    # GPU info, VRAM, Python/PyTorch versions
queue(action="list")  # Running and pending jobs
get_system_stats (action:"logs")(max_lines=50, keyword="error")  # Recent error logs

Verify Node Availability

create_workflow(action="node_info", node_type="KSampler")        # Check specific node
create_workflow(action="node_info", node_type="ControlNetApply")  # Verify custom nodes loaded

Verify Models

list_local_models({ action: "list", model_type: "checkpoints" })   # Installed checkpoints
list_local_models({ action: "list", model_type: "loras" })         # Installed LoRAs
list_local_models({ action: "list", model_type: "controlnet" })    # Installed ControlNets

Quick Reference: Error to Fix

Error Message (partial)	Most Likely Fix
CUDA out of memory	Reduce resolution, use FP8 model; VRAM ladder --lowvram → --novram --cache-none → --reserve-vram N (launch flags)
Expected all tensors on same device	Update custom node, restart ComfyUI
Cannot find node class	Install the node pack, restart ComfyUI
Input contains NaN	Lower CFG, use FP32 VAE, remove LoRAs
expected scalar type Float but found Half	Use FP32 VAE, or --force-fp32
No such file or directory (model)	Check filename, re-download model
invalid header (safetensors)	Re-download — file is corrupted
CUDA error: no kernel image	Reinstall PyTorch with matching CUDA version
Black images, no error	Check denoise > 0, cfg > 0, steps > 0, prompt not empty
Image looks garbled/noisy	Wrong model+VAE combo, wrong sampler settings
Connection refused on port 8188	ComfyUI not running, or using Desktop (port 8000)
Prompt outputs failed validation	Node inputs don't match schema — check create_workflow (action:"node_info")

Sources

• Official: none found as a vendor error catalog. Launch-flag names cross-check against comfyui-launch-flags (upstream cli_args.py).
• Empirical: error→fix table from observed ComfyUI failures.