Error messages

Prompt outputs failed validation

Prompt outputs failed validation

You press Run, nothing generates, and a popup (or the console) shows something like this:

Prompt outputs failed validation:
CheckpointLoaderSimple:
- Value not in list: ckpt_name: 'v1-5-pruned-emaonly-fp16.safetensors' not in []

That exact text is from issue #8826, and it's what a lot of people meet on their first run. The headline is generic. Everything useful is on the lines under it. We counted 174 issues in the ComfyUI repo quoting "Prompt outputs failed validation" as of September 25, 2026, and 96 of them also contain "Value not in list".

What ComfyUI checked

Before it runs anything, ComfyUI 0.37.0 walks back from every output node (Save Image, Preview Image and the like) and checks each input on the way. If no output passes, the server refuses the whole job with this message and an HTTP 400. The console shows the same thing in more detail:

Failed to validate prompt for output 9:
* CheckpointLoaderSimple 4:
  - Value not in list: ckpt_name: 'v1-5-pruned-emaonly-fp16.safetensors' not in []
Output will be ignored

The number after the node type is the node's ID. Recent versions of the web interface retitle the popup "Prompt validation failed" and outline the broken nodes on the canvas, but it's the same check.

"Value not in list" means a missing file

The node wants a file by name and ComfyUI can't find a file by that name in the folder it looks in. The bit in square brackets is what it did find. not in [] means that folder is empty, which on a fresh install is expected, since ComfyUI doesn't ship models. For the default workflow, a ComfyUI team member (comfyui-wiki) told people in #8826 to download v1-5-pruned-emaonly-fp16.safetensors from Comfy-Org's Hugging Face archive and put it in ComfyUI/models/checkpoints.

A non-empty list usually means the workflow was built with a file you don't have under that exact name. Issue #9973 is a clean example: the workflow asked for wan2.1_vace_14B_fp16.safetensors and the machine had only the 1.3B version. Either download the named file or pick yours from the dropdown.

The folder matters. Load Checkpoint reads models/checkpoints, while the separate diffusion model loader reads models/diffusion_models. The same team member explained that all-in-one checkpoints carry their own text encoder and VAE, and many newer models don't, so they need their own loaders and files. After moving a file, press R (the default shortcut for refreshing node definitions) so the dropdowns re-read the folders.

If the file is there and still isn't listed, ltdrdata, another ComfyUI maintainer, suggested in the same thread disabling all custom nodes and testing again.

The other lines you might see

• Required input is missing: an input socket has nothing plugged into it. Connect it, or delete the node if it's leftover.
• Value ... smaller than min of ... (or bigger than max of): a number widget holds something outside the range the node accepts.
• Return type mismatch between linked nodes: two wired-together nodes disagree about the type of what passes between them, which custom nodes and hand-edited API workflows can produce.
• Node '...' not found. The custom node may not be installed.: the workflow uses a custom node you don't have. Install that node pack and restart ComfyUI.