FAQ

Skills: .agents/skills/cosmos3-setup/SKILL.md · .agents/skills/cosmos3-inference/SKILL.md

A catch-all collection of frequently asked questions, tips, and troubleshooting for the Cosmos3 package. Can't find what you need? Check setup.md for installation issues or inference.md for inference details.

To add a new entry, append it under the most relevant section — or under Miscellaneous if nothing fits.

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Table of Contents

• Setup and Installation
• Configuration and Defaults
• Inference
• Training
• Tips and Tricks
• Miscellaneous

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Setup and Installation

Q: I get ImportError: cannot import name '_functionalization' from 'torch._C' inside an NGC container

Clear the library path before running anything:

export LD_LIBRARY_PATH=''

This is needed because the NGC PyTorch container ships its own libraries that conflict with the venv-installed versions. See setup.md#pytorch-import-issue.

Q: ModuleNotFoundError: No module named 'cosmos_framework'

Make sure you installed the package:

uv sync --all-extras --group=cu130
source .venv/bin/activate

If already installed, try --reinstall to force a clean state.

Q: Which CUDA version should I use?

CUDA 13.0 is recommended. CUDA 12.8 is also supported. The major version must match between your system CUDA and the installed PyTorch wheels. Check with:

nvidia-smi                                    # system CUDA
python -c "import torch; print(torch.version.cuda)"  # PyTorch CUDA

Q: I get a CUDA / NVIDIA driver mismatch error (e.g. CUDA error: no kernel image is available for execution on the device, libcudart.so.* cannot open, The NVIDIA driver on your system is too old)

The installed PyTorch CUDA wheels do not match your system's NVIDIA driver. Check the driver's reported CUDA version with nvidia-smi, then delete .venv/ and uv sync against the matching group (cu130-train if the driver supports CUDA 13.x; cu128-train if it supports CUDA 12.8):

nvidia-smi                                    # check the "CUDA Version" field (top right)
rm -rf .venv
uv sync --all-extras --group=cu130-train --reinstall   # or --group=cu128-train
source .venv/bin/activate && export LD_LIBRARY_PATH=

Use the inference-only groups (cu130 / cu128) instead if you don't need the training-only dependencies.

Q: How do I download model checkpoints?

Checkpoints are downloaded automatically from Hugging Face during inference. You need:

1. A Hugging Face token with Read permission
2. Accepted NVIDIA Open Model License Agreement
3. HF_TOKEN environment variable set, or uvx hf auth login

Control the download location with HF_HOME (default: ~/.cache/huggingface). If downloads fail, the commands are printed to the console — run them manually to debug. See setup.md#downloading-base-checkpoints.

Q: fatal error: Python.h: No such file or directory

Reinstall uv and the venv from scratch:

curl -LsSf https://astral.sh/uv/install.sh | sh
uv python install --reinstall
rm -rf .venv
uv sync --all-extras --group=cu130 --reinstall
source .venv/bin/activate

Q: How much disk space do I need?

Plan for ~150 GiB free before the first run. A successful first-run inference or training workflow typically consumes:

• Hugging Face cache ($HF_HOME, default ~/.cache/huggingface): ~90 GiB — base checkpoints (e.g. Cosmos3-Nano, Wan2.2 VAE), tokenizers, and any dataset snapshots pulled by training recipes.
• uv cache ($UV_CACHE_DIR, default ~/.cache/uv): ~20 GiB — wheels for torch/CUDA dependencies across the install groups (cu130-train, cu128-train, etc.).
• Run outputs ($IMAGINAIRE_OUTPUT_ROOT, training, or your -o output dir, inference): ~30 GiB per run — config snapshots, DCP checkpoints saved every save_freq iterations, callback outputs, optional wandb files.

Actual sizes scale with the model tier (Cosmos3-Super is larger than Cosmos3-Nano), the dataset, and how many checkpoints you keep. To relocate any of these off the system disk, set the corresponding env var before installation/run (e.g. export HF_HOME=/data/hf, export UV_CACHE_DIR=/data/uv, export IMAGINAIRE_OUTPUT_ROOT=/data/cosmos-runs).

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Configuration and Defaults

Q: Where are the default inference parameters (guidance, shift, num_steps, etc.)?

Per-modality defaults live in JSON files under cosmos_framework/inference/defaults/<mode>/sample_args.json:

Mode	Default file
text2image	cosmos_framework/inference/defaults/text2image/sample_args.json
text2video	cosmos_framework/inference/defaults/text2video/sample_args.json
image2video	cosmos_framework/inference/defaults/image2video/sample_args.json

Action and image/video-to-video modes have parallel files under cosmos_framework/inference/defaults/{image2image,video2video,forward_dynamics,inverse_dynamics,policy}/sample_args.json.

See AGENTS.md for the full config defaults chain.

Q: How do I override a default parameter?

From most temporary to most permanent:

1. CLI flag: --shift 5.0 (per-run, applies to all samples)
2. Sample argument file: set the field in your input JSON (per-sample)
3. Custom defaults file: pass "defaults_file": "my_defaults.json" in your sample argument file (see inference.md#custom-defaults)
4. Built-in default: edit cosmos_framework/inference/defaults/<mode>/sample_args.json (permanent change)

Fields set in the sample argument file take precedence over defaults. CLI flags override both.

Q: What is the shift parameter and which value should I use?

shift controls the time-shift in the UniPC diffusion sampler. Higher values produce more detail but can introduce artifacts. Recommended values:

Model	Recommended shift
Cosmos3-Nano (8B)	10.0 (default)
Cosmos3-Super (32B)	5.0

Q: How do I add a new parameter to the inference pipeline?

1. Add the field to SamplingArgs and SamplingOverrides in cosmos_framework/inference/args.py
2. Add its default to each cosmos_framework/inference/defaults/<mode>/sample_args.json
3. Wire it through OmniSampleOverrides.build_sample() in cosmos_framework/inference/args.py

Q: What does defaults_file do?

It lets you supply a custom JSON file of default values instead of the built-in presets. The format is the same as the files in cosmos_framework/inference/defaults/. Fields in your sample argument file still take precedence over the custom defaults. See inference.md#custom-defaults.

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Inference

Q: How much GPU memory do the models need?

Model	GPU Memory
Cosmos3-Nano (8B)	32 GB
Cosmos3-Super (32B)	128 GB

Q: I get torch.cuda.OutOfMemoryError during inference

Try these in order:

1. Reduce allocator fragmentation — usually the cheapest fix:

   export PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True
   

2. Increase --dp-shard-size to shard model weights across more GPUs via FSDP. Inference auto-picks a value that fits the model at ~75% device memory (see _get_dp_shard_size in cosmos_framework/inference/args.py); passing a larger explicit value drops per-GPU memory at the cost of more all-gather traffic. Requires multi-GPU.

3. Lower --device-memory-utilization (default 0.75). The auto-dp_shard_size formula is ceil(model_memory / device_memory / utilization), so passing e.g. --device-memory-utilization=0.5 forces auto-mode to pick a larger dp_shard_size and leaves more per-GPU headroom for activations / KV cache. Requires multi-GPU.

4. Add --offload-guardrail-models to move the text and video guardrail models to CPU. Frees the GPU memory they would otherwise hold for the full run, at the cost of some extra latency when guardrails are invoked.

See inference.md#torch-cuda-out-of-memory-error for the full troubleshooting section.

Q: What is the difference between latency and throughput parallelism presets?

Preset	What it does	When to use
latency	Spreads each sample across all GPUs	Interactive / real-time use
throughput	One sample per GPU in parallel	Large batch jobs

Q: How do I generate images instead of videos?

Use a text2image input file:

python -m cosmos_framework.scripts.inference -i inputs/omni/t2i.json -o outputs/ --checkpoint-path Cosmos3-Nano

The modality is determined by the input JSON (num_frames=1 for images), not by a separate flag. See inputs/omni/t2i.json for the format.

Q: How many frames can I generate?

Depends on resolution:

Resolution	Max frames
256p	400
480p	300
720p	200

Default is 189 frames at 24 FPS (~7.9 seconds).

Q: What input formats does image-to-video support?

Provide a vision_path pointing to an image (.jpg, .jpeg, .png) or a URL. See inputs/omni/i2v.json for the format.

Q: How do I run online inference with Ray?

Install serve dependencies and start the server:

uv pip install -e ".[serve]"
python -m cosmos_framework.inference.ray.serve --parallelism-preset=latency -o outputs/ray_serve --checkpoint-path Cosmos3-Nano

Then submit requests via curl, the submit CLI, or the Gradio UI.

Q: How do I fix a torch.compile error during inference?

Add a command-line argument --no-use-torch-compile

OR

Delete the torchinductor cache under the /tmp directory, rm -rf /tmp/torchinductor_*

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Q: I get torch.distributed.DistNetworkError: ... port: 29500 ... EADDRINUSE, address already in use

torchrun defaults its rendezvous to port 29500. The error means that port is already taken on the node — usually because another torchrun job (yours or someone else's on a shared node) is still using it.

Pass a different free port with --master-port, placed before -m (it is a torchrun argument, not an inference argument):

torchrun --nproc-per-node=8 --master-port=29501 -m cosmos_framework.scripts.inference \
  --parallelism-preset=throughput \
  -i "inputs/omni/t2i.json" \
  -o outputs/omni_t2i \
  --checkpoint-path Cosmos3-Super-Text2Image \
  --seed=0

Any free port works (e.g. 29501, 29510); give each concurrent job on the same node a distinct port. Alternatively, --rdzv-endpoint=localhost:0 lets torchrun auto-pick a free port.

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Training

Q: I get torch.cuda.OutOfMemoryError during training (SFT)

Knobs are in the recipe TOML under [model], [model.parallelism], and [dataloader_train]. Try in order:

1. Reduce allocator fragmentation — usually the cheapest fix:

   export PYTORCH_ALLOC_CONF=expandable_segments:True
   

   (The _super launch shells already export this — see examples/launch_sft_*_super.sh.)

2. Enable activation checkpointing in [model.activation_checkpointing]:

   • mode = "full" — checkpoint every transformer block (largest memory savings, trades extra recompute for memory).
   • mode = "selective" — per-op SAC, MoT only (smaller savings, smaller overhead). Falls back to no checkpointing on the VLM path.

3. Raise [model.parallelism].data_parallel_shard_degree to shard weights/optimizer state across more ranks via FSDP. Runtime invariant (from cosmos_framework/utils/vfm/parallelism.py:50-52): data_parallel_replicate_degree × data_parallel_shard_degree == WORLD_SIZE always holds — context_parallel_shard_degree and cfg_parallel_shard_degree are overlay axes that share dp rank slots, not separate mesh dims. Use -1 to let data_parallel_shard_degree auto-fill from torchrun world size.

4. Raise [model.parallelism].context_parallel_shard_degree to split the sequence dimension across ranks. Helpful when activations (not weights) drive the OOM — long videos, high resolution.

5. Lower [dataloader_train].max_samples_per_batch to cap samples per micro-batch. None lets the packer's token budget decide; setting an explicit small number trades throughput for headroom.

6. Enable LoRA on a Cosmos3-Nano recipe. Nano recipes are full-finetune by default (lora_enabled = false); setting [model].lora_enabled = true trains low-rank adapters instead of the full weights, dropping optimizer-state memory substantially. The _super recipes (e.g. vision_sft_super) are already LoRA-only, so this lever doesn't apply there.

See docs/training.md for the full SFT setup and TOML reference ([model.activation_checkpointing], [model.parallelism], [dataloader_train] sections).

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Tips and Tricks

Seed reproducibility

Always pass --seed when comparing runs. Without it, a random seed is used each time.

Prompt upsampling

Short prompts produce worse results. Use the built-in prompt upsampler with a vLLM-served Qwen3 model:

python -m cosmos_framework.scripts.upsample_prompts -i "inputs/omni/*.json" -o outputs/upsample_prompts

Batch inference resume

The inference script automatically skips samples whose output files already exist. If a run is interrupted, re-run the same command to resume.

Generate all modalities at once

python -m cosmos_framework.scripts.inference -i "inputs/omni/*.json" -o outputs/ --checkpoint-path Cosmos3-Nano --seed=0

CLI help

All available flags and their current defaults:

python -m cosmos_framework.scripts.inference --help

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Miscellaneous

This section is a catch-all for tips that don't fit elsewhere. Add new entries freely.

Q: What are the example scripts in examples/ for?

They illustrate how the inference logic works under the hood — examples/inference.py shows the low-level model API and examples/inference_pipeline.py shows the pipeline API. For production use, prefer python -m cosmos_framework.scripts.inference.

Q: Where are the Ray Serve config files?

cosmos_framework/inference/ray/configs/latency.yaml and cosmos_framework/inference/ray/configs/throughput.yaml. These configure the Ray Serve deployment with different parallelism strategies.