Build Models

Docs

• Cog reference (single file): https://cog.run/llms.txt
• cog.yaml reference: https://cog.run/yaml
• Python predictor reference: https://cog.run/python
• Examples: https://github.com/replicate/cog-examples
• Template: https://github.com/replicate/cog-template

When to use this skill

• You have model code, weights, or a HuggingFace/GitHub project you want to host on Replicate.
• You're writing or editing a cog.yaml, predict.py, or train.py.
• For pushing a built model to Replicate, see publish-models.
• For running existing Replicate models, see run-models.

Prerequisites

• Docker running locally.
• Cog installed: brew install replicate/tap/cog or sh <(curl -fsSL https://cog.run/install.sh).
• Optional: cog init to scaffold cog.yaml and predict.py.

Project layout

The canonical Replicate model layout:

cog.yaml
predict.py
weights.py                 # optional download helpers
requirements.txt
cog-safe-push-configs/
  default.yaml             # see publish-models skill
.github/workflows/
  ci.yaml
script/                    # github.com/github/scripts-to-rule-them-all
  lint
  test
  push

cog.yaml essentials

A modern config for a GPU model:

build:
  gpu: true
  cuda: "12.8"
  python_version: "3.12"
  python_requirements: requirements.txt
  system_packages:
    - libgl1
    - libglib2.0-0
predict: predict.py:Predictor

Notes:

• Pin Python to a specific minor version, and pin every line in requirements.txt. Floating versions break cold boots.
• Use python_requirements over inline python_packages once the list grows.
• cuda follows your torch wheel (e.g. 12.8 paired with torch==2.7.1+cu128).
• Add train: train.py:train if your model is fine-tunable.
• Add image: r8.im/owner/name to enable bare cog push.

For async predictors with continuous batching:

concurrency:
  max: 32

predict.py essentials

from cog import BasePredictor, Input, Path

class Predictor(BasePredictor):
    def setup(self) -> None:
        """One-time loads. Heavy work goes here, not in predict()."""
        self.model = load_model("weights/")

    def predict(
        self,
        prompt: str = Input(description="Text prompt for generation"),
        seed: int = Input(description="Random seed; leave blank for random", default=None),
        num_steps: int = Input(description="Number of denoising steps", ge=1, le=50, default=20),
        output_format: str = Input(description="Output image format", choices=["webp", "jpg", "png"], default="webp"),
    ) -> Path:
        """Run a single prediction."""
        if not prompt.strip():
            raise ValueError("prompt cannot be empty")
        out = self.model.generate(prompt, seed=seed, steps=num_steps)
        return Path(out)

Input rules:

• Every input needs a description. The description shows up in the model schema and on Replicate's web UI.
• Use ge/le for numeric bounds, choices=[...] for enums, regex= for strings.
• Use cog.Path for file inputs and outputs, never raw bytes.
• Use cog.Secret for any token-like input (HF tokens, API keys), never plain str.
• Provide a default that's inside choices for categorical inputs.
• Validate inputs early in predict() and raise ValueError.

Streaming text output (for LLMs):

from cog import BasePredictor, Input, ConcatenateIterator

class Predictor(BasePredictor):
    def predict(self, prompt: str = Input(description="Prompt")) -> ConcatenateIterator[str]:
        for token in self.model.stream(prompt):
            yield token

Async predictor with continuous batching (paired with concurrency.max in cog.yaml):

from cog import BasePredictor, Input, AsyncConcatenateIterator

class Predictor(BasePredictor):
    async def setup(self) -> None:
        self.engine = await load_async_engine()

    async def predict(
        self,
        prompt: str = Input(description="Prompt"),
    ) -> AsyncConcatenateIterator[str]:
        async for token in self.engine.generate(prompt):
            yield token

Dynamic choices from on-disk assets (e.g. a voices/ directory of audio samples):

from pathlib import Path as _P
AVAILABLE_VOICES = sorted(p.stem for p in _P("voices").glob("*.wav"))

class Predictor(BasePredictor):
    def predict(
        self,
        speaker: str = Input(description="Voice", choices=AVAILABLE_VOICES, default=AVAILABLE_VOICES[0]),
    ) -> Path: ...

Loading weights fast

Cold boot dominates user-perceived latency. Three patterns, ranked by simplicity:

1. Bake weights into the image at build time

Best for small or medium weights (< 5GB) that you want zero-cold-boot for.

For torchvision:

import os
os.environ["TORCH_HOME"] = "."  # set before importing torch
import torch
from torchvision import models

For HuggingFace:

import os
os.environ["HF_HUB_CACHE"] = "./.cache"
os.environ["HF_XET_HIGH_PERFORMANCE"] = "1"

Then download once during cog build (e.g. in a run: step or by running a small fetcher script as part of the build). The weights become part of the image layer.

2. Pull from weights.replicate.delivery with pget

Best for large weights, or when you want to share weights across multiple models. pget is Replicate's parallel HTTP fetcher.

In cog.yaml:

build:
  run:
    - curl -o /usr/local/bin/pget -L "https://github.com/replicate/pget/releases/download/v0.8.2/pget_linux_x86_64"
    - chmod +x /usr/local/bin/pget

In setup():

import subprocess
from pathlib import Path

WEIGHTS_URL = "https://weights.replicate.delivery/default/my-model/weights.tar"
WEIGHTS_DIR = Path("weights")

class Predictor(BasePredictor):
    def setup(self) -> None:
        if not WEIGHTS_DIR.exists():
            # -x extracts tar in-memory; default concurrency is 4 * NumCPU
            subprocess.check_call(["pget", "-x", WEIGHTS_URL, str(WEIGHTS_DIR)])
        self.model = load_from(WEIGHTS_DIR)

For multiple files in one shot:

manifest = "\n".join([
    f"{base}/unet.safetensors weights/unet.safetensors",
    f"{base}/vae.safetensors  weights/vae.safetensors",
    f"{base}/text_encoder.safetensors weights/text_encoder.safetensors",
])
subprocess.run(["pget", "multifile", "-"], input=manifest, text=True, check=True)

3. HuggingFace Hub with hf_transfer

Set HF_HUB_ENABLE_HF_TRANSFER=1 and use huggingface_hub.snapshot_download or from_pretrained. Faster than vanilla HF downloads. Use a cog.Secret input for gated models.

Weight cache for user-supplied weights

For LoRAs or any weights URL the user passes at predict time, use a sha256-keyed disk cache with LRU eviction:

import hashlib, shutil, subprocess
from pathlib import Path

class WeightsDownloadCache:
    def __init__(self, cache_dir: str = "/tmp/weights-cache", min_disk_free_gb: int = 10):
        self.cache_dir = Path(cache_dir)
        self.cache_dir.mkdir(parents=True, exist_ok=True)
        self.min_disk_free = min_disk_free_gb * 1024**3

    def ensure(self, url: str) -> Path:
        key = hashlib.sha256(url.encode()).hexdigest()
        target = self.cache_dir / key
        if target.exists():
            target.touch()  # bump LRU mtime
            return target
        self._evict_until_room()
        subprocess.check_call(["pget", url, str(target)])
        return target

    def _evict_until_room(self) -> None:
        while shutil.disk_usage(self.cache_dir).free < self.min_disk_free:
            entries = sorted(self.cache_dir.iterdir(), key=lambda p: p.stat().st_mtime)
            if not entries:
                return
            entries[0].unlink()

See replicate/cog-flux/weights.py for a production version that handles HF, CivitAI, Replicate, and arbitrary .safetensors URLs.

Multi-LoRA composition

Reload only when the URL changes; compose two LoRAs with separate scales:

class Predictor(BasePredictor):
    def setup(self) -> None:
        self.pipe = load_base_pipeline()
        self.loaded = {"main": None, "extra": None}

    def _ensure_lora(self, slot: str, url: str | None) -> None:
        if url == self.loaded[slot]:
            return
        if self.loaded[slot] is not None:
            self.pipe.unload_lora_weights(adapter_name=slot)
        if url:
            path = self.cache.ensure(url)
            self.pipe.load_lora_weights(str(path), adapter_name=slot)
        self.loaded[slot] = url

    def predict(
        self,
        prompt: str = Input(description="Prompt"),
        lora_url: str = Input(description="Primary LoRA URL", default=None),
        lora_scale: float = Input(description="Primary LoRA scale", ge=0.0, le=2.0, default=1.0),
        extra_lora_url: str = Input(description="Optional second LoRA URL", default=None),
        extra_lora_scale: float = Input(description="Second LoRA scale", ge=0.0, le=2.0, default=1.0),
    ) -> Path:
        self._ensure_lora("main", lora_url)
        self._ensure_lora("extra", extra_lora_url)
        adapters = [s for s, u in self.loaded.items() if u]
        scales = [lora_scale if s == "main" else extra_lora_scale for s in adapters]
        if adapters:
            self.pipe.set_adapters(adapters, adapter_weights=scales)
        return Path(self.pipe(prompt).images[0].save("/tmp/out.png"))

Cold-boot tricks

From production diffusion models like replicate/cog-flux and replicate/cog-flux-kontext:

• Set perf flags once in setup():

  import torch
  torch.set_float32_matmul_precision("high")
  torch.backends.cuda.matmul.allow_tf32 = True
  torch.backends.cudnn.benchmark = True
  

• Compile and warm up:

  self.model = torch.compile(self.model, dynamic=True)
  _ = self.predict(prompt="warmup", num_steps=1)  # absorbs compile cost in setup
  

• Load big weights with meta device + assign=True to avoid double-allocating:

  with torch.device("meta"):
      model = build_model_skeleton()
  state = torch.load("weights.pt", map_location="cpu")
  model.load_state_dict(state, assign=True)
  

• Share VAE / text encoder across multiple pipelines (e.g. base + img2img + inpaint) instead of loading three copies.
• For fp8/int8, save quantized weights ahead of time and load directly; don't quantize at boot.

Local development

cog init                                    # scaffold cog.yaml + predict.py
cog predict -i prompt="hello"               # build + run a single prediction
cog predict -i image=@input.jpg -o out.png  # file inputs and outputs
cog serve -p 8393                           # HTTP server matching production
cog exec python                             # interactive shell inside the build env

Building

cog build -t my-model
cog build --separate-weights -t my-model    # weights in their own image layer
cog build --secret id=hf,src=$HOME/.hf_token -t my-model

Tips:

• Use --separate-weights for any model with weights > ~1GB. It speeds up cold boots and registry pushes.
• Use --mount=type=cache,target=/root/.cache/pip in run: steps to cache pip across builds.
• Use --secret instead of ARG to keep tokens out of image history.
• The default Cog base image (--use-cog-base-image=true) is faster than rolling your own.

Training

If your model supports fine-tuning, add train: train.py:train to cog.yaml and write a train() function that returns TrainingOutput(weights=Path("model.tar")). The predictor then accepts the URL via setup(self, weights) or the COG_WEIGHTS env var. See https://cog.run/training and replicate/flux-fine-tuner for a full example.

Guidelines

• Keep setup() for one-time loads; keep predict() fast and deterministic in shape.
• Pin Python and every dependency. Use numpy<2 if your torch is older.
• Always describe every input. Schemas without descriptions are unusable on the web UI.
• Use cog.Path for files and cog.Secret for tokens.
• Pin pget to a specific release (v0.8.2) for reproducibility.
• Set HF_HUB_ENABLE_HF_TRANSFER=1 whenever you call HuggingFace Hub.
• Set TRANSFORMERS_OFFLINE=1 after weights are loaded to prevent runtime HF lookups.
• Test with cog predict before pushing. If it doesn't work locally, it won't work in production.

Production references

• https://github.com/replicate/cog-examples — minimal patterns (resnet, hello-world, streaming, training)
• https://github.com/replicate/cog-template — scaffolder for new model repos
• https://github.com/replicate/cog-flux — multi-variant FLUX models, weights cache, fp8 + torch.compile
• https://github.com/replicate/cog-flux-kontext — meta-device loading, warmup compilation
• https://github.com/replicate/cog-vllm — async LLM server with continuous batching, training-as-packaging
• https://github.com/replicate/cog-comfyui — ComfyUI workflows as a Cog model, custom-node helpers
• https://github.com/replicate/flux-fine-tuner — multi-LoRA composition, shared pipeline components
• https://github.com/replicate/vibevoice — TTS with dynamic choices, minimal cog.yaml
• https://github.com/replicate/pget — parallel weights fetcher