ckptkit

The missing Swiss Army knife for model checkpoints.

ckptkit inspects, diffs, validates, and merges model checkpoints without loading them into GPU memory. Parse SafeTensors headers in milliseconds, compare checkpoints after fine-tuning, merge LoRA adapters, and validate file integrity — all from the command line or Python.

Why ckptkit?

Working with model weights means dealing with:

• "What layers are in this checkpoint?" → ckptkit info
• "What changed after fine-tuning?" → ckptkit diff
• "Is this download corrupt?" → ckptkit validate
• "Merge this LoRA adapter into the base" → merge_lora_state_dicts()
• "Show me parameter counts per layer" → ckptkit stats

mergekit handles model merging (TIES, DARE, SLERP), but nobody built the everyday checkpoint utility. ckptkit is that tool.

Install

pip install ckptkit

With SafeTensors support (recommended):

pip install ckptkit[safetensors]

With PyTorch support:

pip install ckptkit[torch]

Everything:

pip install ckptkit[all]

CLI

Inspect

# See what's inside a checkpoint
ckptkit info model.safetensors

# JSON output for scripts
ckptkit info model.safetensors --json | jq '.n_parameters'

Diff

Compare two checkpoints — see what changed during fine-tuning:

ckptkit diff base_model.safetensors finetuned_model.safetensors

Validate

Check for corruption before a long training run:

ckptkit validate model.safetensors
# ✓ model.safetensors: valid (safetensors)

Stats

ckptkit stats model.safetensors

Python API

Inspect

from ckptkit import inspect

info = inspect("model.safetensors")
print(f"Parameters: {info.n_parameters:,}")
print(f"Tensors: {info.n_tensors}")
print(f"Format: {info.format.value}")

for t in info.tensors[:5]:
    print(f"  {t.name}: {t.shape} {t.dtype.value} ({t.numel:,} params)")

Diff

from ckptkit import diff, format_diff

result = diff("base.safetensors", "finetuned.safetensors")
print(f"Changes: {result.n_changes}")
print(f"Identical: {result.n_identical} / {result.n_shared}")

for entry in result.entries:
    print(f"  {entry.change_type}: {entry.tensor_name} — {entry.details}")

Merge LoRA

import torch
from ckptkit import merge_lora_state_dicts

base = torch.load("base_model.bin", map_location="cpu")
adapter = torch.load("adapter_model.bin", map_location="cpu")

merged = merge_lora_state_dicts(base, adapter, alpha=1.0)
torch.save(merged, "merged_model.bin")

Validate

from ckptkit import validate

result = validate("model.safetensors")
if not result.valid:
    for issue in result.issues:
        print(f"  {issue.severity}: {issue.message}")

Stats

from ckptkit import inspect, stats_from_info

info = inspect("model.safetensors")
stats = stats_from_info(info)

print(f"Total size: {stats.total_size_human}")
for dtype, count in stats.dtype_counts.items():
    print(f"  {dtype}: {count:,} parameters")

Format support

Format	Inspect	Diff	Validate	Merge
SafeTensors	✓ (header-only, fast)	✓	✓ (full integrity)	✓
PyTorch (.bin/.pt)	✓ (requires torch)	✓	basic	✓

How it works

SafeTensors inspection is fast because the format puts all tensor metadata (names, shapes, dtypes, offsets) in a JSON header at the start of the file. ckptkit reads only the first few KB, never loading the actual weight data.

LoRA merging performs base_weight += alpha * (lora_B @ lora_A) for each matched layer pair, with automatic key resolution for common adapter formats (PEFT, HuggingFace).

See Also

Part of the stef41 LLM toolkit — open-source tools for every stage of the LLM lifecycle:

Project	What it does
tokonomics	Token counting & cost management for LLM APIs
datacrux	Training data quality — dedup, PII, contamination
castwright	Synthetic instruction data generation
datamix	Dataset mixing & curriculum optimization
toksight	Tokenizer analysis & comparison
trainpulse	Training health monitoring
quantbench	Quantization quality analysis
infermark	Inference benchmarking
modeldiff	Behavioral regression testing
vibesafe	AI-generated code safety scanner
injectionguard	Prompt injection detection

License

Apache-2.0