PyBytecode v3 — 1.5B

Turns Python 3.12 bytecode back into Python source. Hand it a disassembled code object, get source code back.

The unusual part: you can check every answer. Recompile what the model wrote and compare it against the bytecode you started with — if they match, that file is exactly right, and you know it without trusting an accuracy number.

Weights are Apache-2.0. A GGUF build ships alongside for llama.cpp / LM Studio / Ollama.

Quickstart

from transformers import AutoModelForCausalLM, AutoTokenizer

tok = AutoTokenizer.from_pretrained("BlazingCustoms/pybytecode-v3-1.5b")
model = AutoModelForCausalLM.from_pretrained(
    "BlazingCustoms/pybytecode-v3-1.5b", torch_dtype="bfloat16", device_map="auto")

INSTRUCTION = ("Decompile this Python 3.12 bytecode disassembly back into the original Python "
               "source code. Output only the source code.")

# `disasm` comes from harness/pybytecode_core/rep.py: disassemble_v2(code_object)
msgs = [{"role": "user", "content": f"{INSTRUCTION}\n\n{disasm}"}]
batch = tok.apply_chat_template(msgs, add_generation_prompt=True,
                                return_tensors="pt", return_dict=True).to(model.device)
prediction = tok.decode(model.generate(**batch, max_new_tokens=2048,
                                       do_sample=False)[0][batch["input_ids"].shape[1]:],
                        skip_special_tokens=True)

Greedy decoding (temperature 0) for a single shot; temperature ~0.8 when you sample several candidates.

Checking the answer

Compile the model's output and compare the resulting code object to the one you were decompiling. Same code object means same behaviour, so a match tells you this answer is correct:

from harness.pybytecode_core.verify import code_fingerprint

def certified(prediction: str, reference_code_object) -> bool:
    got = compile(prediction, "<pred>", "exec", dont_inherit=True, optimize=0)
    return code_fingerprint(got) == code_fingerprint(reference_code_object)

Two things follow. A failed check means "not confirmed", not "wrong" — a correct rewrite that compiles differently (a while where the original had a for) won't match, so the accuracy figures below are a floor, not an estimate. And because the check is cheap and reliable, sampling several answers and keeping the first one that passes is a real gain rather than a nicer guess.

Results

On the benchmark published with this model:

The benchmark is csn-3.12-licensed: 600 real functions from 117 GitHub repositories, compiled to 3.12 bytecode, shipped with the model. The base model before fine-tuning gets essentially none of them, so this is not something a general code model can guess its way through.

Full numbers, confidence intervals, method, per-budget curve and the comparison with other systems: EVAL.md.

PyLingual is another system that does this task, by symbolic reconstruction rather than generation. On our earlier benchmarks it scores about the same as we do, and the two miss on different inputs — so running both and keeping whichever answer passes the check gets you more than either alone. Numbers in EVAL.md.

When it works well, and when it doesn't

It is good on individual functions and gets much worse on long ones. Size is measured in disassembly lines — how long the input you hand the model is. One line tells you:

from harness.pybytecode_core.rep import disassemble_v2
rep_lines = disassemble_v2(code_object).count("\n")

Below ~100 lines it is on home ground. Accuracy starts dropping around 200, and above ~400 lines nothing certified at all, even with 32 tries. Sampling more buys roughly one bucket of headroom; it does not remove the limit. For big units, a symbolic decompiler is the better tool. The full seven-bucket curve is in EVAL.md.

Limits

• Long inputs. The table above is the honest specification: trained on functions, not modules, and it fails above ~400 disassembly lines.
• Python 3.12 only. Trained and measured on 3.12; the checker refuses other minor versions by design.
• If the .pyc was built with -O, compile at the same level or the check will not match. Wrong level collapses to ~24%, so try all three — it costs three compiles. At -O and above, docstrings aren't in the .pyc at all, so docstring recovery can't be confirmed against one.
• A .pyc built by someone else can fail the check even when the answer is right — about 0.33% of the time, because CPython patch releases compile the same source differently. It always fails in the safe direction: "unknown" about a correct answer, never "confirmed" about a wrong one.
• It has not been shown to work on real malware. On the one packed sample we tried, the entry-point module produced nothing certifiable. Extraction worked; decompiling the actual malware logic did not.
• Untested: Python 3.13, Nuitka, non-CPython builds, obfuscated bytecode.

Details on all of these in EVAL.md and ORACLE-LIMITS.md.

Model details

Fine-tuned from Qwen/Qwen2.5-Coder-1.5B-Instruct (Apache-2.0) with LoRA on 48,196 pairs of Python 3.12 disassembly → source, adapter merged. The corpus was filtered to permissive licences before training and is not redistributed — per-row attribution was not retained, so shipping it would strip required notices. Lineage: DATA-CARD-training-corpus.md. Training settings: EVAL.md.

Licence

Apache-2.0. See LICENSE and NOTICE.

Derived from Qwen/Qwen2.5-Coder-1.5B-Instruct, which is Apache-2.0. Under Apache-2.0 §4 we ship the licence, retain attribution, and state our changes (LoRA fine-tune, adapter merged; no architecture, vocabulary or tokenizer change). The same obligations pass to you if you redistribute these weights or build derivatives.

Decompilation has obvious dual use. Apache-2.0 imposes no field-of-use restriction and we have not added one. Complying with the law where you operate is your responsibility.

Citation

@software{pybytecode2026,
  title  = {PyBytecode: verified neural decompilation for Python 3.12 bytecode},
  author = {Blazing Customs},
  year   = {2026},
  note   = {Fine-tuned from Qwen2.5-Coder-1.5B-Instruct},
  url    = {https://huggingface.co/BlazingCustoms/pybytecode-v3-1.5b}
}