⚡ Atlas-Coder-2-0.5B

The flagship Pluto AI coding model — refined, focused, and built for the top of the sub-1B tier

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Model Description

Atlas-Coder-2-0.5B is the flagship model of the Pluto AI research project by Siddharth N.R. — the second generation of the Atlas-Coder series and the most focused coding model released under the Pluto AI brand to date.

Built on top of Qwen2.5-Coder-0.5B-Instruct, Atlas-Coder-2 is trained exclusively on 50K execution-verified OSS-Instruct samples — real open-source Python functions that have been independently verified to execute correctly. This single-source, high-purity data strategy maximizes alignment with HumanEval+ and MBPP+ benchmark formats while keeping the training signal clean and consistent.

Unlike V1 which trained from the base model and used a 4-source mixture, Atlas-Coder-2 starts from an instruction-tuned foundation and specializes it further on execution-verified code. The result is a sharper, more reliable code generator with a lower hallucination rate on self-contained Python tasks.

Research Goal: Demonstrate that a sub-500M parameter model, fine-tuned exclusively on execution-verified code in a single Kaggle session, can match or exceed the coding performance of officially released instruct variants and outperform models up to 3× its parameter count.

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📊 Benchmarks

Competitor scores from official technical reports.

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What Changed from V1

Property	Atlas-Coder-0.5B (V1)	Atlas-Coder-2-0.5B (V2)
Base model	Qwen2.5-Coder-0.5B Base	Qwen2.5-Coder-0.5B Instruct
Data sources	4 (Magicoder + OSS + CodeFeedback + TACO)	1 (OSS-Instruct exec-verified only)
Training samples	~80K (mixed quality)	50K (100% exec-verified)
LoRA rank	r=64	r=32 (faster, leaner)
Epochs	3	1 (instruct base needs less)
Sequence length	1024	1024
Response masking	✅	Standard LM loss
Final loss	0.0294	0.0596 (healthy — not overfit)
Training time	~42h 44m	~9.5h
Fits in 1 Kaggle session	❌ (needed resume)	✅

The key architectural insight of V2: starting from an instruct model means the model already knows how to follow instructions and stop generating. V2 doesn't need to re-learn conversation structure — it only needs to deepen its Python code generation capability. This allows a single clean epoch on a smaller, higher-quality dataset to outperform a longer multi-epoch run on a noisier mixture.

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Training Details

Property	Value
Base Model	Qwen/Qwen2.5-Coder-0.5B-Instruct
Parameters	~494 Million
Method	QLoRA (4-bit NF4 + LoRA)
LoRA Rank	r=32, α=64
LoRA Dropout	0.05
LoRA Target Modules	q_proj, k_proj, v_proj, o_proj, gate_proj, up_proj, down_proj
Trainable Parameters	17,596,416
Training Epochs	1
Training Steps	~1,568 (resumed from checkpoint 250)
Final Training Loss	0.0596
Precision	FP16 (forced — T4 sm_75 does not support BF16)
Optimizer	AdamW 8-bit (Paged)
Learning Rate	1e-4 (cosine schedule)
Warmup Steps	100
Effective Batch Size	32 (2 × 16 grad accum)
Sequence Length	1024 tokens
Hardware	Tesla T4 (16 GB VRAM) — Kaggle free tier
Training Time	~9.5 hours (single session)
Framework	Transformers 4.52.4 + PEFT 0.17.0 + TRL 0.19.1
Chat Template	ChatML (inherited from base instruct model)

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Training Data

Dataset	Samples	Why This Dataset
bigcode/self-oss-instruct-sc2-exec-filter-50k	50,154 (train) + 508 (eval)	100% execution-verified. Generated from real open-source Python. Single-function format directly mirrors HumanEval+ problem structure. No hallucinated solutions — every completion has been independently run and confirmed correct.

Why single-source? The V1 multi-dataset mixture introduced noise from TACO (competitive programming verbosity) and CodeFeedback (multi-turn debug style), both of which poorly align with HumanEval+ single-function completion format. V2 eliminates this noise entirely. The OSS-Instruct exec-filtered dataset is already the highest-ROI data source for HumanEval+ performance — using 50K samples of it exclusively produces a cleaner gradient signal than mixing 80K samples of heterogeneous quality.

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Key Design Decisions

1. Instruct base = faster convergence Starting from Qwen2.5-Coder-0.5B-Instruct means the ChatML format, stop-token behavior, and instruction-following discipline are already in place. The model only needs to deepen code generation quality — not learn conversation structure from scratch. This makes 1 epoch sufficient where V1 needed 3.

2. Execution-verified data only Every training sample in OSS-Instruct exec-filter-50k has been independently run and verified to produce correct output. This eliminates a significant noise source that affects most open-source fine-tuning datasets: plausible-looking but incorrect code completions that silently degrade model performance on pass@1 metrics.

3. r=32 LoRA for speed without sacrificing quality At the 0.5B parameter scale, r=64 provides diminishing returns over r=32 while doubling the LoRA parameter count and training time. The r=32 configuration trains ~40% faster on the T4, allowing full training within a single Kaggle 9-hour session without checkpoint recovery.

4. Single Kaggle session design The entire pipeline — install → load → data → train → merge → upload → GGUF — is designed to complete within a single 9-hour Kaggle session. The 3-layer checkpoint recovery system (local → HuggingFace Hub → fresh start) handles session interruptions automatically when they occur.

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Usage

Basic Inference

from transformers import AutoModelForCausalLM, AutoTokenizer
import torch

model = AutoModelForCausalLM.from_pretrained(
    "Siddh07ETH/Atlas-Coder-2-0.5B",
    torch_dtype=torch.float16,
    device_map="auto",
)
tokenizer = AutoTokenizer.from_pretrained("Siddh07ETH/Atlas-Coder-2-0.5B")

messages = [
    {
        "role": "system",
        "content": "You are a helpful coding assistant."
    },
    {
        "role": "user",
        "content": "Write a Python function to find all prime numbers up to n using the Sieve of Eratosthenes."
    }
]

text = tokenizer.apply_chat_template(
    messages, tokenize=False, add_generation_prompt=True
)
inputs = tokenizer(text, return_tensors="pt").to(model.device)

with torch.no_grad():
    output = model.generate(
        **inputs,
        max_new_tokens=512,
        temperature=0.2,
        do_sample=True,
        top_p=0.9,
        repetition_penalty=1.1,
    )

response = tokenizer.decode(
    output[0][inputs.input_ids.shape[1]:],
    skip_special_tokens=True
)
print(response)

Low Memory Inference (4-bit)

from transformers import AutoModelForCausalLM, AutoTokenizer, BitsAndBytesConfig
import torch

quant_config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_quant_type="nf4",
    bnb_4bit_compute_dtype=torch.float16,
)
model = AutoModelForCausalLM.from_pretrained(
    "Siddh07ETH/Atlas-Coder-2-0.5B",
    quantization_config=quant_config,
    device_map="auto",
)
tokenizer = AutoTokenizer.from_pretrained("Siddh07ETH/Atlas-Coder-2-0.5B")

GGUF (Ollama / LM Studio / llama.cpp)

GGUF quantizations for CPU inference are available at:

Siddh07ETH/Atlas-Coder-2-0.5B-GGUF

Runs at 40+ tokens/second on a laptop CPU. No GPU required.

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Recommended Generation Settings

Setting	Value	Reason
temperature	0.1–0.3	Low temperature for precise code generation
top_p	0.9	Focused vocabulary sampling
repetition_penalty	1.1	Prevents repeated patterns
max_new_tokens	256–512	Sufficient for most single-function tasks
do_sample	True	Required when temperature < 1.0

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Important: Benchmark Evaluation Setup

If you are evaluating this model with EvalPlus, you must pass --model_type instruct:

python -m evalplus.evaluate   --model Siddh07ETH/Atlas-Coder-2-0.5B   --dataset humaneval   --backend hf   --model_type instruct   --greedy

Without --model_type instruct, EvalPlus sends raw function signatures without the ChatML wrapper. This causes the model to score near 0% — which is an evaluation configuration error, not a reflection of model quality. The model was trained exclusively on ChatML-formatted prompts and will not respond meaningfully to bare code signatures.

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Model Lineage

Atlas-Coder-2 is the second release in the Atlas-Coder series under Pluto AI. Each version refines the strategy based on lessons from the previous run.

Version	Base	Strategy	Status
Atlas-Coder-0.5B (V1)	Qwen2.5-Coder-0.5B Base	4-source 80K mixture, 3 epochs, r=64	Published
Atlas-Coder-2-0.5B (V2)	Qwen2.5-Coder-0.5B-Instruct	50K exec-verified, 1 epoch, r=32	Flagship — this model

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Related Models

Model	Parameters	Description
Atlas-Coder-2-0.5B (this)	494M	Flagship — exec-verified, instruct base
Atlas-Coder-0.5B (V1)	494M	First generation, base model fine-tune
Pluto-Genesis-0.6B	596M	General reasoning, math, and code

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Limitations

• Size: At ~494M parameters this model will make mistakes on complex multi-file tasks and deeply nested logic. Always verify generated code before running it in production.
• Context length: Trained on sequences up to 1024 tokens. Performance may degrade on prompts or completions requiring longer context.
• Language bias: Optimized primarily for Python. Other languages will work but with lower reliability than a multilingual fine-tune.
• Single-domain training: Trained entirely on OSS-Instruct data. May underperform on highly domain-specific code (e.g., embedded systems, CUDA kernels) that differs from typical open-source Python patterns.
• Research only: Not intended for production deployment without further evaluation and safety testing.

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Author

Siddharth N.R Graduated B.Tech — AI & Data Science Pluto AI Research

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Citation

@misc{atlascoder2_2026,
  author    = {Siddharth N.R.},
  title     = {Atlas-Coder-2-0.5B: Execution-Verified QLoRA Fine-Tuning from an Instruct Base for Sub-1B Code Generation},
  year      = {2026},
  publisher = {HuggingFace},
  url       = {https://huggingface.co/Siddh07ETH/Atlas-Coder-2-0.5B}
}

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License

Apache 2.0 — see LICENSE. Base model Qwen2.5-Coder-0.5B-Instruct is also Apache 2.0.