merlin-llm 0.1.6

Merlin — a fast local LLM for agentic coding on Apple Silicon

 Merlin — Specialized Agentic Coding Model

A 3B language model built from scratch exclusively for agentic coding — not a general assistant, not a fine-tuned GPT. Every training token, every design decision, every protocol token is optimized for one job: executing code tasks fast, locally, and at scale.

Runs on any MacBook. No API key. Your code never leaves your machine.

→ tsuberim.github.io/merlin

The Idea

LLM agents spend most of their tokens on execution, not reasoning — grep a file, run a test, rename a function. Today that all hits a frontier model at $1–10/M tokens.

Merlin is the brute-force execution layer beneath a smarter orchestrator. A frontier model (Claude, GPT-4) plans; Merlin executes — locally, in parallel, at zero marginal cost.

Mode	Workers	Cost
Local	1, on your MacBook	$0 — always
Hosted	100–1000 via GPU batching	pay-per-task

Design

Choice	What	Why
Pre-trained from scratch	100B tokens — code, bash, agentic traces, commits	No proprietary distillation; weights are commercially clean
Custom tokenizer	32K BPE + 18 agent protocol special tokens	Tool-call protocol is first-class, not bolted on
6K context window	Sized for one large Python file + agent overhead	Not a general-purpose model
RL post-training	GRPO on verifiable bash/filesystem rewards	Ground truth without a judge model
MLX int4 inference	~1.5 GB weights, >500 tok/s on M3	Fits any M-series Mac

Agent Protocol

18 special tokens define the tool-call format — the model learns to emit and parse tool calls natively:

<|task|> Read src/main.py and return the function names.
<|think|> I need to read the file first.<|/think|>
<|tool_call|><|tool_name|>read_file<|tool_args|>{"path": "src/main.py"}<|/tool_call|>
<|tool_result|>def train(): ...\ndef evaluate(): ...<|/tool_result|>
<|answer|> train, evaluate

Architecture

GPT-style decoder-only. RMSNorm, SwiGLU, GQA (n_kv_head=8), no bias, weight tying, pre-norm.

Config	Params	n_embd	n_head	n_layer	block_size
tiny	~1.6M	32	2	2	64
medium	~21M	256	8	8	512
base (330M)	~330M	1024	16	16	2048
3b	~3.17B	3072	24	20	4096

Corpus

~100B tokens across 7 sources. Two-phase curriculum: 80B general mix → 20B upweighted traces + instruction data.

Source	Share
Stack v2 — Python	~38%
Stack v2 — Bash / Markdown	~8%
Agentic traces (synthetic)	~15%
GitHub commits + issues	~11%
Stack Overflow	~10%
Math + instruction mix	~12%
tldr pages	<1%

v0 corpus (1.19B tokens): tsuberim/merlin-corpus-v0 Tokenizer: tsuberim/merlin-tokenizer-v0

Status

Milestone	Status
Agentic protocol + eval harness (49 tasks, 47% on 3B baseline)	✅ Done
Custom BPE tokenizer (32K vocab, 20 special tokens)	✅ Done
Data pipeline (download → tokenize → pack)	✅ Done
v0 corpus on HuggingFace (1.19B tokens)	✅ Done
E2E training loop, 330M model on H100	✅ Done
SFT infrastructure	✅ Done
Repo scanning pipeline (clone + pytest → passing repos)	🔄 In progress
Agentic trace generation (target: 200K traces)	⏸ Planned
Full 100B token corpus	⏸ Planned
3B pre-training run	⏸ Planned
RL post-training (GRPO on verifiable rewards)	⏸ Planned
MLX int4 3B model release	⏸ Planned

Stack

Role	Tool
Training	PyTorch + CUDA (NVIDIA H100)
Inference	MLX (Apple Silicon / Metal)
Cloud compute	Modal
Tokenizer	HuggingFace tokenizers (BPE, Rust)
Trace generation	vLLM + Qwen2.5-Coder-32B
Observability	W&B
Datasets + models	HuggingFace Hub

Setup

python3 -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt

License

MIT