mlxrl

Fast on-policy MLX RL for Apple Silicon, including single-turn GRPO-family RL and multi-turn GiGPO; not a general RL framework, not preference tuning, and not distributed training.

mlxrl is a small, single-process RL post-training library for LLMs on Apple Silicon. It is built around one idea: GRPO on MLX should be a fast batched rollout path with a thin loss and optimizer step on top, not a framework.

The current implementation targets QLoRA GRPO/GiGPO on local 4-bit MLX models. It reuses mlx-lm model loading, LoRA layers, KV caches, and sampling utilities, and keeps generation and training in one Python process with one model object.

mlxrl is pre-1.0. The correctness gates are stable, but import APIs and config fields may change before a 1.0 release.

Quickstart

git clone https://github.com/inductiveML/mlxrl.git
cd mlxrl
UV_CACHE_DIR=.uv-cache uv sync --all-groups
UV_CACHE_DIR=.uv-cache uv run mlxrl train \
--config examples/qwen3_0_6b_grpo.toml \
--available-memory-gb 48

For the measured 9B-on-48GB shape, use the checkpointed G=2 config:

UV_CACHE_DIR=.uv-cache uv run mlxrl train \
--config examples/qwen35_9b_g2_checkpoint.toml \
--available-memory-gb 48 \
--dry-run

What Works

• Batched group rollouts with MLX-LM KV caches and sampling.
• Full-forward old-policy logprob recompute for training-time pi_old.
• Adapter-disabled reference policy on the same model object.
• GRPO, Dr. GRPO, DAPO, and GSPO loss variants.
• RLOO (REINFORCE Leave-One-Out) as a critic-free rollout objective.
• Multi-turn agentic rollout through an Environment / EnvFactory protocol.
• GiGPO with episode-level and anchor-state step-level advantages.
• Full-forward trajectory action-logprob recompute for 4-bit correctness.
• QLoRA injection on dense and heterogeneous/hybrid layer stacks.
• Qwen3.5-style hybrid support via MLX-LM auto LoRA targeting, including DeltaNet linear_attn.in_proj_* and dense attention q/k/v/o_proj.
• Per-layer gradient checkpointing through mlx_lm.tuner.trainer.grad_checkpoint for linear-attention/DeltaNet backward memory.
• Micro-batched gradient accumulation for token-mean policy losses.
• beta == 0 reference-forward skip.
• Phase 4 benchmark harness for mlxrl, mlx-tune, mlx-lm-lora, and mlx-lm.

Install

Install the PyPI distribution:

pip install inductive-mlxrl

The Python import package and CLI command are still mlxrl:

mlxrl --help

Source installs are also supported:

UV_CACHE_DIR=.uv-cache uv sync --all-groups

Run commands through the local environment:

UV_CACHE_DIR=.uv-cache uv run mlxrl --help

Python 3.11+ is required. The MLX runtime path is supported on Apple Silicon with Python 3.11 through 3.14, matching the current MLX wheel support window. Runtime dependencies are intentionally small: mlx and mlx-lm on macOS arm64. Development dependencies include pytest, ruff, pyright, mlx-tune, and mlx-lm-lora for comparison benchmarks. The PyPI distribution name is inductive-mlxrl; the import package and console script remain mlxrl.

Quick Smoke Tests

Dense Qwen3 0.6B:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase0-smoke \
--model mlx-community/Qwen3-0.6B-4bit \
--prompt "What is 2+2?"

Hybrid Qwen3.5 9B with rank-16 LoRA:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase0-smoke \
--model mlx-community/Qwen3.5-9B-MLX-4bit \
--rank 16 \
--scale 2.0 \
--prompt "What is 2+2?"

The smoke gate prints the model id, layer count, LoRA target keys, per-layer LoRA module counts, total/trainable parameter counts, and logits shape. It fails if any trainable leaf is not lora_a or lora_b.

Training Commands

Toy hand-computed GRPO math gate:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase1-toy-gate

Small built-in GSM8K-style run:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase1-gsm8k \
--model mlx-community/Qwen3-0.6B-4bit \
--steps 20 \
--group-size 4 \
--max-tokens 64

Config-driven run:

UV_CACHE_DIR=.uv-cache uv run mlxrl train \
--config examples/qwen3_0_6b_grpo.toml \
--available-memory-gb 48

Reference multi-turn GiGPO run against the built-in recurring-state env:

UV_CACHE_DIR=.uv-cache uv run mlxrl train \
--config examples/qwen3_0_6b_gigpo_recurring.toml \
--available-memory-gb 48

The config schema validates model id, quant bits, group size, completion/prompt lengths, checkpointing granularity, iogpu.wired_limit_mb, optimizer settings, algorithm hyperparameters, KL beta, and seed before a model is loaded. CLI overrides such as --steps, --group-size, --max-tokens, --algorithm, --beta, and --seed apply on top of the file.

For DeltaNet / linear-attention models, enable per-layer checkpointing:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase1-gsm8k \
--model mlx-community/Qwen3.5-9B-MLX-4bit \
--rank 16 \
--scale 2.0 \
--checkpoint-completion-forward \
--steps 1 \
--group-size 2 \
--max-tokens 256

Despite the historical CLI name, --checkpoint-completion-forward now enables per-transformer-block checkpointing at model setup. The old whole-model mx.checkpoint(...) wrapper was removed because it does not cap DeltaNet's per-layer scan memory.

Phase 2 rollout equivalence check:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase2-equivalence \
--model mlx-community/Qwen3-0.6B-4bit \
--group-size 4 \
--max-tokens 32 \
--compile-decode-step \
--batch-groups

Import API

Minimal model setup:

frommlxrl.policyimportLoRAConfig, load_policy_with_loramodel, tokenizer, report=load_policy_with_lora(
model_id="mlx-community/Qwen3.5-9B-MLX-4bit",
config=LoRAConfig(
rank=16,
scale=2.0,
dropout=0.0,
grad_checkpoint=True,
),
)

One optimizer step:

importmlx.optimizersasoptimfrommlxrl.algoimportGRPOAlgorithmfrommlxrl.trainimportbatch_from_rollouts, optimizer_stepoptimizer=optim.Adam(learning_rate=1e-5)
algorithm=GRPOAlgorithm()
batch=batch_from_rollouts(
model=model,
completions=completions,
rewards=rewards,
group_size=4,
pad_token_id=pad_token_id,
algorithm=algorithm,
compute_reference=beta!=0.0,
)
metrics=optimizer_step(
model=model,
optimizer=optimizer,
batch=batch,
beta=beta,
pad_token_id=pad_token_id,
algorithm=algorithm,
use_checkpoint=True,
micro_batch_size=2,
)

Multi-turn GiGPO uses the additive trajectory path:

frommlxrl.algoimportGiGPOAlgorithmfrommlxrl.envimportRecurringStateTextEnvfrommlxrl.rolloutimportgenerate_agentic_trajectoriesfrommlxrl.trainimportbatch_from_trajectories, optimizer_step_trajectoryalgorithm=GiGPOAlgorithm(omega=1.0, gamma=1.0)
trajectories=generate_agentic_trajectories(
model=model,
tokenizer=tokenizer,
env_factory=lambdatask, seed, group: RecurringStateTextEnv(
task=str(task),
max_turns=3,
),
tasks=["finish the task"],
group_size=4,
sampling=sampling,
rollout_mode="parallel_per_turn",
)
batch=batch_from_trajectories(
model=model,
trajectories=trajectories,
group_size=4,
pad_token_id=pad_token_id,
algorithm=algorithm,
compute_reference=beta!=0.0,
)
metrics=optimizer_step_trajectory(
model=model,
optimizer=optimizer,
batch=batch,
beta=beta,
pad_token_id=pad_token_id,
algorithm=algorithm,
)

micro_batch_size=0 keeps the original whole-batch path. Micro-batching is currently exact for token-mean policy losses: base GRPO, DAPO, GSPO token mode, RLOO, and Dr. GRPO with loss_reduction="token_mean". Sequence-reduced losses should keep micro_batch_size=0.

ECHO World Modeling

ECHO is an additive GiGPO trainer feature for tagged multi-turn trajectories. Producer code may attach token_roles to each Trajectory, aligned to full_token_ids: 0 = MASKED, 1 = ACTION, 2 = ECHO. ACTION tokens keep the normal GiGPO policy-gradient loss. ECHO tokens add an independently normalized SFT term, alpha * NLL, in the same forward/backward pass.

echo_alpha defaults to 0.0, which leaves current GiGPO behavior untouched. For early world-modeling, start small, roughly 0.005 to 0.05, and consider echo_schedule = "linear_taper_to_zero" with echo_taper_steps = N so training returns to pure RL. alpha is model-size-sensitive; too-high values can overfit tool responses or collapse RL.

ECHO is content-agnostic by design. mlxrl never inspects token text to decide what is an environment or tool response. The rollout producer owns the tags: MASKED tokens have zero gradient, ACTION tokens receive the RL advantage, and ECHO tokens receive only the SFT loss with no KL penalty or importance ratio. The trainer logs loss_echo and token-level echo_acc separately from the ACTION loss.

Policy Semantics

• The base model is frozen before LoRA injection.
• Only LoRA adapter leaves are trainable.
• Reference logprobs are computed by temporarily disabling adapters on the same model object; there is no second reference model in memory.
• Old-policy logprobs are recomputed with a full forward for the training batch. Rollout-time logprobs are captured for inspection, but 4-bit sequential decode and full-forward prefill are not numerically identical on hybrid/quantized models, so recompute remains the default training semantics.
• When beta == 0, the reference forward is skipped and the policy logprobs are used as a zero-KL placeholder.
• PPO, DPO, and ORPO are intentionally out of scope. PPO needs a separate critic and value forward; DPO/ORPO are offline preference objectives with no rollout phase. mlxrl is critic-free, on-policy, and rollout-based by design.

Algorithms

Concrete algorithms implement the small Algorithm protocol: compute advantages, optionally filter a prepared batch, then compute a loss from policy, old-policy, and reference logprobs. rollout/, policy/, and train/ do not import concrete algorithm implementations.

algorithm	defining behavior
GRPO	group-normalized rewards, token-level importance ratio
Dr. GRPO	centered or normalized rewards with decoupled length reduction
DAPO	asymmetric low/high clipping plus optional dynamic zero-advantage group filtering
GSPO	sequence-level, length-normalized importance ratio and clipping
RLOO	leave-one-out group baseline, no critic, no std-normalized advantage
GiGPO	multi-turn episode advantage plus anchor-state step advantage

Memory Preflight

mlxrl train can estimate memory before loading the model:

UV_CACHE_DIR=.uv-cache uv run mlxrl train \
--config examples/qwen3_0_6b_grpo.toml \
--available-memory-gb 48 \
--dry-run

The estimator is calibrated to measured anchors: 6.245 GB for Qwen3-0.6B/G4/prompt≈19/T256, 25.9 GB for Qwen3.5-9B/G2/seq609/per-layer-checkpointed, 45.9 GB for Qwen3.5-9B/G4/seq609/per-layer-checkpointed, and 36 GB for Qwen3.5-9B/G2/seq128/no-checkpoint. For hybrid 9B no-checkpoint long-sequence configs, it reports an OOM-risk lower bound rather than a fake precise peak. Multi-turn configs include max_turns and max_observation_len in the estimated trajectory length and are labeled as estimated OOM-risk shapes until new measured anchors are added. For an obviously too-large Qwen3.5-9B/G8/prompt97/T512/no-checkpoint config on 48 GB, it flags the run and suggests the measured-boundary fallback around G4/T512/checkpointed.

Benchmarks

Local M4 Max Phase 4 snapshot:

• 454 rollout tok/s on Qwen3-0.6B GRPO with G=4 and 256-token completions.
• 0.283 end-to-end it/s with full mlxrl training semantics.
• 3.2x faster rollout and 2.2x higher end-to-end it/s than mlx-tune v0.5.1 on the same run shape.
• 1.3x faster rollout than sequential mlx-lm generation at G=4.

These are the two-pass means from benchmarks/results/gate5_full_reconciled.md, run with MLX 0.31.2, MLX-LM 0.31.3, mlx-community/Qwen3-0.6B-4bit, 100 measured steps with 5 warmup steps discarded:

target	comparison	rollout tok/s	grad s/step	samples/s	it/s	peak GB
mlxrl	apples-to-apples GRPO	454.1	1.282	1.133	0.283	6.25
mlx-lm	generation-only, G=1	347.0	-	1.355	-	0.52
mlx-lm-g4	generation-only, sequential G=4	349.7	-	1.366	-	0.52
mlx-tune	package-speed reference	142.2	0.502	0.519	0.130	6.16
mlx-lm-lora	package-speed reference	557.9	0.592	1.648	0.412	5.32

mlx-lm-lora reports higher raw package-speed throughput in this snapshot, but its benchmarked path is not the same training problem as mlxrl's live old-policy/reference semantics and completion-loss masking. That is the honest case where mlxrl is not faster; the apples-to-apples comparison label is reserved for mlxrl's own semantic path. On the 9B Noether real workload, the checkpointed MLX path measured about 6x faster than the previous torch-MPS path; that workload is separate from the public Phase 4 package-speed harness.

Run the Phase 4 harness:

UV_CACHE_DIR=.uv-cache uv run python benchmarks/run_phase4.py run \
--targets mlxrl,mlx-lm,mlx-tune,mlx-lm-lora \
--model mlx-community/Qwen3-0.6B-4bit \
--steps 100 \
--warmup-steps 5 \
--group-size 4 \
--max-tokens 256 \
--passes 2 \
--output benchmarks/results/phase4.jsonl \
--summary benchmarks/results/phase4.md \
--allow-missing-baselines

The harness reports synchronized rollout tok/s, gradient seconds per step, samples/s, it/s, and peak MLX memory. mlx-lm targets are generation-only; external package targets are useful speed references but may not match mlxrl training semantics.

Development

See CONTRIBUTING.md and DESIGN.md before adding algorithms or changing rollout/logprob semantics.

Run the quality gates:

UV_CACHE_DIR=.uv-cache uv run pytest
UV_CACHE_DIR=.uv-cache uv run ruff check .
UV_CACHE_DIR=.uv-cache uv run pyright

MLX lazy evaluation matters. Any mx.eval(...) or mx.synchronize() in this repo should mark a real boundary: sampled token append/EOS checks, logprob freezing before adapter mutation, per-micro-batch graph release, optimizer updates, or benchmark timing boundaries.

Layout

mlxrl/
rollout/ # batched group generation
env/ # agentic environment protocols and reference envs
policy/ # model loading, LoRA setup, logprob passes
algo/ # GRPO-family advantages and losses
train/ # value_and_grad and optimizer integration
data/ # toy GSM8K data and rewards
cli.py
tests/
benchmarks/

Non-Goals

• No inference server or second model copy.
• No CUDA or torch fallback.
• No distributed training.
• No broad RL framework abstractions beyond the small algorithm interface.