Reinforcement learning of Gemma4-2B

Open in Databricks

Post-train the Gemma4-2B (unsloth/gemma-4-E2B-it) large language model (LLM) with reinforcement learning on AI Runtime (serverless GPU). This example uses Group Relative Policy Optimization (GRPO) to teach the model to solve Sudoku puzzles: the model writes a solver strategy, and reward functions score it for producing valid, non-cheating solutions. It runs end-to-end on a single H100 GPU and shows you how to:

Load Gemma4-2B with Low-Rank Adaptation (LoRA) adapters using Unsloth for memory-efficient reinforcement learning
Define a reinforcement learning environment and reward functions that score the model's generated strategies
Train the policy with the GRPO trainer (built on Transformer Reinforcement Learning (TRL))
Run inference with the trained model and save the LoRA adapters

Key concepts:

GRPO: A reinforcement learning algorithm that optimizes a policy from group-relative rewards, without training a separate value model
LoRA: Trains a small set of adapter weights instead of the full model to reduce memory use
Unsloth: A library for memory-efficient LLM fine-tuning and reinforcement learning

note

This example requires the AI Runtime environment version 6 or above.

Connect to serverless GPU compute

This notebook requires serverless GPU compute. To connect:

Click the notebook's compute selector in the top right and select Serverless GPU.
On the right side, click the environment button.
Select H100 as the Accelerator.
Choose AI v6 from the base environment.
Click Apply.

Task: solve Sudoku with reinforcement learning

The goal is to make Gemma4-2B learn to solve Sudoku puzzles using GRPO. The model devises a strategy to fill in empty cells, and the reward functions score it for correct placements and for completing valid puzzles.

Install libraries

This example uses Unsloth for memory-efficient reinforcement learning on Gemma4-2B. The next cell installs it along with its supporting dependencies.

Python
%pip install unsloth==2026.9.4

Python
%%capture
!pip install --no-deps --upgrade timm # For Gemma 4 vision/audio

Load Gemma4-2B with Unsloth

Python
from unsloth import FastVisionModel
import torch
max_seq_length = 4096 # Can increase for longer reasoning traces
lora_rank = 32 # Larger rank = smarter, but slower

gemma4_models = [
    # Gemma-4 instruct models:
    "unsloth/gemma-4-E2B-it",
    "unsloth/gemma-4-E4B-it",
    "unsloth/gemma-4-31B-it",
    "unsloth/gemma-4-26B-A4B-it",
    # Gemma-4 base models:
    "unsloth/gemma-4-E2B",
    "unsloth/gemma-4-E4B",
    "unsloth/gemma-4-31B",
    "unsloth/gemma-4-26B-A4B",
] # More models at https://huggingface.co/unsloth

model, tokenizer = FastVisionModel.from_pretrained(
    model_name = "unsloth/gemma-4-E2B-it",
    max_seq_length = max_seq_length,
    load_in_4bit = False, # False for LoRA 16bit
    fast_inference = False, # Enable vllm fast inference
)

For efficient reinforcement learning, this example uses LoRA to train adapter weights instead of the full model, which reduces memory use.

Python
model = FastVisionModel.get_peft_model(
    model,
    r = lora_rank, # Suggested values: 8, 16, 32, 64, or 128
    target_modules = [
        "q_proj", "k_proj", "v_proj", "o_proj",
        "gate_proj", "up_proj", "down_proj",
    ],
    lora_alpha = lora_rank*2, # *2 speeds up training
    use_gradient_checkpointing = "unsloth", # Reduces memory usage
    random_state = 3407,
)

Implement the Sudoku game

The Sudoku environment accepts a strategy that returns a (row, column, value) tuple for each move.

Python
from dataclasses import dataclass, field
from typing import List, Tuple, Optional
import random
import copy

def _is_valid_placement(board: List[List[int]], row: int, col: int, num: int) -> bool:
    """Check if placing num at (row, col) is valid."""
    # Check row
    if num in board[row]:
        return False

    # Check column
    if num in [board[r][col] for r in range(9)]:
        return False

    # Check 3x3 box
    box_row, box_col = 3 * (row // 3), 3 * (col // 3)
    for r in range(box_row, box_row + 3):
        for c in range(box_col, box_col + 3):
            if board[r][c] == num:
                return False

    return True

def _solve_sudoku(board: List[List[int]]) -> bool:
    """Solve sudoku using backtracking (for puzzle generation)."""
    for row in range(9):
        for col in range(9):
            if board[row][col] == 0:
                for num in range(1, 10):
                    if _is_valid_placement(board, row, col, num):
                        board[row][col] = num
                        if _solve_sudoku(board):
                            return True
                        board[row][col] = 0
                return False
    return True

def _generate_complete_board(rng: random.Random) -> List[List[int]]:
    """Generate a complete valid Sudoku board."""
    board = [[0 for _ in range(9)] for _ in range(9)]

    # Fill diagonal 3x3 boxes first (they don't affect each other)
    for box in range(3):
        nums = list(range(1, 10))
        rng.shuffle(nums)
        for i in range(3):
            for j in range(3):
                board[box * 3 + i][box * 3 + j] = nums[i * 3 + j]

    # Solve the rest
    _solve_sudoku(board)
    return board

@dataclass
class SudokuGame:
    difficulty: int = 40  # Number of cells to remove (20 = easy, 40 = medium, 50 = hard)
    seed: Optional[int] = None
    _rng: random.Random = field(init = False, repr = False)
    _board: List[List[int]] = field(init = False, repr = False)
    _solution: List[List[int]] = field(init = False, repr = False)
    _initial_board: List[List[int]] = field(init = False, repr = False)
    _moves: int = field(default = 0, init = False, repr = False)
    _state: str = field(default = "ongoing", init = False, repr = False)

    def __post_init__(self):
        self._rng = random.Random(self.seed)

        # Generate complete board
        complete_board = _generate_complete_board(self._rng)
        self._solution = copy.deepcopy(complete_board)

        # Remove cells to create puzzle
        self._board = copy.deepcopy(complete_board)
        cells = [(r, c) for r in range(9) for c in range(9)]
        self._rng.shuffle(cells)

        for r, c in cells[:self.difficulty]:
            self._board[r][c] = 0

        self._initial_board = copy.deepcopy(self._board)
        self._update_state()

    def board(self) -> List[List[int]]:
        """Return current board state."""
        return [row[:] for row in self._board]

    def initial_board(self) -> List[List[int]]:
        """Return initial puzzle state."""
        return [row[:] for row in self._initial_board]

    def state(self) -> str:
        """Return game state: 'ongoing', 'success', or 'failed'."""
        return self._state

    def moves(self) -> int:
        """Return number of moves made."""
        return self._moves

    def place_number(self, row: int, col: int, num: int) -> bool:
        """Place a number on the board. Returns True if valid move."""
        # Validate input
        if not (0 <= row < 9 and 0 <= col < 9):
            self._state = "failed"
            return False

        if not (1 <= num <= 9):
            self._state = "failed"
            return False

        # Can't modify initial cells
        if self._initial_board[row][col] != 0:
            self._state = "failed"
            return False
        if self._board[row][col] != 0:
            self._state = "failed"
            return False
        # Check if placement is valid
        if not _is_valid_placement(self._board, row, col, num):
            self._state = "failed"
            return False

        # Place number
        self._board[row][col] = num
        self._moves += 1
        self._update_state()
        return True

    def _update_state(self) -> None:
        """Update game state based on current board."""
        # Check if puzzle is complete
        if all(self._board[r][c] != 0 for r in range(9) for c in range(9)):
            # Verify solution is correct
            if self._board == self._solution:
                self._state = "success"
            else:
                self._state = "failed"
        else:
            self._state = "ongoing"

    def pretty(self, colors: bool = True) -> str:
        """Pretty print the Sudoku board."""
        RESET = "\x1b[0m"
        INITIAL = "\x1b[38;5;45m"   # Cyan for initial numbers
        PLACED = "\x1b[38;5;226m"    # Yellow for placed numbers
        EMPTY = "\x1b[38;5;239m"     # Gray for empty cells

        lines = []
        lines.append("┌───────┬───────┬───────┐")

        for row in range(9):
            row_str = "│ "
            for col in range(9):
                num = self._board[row][col]

                if colors:
                    if num == 0:
                        row_str += f"{EMPTY}.{RESET}"
                    elif self._initial_board[row][col] != 0:
                        row_str += f"{INITIAL}{num}{RESET}"
                    else:
                        row_str += f"{PLACED}{num}{RESET}"
                else:
                    row_str += str(num) if num != 0 else "."

                if col % 3 == 2:
                    row_str += " │ "
                else:
                    row_str += " "

            lines.append(row_str.rstrip())

            if row == 8:
                lines.append("└───────┴───────┴───────┘")
            elif row % 3 == 2:
                lines.append("├───────┼───────┼───────┤")

        return "\n".join(lines)

Test the Sudoku environment:

Python
# Create an easy puzzle
game = SudokuGame(difficulty = 30, seed = 42)
print("Initial puzzle:")
print(game.pretty())
print(f"\nState: {game.state()}, Moves: {game.moves()}")

Python
game

Try making some moves:

Python
# Make a valid move
game.place_number(0, 1, 7)
print("\nAfter placing 7 at (1,0):")
print(game.pretty())
print(f"State: {game.state()}, Moves: {game.moves()}")

An action outside the allowed action space causes the game to enter the failed state. The game then rejects subsequent actions.

Set up the reinforcement learning environment

Run strategies with time limits to prevent infinite loops.

Python
from typing import Callable
from unsloth import execute_with_time_limit

def _execute_strategy(strategy: Callable, game: SudokuGame):
    """Execute a strategy function on a Sudoku game."""
    assert callable(strategy)

    max_moves = 100
    valid_moves = 0  # Track successful moves

    while game.state() == "ongoing" and valid_moves < max_moves:
        try:
            board = game.board()
            initial = game.initial_board()
            result = strategy(board, initial)

            # Validate result format
            if not isinstance(result, (tuple, list)) or len(result) != 3:
                # Invalid format = immediate fail, but return valid moves made
                return valid_moves, "failed"

            row, col, num = result

            # Validate types
            if not all(isinstance(x, int) for x in [row, col, num]):
                return valid_moves, "failed"

            # Try to place number
            success = game.place_number(row, col, num)

            if success:
                valid_moves += 1  # Count this valid move
            else:
                # Invalid move = game fails, but return valid_moves made so far
                return valid_moves, "failed"

        except Exception:
            return valid_moves, "failed"

    if valid_moves >= max_moves and game.state() == "ongoing":
        return valid_moves, "failed"

    return valid_moves, game.state()

Apply a 10-second timeout to allow longer strategies while preventing infinite loops.

Python
@execute_with_time_limit(10)
def execute_strategy(strategy: Callable, game: SudokuGame):
    """Execute strategy with 10 second time limit."""
    return _execute_strategy(strategy, game)

Test with a simple strategy:

Python
def simple_strategy(board, initial):
    """Simple strategy: fill first empty cell with 1."""
    for r in range(9):
        for c in range(9):
            if board[r][c] == 0 and initial[r][c] == 0:
                return (r, c, 7)
    return (0, 0, 7)

game = SudokuGame(difficulty = 30, seed = 42)
try:
    moves, state = execute_strategy(simple_strategy, game)
    print(f"Moves: {moves}, State: {state}")
except TimeoutError as e:
    print(f"Timed out: {e}")

Python
print(game.pretty())

Run generated code

Before running a generated Python function, verify that it does not access disallowed global variables or external modules. This check helps prevent reward hacking.

The following code passes check_python_modules because it does not import external modules:

Python
from unsloth import check_python_modules, create_locked_down_function

# Test safe code
sample = """
def strategy(board, initial):
    for r in range(9):
        for c in range(9):
            if board[r][c] == 0:
                return (r, c, 1)
    return (0, 0, 1)
"""

ok, info = check_python_modules(sample)
print("Safe Python code?", ok)
print(info)

The following code imports numpy, so check_python_modules rejects it:

Python
sample = """
def strategy(board, initial):
    import numpy as np
    return (0, 0, 1)
"""

ok, info = check_python_modules(sample)
print("Safe Python code?", ok)
print(info)

Set up the data and reinforcement learning task

Create a prompt that instructs the model to generate a Sudoku-solving strategy. You can adapt this prompt for other reinforcement learning tasks.

Python
prompt = """
Create a Sudoku solving strategy using only native Python built-in functions without any import statements.
You are given two lists of lists (9x9 grids):
- board: current state (0 means empty)
- initial: starting puzzle (0 means was empty, numbers are fixed)

Return a tuple (row, col, number) for the next move.
- row: 0-8 (row index)
- col: 0-8 (column index)
- number: 1-9 (digit to place)

Only place numbers in cells that are BOTH empty in initial AND empty in board (initial[row][col] == 0 AND board[row][col] == 0)
Use Sudoku rules: no duplicates in rows, columns, or 3x3 boxes.
Enclose the function in a Python Markdown code block.
All helper functions must be inside def strategy. Output only the function.
""".strip()

print(prompt)

Generate a baseline response before reinforcement learning:

Python
text = tokenizer.apply_chat_template(
    [{"role": "user", "content": prompt.strip()}],
    tokenize = False,
    add_generation_prompt = True,
)

from transformers import TextStreamer
print("=" * 50)
print("BASE MODEL OUTPUT (before RL training):")
print("=" * 50)

inputs = tokenizer(
    text = text,
    add_special_tokens = False,
    return_tensors = "pt",
).to("cuda")

text_streamer = TextStreamer(tokenizer, skip_prompt = True)
result = model.generate(**inputs, streamer = text_streamer, max_new_tokens = 128,
                        use_cache = True, temperature = 1.0, top_p = 0.95, top_k = 64)

Define reward functions

Define extract_function to extract a function from a Markdown code block.

Then define three reward functions:

function_works rewards the model when the strategy is a valid Python function.
no_cheating penalizes functions that import external modules.
strategy_succeeds rewards generated strategies for making valid moves and solving the Sudoku puzzle.

Python
def extract_function(text):
    """Extract Python function from markdown code blocks."""
    if text.count("```") >= 2:
        first = text.find("```") + 3
        second = text.find("```", first)
        fx = text[first:second].strip()
        fx = fx.removeprefix("python\n")
        fx = fx[fx.find("def"):]
        if fx.startswith("def strategy(board, initial):"):
            return fx
    return None

Reward 1: Validate the function

Checks whether the generated code is valid Python and runs successfully.

Python
def function_works(completions, **kwargs):
    """Reward for generating valid executable Python code."""
    scores = []
    for completion in completions:
        score = 0
        response = completion[0]["content"]
        function = extract_function(response)

        if function is not None:
            ok, info = check_python_modules(function)

        if function is None or "error" in info:
            score = -2.0  # Invalid function
        else:
            try:
                new_strategy = create_locked_down_function(function)
                score = 1.0  # Valid function
            except:
                score = -1.0  # Function has errors

        scores.append(score)
    return scores

Reward 2: Prevent cheating

Penalizes functions that import external libraries.

Python
def no_cheating(completions, **kwargs):
    """Penalize use of external imports."""
    scores = []
    for completion in completions:
        response = completion[0]["content"]
        function = extract_function(response)

        if function is not None:
            ok, info = check_python_modules(function)
            scores.append(1.0 if ok else -20.0)  # Heavy penalty for cheating
        else:
            scores.append(-1.0)  # Failed to create function

    return scores

Reward 3: Reward successful strategies

Rewards strategies that successfully solve Sudoku puzzles.

Python
import numpy as np

global PRINTER
PRINTER = 0

def strategy_succeeds(completions, **kwargs):
    """Reward valid moves even if strategy eventually fails."""
    global PRINTER
    scores = []

    seed = np.random.randint(10000)
    difficulty = 40
    for completion in completions:
        printed = False
        response = completion[0]["content"]
        function = extract_function(response)

        if PRINTER % 5 == 0:
            printed = True
            print("\n" + "=" * 60)
            print(function)
            print("=" * 60)
        PRINTER += 1

        if function is not None:
            ok, info = check_python_modules(function)

        if function is None or "error" in info:
            scores.append(0)
            continue

        try:
            new_strategy = create_locked_down_function(function)
        except:
            scores.append(0)
            continue

        try:
            game = SudokuGame(difficulty = difficulty, seed = seed)
            valid_moves, game_state = execute_strategy(new_strategy, game)
            if valid_moves == difficulty:
                game_state = "success"

            print(f"\n Valid moves: {valid_moves}, Final state: {game_state}")

            if not printed:
                print("Strategy:")
                print(function[:200] + "..." if len(function) > 200 else function)

            print("\nFinal board:")
            print(game.pretty())

            if game_state == "success":
                scores.append(30.0)  # Solved the puzzle
            elif valid_moves > 0:
                # Reward based on valid moves made before failure
                # Each valid move is worth 0.2 points
                reward = valid_moves * 0.2
                scores.append(reward)
            else:
                scores.append(-2.0)  # Failed immediately with no valid moves

        except TimeoutError:
            print("Timeout")
            scores.append(-1.0)
        except Exception as e:
            print(f"Exception: {str(e)[:100]}")
            scores.append(-3.0)

    return scores

Prepare the dataset

Create the training dataset.

Python
from datasets import Dataset

dataset = Dataset.from_list([
    {
        "prompt": [{"role": "user", "content": prompt.strip()}],
        "answer": 0,
    }
] * 1000)

maximum_length = len(tokenizer.apply_chat_template(
    [{"role": "user", "content": prompt.strip()}],
    add_generation_prompt = True
))

print(f"Maximum prompt length: {maximum_length}")
print("\nDataset sample:")
print(dataset[0])

Train the model

Configure the GRPO trainer. For other supported algorithms and configuration options, see the Unsloth reinforcement learning documentation.

Python
# Leave room for the prompt (plus 1 token safety margin)
max_completion_length = max_seq_length - (maximum_length + 1)

from trl import GRPOConfig, GRPOTrainer
training_args = GRPOConfig(
    temperature = 1.0,
    learning_rate = 5e-5,
    weight_decay = 0.001,
    warmup_ratio = 0.1,
    lr_scheduler_type = "linear",
    optim = "adamw_8bit",
    logging_steps = 1,
    per_device_train_batch_size = 1,
    gradient_accumulation_steps = 2, # Increase to 4 for smoother training
    num_generations = 2, # Decrease if out of memory
    max_completion_length = max_completion_length,
    # num_train_epochs = 1, # Set to 1 for a full training run
    max_steps = 60,
    save_steps = 100,
    report_to = "none", # Can use Weights & Biases, TrackIO
    output_dir = "outputs",
    epsilon = 0.2,
    epsilon_high = 0.28, # one sided
    delta = 1.5, # two sided
    loss_type = 'bnpo',
    mask_truncated_completions = True
    # For optional training + evaluation
    # fp16_full_eval = True,
    # per_device_eval_batch_size = 4,
    # eval_accumulation_steps = 1,
    # eval_strategy = "steps",
    # eval_steps = 1,
)

Run the trainer and monitor the reward column. The configured 60-step run is a short demonstration, so reward values can remain low or fluctuate throughout training.

Step	reward	reward_std	completion_length	kl
1	0.125000	0.000000	200.000000	0.000000
2	0.072375	0.248112	200.000000	0.000000
3	-0.079000	0.163776	182.500000	0.000005

Python
# For optional training + evaluation
# new_dataset = dataset.train_test_split(test_size = 0.01)

trainer = GRPOTrainer(
    model = model,
    processing_class = tokenizer,
    reward_funcs = [
        function_works,
        no_cheating,
        strategy_succeeds,
    ],
    args = training_args,
    train_dataset = dataset,

    # For optional training + evaluation
    # train_dataset = new_dataset["train"],
    # eval_dataset = new_dataset["test"],
)

Start the 60-step training run:

Python
trainer.train()

Save the trained LoRA adapters:

Python
model.save_pretrained("gemma_4_lora")  # Local saving
tokenizer.save_pretrained("gemma_4_lora")

Verify that the LoRA adapter weights are nonzero:

Python
from safetensors import safe_open

tensors = {}
with safe_open("gemma_4_lora/adapter_model.safetensors", framework = "pt") as f:
    # Verify both A and B are non zero
    for key in f.keys():
        tensor = f.get_tensor(key)
        n_zeros = (tensor == 0).sum() / tensor.numel()
        assert(n_zeros.item() != tensor.numel())

Run inference

Generate a response with the trained model:

Python
text = tokenizer.apply_chat_template(
    [{"role": "user", "content": prompt.strip()}],
    tokenize = False,
    add_generation_prompt = True,
)

from transformers import TextStreamer

_ = model.generate(
    **tokenizer(images = None,text = text, return_tensors = "pt").to("cuda"),
    temperature = 1.0,
    max_new_tokens = 512,
    streamer = TextStreamer(tokenizer, skip_prompt = False),
)

Next steps

Now that you've post-trained Gemma4-2B with GRPO, you can:

Deploy the model: Deploy custom models
Explore more post-training examples: Post-training OSS models (LLMs)
Optimize serverless GPU usage: Best practices for AI Runtime
Troubleshoot issues: Troubleshoot issues on AI Runtime

Example notebook

Reinforcement learning of Gemma4-2B

Open notebook in new tab Open in Databricks

Connect to serverless GPU compute​

Task: solve Sudoku with reinforcement learning​

Install libraries​

Load Gemma4-2B with Unsloth​

Implement the Sudoku game​

Set up the reinforcement learning environment​

Run generated code​

Set up the data and reinforcement learning task​

Define reward functions​

Reward 1: Validate the function​

Reward 2: Prevent cheating​

Reward 3: Reward successful strategies​

Prepare the dataset​

Train the model​

Run inference​

Next steps​

Example notebook​

Reinforcement learning of Gemma4-2B

Connect to serverless GPU compute

Task: solve Sudoku with reinforcement learning

Install libraries

Load Gemma4-2B with Unsloth

Implement the Sudoku game

Set up the reinforcement learning environment

Run generated code

Set up the data and reinforcement learning task

Define reward functions

Reward 1: Validate the function

Reward 2: Prevent cheating

Reward 3: Reward successful strategies

Prepare the dataset

Train the model

Run inference

Next steps

Example notebook