gemma4-26b-a4b-code-trainer-v11-farca

LoRA adapter for google/gemma-4-26B-A4B-it, reinforcement-learned with FARCA-GRPO (Fact-Aligned Reliability-Aware Credit Assignment + Group Relative Policy Optimization) on a combined rule-based and NLI reward signal. This is the V11 FARCA release — the first RL stage in the Gemma pipeline, trained on top of the merged DAPT + SFT adapter chain.

Part of the Code-Trainer / RTPI pipeline (GitHub).

Model architecture notes

Gemma 4 26B-A4B is a Mixture-of-Experts model: 128 experts + 1 shared expert, 8 active per layer, 30 layers. Total parameters: 25.8B; active per forward pass: 3.8B.

LoRA targeting constraint: the routed expert FFN layers use 3D nn.Parameter tensors that PEFT cannot target. LoRA is applied only to shared attention + shared MLP modules. All learning rates are halved vs the Qwen pipeline for MoE routing stability.

Gemma4ClippableLinear: modules must be unwrapped before PEFT operations (handled by src/utils.py:unwrap_clippable_linear).

What is FARCA-GRPO

FARCA extends standard GRPO with two additional reward components:

1. Fact-aligned claim extraction — extracts verifiable claims from each completion (rule-based backend for speed and determinism).
2. Reliability-aware credit assignment — blends NLI-based factuality scoring with rule-based tool-call formatting reward, weighted by configurable nli_weight and rule_weight parameters.

The advantage estimate mixes standard GRPO group-relative advantages with FARCA's per-claim reliability scores, controlled by mu (mixing coefficient) and tau (temperature). A warmup_steps ramp (0 to 1) prevents advantage variance spikes during early training — critical for MoE routing stability.

Adapter chain

The V11 FARCA adapter cannot be loaded directly on the base model — it expects the DAPT and SFT adapters to be merged first:

google/gemma-4-26B-A4B-it
  └─ merge: cmndcntrlcyber/gemma4-26b-a4b-dapt-offsec              (DAPT)
      └─ merge: cmndcntrlcyber/gemma4-26b-a4b-code-trainer-aggressive-full1  (SFT)
          └─ merge: cmndcntrlcyber/gemma4-26b-a4b-code-trainer-vision-sft  (Vision SFT)
              └─ LoRA: cmndcntrlcyber/gemma4-26b-a4b-code-trainer-v11-farca  (this adapter)

For deployment, all four are merged into the base and quantized to GGUF (see gemma26b-offsec-coder-gguf).

Training data

• Prompt dataset: cmndcntrlcyber/code-trainer-v10-grpo-prompts
• Max prompts: 400 (reduced from Qwen's 500 for budget constraints)

Each prompt is formatted via apply_chat_template with the Nexus system prompt and full tool definitions.

Reward function

FARCA blends two reward signals:

FARCA parameters

Training procedure

Intended use

• Direct use: load the full adapter chain on top of google/gemma-4-26B-A4B-it for instruction-following code generation, tool calling, and multi-turn agent behaviour with improved <tool_call> tag formatting and factual grounding.
• Downstream: merge the full adapter chain into the base model, then apply DPO (gemma4-26b-a4b-code-trainer-v10-dpo) and quantize to GGUF.
• Out of scope: this adapter was not trained for safety alignment or non-code tasks. The FARCA-GRPO stage optimizes formatting and factual grounding, not task completion.

Deployment notes

• Inference target: RTX 5060 Ti 16 GB (after full merge + GGUF quantization).
• Context length: 4,096 tokens recommended; trained at 768 max completion length but the base model supports 256K.

How to use

import torch
from peft import PeftModel
from transformers import AutoModelForCausalLM, AutoTokenizer
from src.utils import unwrap_clippable_linear

base_id = "google/gemma-4-26B-A4B-it"
dapt_id = "cmndcntrlcyber/gemma4-26b-a4b-dapt-offsec"
sft_id = "cmndcntrlcyber/gemma4-26b-a4b-code-trainer-aggressive-full1"
farca_id = "cmndcntrlcyber/gemma4-26b-a4b-code-trainer-v11-farca"

tokenizer = AutoTokenizer.from_pretrained(base_id)
model = AutoModelForCausalLM.from_pretrained(
    base_id, torch_dtype=torch.bfloat16, device_map="auto",
)
unwrap_clippable_linear(model)

# Merge DAPT and SFT adapters into base weights
model = PeftModel.from_pretrained(model, dapt_id)
model = model.merge_and_unload()
model = PeftModel.from_pretrained(model, sft_id)
model = model.merge_and_unload()

# Load FARCA-GRPO adapter (active LoRA)
model = PeftModel.from_pretrained(model, farca_id)
model.eval()

messages = [
    {"role": "user", "content": "Read the file main.py and summarise its structure."},
]
inputs = tokenizer.apply_chat_template(
    messages, return_tensors="pt", add_generation_prompt=True,
).to(model.device)
out = model.generate(inputs, max_new_tokens=512, do_sample=False)
print(tokenizer.decode(out[0][inputs.shape[1]:], skip_special_tokens=True))

Limitations

• Shared layers only. LoRA cannot target routed expert FFN (3D nn.Parameter).
• No safety tuning. Inherits the base model's safety properties.
• Budget-constrained. 400 prompts (vs Qwen's 500) due to larger model throughput overhead.
• FARCA warmup required. Removing warmup_steps on MoE can cause advantage variance spikes and training instability.

Reproducibility

• Code: github.com/cmndcntrlcyber/code-trainer-pipeline (src/phase4c_rl/)
• Training launch:

  python -m src.phase4c_rl.scripts.launch_farca_grpo \
      --config src/config/pipeline-gemma26b.yml --wait