---
language:
- ny
- en
license: cc-by-4.0
library_name: transformers
pipeline_tag: translation
base_model: Helsinki-NLP/opus-mt-ny-en
tags:
- translation
- marian
- nyanja
- chichewa
- low-resource
- domain-adaptation
- warm-up
- encoder-mlm
model-index:
- name: nyanja_english_warmup
  results: []
---

# Nyanja → English Encoder Warm-Up Model

This is an **intermediate "warm-up" checkpoint**, not a finished translation system. It is
[`Helsinki-NLP/opus-mt-ny-en`](https://huggingface.co/Helsinki-NLP/opus-mt-ny-en) whose
**encoder** has undergone masked-language-model (MLM) adaptation on monolingual Nyanja
(Chichewa) text, with the decoder frozen. The goal is to adapt the encoder's understanding of
Nyanja before a downstream supervised fine-tuning stage on parallel data.

> **Important:** This model has only seen *monolingual* Nyanja during this stage. Monolingual
> adaptation does **not**, on its own, teach the Nyanja↔English mapping, so this checkpoint
> should **not** be expected to have higher translation quality (BLEU/chrF++) than the base
> model. It is intended to be **fine-tuned on parallel ny–en data** before evaluation or use.

## Model details

| | |
|---|---|
| Base model | `Helsinki-NLP/opus-mt-ny-en` (MarianMT) |
| Direction | Nyanja (Chichewa) → English |
| Warm-up objective | Encoder masked language modeling (mask 15%, predict masked tokens only) |
| Trainable | Encoder only — decoder frozen |
| Vocabulary | **59,812** (native 59,811 + one added `<mask>` token; embeddings resized) |
| Hidden size | 512 |
| Parameters | ~75M total (~19.2M trained) |

> **Vocab note / integration gotcha:** because MLM needs a mask token (MarianMT has none), a
> `<mask>` token was added and the embeddings resized to **59,812**. This model is therefore
> **not drop-in compatible** with a vanilla `opus-mt-ny-en` (59,811). When you fine-tune or load
> it downstream, use **this repo's tokenizer** so the sizes match.