# Classification, Routing and Few-Shot Labelling — Embeddings & Vector Search

Source: https://www.geekswithgeeks.com/en/embeddings/u-classify

> Use embeddings as features for cheap, fast classifiers.

## A small classifier on top of vectors

Embeddings turn text classification into a simple problem. Embed labelled examples and train a lightweight classifier (logistic regression, k-nearest neighbours) on the vectors: it needs far fewer labels than training a text model from scratch, runs in microseconds, and is easy to retrain. A zero-training variant is **nearest-prototype routing**: embed a few example sentences per category, average them into a **centroid**, and assign a new text to the nearest centroid, with a similarity threshold below which the answer is "unknown / send to a human". This works well for intent routing, tagging and triage. Check accuracy per class with a confusion matrix on held-out examples, and watch for classes that the embedding does not separate.

## Nearest-centroid routing (illustrative)

Assumes `embed()` returns unit vectors; not run here because it needs a real embedding model.

```python
import numpy as np

examples = {
    "billing":   ["I was charged twice", "refund my invoice"],
    "technical": ["the app crashes on start", "cannot log in"],
}
centroids = {}
for label, texts in examples.items():
    c = np.mean([embed(t) for t in texts], axis=0)
    centroids[label] = c / np.linalg.norm(c)

def route(text, threshold=0.45):
    v = embed(text)
    label, score = max(((l, float(v @ c)) for l, c in centroids.items()), key=lambda x: x[1])
    return label if score >= threshold else "unknown"          # below threshold: send to a human
```

**Quiz:** Why include a similarity threshold in nearest-centroid routing?

- [ ] To avoid embeddings
- [ ] To make vectors longer
- [x] So off-topic inputs become "unknown" instead of a forced wrong label
- [ ] To speed up training

*Answer:* So off-topic inputs become "unknown" instead of a forced wrong label. Nearest-neighbour methods always return something; a threshold allows abstaining.