Your AI Learning Environment

Learn anything.Your way.

Lenni reads your current skills, builds a personalised learning plan with chapters and exercises, teaches you with the AI you choose, and tracks your growth — all in one place.

See how it works

20 free credits · No subscription · Pay as you learn

lenni.app/paths/ai-engineering/lesson/3-3

Chapter 3 · Lesson 3IN PROGRESS

Backpropagation & Gradient Flow

Backpropagation is the algorithm that makes neural networks learn. It computes how much each weight in the network contributed to the final error, then nudges every weight in the direction that reduces it.

The forward pass

Data flows forward through every layer. Each neuron computes a weighted sum of its inputs and passes the result through an activation function. The final layer produces a prediction.

Input

Hidden 1

Hidden 2

Output

Computing the loss

After the forward pass, we compare the prediction ŷ to the true label y using a loss function. Mean squared error is common for regression:

L = ½ · (ŷ − y)2

The backward pass

Backprop uses the chain rule to compute the gradient of the loss with respect to every weight — layer by layer, working backwards from the output.

◆

Key insight: The gradient tells each weight: "move this much, in this direction, to reduce the loss." That's the entire learning signal.

The problem

AI can answer your questions. But can it teach you?

ChatGPT, Claude, Gemini and other AI models are great at explaining things. But learning is more than asking questions — you need a clear path, the right resources, practice, feedback, and a way to know how far you've come.

A new chat every time, with no memory of what you've already learned

Answers with no structure — no path, no order, no next step

No way to test whether it actually sank in

No sense of progress, so you never know how far you've come

The Lenni way

More than an AI chatbot.

Chat with AI and you get an answer. Learn with Lenni and you build knowledge — Lenni turns AI into a structured learning experience.

A chat with AI

Heres your answer.

One-off, disconnected replies
No memory of your goal
No sense of your progress
You have to structure it yourself

→

Learning with Lenni

Lets build your path.

Built from your goal and current level
One place for your whole journey
Tracks what you've learned and proven
Quizzes and exercises to validate it

What Lenni does

Everything you need, nothing you have to plan yourself.

◆

A path built from you

Lenni starts from what you already know and plots the shortest real path to your goal — no generic curriculum.

◇

Powered by the best AI models

Lenni uses top-tier AI models to generate lessons, quizzes, and exercises tailored to your exact level.

✎

Learn, then prove it

Every topic ends with a quiz or exercise Lenni generates for you, so you validate what you learned, not just read it.

○

Progress you can see

Completed, current, and upcoming topics update automatically, so you always know exactly where you stand.

△

One place, not one chat

No more starting a new conversation every time. Your goal, resources, practice, and history all live together.

▣

Learn almost anything

Technology, business, creative skills, languages — tell Lenni the goal and it builds the path around it.

Skill detection

Lenni meets you where you are.

Connect your GitHub, LinkedIn, or resume — or take a quick adaptive test. Lenni analyses your actual skills and builds a path that starts from your real level, not a generic beginner course.

GitHub repos & language detection
LinkedIn work history & seniority
Resume parsing for skills & education
Adaptive skill test if you prefer

SKILL LEVEL DETECTION

◈GitHubREADING

12 public repos

Python · JS · Rust

ML projects detected

◉LinkedIn

3 yrs experience

Software Engineer

2 ML-related roles

▤Resume

BSc Computer Science

PyTorch · TensorFlow

Deployed 2 models

◎Skill test

Scored 74 / 100

Strong: data structures

Weak: optimisers

LENNI CONCLUSION

Intermediate level detected. Your path starts at Chapter 2, skipping basics you already know.

YOUR LEARNING PLAN · AI-generated

✓

Foundations

4 lessons · 2 hrs

✓

Core Concepts

5 lessons · 3 hrs

Applied PracticeIN PROGRESS

6 lessons · 3.5 hrs

Advanced Topics

4 lessons · 2.5 hrs

Final Project

2 lessons · 1 hrs

21 lessons · ~12 hours total40% complete

Learning plan

A structured path, not a pile of links.

Lenni generates a complete curriculum broken into chapters and lessons, with estimated time for each. The plan adapts as you learn — finish a chapter early, and the next one updates automatically.

Chapters and lessons generated for your goal
Time estimates per topic
Plan updates as you complete lessons
Skip what you already know

AI lessons + chat

Learn, then ask. Without losing your place.

Each lesson is generated by your chosen AI model, tailored to your level. Got a question mid-lesson? Just ask — Lenni answers in context and keeps the lesson on track.

Lessons generated to your exact level
Ask questions without losing context
Works with any AI provider
All history saved — revisit anytime

Chapter 4 · Lesson 1

Attention Mechanisms

Chat to ask anything

Attention lets the model weigh every token against every other — that's why Transformers capture long-range dependencies so well.

Why can't RNNs do the same thing?

RNNs pass context through a hidden state, so distant information gets diluted. Attention looks at everything at once, no forgetting.

That makes sense. What's the time complexity trade-off?

Ask anything about this lesson…↑

EXERCISE · Chapter 4

AI-generated quiz

Q 2 of 5

What is the time complexity of the self-attention mechanism with respect to sequence length?

Select an answer to see Lenni's feedback.

Exercises & evaluation

Don't just read it. Prove you learned it.

Every chapter ends with AI-generated quizzes and exercises. Lenni evaluates your answers, explains what you got wrong, and only marks a topic complete when you've actually demonstrated understanding.

Quizzes generated per chapter
Instant AI feedback on every answer
Retake until you pass
Score history kept on your profile

Progress tracking

See exactly how far you've come.

Lenni tracks every lesson, quiz score, and learning streak. Your dashboard shows which skills are growing, where you're strongest, and what needs more attention — so you always know what to do next.

Lessons and quiz scores tracked automatically
Skill growth per topic
Daily activity and streak
Full history across all your paths

PROGRESS TRACKER

Lessons

Quizzes

82%

Avg score

Streak

DAILY ACTIVITY

SKILL GROWTH

ML Theory

82%

Python / NumPy

70%

Model Training

55%

Evaluation

38%

How it works

From goal to mastery in four steps.

Enterprise-grade AI, accessible to everyone.

Lenni uses top-tier AI models to generate personalised lessons, quizzes, and exercises. No configuration needed — just start learning.

Claude & GPT-4

Industry-leading models power your lessons and exercises.

Personalised to you

AI adapts content to your level, pace, and learning style.

No setup required

We handle the AI infrastructure. You just focus on learning.

Always improving

New models and features are added as Lenni grows.

Pricing

Pay for what you use.

Start free. Buy credits when you need them. No subscription required.

CREDIT PACKS

Starter

Enough for one full learning path.

100 credits · never expire

100 credits
1 credit = 1 lesson, quiz, or exercise
Credits never expire
Use across any learning path

Questions people actually ask.

No. Lenni starts from your current level, whatever that is, and builds the path from there — beginner or advanced.

1 credit = 1 lesson, quiz, or exercise. You get 20 free credits to start. Buy more packs anytime — credits never expire.

Skill detection, curriculum generation, and progress tracking are always free. Consuming content (lessons, quizzes, exercises) costs 1 credit each.

Yes. Your first learning path is free. Set a new goal anytime, or add additional paths for $5 each.

A chat gives you an answer and forgets it. Lenni keeps a persistent path, tracks what you've actually learned, and knows what's next — closer to a structured course than a chat window.

Yes — you get 20 free credits, 1 learning path, skill detection, and curriculum generation. No credit card required.

Almost anything — programming languages, AI engineering, cybersecurity, product management, marketing, entrepreneurship, and more. If you can name the goal, Lenni can build the path.

Start learning something new.

Choose a goal. Lenni builds your path. You master it.