Diffhall Hall Pass $1.95/moFree during preview
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Rooms

01Ask for work you can check 02Read the whole diff 03Keep secrets out of the code 04Treat every input as hostile 05Check who is asking 06Check that it exists 07Make tests able to fail 08Plan for when it breaks

Hall Pass $1.95/month or $19.50/year · one room $1.00 · Free during preview

Self-study for people who build with AI

Read what
the AI wrote.

You ask, the model writes, it runs. Diffhall teaches the step in between: reading the change before you accept it. Eight short rooms and sixteen review benches, where you tick the lines you would send back and see what you caught.

Take the first bench Walk the rooms

Hall Pass $1.95/month or $19.50/year · one room $1.00 · Free during preview. Rooms 1 and 2 and the first bench of every room are always free.

Room 02 · Bench Greet the user by first name
  1. 1 async function signup(req, res) {
  2. 2- if (!isEmail(req.body.email)) return res.status(400).send('Bad email');
  3. 3 const user = await db.users.create({
  4. 4 email: req.body.email,
  5. 5+ firstName: req.body.firstName,
  6. 6 passwordHash: await hash(req.body.password),
  7. 7 });
  8. 8+ console.log('new user', req.body);
  9. FlaggedThis logs the whole request body, password included.
  10. 9+ const name = user.firstName || 'there';
  11. 10+ await sendWelcome(user.email, `Welcome, ${name}!`);
  12. 11 return res.status(201).send({ id: user.id });

1 of 2 caught0 false flags

Directory

Diffhall · 8 rooms · 16 benches

  1. 01Ask for work you can checkFree
  2. 02Read the whole diffFree
  3. 03Keep secrets out of the code1 free + 1
  4. 04Treat every input as hostile1 free + 1
  5. 05Check who is asking1 free + 1
  6. 06Check that it exists1 free + 1
  7. 07Make tests able to fail1 free + 1
  8. 08Plan for when it breaks1 free + 1

How a bench works

  1. 1

    Read the ask

    Each bench starts with what you asked the AI for: a prompt, a plan, a diff, a config file, a set of tests.

  2. 2

    Tick the lines

    Tick every line you would not accept. Tap the line on a phone; no typing, no time limit.

  3. 3

    Hand in your read

    See what you caught, what you missed and what you flagged by mistake, with a note on every planted problem.

Why reading matters

21.7%

of the packages suggested by open-source code models in a 2024 study did not exist; 5.2% for commercial models. Invented names can be registered by anyone. Spracklen et al., arXiv 2406.10279

A01

Broken access control is the first entry in the OWASP Top 10:2025, and the bug that passes every test you run as yourself. OWASP Top 10:2025

None

is the default timeout in Python's Requests library: a request does not time out unless you set one, and without one it can hang for minutes or more. Requests docs

Sources checked 2026-10-15.

Admission

Walk-in

Free

  • Every room's reading and sources
  • Rooms 1 and 2 in full (4 benches)
  • The first bench of every other room

Hall Pass

$1.95/month

or $19.50 a year (two months free)

  • All 16 benches, as often as you like
  • Your notebook: every read kept, on any device, by private link
  • The printable Review Card

Free during preview

One room

$1.00

paid once, kept

  • Both benches of the room you choose
  • Your reads of it kept in your notebook

Free during preview

Preview: payments are not switched on yet, so every room and bench is open to everyone and nothing is charged. The prices above are the prices that will apply when checkout opens; nothing renews or charges without you choosing it then.

A hall, not a college

Diffhall is a self-study website. It does not enrol anyone, award degrees, credits or grades, or issue certificates, and nothing here counts toward any qualification. The rooms are short readings that name their sources; the benches are practice. What you take away is a habit.