Validation, serialization, and the trust boundary
By the end of today you can identify every trust boundary in a system and say what must be validated at each, and explain why validating in the browser protects nobody.
YesterdayOn Day 36 Pydantic rejected bad input for you. Today you learn what it was actually protecting and where else you need the same protection.
TomorrowTomorrow, the boundary that decides who someone is and what they may do.
Why this matters
Almost every serious security failure is data crossing a boundary without being checked. This one idea covers SQL injection, prompt injection on Day 89, and unsafe tool arguments on Day 78.
- Trust boundaries
- Input validation
- Serialising outward
- Defence in depth
Learn it
75 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
3 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
45 minAttack your own books API with curl: send wrong types, omit required fields, send a 10MB string, send extra fields you never declared, and send a negative price. Record what got through. Then add validation to close every hole, including at least one business rule Pydantic does not give you for free.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
Free · sign inRate yourself against five named criteria per concept. Completion and mastery are tracked separately, and anything you rate shakily comes back automatically on a spaced schedule.
Recap
- 01A trust boundary is anywhere data arrives from outside your control
- 02Client-side validation is a courtesy to honest users, not a defence
- 03Structure is free from the framework; meaning never is
- 04Whitelist outward, because blacklists leak by default
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.