Most full-stack developers are stronger on one side of the stack, and the first screen's job is to find out which. In about 20 minutes, confirm which stack they have actually used and whether they can follow one feature from the database to the screen, so you can match their stronger side to what the client needs.
What does a full-stack developer do?
A full-stack developer works on both halves of a web application. The Bureau of Labor Statistics describes the two halves separately: front-end developers "create the technical features for a website's look," and back-end developers "are responsible for the overall technical construction of the website." A full-stack developer does some of each, usually with one front-end framework (React, Angular or Vue), one back-end language (JavaScript with Node.js, C#, Java or Python) and a database.
Which skills should you check at each level?
| Level | What they should be able to talk about |
|---|---|
| Junior (0 to 2 years) | One front-end framework and one back-end language, building a small feature end to end with help, basic SQL, Git |
| Mid-level (2 to 5 years) | Owning features across the screen, the API and the database; login and permissions; database changes; tests on both sides; deployment basics |
| Senior (5+ years) | Designing how the parts fit, performance from browser to database, security, choosing tools, mentoring, cloud hosting decisions |
What screening questions should you ask a full-stack developer?
Name the client's stack in each question where you can.
Describe one feature you built, from the database up to the screen.
This is the core full-stack question: one story that crosses every layer.
Strong answer: covers each step in order, such as a new table or column, the API endpoint that reads and writes it, the screen that calls the API, validation on both sides, tests and how it was deployed.
Weak answer: lots of detail on one layer, and "then it connects to the back end" for the rest.
Which side of the stack are you stronger on, and what did you last have to learn on the other side?
Honest candidates answer this easily, and the answer tells you where to probe.
Strong answer: picks a side without hesitation and gives a specific example of learning the other, such as writing their first database migration or fixing a CSS layout bug.
Weak answer: "I'm equally strong on everything."
A user says a form saves, but the data never shows up. How do you track it down?
Debugging across layers is the daily work of a full-stack developer.
Strong answer: checks the browser's network tab to see whether the request went out and what came back, then the server logs, then the database, narrowing it down one layer at a time.
Weak answer: "I'd look through the code."
How does login work in an app you built?
Login involves both halves of the app, and it's easy to get wrong.
Strong answer: describes where passwords are stored (hashed, never in plain text), whether the app uses session cookies or tokens and where those are kept, and an outside login provider if they used one. Senior candidates mention protecting against common attacks such as cross-site scripting.
Weak answer: "We used JWT," and nothing more.
You need to add a column to a busy production table. How do you roll that out safely?
Database changes are where full-stack developers can cause outages.
Strong answer: a migration script in version control (for example with Entity Framework, Flyway, Alembic or Prisma), a change that old and new code can both handle, the order of deploying the database and the code, and a backup or rollback plan.
Weak answer: "I'd run the ALTER TABLE command on production."
How do you design an API that both a web app and a mobile app use?
This shows back-end judgment beyond wiring one screen to one endpoint.
Strong answer: consistent endpoints, versioning so old mobile apps keep working, paging for long lists, a standard error format and documentation (often OpenAPI). They may compare REST and GraphQL with a reason.
Weak answer: "I make endpoints as each screen needs them."
Tell me about a bug where the cause was in a different layer than it first looked.
People who have worked across the stack have a story like this ready.
Strong answer: a specific story, such as a slow page blamed on the front end that turned out to be a missing database index, with how they found it.
Weak answer: no story, or a story where someone else found the cause.
What red flags should you note?
- The resume lists four front-end frameworks and four back-end languages, all at the same level.
- Every project is a tutorial clone (a social media clone, a to-do app) for a mid-level or senior role.
- They can't explain what happens between clicking a button and the data reaching the database.
- Every back-end question gets "the back-end team handled that," or every front-end question gets "the designer did that."
- The stack in their answers doesn't match the stack on the resume.
Note these and probe them in your own call. If you're unsure the person on the call is the person on the resume, read how to spot a proxy interview.
How do you run this screen with Simplehire?
Enter the client's stack as the role's required skills, one per item, for example React, Node.js, PostgreSQL, REST API design and AWS. Listing them separately gives the interview both sides of the stack to ask about, and the report's Skills demonstrated X/Y count reflects both. The candidate talks with our AI interviewer in the browser, and you get the report as soon as they finish.
Basic ($2 per screen, about 15 minutes) works for first screens. Adaptive ($3, about 20 minutes) is the one we'd use before submitting a full-stack candidate, because follow-up questions on the candidate's answers show how much is behind each side, and each answer gets an AI assessment. Advanced ($5, about 30 minutes) fits senior or client-critical roles and adds an optional coding round, scored separately.
The report gives a verdict, a Screen summary, Strengths and Areas to probe; put Strengths beside Areas to probe to see which side of the stack carries the candidate. You make the call on who moves forward. Proctoring runs on every screen, and every alert is reviewed by a person. Basic screens are free for the first 25 each month (pricing).
For a closer look at one side, use the React developer questions, the Python developer questions or the .NET developer questions.
Frequently asked questions
What does full stack mean on a resume?
It means the candidate has worked on both the front end (the screens users see) and the back end (servers, APIs and databases). It doesn't say which stack. React with Node.js, Angular with .NET and React with Python or Java are all called full stack, so ask which one.
Are most full-stack developers equally strong on both sides?
Don't assume so. Many have spent most of their time on one side. That's fine as long as the stronger side matches what the client needs, which is why the screen should find out which one it is.
How long should a first full-stack screen take?
About 20 minutes: a few minutes on their stack, one question that walks a feature from database to screen, a question for each side of the stack, and one debugging question. Save system design for the technical round.
