Stone Software Engineer Interview Guide

The 60-second brief

A Software Engineer at Stone is a core builder of the financial technology infrastructure that powers millions of merchants across Brazil. Stone has evolved far beyond a simple card-processing company; it is a massive tech ecosystem encompassing payments, banking, credit, and specialized software solutions. As an engineer here, you will design, deploy, and scale high-throughput systems where low latency, high availability, and absolute correctness are non-negotiable. Your work will directly impact the daily operations of Brazilian entrepreneurs. Whether you are optimizing transaction processing pipelines, building robust APIs, integrating generative AI chatbots, or scaling mobile payment applications, you will solve complex distributed systems problems.

This guide is scoped to a Software Engineer candidate at Stone.

Focus areas

• System Design
• Arquitetura de software
• Kotlin

The Stone interview process

Stone candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Recruiter Conversation

Initial conversation with a recruiter to align expectations and assess general fit.

What to demonstrate

• Initial conversation with a recruiter to align expectations and assess general fit
• Depth in System Design

How to prepare

• Be able to walk your CV end to end in two minutes, and say why this company specifically.
• Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Technical Stages

One or two technical rounds focusing on coding abilities, system design knowledge, and past project experiences.

What to demonstrate

• One or two technical rounds focusing on coding abilities, system design knowledge, and past project experiences
• Depth in System Design

How to prepare

• Answer aloud and timed: Given an array of integers, find the contiguous subarray which has the largest sum and return its sum.
• Answer aloud and timed: Reverse a linked list iteratively and recursively, explaining the trade-offs of each approach.

Cultural Fit Interview

Interview to ensure your working style aligns with Stone's core organizational values.

What to demonstrate

• Interview to ensure your working style aligns with Stone's core organizational values
• Depth in System Design

How to prepare

• Answer aloud and timed: Explain how you would implement a queue using two stacks.
• Answer aloud and timed: Design a URL shortener service like Bitly, focusing on high-read throughput and database selection.

Where candidates lose points

Communicate your thought process

During both coding and system design interviews, talk through your reasoning. Explain the trade-offs of the solutions you consider, as interviewers value your problem-solving approach just as much as the final output.

Structure your behavioral answers

Use the STAR method (Situation, Task, Action, Result) to keep your responses concise, structured, and focused on your personal contributions and the impact of your actions.

Showcase your curiosity

Don't hesitate to ask thoughtful questions about Stone's engineering culture, technical challenges, and team structure at the end of your interviews. It demonstrates genuine interest and engagement.

Do not attempt to overcomplicate your system designs

Focus on simplicity first, and only introduce complex architectures (like microservices or event streaming) when the scale or business requirements genuinely justify them.

Questions & practice

Write a function to determine if a string has all unique characters, optimizing for both time and space comple

Medium · Coding and Algorithms

Write a function to determine if a string has all unique characters, optimizing for both time and space complexity.

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

Given an array of integers, find the contiguous subarray which has the largest sum and return its sum.

Medium · Coding and Algorithms

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

Reverse a linked list iteratively and recursively, explaining the trade-offs of each approach.

Medium · Coding and Algorithms

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

Explain how you would implement a queue using two stacks.

Medium · Coding and Algorithms

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

Explain the difference between optimistic and pessimistic locking, and how they prevent race conditions.

Medium · Technical Concepts and Domain Knowledge

Approach

1. Say what the runtime actually does before reasoning about the code.
2. Name what is shared across threads and what owns each piece of state.
3. Identify the window where an invariant is briefly untrue.
4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

• What happens if two callers reach this at the same time?
• Where could this allocate more than you expect?

What is the difference between SQL and NoSQL databases, and when would you choose one over the other for a fin

Medium · Technical Concepts and Domain Knowledge

What is the difference between SQL and NoSQL databases, and when would you choose one over the other for a financial ledger?

Approach

1. Name the grain you start from and join outward from it.
2. Check whether any join is one-to-many before aggregating, or the sums inflate.
3. Say which index the query would use, and what makes it unusable.
4. Handle the rows that do not match: that is usually the actual question.

Follow-up

• How does the query change if that join becomes one-to-many?
• What happens to this when the table is ten times larger?

Explain the concepts of database indexing and how an improper index can degrade write performance.

Medium · Technical Concepts and Domain Knowledge

Approach

1. Name the grain you start from and join outward from it.
2. Check whether any join is one-to-many before aggregating, or the sums inflate.
3. Say which index the query would use, and what makes it unusable.
4. Handle the rows that do not match: that is usually the actual question.

Follow-up

• How does the query change if that join becomes one-to-many?
• What happens to this when the table is ten times larger?

Implement a basic rate limiter for an API endpoint using an algorithm of your choice.

Medium · Coding and Algorithms

Approach

1. Say who the caller is and what they do when the call fails halfway.
2. Define the identity of a request so a retry cannot double-apply it.
3. Separate accepted, pending, failed and confirmed; they are different facts.
4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

• What happens if the caller retries after a timeout?
• How does a client discover it is on an old version of this contract?

Design a URL shortener service like Bitly, focusing on high-read throughput and database selection.

Medium · System Design and Architecture

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How would you architect a real-time notification system that handles millions of push notifications and SMS me

Medium · System Design and Architecture

How would you architect a real-time notification system that handles millions of push notifications and SMS messages daily?

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

Design the backend architecture for a payment gateway, ensuring transaction consistency and handling network t

Medium · System Design and Architecture

Design the backend architecture for a payment gateway, ensuring transaction consistency and handling network timeouts gracefully.

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How would you structure a chat application's backend to support offline messaging and synchronization?

Medium · System Design and Architecture

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

Explain how you would transition a monolithic application to a microservices architecture using message broker

Medium · System Design and Architecture

Explain how you would transition a monolithic application to a microservices architecture using message brokers like Kafka or SQS.

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How do you implement observability and tracing in a microservices environment? What metrics are critical to mo

Medium · Technical Concepts and Domain Knowledge

How do you implement observability and tracing in a microservices environment? What metrics are critical to monitor?

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How do you ensure secure communication between internal microservices?

Medium · Technical Concepts and Domain Knowledge

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

Exports duplicate a row range about once a week

Hard · leases · clock skew · idempotency

Roughly once a week an export writes a file containing a duplicated range of rows. The affected job_run rows show attempt = 1, status = succeeded, one started_at, and a lease_owner naming a different host from the one whose logs show the job starting. Leases last 30 seconds and are heartbeated every 10 from inside the handler; lease_expires_at is computed on the worker and compared against the database's now(). Find the mechanism, and give a fix that holds even if you cannot fix the clocks.

Approach

1. Start from the fact that eliminates the obvious answer. attempt = 1 means no retry was recorded, so this is not a re-run after failure; two workers ran the same row concurrently and the takeover path never touched the counter. lease_owner naming a host other than the one that started the job is the same statement from the other side.
2. Enumerate the mechanisms that cause a premature takeover, then find the signal that separates them. Either the lease genuinely expired because the heartbeat did not fire, which is what happens when the heartbeat runs on the handler's own thread and the handler makes a long blocking call, or it only appeared expired because two clocks disagree, since lease_expires_at is written from the worker's clock and evaluated against the database's. The discriminator is the distribution: incidents clustered on the longest exports indict the heartbeat, incidents clustered on one host indict skew. Measure both, and measure each host's offset against the database directly.
3. Read the reclaim query precisely. In PostgreSQL now() is transaction start time, not statement time, so a reclaimer holding a long transaction compares against an older timestamp than expected; clock_timestamp() is the statement-time function. This is worth ruling in or out before you redesign anything, because it changes which rows look expired.
4. Remove the second clock rather than trying to synchronise it. Issue and extend the lease in the database, with lease_expires_at = now() + interval '30 seconds' in both the claim and the heartbeat, so exactly one clock is ever compared and worker skew stops mattering to this predicate.

Follow-up

• The displaced worker has already streamed half the file to object storage. What makes that side effect safe to repeat?
• You now count takeovers. What alert fires on that counter, and at what threshold?

Suggested practice checklist

Built from the rounds and topics Stone candidates report.

Day 1 — Map the Stone loop

• Write out the reported sequence: Recruiter Conversation, Technical Stages, Cultural Fit Interview.
• For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 3 reported rounds, with the weakest marked.

Day 2 — Work System Design

• Spend the session on System Design, which Stone candidates report being tested on.
• Write one worked example in System Design and time yourself on it.

Deliverable: One timed worked example in System Design.

Day 3 — Work Arquitetura de software

• Spend the session on Arquitetura de software, which Stone candidates report being tested on.
• Write one worked example in Arquitetura de software and time yourself on it.

Deliverable: One timed worked example in Arquitetura de software.

Day 4 — Work Kotlin

• Spend the session on Kotlin, which Stone candidates report being tested on.
• Write one worked example in Kotlin and time yourself on it.

Deliverable: One timed worked example in Kotlin.

Day 5 — Answer out loud: Coding and Algorithms

• Answer aloud, timed: Write a function to determine if a string has all unique characters, optimizing for both time and space complexity.
• Answer aloud, timed: Implement a basic rate limiter for an API endpoint using an algorithm of your choice.

Deliverable: Spoken answers to 2 reported Coding and Algorithms question(s), under time.

Day 6 — Answer out loud: System Design and Architecture

• Answer aloud, timed: Design a URL shortener service like Bitly, focusing on high-read throughput and database selection.
• Answer aloud, timed: How would you architect a real-time notification system that handles millions of push notifications and SMS messages daily?

Deliverable: Spoken answers to 2 reported System Design and Architecture question(s), under time.

Day 7 — Answer out loud: Technical Concepts and Domain Knowledge

• Answer aloud, timed: What is the difference between SQL and NoSQL databases, and when would you choose one over the other for a financial ledger?
• Answer aloud, timed: Explain the concepts of database indexing and how an improper index can degrade write performance.

Deliverable: Spoken answers to 2 reported Technical Concepts and Domain Knowledge question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Describe a time when you made a technical decision that resulted in a failure. How did you handle it, and what

Medium · Behavioral and Cultural Fit

Describe a time when you made a technical decision that resulted in a failure. How did you handle it, and what did you learn?

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

How do you handle a situation where you disagree with a product manager or a tech lead on a technical approach

Medium · Behavioral and Cultural Fit

How do you handle a situation where you disagree with a product manager or a tech lead on a technical approach?

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Tell me about a complex project you owned from start to finish. How did you manage stakeholders and ensure del

Medium · Behavioral and Cultural Fit

Tell me about a complex project you owned from start to finish. How did you manage stakeholders and ensure delivery?

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Give an example of how you mentored a junior engineer or helped improve your team's overall engineering practi

Medium · Behavioral and Cultural Fit

Give an example of how you mentored a junior engineer or helped improve your team's overall engineering practices.

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

How do you prioritize technical debt versus delivering new features under tight deadlines?

Medium · Behavioral and Cultural Fit

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?