Question 1
Difficulty: hard
How would you design a smart contract for a token sale that is secure, auditable, and easy to upgrade later?
Sample answer
I would start by separating the sale logic from the token logic so each part has a clear responsibility. For the sale contract, I’d keep the state simple: pricing, caps, whitelist rules if needed, purchase tracking, and a clear finalization path. Security would come first, so I’d avoid unnecessary external calls, use established libraries, and include protections like reentrancy guards and explicit access control. For auditability, I’d emit detailed events for purchases, refunds, and admin actions so every important state change is easy to trace. If upgrades are required, I’d use a proxy pattern only when the business case justifies it, because upgradeability adds risk. I’d also make sure admin powers are limited and transparent, with timelocks or multisig control where possible. Before deployment, I’d test edge cases thoroughly, especially around token decimals, funding caps, and paused states.
Question 2
Difficulty: medium
Tell me about a time you found and fixed a security issue in blockchain code before it reached production.
Sample answer
In a previous project, I was reviewing a staking contract and noticed that rewards were being calculated after the external transfer call. That created a reentrancy risk if the reward token had unexpected behavior or if the contract was extended later. I raised it during review and changed the flow so the internal accounting updates happened before any token transfer. I also added a reentrancy guard as a second layer of protection. While testing the fix, I found a related issue where rounding differences could accumulate and give slightly inconsistent rewards over time, so I adjusted the math to use safe precision rules and documented the edge cases. What I learned from that experience is that blockchain security is never just about one bug. It’s about understanding how small design choices can become attack surfaces. I now treat every contract review like a potential audit finding and look for failure paths early.
Question 3
Difficulty: easy
How do you decide whether to build on Ethereum, a Layer 2, or another blockchain platform?
Sample answer
I decide based on the product’s real constraints, not just what is popular. If the application needs strong ecosystem support, deep liquidity, and broad tooling, Ethereum is often the safest starting point. If transaction cost and throughput are major concerns, I would seriously consider a Layer 2 because it can make the user experience much more practical, especially for frequent interactions like gaming, trading, or micropayments. For other chains, I look at the developer tooling, finality, validator model, bridge risk, and the size of the target user base. I also think about long-term maintainability. A chain with great performance but weak tooling can slow the team down. I’ve found it helps to map the business requirements first, then compare security assumptions, fees, and integration effort. In practice, I’d build a small proof of concept before committing, because the best technical choice is the one that matches both product needs and operational reality.
Question 4
Difficulty: medium
Describe how you would test a smart contract before deployment.
Sample answer
I use a layered testing approach because smart contract bugs tend to be expensive. First, I write unit tests for the core functions and make sure the expected state transitions happen exactly as intended. Then I add edge-case tests for access control, invalid inputs, boundary values, and failure paths. If the contract depends on multiple components, I create integration tests to verify how they behave together, especially around token transfers, oracle reads, or cross-contract calls. I also like property-based testing because it helps surface scenarios I wouldn’t think of manually, such as repeated operations or unusual ordering. For higher-risk contracts, I run static analysis and review gas usage to spot inefficiencies. On top of that, I would deploy to a testnet or local fork and simulate realistic usage patterns. My goal is to catch issues before audit, not after. Testing is not just validation for me; it is part of the design process.
Question 5
Difficulty: medium
What would you do if a product manager asked for a blockchain feature that you believed was too risky to ship as designed?
Sample answer
I would push back respectfully, but I wouldn’t just say no. I’d start by explaining the specific risk in practical terms, such as how the design could expose funds, complicate auditing, or create a support burden for users. Then I would propose safer alternatives that preserve the business goal. For example, if they wanted irreversible automation with too much admin control, I might suggest a phased rollout, limited beta, or a simpler version with fewer attack surfaces. I think it’s important to translate technical risk into product impact, because that helps non-technical stakeholders make better decisions. If needed, I’d bring in examples from prior incidents or known failure patterns to make the issue concrete. I’ve found that product teams usually appreciate strong opinions when they come with options. My job is to protect the user and the company while still helping the team move forward efficiently.
Question 6
Difficulty: easy
How do you handle gas optimization without making the contract harder to read or safer?
Sample answer
I treat gas optimization as a second-pass activity, not the starting point. My first priority is correctness, clarity, and security. Once the contract is stable, I look for improvements that do not change behavior or reduce readability too much. That includes minimizing storage writes, caching repeated values in memory, avoiding unnecessary loops over large arrays, and choosing appropriate data types when it makes a measurable difference. I also pay attention to struct layout and event design because they can affect both gas and observability. What I avoid is over-optimizing in ways that make the code fragile or obscure. Saving a few gas units is not worth introducing a maintenance problem or an audit headache. In practice, I document any optimization that looks non-obvious and make sure the team understands the tradeoff. Good gas work should make the contract efficient, but the code should still be understandable to another engineer six months later.
Question 7
Difficulty: medium
Tell me about a time you had to work with auditors or external security reviewers.
Sample answer
I worked on a DeFi integration where we knew the contract would face a formal audit before launch. I treated the audit as a collaboration, not a checkpoint. Before submitting, I cleaned up the codebase, wrote clear comments for any unusual logic, and prepared a simple document describing the contract’s intended behavior and assumptions. When the auditors returned findings, I grouped them into categories: critical, logic-related, and improvement suggestions. That helped the team prioritize changes instead of reacting randomly. I also sat with the auditors on a few follow-up calls to clarify how certain state transitions were supposed to work. One useful lesson was that many findings came from ambiguity rather than deep bugs, so better documentation could have prevented some back-and-forth. After the fixes, I re-ran the test suite and added regression tests for every issue raised. I like working with auditors because they make the code stronger and expose blind spots early.
Question 8
Difficulty: hard
How would you approach integrating an off-chain API or oracle into a smart contract system?
Sample answer
I would be very careful, because once a contract depends on off-chain data, the trust model changes. First, I’d define exactly what the contract needs from the oracle and how often it needs updates. Then I’d evaluate whether the data should be fetched through a decentralized oracle network, a signed message flow, or a custom service, depending on the level of trust and latency requirements. I would never assume the data is always correct. The contract should validate freshness, handle stale or missing responses, and include fallback behavior where possible. I’d also think about manipulation resistance, especially for price feeds or anything with financial impact. On the implementation side, I’d keep the oracle interface narrow and make it easy to replace if the provider changes. I’d also design tests that simulate delayed or malformed data, because those are the scenarios that tend to break production systems. The key is to make the trust assumptions explicit from day one.
Question 9
Difficulty: hard
Describe a situation where you had to debug a blockchain transaction that failed in production.
Sample answer
A user-reported failure turned out to be a subtle allowance issue in a token flow. The transaction was reverting only for certain wallets, which made it look inconsistent at first. I traced the execution using event logs and a forked local environment, then replayed the failing transaction with the same parameters. That showed the problem came from an assumed token standard behavior that did not hold across all assets we supported. Instead of handling every token the same way, the contract needed to account for non-standard return values and transfer patterns. I patched the logic to be more defensive and added tests for common edge cases across tokens with different behaviors. I also updated the frontend error messages so users would get clearer feedback instead of a generic revert. What I took from that incident is that production debugging on-chain requires patience and good observability. Transaction traces, logs, and reproducible state are essential for finding the root cause quickly.
Question 10
Difficulty: easy
Why do you want to work as a Blockchain Developer, and what kind of products are you most interested in building?
Sample answer
I’m interested in blockchain development because it combines software engineering with systems that have real financial and operational consequences. I like building things where correctness, transparency, and trust matter just as much as features. What keeps me engaged is that the work forces you to think carefully about state, incentives, and failure modes, not just code paths. The products I’m most interested in are the ones that solve genuine user problems rather than using blockchain for its own sake. That could be payments, asset tracking, identity, tokenized access, or DeFi infrastructure, as long as the design has a clear reason to be on-chain. I also enjoy working on developer tools because better tooling improves the whole ecosystem. What motivates me most is shipping systems that are dependable enough for people to use confidently. I want to be on a team that values rigorous engineering, fast learning, and practical product judgment.
