Question 1
Difficulty: medium
Walk me through how you would design a smart contract from requirements to deployment.
Sample answer
I start by turning the business goal into a very explicit technical spec: what state the contract must hold, who can call each function, what assets move, and what failure cases need to be prevented. From there I define the core invariants first, because those guide every implementation choice. I usually sketch the contract interfaces, write test cases before code where possible, and think through edge conditions like reentrancy, authorization, rounding, and upgrade needs. Once the basic design is clear, I implement in small pieces and validate each function with unit tests and fork tests. I also review gas costs early instead of treating optimization as a final step. Before deployment, I run static analysis, peer review, and ideally a testnet rehearsal with realistic parameters. In practice, the biggest mistakes come from ambiguous requirements, so I spend extra time clarifying assumptions before writing code.
Question 2
Difficulty: hard
How do you protect a smart contract against common vulnerabilities like reentrancy, access-control bugs, and integer issues?
Sample answer
I treat security as part of the design, not a final checklist. For reentrancy, I prefer the checks-effects-interactions pattern and add a reentrancy guard when external calls are unavoidable. For access control, I make permissions explicit and simple rather than relying on scattered modifiers that are hard to audit. I also try to avoid giving a single account too much power unless the product truly requires it, and if it does, I design a safer admin flow with timelocks or multisig. On integer issues, modern Solidity helps with overflow protection, but I still watch for precision loss, division edge cases, and rounding logic in accounting functions. I also pay attention to external dependencies like oracle feeds, token standards, and upgrade hooks, because many exploits happen at the boundaries. My general rule is to minimize trust, minimize complexity, and test every assumption aggressively.
Question 3
Difficulty: medium
Tell me about a time you found a bug late in the development process. How did you handle it?
Sample answer
In one project, I caught a critical issue during final testing where a fee calculation worked correctly for normal transfers but broke in an edge case involving small token amounts. The bug was subtle because the main path passed all tests, and it only showed up when amounts were near the rounding threshold. Once I confirmed the issue, I paused deployment immediately and wrote a focused test suite around the affected logic to reproduce it consistently. I then reviewed similar functions to make sure the same assumption was not repeated elsewhere. After fixing the logic, I had another developer review the patch and reran the full suite, including fork tests and integration tests. I also documented the root cause so the team could spot similar risks earlier in future work. I’d rather delay release than ship something that silently behaves incorrectly with user funds.
Question 4
Difficulty: medium
How do you write tests for smart contracts, and what do you consider a strong test suite?
Sample answer
A strong smart contract test suite goes beyond checking happy paths. I want tests that prove the contract’s intended behavior, catch regressions, and expose edge cases that could affect funds or permissions. I usually cover unit tests for each function, integration tests for multi-step flows, and negative tests for unauthorized calls, paused states, and invalid inputs. I also like property-based testing or fuzzing for arithmetic-heavy logic, because those tools can uncover patterns I might not think to write manually. If the contract interacts with external systems, I simulate those dependencies and test failure modes, not just success cases. For production-critical code, I also run fork tests against a live chain state to see how the contract behaves in realistic conditions. My benchmark is simple: if I changed a line in the contract, would the test suite quickly tell me what broke and why?
Question 5
Difficulty: medium
What is your approach to gas optimization without making the code less maintainable?
Sample answer
I optimize gas only where the savings justify the tradeoff, because unreadable code creates its own cost in audits, maintenance, and future feature work. My approach is to measure first, then optimize the highest-impact paths. I look at storage layout, unnecessary writes, repeated computations, and expensive external calls. I also use smaller data types only when they actually reduce storage or packing costs and when they don’t make the code awkward. Sometimes the biggest win is architectural, like reducing how often a function needs to touch storage rather than shaving a few units off an opcode. I avoid clever micro-optimizations that make the code harder to audit unless the contract is extremely performance-sensitive. A good rule is: make the contract safe and clear first, then profile and tune the functions that matter most. That keeps the code understandable while still respecting user transaction costs.
Question 6
Difficulty: hard
How would you explain the tradeoffs between an upgradable and a non-upgradable smart contract?
Sample answer
A non-upgradable contract gives users a very clear trust model: the code is fixed, so they know exactly what they are interacting with. That simplicity is great for security and auditing, but it also means bugs or missing features are permanent. An upgradable contract gives the team flexibility to fix issues, improve logic, or adapt to regulation and product changes, but it introduces extra complexity and trust assumptions. You now have to secure the upgrade path, governance, admin keys, and storage compatibility. In practice, I choose based on the product’s risk profile. For core financial logic, I prefer minimizing upgrade power and putting strong controls around any upgrade mechanism, such as multisig approval, timelocks, and transparent announcements. If upgrades are necessary, I design the system so the upgrade surface is small and well-tested. The key is being honest about what users are trusting and making that trust model explicit.
Question 7
Difficulty: medium
Describe how you would investigate a failed transaction in a deployed smart contract.
Sample answer
I’d start by reproducing the failure in the same environment if possible, because the chain state often matters more than the code itself. Then I’d inspect the revert reason, transaction trace, event logs, and the exact inputs that were used. If it’s a production issue, I’d compare the failing transaction against a successful one to isolate the difference in state or parameters. I also check whether the failure comes from the contract itself or from an external dependency such as an oracle, token contract, or missing allowance. If the issue is intermittent, I look for time-based logic, state changes between transactions, or front-running effects. Once I identify the root cause, I confirm whether it’s a user error, expected behavior, or an actual bug. For a true bug, I’d patch it carefully, add a regression test, and document the impact clearly so product and support teams can communicate accurately to users.
Question 8
Difficulty: medium
How do you decide whether to use Solidity patterns like libraries, inheritance, or interfaces in a project?
Sample answer
I choose patterns based on clarity, reuse, and auditability rather than habit. Interfaces are useful when I need to define a stable external contract boundary or interact with other contracts without caring about implementation details. Libraries are a good fit for reusable logic that doesn’t need its own storage, especially utility code that would otherwise be duplicated across contracts. Inheritance can be helpful for shared behavior, but I use it carefully because deep inheritance trees can make code harder to reason about and audit. If multiple contracts need the same behavior, I first ask whether composition would be cleaner. I try to avoid clever abstractions that save a few lines but hide important control flow. For smart contracts, readability is a security feature, so I prefer the simplest design that still supports the product. The best pattern is the one the next engineer and the auditor can understand quickly.
Question 9
Difficulty: hard
What steps would you take before approving a smart contract for mainnet deployment?
Sample answer
Before mainnet deployment, I want confidence from several angles, not just one passing test suite. I’d confirm the requirements and expected behavior one last time, then verify that the code has been reviewed by at least one other developer and, ideally, an external auditor for critical systems. I’d make sure the test coverage includes unit, integration, edge-case, and failure-path scenarios, and I’d run the latest version through static analysis tools to catch common issues. If the contract depends on external contracts, oracles, or protocol assumptions, I’d validate those integrations on a testnet or fork. I’d also review deployment scripts carefully, because a correct contract can still fail if the deployment process is wrong. Finally, I’d check operational readiness: admin keys secured, upgrade and pause mechanisms understood, monitoring in place, and a rollback or incident plan documented. Mainnet should feel boring, and that only happens when preparation is thorough.
Question 10
Difficulty: easy
Why do you want to work as a Smart Contract Developer, and what do you enjoy most about this type of work?
Sample answer
I’m interested in smart contract development because it sits at the intersection of engineering, security, and product design. The code has real consequences, so there’s no room for vague thinking or sloppy implementation. I like that every decision needs to be justified technically and economically, especially when user funds or protocol behavior are involved. What I enjoy most is the combination of precision and creativity: you need to build something that is deterministic and secure, but also practical enough for users and the business. I also like the collaborative side of the role, because the best outcomes usually come from working closely with product, security, and backend teams to simplify the system. For me, this is the kind of work where good engineering discipline really matters. You can see the impact directly, and that makes the responsibility motivating rather than intimidating.
