Brex·Software Engineer·Take-home Assignment·Intermediate

Intermediate

Jul 2026

Summary

Brex take-home for a software engineering role. You build a small full-stack app that handles monetary transactions, and they want the whole thing: API, frontend, persistence, validation, tests, and a README. More involved than it sounds.

Questions Asked (1)

Using an AI coding assistant, build a minimal full-stack web application that supports basic monetary transactions and account balances. It should expose a REST or GraphQL API for creating credit and debit transactions, persist data, include a simple frontend to submit and view transactions, validate inputs, prevent negative balances, and come with basic tests and a README.

System DesignAPI & IntegrationsTechnical Trade-offs

Author's notes

The scope feels contained until you actually sit down with it.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then outline a minimal architecture that separates concerns (API, persistence, frontend). Emphasize how you would use an AI coding assistant to accelerate development while ensuring correctness through tests and validation. Conclude by discussing trade-offs and potential improvements.

Pro tip: Demonstrate awareness of financial correctness by highlighting idempotency and atomicity in transaction handling, and mention how you'd use the AI assistant to generate test cases for edge conditions like concurrent debits.

1. Clarify Requirements and Scope

Ask clarifying questions about expected scale, authentication, and whether transactions need to be idempotent. Define the minimal viable features: create credit/debit, view balance, list transactions.

2. Design the Architecture

Propose a simple stack (e.g., Node.js/Express + SQLite + React) and outline the API endpoints (POST /transactions, GET /balance, GET /transactions). Discuss data model: accounts and transactions tables with foreign key and constraints.

3. Implement Core Logic with AI Assistance

Explain how you'd use an AI coding assistant to generate boilerplate, API handlers, and frontend components. Emphasize manual review for critical sections like balance validation and transaction atomicity.

4. Ensure Correctness and Validation

Describe input validation (e.g., positive amounts, valid account IDs) and business rule enforcement (prevent negative balances). Mention using database transactions to ensure atomicity and idempotency keys to avoid duplicate submissions.

5. Test and Document

Outline a testing strategy: unit tests for validation logic, integration tests for API endpoints, and end-to-end tests for the frontend. Include a README with setup instructions, API documentation, and design decisions.

Key Points to Mention

Use of database transactions to ensure atomicity when updating balances and recording transactions.
Implementation of idempotency keys to prevent duplicate transactions from retries.
Input validation and sanitization to prevent injection and ensure data integrity.
Trade-offs between using an AI assistant for speed vs. manual review for critical financial logic.
Testing approach: unit, integration, and end-to-end tests, including edge cases like concurrent debits.
Clear separation of concerns: API layer, business logic, and persistence, with a simple frontend.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.