Brex

Brex is a business banking and spend management platform that has built a genuinely developer-accessible API on top of its financial infrastructure. For agents that need to automate expense workflows, issue virtual cards for controlled spending, or surface transaction data for financial reporting, Brex's API provides direct access to business financial operations without the abstraction layers that traditional banks impose. This is particularly valuable for startups and technology companies that are already Brex customers.

The API covers transactions, expenses, accounts, cards, team members, and budgets. The card issuance and control capabilities are particularly powerful for agents — creating virtual cards with specific spending limits and merchant restrictions enables precise programmatic control over business spending. That level of control supports use cases like automated vendor payments, employee expense card provisioning, and budget-constrained operational spending.

Authentication uses OAuth 2.0 for user-delegated access and API keys for server-to-server workflows. The OAuth model means agents can act on behalf of specific users with appropriate permissions, while service-to-service keys enable automation without user session involvement. Financial API access requires careful permission scoping — agents should request only the data access they need.

Reliability for financial operations is high-stakes — failed card transactions or missed expense submissions have direct business impact. Brex maintains banking-grade reliability standards for its core financial operations. API operations that modify financial state (create expense, issue card) should implement idempotency key patterns to prevent duplicate transactions on retry.

Documentation is developer-friendly and reflects Brex's technology-first culture. The API reference is thorough, and the conceptual guides explain the financial data model clearly. Teams integrating agents with Brex for expense automation or financial reporting will find the docs sufficient for production integration.