Financial APIs: What They Are and How They Work

In the past, a finance team would have some hoops to jump through in order to issue corporate cards to their team members. You’d have to request a set number of cards through your existing bank, wait for them to ship, and establish flat spending controls that apply to each of them. Nowadays, some platforms allow companies to issue unlimited virtual cards to their employees, each with custom spend limits and merchant restrictions. This is possible thanks to the development of financial APIs.

An API, or application programming interface, allows two software systems to exchange data and trigger actions. In finance, that exchange can involve bank accounts, payment orders, credit limits, and KYC verification profiles rather than generic application data.

This article covers what financial APIs are, how they work, and which leading providers currently offer them. If you’re looking to take advantage of APIs within your banking platform, try taking a look at Slash.¹ Slash is a neobank that uses financial APIs to connect accounts, corporate cards, payment rails, and more together on the same dashboard.

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API Glossary

API (application programming interface): A set of defined rules and protocols that lets one software system request data or actions from another.
Protocol: The agreed-upon standard governing how data is formatted and transmitted between systems. Overall, HTTPS is the dominant transport protocol for external financial APIs.
Open finance: A framework in which banks and financial institutions share customer-permissioned data with third parties through standardized APIs.
Open APIs: APIs made available to any qualified third-party developer, as opposed to a single partner or internal use only.
REST (Representational State Transfer): One of the most common architectural styles for building external APIs. REST APIs use standard HTTP methods and typically exchange data as JSON, making them relatively straightforward to integrate.
Webhooks: Outbound notifications sent from one system to another when a specific event occurs, such as a card authorization being approved or a payment settling. Unlike a standard API call where you ask for data, webhooks push data to you automatically.
Know-Your-Customer: A mandatory due diligence process used by banks and fintechs to verify the identities of their clients and assess their risk.

What Are Financial APIs?

Financial APIs are specialized APIs designed to exchange money and compliance-sensitive data between banks, fintech platforms, and businesses. While generic APIs are used for standard mobile apps and websites, financial APIs may be in charge of handling bank accounts, FX quotes, credit limits, KYC verification results, and payments. These sorts of things come with regulatory implications that most other scenarios don’t.

These types of APIs are central to “open banking”, which is a term used to describe frameworks that give consumers the right to share bank data with third parties. Until recently, this wasn’t even technically legal. In the United States, consumers weren’t granted the right to connect their financial data with other apps and services until a 2024 addition to the Dodd-Frank Act.

Now that they’re more common, financial APIs exist to make card payments settle in seconds, sync bank accounts to other systems, and enable instant account verification before a payment. Businesses that use these sorts of APIs can speed up some of their money management processes without sacrificing accuracy or crossing any regulatory lines.

How financial APIs work

Let’s take a look at a quick example, along with some of the technical details. If a finance team wants to initiate a $40,000 vendor payment through their treasury management system, they’ll likely need an API that connects that system to their bank. They submit the payment on their treasury system’s dashboard, which typically appears as an HTTPS POST request containing the amount, recipient, currency, and reference number.

The API validates the request, confirms sufficient funds, and runs compliance checks. If everything clears, the payment is queued and the API returns a transaction ID and status.

For one-off actions like "create payment" or "issue card," REST APIs are often used. For ongoing event streams, though, REST would require continuous polling. WebSocket connections maintain an open, two-way channel so events push through as they happen. Webhooks take a different approach, as the API sends an outbound HTTP POST to a URL you specify whenever a defined event occurs, such as a card authorization or a payment failure. That means your system can react without polling 24/7.

The standard in finance

Slash goes above with better controls, better rewards, and better support for your business.

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Comparing Leading Financial API Providers

Not all financial APIs do the same thing. Some handle specific pieces of a business’s banking needs, while others offer connections across a wide range of tools. Here are some of the most popular financial platforms that build their products using APIs:

Slash

Slash uses APIs to cut down on the software fragmentation that today’s businesses often struggle with. Finance teams can get caught working between banking portals, card providers, and accounting systems with different data stuck in each. Slash was built to connect them.

The Slash platform combines accounts, cards, payments, crypto, treasury, and forecasting on one dashboard, making money movement easier and financial data more accessible in-the-moment. APIs also allow Slash to integrate two-ways with accounting platforms like QuickBooks Online, Xero, Sage Intacct, and NetSuite. This level of flexibility can help businesses reduce transcription mistakes and move more efficiently.

Strength: Accessibility and centralization. Busy finance teams can work from a single platform without having to manage separate logins and manually copy data from one system to another.

Stripe

Stripe itself connects with a wide variety of other applications through APIs, including accounting platforms like QuickBooks, e-commerce solutions like Shopify, and marketing apps like MailChimp. At its core, Stripe processes card payments and ACH transfers, with support for 100+ payment methods across 135+ currencies. Beyond their payments, Stripe also offers treasury accounts and corporate cards with programmable spend controls. However, their treasury options don’t come with any annualized yield, and each card costs a small fee to issue.

Strength: A developer-friendly environment. Stripe offers a lot of documentation that helps users connect Stripe to their current applications and crack into their API for custom-built workflows.

Plaid

Plaid's APIs give fintech applications access to consumer and business bank account data with user permission. Their core products include:

Auth, which verifies account and routing numbers for ACH payments
Transactions, which returns up to 24 months of categorized transaction history
Balance, which checks real-time account balances before initiating a transfer
Identity, which verifies user identities for the sake of KYC

Plaid also offers an API specification called Core Exchange, which lets banks publish FDX-compliant APIs so their customers can connect to different apps using Plaid's network. While Plaid’s built to read data, it doesn't actually execute financial operations. Moving money through Plaid requires its Transfer product, which is newer and fairly limited compared to dedicated payment rails.

Strength: Coverage and reach. Plaid connects to over 12,000 financial institutions across several countries, which outpaces the rest of its competitors. As a result, it’s a popular choice for bank account linking on many everyday apps and services.

Wise Platform

Wise Platform is the API layer built on Wise's consumer transfer product. It gives banks and businesses configurable access to international money transfers, multi-currency accounts, and foreign exchange at Wise's mid-market rates. Their API covers account creation across 80+ currencies, payment initiation to 160+ countries, real-time FX quotes, and balance management. As helpful as it is for cross-border payments, though, it doesn’t issue cards or provide standard banking infrastructure.

Strength: FX pricing and geographic coverage. With more than 160 supported countries, the applications that connect to Wise can move money across most borders with a solid mid-market exchange rate.

Yodlee

Yodlee, which is now part of Envestnet, is a fintech company that specializes in data aggregations and analytics. Their APIs can connect to over 19,000 global sources, including banks and wealth management platforms. Financial institutions can use Yodlee to give users a consolidated view of their full financial picture, including banking, investment, credit, and loan accounts. They also offer a tool called FastLink, which handles authentication flows for account linking.

Yodlee’s core audience is mid-sized financial institutions that want to create deeper banking experiences for their customers. Costs are negotiated based on your specific use case, but you can often expect monthly API access and base platform fees to reach $1,000-$2,000 monthly.

Strength: Enterprise depth and wealth management coverage. Yodlee's combination of data connection and vertical-specific products, particularly for wealth platforms and lenders, is its key advantage. They’ve also been around since 1999, making them a pioneer in the financial services space.

Marqeta

Marqeta is a card issuing platform first and foremost. Its open API lets companies create and configure physical and virtual payment cards that come with authorization-level spend controls. These cards can also come with Just-In-Time (JIT) funding, which is a model where each transaction triggers a real-time check against the company's own ledger before the card is authorized. Essentially, the account is funded in real-time as the transaction processes, and the issuing company can approve or deny individual purchases as it sees fit. While Marqeta’s card features are impressive, it doesn’t come with bank accounts or payment processing tools, meaning you’ll have to pair their product with a separate bank partner.

Strength: Card controls and authorization-level control. For businesses closely monitoring their liquidity and margins, Marqeta’s Just-In-Time funding feature gives their card program a leg up.

Automate Your Financial Workflows with Slash

The platforms we’ve looked at offer particular strengths across services like global payments, card spend, and data aggregation. They’re great for businesses looking for only one of those features – but many leaders need multiple solutions, especially when they’re trying to grow quickly and get a head start on their rivals. Slash’s API tools bring almost every banking feature you could ask for together on one platform.

Here are some of the features our APIs give users access to:

Accounting integrations: Slash’s APIs let the platform connect transaction, card, and account data with accounting platforms like QuickBooks Online, Sage Intacct, NetSuite, and Xero. This means your spending and balance data exists in the exact same way in two different locations.
Working capital: APIs can surface real-time financial activity and account data that help power working-capital products, from eligibility checks to funding workflows. This allows capital offers to be tied directly to a business’s operating activity on Slash.⁵ Our short-term financing comes with flexible 30-, 60-, or 90-day repayment terms.
Slash Visa® Platinum Cards: Platforms can issue, manage, and monitor cards with granular spend controls thanks to the help of Slash’s card APIs.
Treasury: Our APIs provide programmatic access to balances and transfers, enabling treasury tools to move funds, monitor liquidity, and manage cash across accounts.⁶ Teams can build automated cash-management workflows on top of our banking infrastructure. Currently, users can earn up to 3.80% annualized yield on their idle funds.
Crypto and stablecoins: Slash’s APIs can connect fiat accounts with crypto and stablecoin flows, supporting on-ramp, off-ramp, and Global USD-style money movement.⁴

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Frequently Asked Questions

Can you use multiple financial API providers together?

Sure. A stack could combine Plaid for bank account verification, Stripe for payment processing, and Marqeta for card issuance. However, each provider requires its own authentication, its own error handling, its own webhook subscriptions, and its own versioning management.

Do I need to understand coding to use a financial API?

Not necessarily. Many simple functions, like card issuance, are built into a platform in a user-friendly way. However, if you’re looking to use an API to build a custom workflow, you might need to write some scripts.

Are financial APIs safe to use?

Just about 100% of the time, yes. They act as controlled gateways, allowing applications to communicate without exposing your raw login credentials to the third-party app. While security lapses and hacks are possible, they’re no more prone to vulnerabilities than most banking products.