Composability

Composability works because most DeFi protocols are open-source smart contracts with standardized interfaces, such as the ERC-20 token standard or the newer ERC-4626 vault standard, so any contract on the same blockchain can call another protocol's public functions directly, without a partnership agreement or approval from its creators.

In practice, the output of one protocol becomes the input of the next. A user can supply DAI to a lending market, take the interest-bearing token that results, trade it on a decentralized exchange, then deposit the proceeds into a yield vault, all executed atomically within a single transaction. Yearn Finance vaults use exactly this pattern, routing deposits through Curve to earn trading fees and staking the resulting liquidity token in Convex for extra rewards, layering two other protocols' functionality without needing either team's cooperation. This permissionless stacking is what lets a single new protocol unlock thousands of new product combinations overnight.

Composability is largely a feature of activity happening on one chain, most prominently Ethereum, where the bulk of DeFi's total value locked sits and where protocols share the same execution environment. Moving assets across separate blockchains still requires bridges or dedicated interoperability tooling rather than a native function call.

The same interconnection that speeds up innovation also concentrates risk. If a foundational protocol is exploited, for instance through a flash loan that manipulates a price oracle, every application built on top of it can be affected within the same transaction, a failure mode security researchers call composability risk or cascading contagion. Auditors and developers increasingly treat every dependency in a protocol's stack as part of its own attack surface.