DeFi bots can fail for reasons that have little to do with their trading strategy: an API limit, a stale quote, an unsupported chain, unusual token behavior, or an execution price that exceeds the bot’s risk limits. Treat these as separate failure classes. The examples below draw mainly on Uniswap and EVM-style integrations; exact APIs, transaction mechanics, and protections vary by protocol and chain.
1. Treating every API error the same
An API error is not automatically a reason to retry. A malformed request or failed authentication needs a correction; a rate limit or temporary service outage may clear with time. Retrying every response immediately can amplify the problem.
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Uniswap documents a default rate limit of 6 requests per second for most API keys. That is a Uniswap-specific default, not a general DeFi limit, and the account’s provisioned limit should be checked against current documentation. Requests that exceed the applicable limit can receive HTTP 429 responses.
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- Throttle requests to stay within the applicable provider limit.
- Handle HTTP 400 and 401 as request or authentication problems; fix the cause rather than retrying unchanged.
- For HTTP 429, pause requests from the affected key before retrying. Use backoff for retryable temporary errors, including service unavailability such as HTTP 503.
- Handle HTTP 500 and other server failures according to the provider’s documented retry guidance.
- Log status codes and request IDs where available. Use machine-readable error codes for branching, not changeable human-readable messages.
2. Treating a quote or simulation as a guarantee
A quote estimates a possible trade; a simulation checks a proposed transaction under particular conditions. Neither guarantees that the transaction will succeed or execute at the quoted price. Uniswap notes that gas conditions and slippage can change between a swap response and transaction placement.
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How to fix it
- Request a fresh quote close to execution and reconsider stale or failed quotes instead of blindly resubmitting them.
- Set an acceptable minimum output. For contract-based swaps, Uniswap’s guidance is to enforce slippage limits through
amountOutMinimumor use an on-chain price oracle. - Use a transaction deadline if the protocol supports one; the exact mechanism is protocol-dependent.
- Handle a failed simulation as a warning, not proof that the transaction will fail—or succeed—on-chain.
3. Skipping chain and liquidity checks
A correctly formed transaction can still be unusable if the selected protocol does not support the target chain or the requested pair lacks liquidity. Support can change, so assumptions that were valid at deployment may become stale.
How to fix it
- Before requesting a quote or submitting a trade, verify the chain ID and confirm that the protocol is available on that chain.
- Validate the token addresses for the selected chain; a token address on one network does not establish that the same asset is available on another.
- Check that the pair has liquidity sufficient for the intended trade.
- Make each check a preflight condition and return a specific failure reason rather than repeatedly submitting an unsupported or unavailable trade.
4. Assuming every token and approval behaves alike
Token contracts do not all follow the same assumptions. Uniswap support identifies variable token fees, unsellable tokens, nonstandard approval functions, and token math issues—including rebasing or fee-on-transfer behavior—as possible causes of failed swaps. An approval succeeding therefore does not establish that a later swap will work as expected.
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How to fix it
- Inspect the failure and classify whether it concerns approval, transfer, token behavior, or another part of the swap.
- Account for token-specific transfer fees and approval behavior when constructing and evaluating transactions.
- Do not widen slippage automatically to mask an unexplained failure.
- Recognize that some tokens may be unsellable by design or because of malicious contract logic; repeated retries cannot guarantee a remedy.
5. Setting unsafe slippage limits or overlooking transaction risks
Slippage limits create a trade-off. A tight limit can reject a trade when the market moves; a permissive limit allows execution at a worse price. On public chains, transaction ordering can also expose pending trades to front-running or sandwiching. A slippage limit constrains an acceptable execution price but does not eliminate transaction-ordering or contract risks.
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How to fix it
- Choose a bounded execution price based on the strategy and market conditions rather than using a permissive setting by default.
- For contract-based strategies, enforce a minimum output or use an oracle as appropriate to the design.
- Consider transaction-protection options only when they are supported by the relevant chain and transaction path; no one protection feature applies universally.
- For custom contracts, account for custom-math errors, external state changes, and operational failures. Where the design permits, include limits and a way to stop trading.
Preflight checks before a bot submits a swap
- The API request is authenticated, valid, and within the provider’s current rate limit.
- The chosen chain and protocol support the trade, and the pair has liquidity.
- The quote is fresh enough for the strategy, and the transaction has an acceptable minimum output.
- The token’s transfer and approval behavior is understood well enough for the planned swap.
- Failure handling distinguishes a fixable request error from a stale quote, unavailable market, or token-specific problem.
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