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DeFi bots can fail before a transaction is sent, during execution, or because the token or contract behaves differently than expected. The most useful fixes are to classify API errors, treat quotes as temporary estimates, validate chain and liquidity, account for token-specific behavior, and set bounded execution limits. The examples below focus mainly on Uniswap and EVM-style integrations; they are not universal rules for every protocol or chain.
1. Treating API errors alike and ignoring rate limits
An API error should determine what the bot does next. Retrying a bad request or invalid credential will not fix it, while a temporary service failure may be worth retrying after a delay. Uniswap says most API keys have a default limit of 6 requests per second, though the limit for a particular key can differ. Exceeding the provisioned limit can return HTTP 429. Check the current limit for your key in the Uniswap API documentation and its getting-started guidance.
- 400: Check the request structure and required fields.
- 401: Check authentication and credentials.
- 429: Slow or pause requests for that key, then retry in line with the provider’s guidance.
- 500 or 503: Treat as potentially temporary and retry with backoff rather than sending repeated requests immediately.
Throttle requests before they exceed the limit. Log request IDs and error codes so you can diagnose failures, and use the provider’s machine-readable code to classify them; human-readable error details can change. Keep validation and authentication failures out of the transient-retry loop.
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2. Treating a quote or simulation as a guarantee
A quote describes an expected trade under conditions at the time it was generated. A simulation can help identify problems, but neither guarantees an on-chain swap will succeed. Gas conditions and market prices can change between the response and transaction placement, as Uniswap’s trading overview explains.
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- Request a fresh quote close to execution rather than relying on an old response.
- Set an acceptable minimum output for the trade. For contract-based swaps, Uniswap identifies
amountOutMinimumor an on-chain price oracle as safeguards. - Use a transaction deadline if the protocol supports one; the mechanism varies by protocol.
- If the quote is stale or execution fails, reassess the trade instead of blindly retrying it.
A minimum output can bound how poor an execution you will accept, but it cannot ensure the transaction succeeds if conditions move beyond that bound.
3. Skipping chain and liquidity checks
A correctly formed transaction can still fail if the selected protocol is unavailable on the target chain or the trading pair has no usable liquidity. Uniswap’s integration guidance recommends checking both protocol availability and pair liquidity; supported-chain lists can change.
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Make these explicit preflight checks before submitting a trade:
- Confirm the chain ID matches the bot’s intended network.
- Confirm the protocol supports that chain at deployment time.
- Validate the token contract addresses for that network.
- Check that liquidity is available for the pair.
When a check fails, stop and report which condition is missing. Repeatedly submitting the same transaction cannot make an unsupported protocol or unavailable market usable.
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4. Assuming every token and approval works the same way
A successful approval does not prove that a swap will work as expected. Tokens can charge transfer fees, use nonstandard approval functions, rebase balances, or behave differently in their transfer math. Some tokens may be unsellable because of their design or malicious contract logic. Uniswap’s failed-transaction troubleshooting guidance describes these as possible causes of swap failures.
Inspect the failure and identify the token behavior before changing execution settings. Account for token-specific transfer and approval behavior in the integration, and do not automatically increase slippage to force a retry: a fee or contract restriction may be the real cause, and widening the bound can expose the bot to a worse execution.
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5. Using unsafe slippage settings or overlooking MEV and contract risk
Transactions on public chains can be reordered while pending. Uniswap explains that this can expose trades to front-running or sandwiching. Slippage limits help cap the execution you will accept, but they involve a trade-off: a tight bound can cause a transaction to fail after market movement, while a permissive bound allows a worse result. See Uniswap’s swap concepts and v4 security guidance.
For contract-based strategies, Uniswap’s safety note is direct: “When trading from smart contracts, you must enforce slippage limits via amountOutMinimum or use an onchain price oracle.” Choose a bound that fits the strategy and market conditions; there is no universally safe slippage value.
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Slippage settings do not eliminate transaction-ordering or contract risks. The v4 security guidance also discusses custom-math errors, external state drift, and operational failures. Where the contract design permits, include execution limits and a kill switch. Consider transaction-protection options only when they are supported by the chain and execution path you actually use; no single protection feature applies universally.
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