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Beyond return values, contracts communicate through reverts (transactions that fail and undo state) and events (logs emitted on success).

Good tests assert both: that valid actions emit the right events, and that invalid actions revert with the right reason.

The Toolbox adds special Chai matchers for Ethereum, available after importing them (the Toolbox does this automatically):

• .to.be.revertedWith(...)
• .to.be.revertedWithCustomError(...)
• .to.emit(...)
• .to.changeEtherBalance(...)

To check that a transaction fails, wrap the call (without awaiting it first) and use revertedWith:

The string must match the contract's require message exactly.

Modern Solidity uses gas-efficient custom errors. Assert them with revertedWithCustomError:

You pass the contract instance so the matcher can decode the error from the ABI.

Custom errors can carry data. Chain withArgs to assert the values:

This verifies the contract reverted for the exact reason you expect.

Use .to.emit to assert that a transaction emitted an event:

You pass the contract and the event name. This confirms the log was produced.

Combine emit with withArgs to verify the event payload:

This ensures the from, to, and amount fields are exactly correct.

For ETH transfers, changeEtherBalance asserts the net change for an account:

There is also changeTokenBalance for ERC-20 transfers.

Some failures have no message — for example, arithmetic underflow panics or plain revert(). Assert these generically:

For panic codes specifically, use revertedWithPanic.

Testing reverts and events covers the negative and observable paths of your contract:

• Reverts prove your guards (access control, balance checks) actually block bad input.
• Events prove off-chain systems will receive the right notifications.

Skipping these leaves dangerous gaps in coverage.

After a revert, it is good practice to confirm state was untouched, since reverts roll back all changes:

This double-checks the guard truly protected the state.

You learned to assert behavior beyond return values.

• revertedWith checks require messages; revertedWithCustomError checks custom errors.
• withArgs verifies error or event arguments precisely.
• .to.emit confirms events fired.
• changeEtherBalance / changeTokenBalance assert net balance changes.
• Use .to.be.reverted for failures without a reason string.