Summ's advanced reconciliation engine is an updated version of the tax engine that keeps one set of numbers: the balance you actually hold in each account and the balance your tax records say you hold are always the same. When something in your data does not add up, the engine shows you exactly which transaction caused the gap, so you can fix the right thing instead of hunting for it.
If your account is still on the standard engine, you can turn the advanced engine on yourself from Settings, choose the date it applies from, and turn it off again later. This guide covers the switch. For the thinking behind the engine, with diagrams, see How the advanced reconciliation engine works.
What the advanced engine does differently
Every asset you hold is backed by a tax record. If a sale, transfer, or other outgoing transaction would take an account below zero, the engine adds a record with an assumed cost basis for the missing amount (a zero cost basis by default) so the calculation can continue, and it flags the transaction that caused the shortfall.
Uncategorised transactions count. An uncategorised incoming transaction is treated as an acquisition with an assumed cost basis, and an uncategorised outgoing transaction reduces your balance, unless you have changed the uncategorised transaction toggles in your tax settings. On the standard engine both were held aside as unmatched transfers.
Problems point at their source. Transactions that created an assumed-cost-basis record carry an Assumed cost basis warning. Filter the Transactions page by that warning to see everything that needs attention. Fixing the source transaction (for example, categorising it or importing the account it came from) removes the warning on the next recalculation.
Transfers are checked both ways. If you sent more than you received, the difference is treated as a disposal at the market value when you sent it. If you received more than you sent, the extra amount gets an assumed cost basis.
Who sees this setting
The setting appears in Settings › Tax › Advanced when both of these are true:
Your account country is not the United States, the United Kingdom, or Canada. The US has its own version of this engine, and the UK and Canada are not yet supported.
Your account is on the standard engine, or you turned the advanced engine on yourself using this setting.
Most new accounts already start on the advanced engine, so they do not see this setting. If Summ moved your account onto the advanced engine for you and you want to switch back, reach out to Support via the in-app chat.
How to turn the advanced engine on
Open Settings and select the Tax tab.
Click Advanced below the tax settings to expand the advanced section. The engine setting is at the very bottom.
Under Advanced reconciliation engine, click the Enable advanced reconciliation engine row. It reads Off while you are on the standard engine.
Choose when the engine should apply from:
From the beginning recalculates your whole history on the advanced engine. This option is only offered when you have no tax periods yet.
Select a date lets you pick the first day the advanced engine applies. Everything before that day stays on the standard engine. Days inside an existing tax period cannot be selected.
End of latest period is a shortcut for the day after your most recent tax period ends. It is offered instead of From the beginning when you already have tax periods.
Click Save. You will see the message Reconciliation engine turned on. Your report is being recalculated. The row now reads On.
Saving starts a recalculation of your report in the background. Large accounts can take a few minutes.
What happens when you choose a start date
Summ creates an unlocked period that ends the day before your chosen date. You can see it in Settings › Lock periods. It marks where the standard engine stops and the advanced engine starts.
At that boundary the engine checks that your tax records match what you actually held. Where they do not match, it adjusts the opening balances: missing amounts get a record with an assumed cost basis, and surplus records are removed. Transactions whose balance was adjusted show a Migration adjustment info icon next to the balance. See Understanding the balance adjustment in Summ.
Deleting that unlocked period removes the boundary and changes your historical calculations, so leave it in place unless you intend to turn the engine off.
If you chose From the beginning instead, there is no boundary and no migration adjustment. Your full history is recalculated on the advanced engine, and any gaps show up as Assumed cost basis warnings on the transactions that caused them.
How to turn the advanced engine off
Open Settings › Tax › Advanced and click the Enable advanced reconciliation engine row, which reads On.
In the Disable advanced reconciliation engine? dialog, read the warning Your tax numbers may change. and click Disable.
You will see the message Reconciliation engine turned off. Your report is being recalculated.
Turning the engine off removes the unlocked boundary period it created, clears every Assumed cost basis warning, and recalculates your report on the standard engine in the background. Your transactions themselves are not changed.
Locked periods
Locked tax years stay locked. The advanced engine cannot be turned on with a start date that falls inside or before a locked period, and From the beginning is not available while any locked period exists.
You can start the engine on the day after a locked period ends. The locked period stays on the standard engine.
If you lock a period after turning the engine on, the engine cannot be turned off again until that period is unlocked. This protects a return you have already filed.
An existing unlocked period also has to end before your chosen start date. If it does not, you will see You already have a tax period on or after that date. Choose a later start date.
Common scenarios
Fresh start from this financial year. Jordan has a few years of messy history and has not locked anything. Jordan opens the setting, clicks Select a date, picks 1 July, and saves. Everything before 1 July stays as it was on the standard engine, an unlocked period ending 30 June appears on the Lock periods page, and from 1 July onwards every gap points at the transaction that caused it.
Clean account, full history. Priya has one wallet, no tax periods, and wants the most accurate view of her whole history. She chooses From the beginning. Her report is recalculated, and the only warnings she sees are two Assumed cost basis warnings on staking withdrawals from a validator she never imported. She adds the validator account and the warnings disappear.
Already filed last year. Sam locked the period ending 30 June 2025 after filing. Sam opens the setting and the picker only allows dates from 1 July 2025 onwards. Sam clicks End of latest period and saves. The locked period is untouched and the advanced engine applies from 1 July 2025. Later, Sam locks the period ending 30 June 2026 and files. From then on the engine cannot be turned off unless that period is unlocked first.
Frequently asked questions
Can I go back to the standard engine?
Yes, if you turned the advanced engine on yourself using this setting and you have not locked a period since. Turning it off deletes the boundary period, clears the assumed-cost-basis warnings, and recalculates your report. You can turn it on again later, with a different start date if you like.
Will my transactions change?
No. The engine changes how your tax records and balances are calculated from your transactions. It does not edit, add, or delete the transactions you imported.
Why does the setting say the engine is on, but I cannot see a way to turn it off?
Accounts that were created on the advanced engine, or that Summ moved onto it, do not have the self-serve switch. If you need to change engines, reach out to Support via the in-app chat.
What is an assumed cost basis?
When the engine has to create a record for an asset it has no purchase history for, it gives that record an assumed cost basis, which is zero by default. A zero cost basis means the full sale proceeds count as gain, so these records can make your gains look higher than they really are. Fix the transaction the warning points at, usually by importing a missing account or categorising an incoming transaction, and the record is replaced with the real cost. How the advanced reconciliation engine works explains this in depth.
Why do I see a Migration adjustment on some balances?
You turned the engine on from a start date, and at that date your tax records did not match what you actually held. The adjustment brings them into line. The balance adjustment guide explains what to do about it.
Does turning the engine off undo a migration adjustment?
Yes. The boundary period is removed and your report is recalculated on the standard engine, so the adjustment no longer applies.
Why can I not pick the date I want?
The start date has to be after every tax period you already have, locked or unlocked. Days that are not allowed are disabled in the picker. Delete or move the unlocked period first, or choose End of latest period.
Does this cost extra or need a specific plan?
No. The setting is available on every plan for accounts in eligible countries.
Good to know
Turning the engine on or off always triggers a full recalculation of your report. Download a copy of your current reports first if you want to compare before and after.
Clearing your data does not change which engine your account uses.
Assumed-cost-basis records only affect your tax records. They never change the actual assets you hold.
You cannot pick a date in the future.
If you have any questions, reach out to our Support Team via the in-app chat. 😇