Inside UBS’s landmark testing challenge

Michiel Willems
August 6, 2026

The integration of Credit Suisse into UBS has been described as one of the most complex mergers in banking history.

It is also one of the largest software testing and assurance programmes ever undertaken by a global financial institution, even though testing rarely dominated the public narrative surrounding the deal.

Behind the account migrations, legal-entity combinations, infrastructure closures and planned cost savings lies a fundamental technology challenge: moving customers, products and data away from Credit Suisse systems while preserving service continuity, financial accuracy, security and regulatory control.

UBS has now provided unusually clear confirmation of the scale of testing required.

Ahead of the first major Swiss migration wave in the second quarter of 2025, the bank said it had performed “robust and extensive technology testing and rehearsals.” It also increased support capacity in branches and contact centres in preparation for the migration.

‘Testing and rehearsals’

The choice of the word rehearsals is worth paying attention to. A migration of this kind cannot be validated solely through conventional functional testing in a laboratory.

It requires teams to simulate the sequence of the migration, validate operational procedures, confirm dependencies, prepare customer-support teams and establish what should happen if a step fails. The production event itself becomes the final execution of a process that has already been repeatedly tested.

By the end of 2025, UBS had moved every client account booked outside Switzerland and 85 per cent of Swiss-booked accounts onto UBS platforms. It had also completed the integration of Asset Management, including the final portfolio migrations.

UBS said the remaining Swiss client transfers were expected by the end of September 2026, while the wider Credit Suisse integration was expected to be substantially completed by the end of this year.

Each of those migrations potentially touches customer records, account structures, transaction histories, permissions, reporting processes, regulatory data and connections to other bank systems. The testing challenge is therefore multidimensional.

Data has to arrive completely and accurately, while applications receiving the migrated accounts must process them correctly.

Downstream services must continue to recognise customers and products. Access rights must remain appropriate and transactions must reconcile.

Moreover, customer-facing channels must display the correct information. Operational teams must be able to detect and resolve exceptions.

UBS has not published a detailed testing architecture or the number of test cases executed so it would be speculative to assign specific test volumes or claim that a particular testing tool was used.

However, the bank’s own disclosures did establish that technology testing and migration rehearsals were central to its preparation. They also show why regression and integration assurance would have been unavoidable.

Credit Suisse was not being connected to UBS as a single application. UBS was combining businesses while transferring accounts and products, simplifying legal entities, decommissioning applications and preserving services across several regions.

By the end of the first quarter of 2025, the Non-core and Legacy division had decommissioned almost 10 per cent of its applications during that quarter and 48 per cent since the division’s creation.

By the end of last year, UBS said approximately 73 per cent of applications in Non-core and Legacy had been decommissioned.

Application retirement is frequently treated as a cost-saving exercise. From a QA perspective, it is also a dependency-testing exercise.

Before an application can be switched off, the bank needs confidence that necessary functions and data have moved elsewhere, that remaining services no longer rely on it and that regulatory or historical information will remain accessible.

The absence of an immediate failure does not prove that a decommissioning was successful. A hidden dependency may only emerge during a month-end process, annual report, unusual market event, customer complaint or regulatory request.

Legacy retirement therefore requires careful analysis of interfaces, data lineage and operational usage, followed by testing that confirms the replacement environment can support both normal and exceptional activity.

Integration risks

UBS’s annual report makes clear that the bank views operational stability as a central integration risk. The bank said the success of the transaction depended partly on combining the two firms “rapidly and effectively, while maintaining stability of operations and high levels of service to customers of the combined franchise.”

That requirement fundamentally shapes the testing strategy. Speed cannot be assessed independently from stability. A migration completed on schedule but followed by transaction errors, lost access or sustained customer-service disruption would not represent successful delivery.

The quality bar is therefore not merely technical correctness. It is the preservation of an operating bank throughout a period of extensive technological and organisational change.

Regulatory and control concerns make that more complex.

UBS has been remediating material weaknesses in Credit Suisse’s internal control over financial reporting. The bank said it integrated the Credit Suisse control framework into UBS processes, reviewed systems and internal controls connected to the integration and implemented additional controls to reflect the increased complexity.

This means technology migration and control migration cannot be entirely separated.

When data, calculations or reporting processes move between platforms, teams must determine whether the controls attached to those processes remain effective. They also need evidence that systems produce complete, accurate and appropriately governed information after migration.

Testing in this context becomes part of control assurance. The programme also covers the transfer of businesses and products between legal entities.

UBS reported that Credit Suisse International transferred substantially all its residual business and related products to UBS AG London Branch and UBS Europe SE.

A transfer of that nature may require coordinated validation across product processing, contractual records, reporting, risk calculations and customer servicing. The technical result must correspond with the legal and operational result.

This is why large banking integrations cannot rely on testing teams working in isolation. Business experts need to validate product behaviour. Operations teams need to rehearse procedures.

Risk and compliance teams need to confirm controls, while data teams need to reconcile records and infrastructure teams need to monitor capacity and stability. Customer-facing teams need to prepare for questions and exceptions.

UBS’s decision to increase branch and contact-centre support ahead of the migration illustrates that broader operational approach. The integration also highlights the importance of sequencing.

Accounts cannot simply be moved because a destination platform is technically available. The bank must coordinate customer communication, data readiness, legal structures, operational support and technology dependencies.

Regression testing

Each migration wave creates new combinations of legacy and target systems that must continue to operate together until the transition is complete.

That makes regression testing particularly challenging.

A function that worked before one wave may behave differently after data or products have moved. Interfaces need to work while some customers remain on former Credit Suisse platforms and others have transferred to UBS. Reporting and operational controls may temporarily span both environments.

The test estate itself therefore changes throughout the programme. UBS has not publicly disclosed defect rates, automation coverage or the number of failed rehearsals. The absence of those details limits any attempt to evaluate the programme as a conventional QA case study.

What the bank has disclosed is nevertheless significant. It confirms that extensive technology testing and rehearsals preceded the main Swiss migration.

It also showed that migrations and application decommissioning proceeded in waves and it has linked successful integration directly to operational stability and customer service. It also demonstrated that control remediation and technology consolidation were being carried out together.