It depends on what the contract defined as the result and on who warned whom. If the supplier could, exercising professional care, have identified that your materials or instructions for the integration were unsuitable and failed to warn you, it is liable for defects caused by their use. If it warned you and you insisted in writing on the original solution, it is not liable for those defects. If the integration depends on cooperation which, under the contract, you were to procure, even through another supplier, the implementation supplier is not in delay in so far as it cannot perform for want of that cooperation.
A new order management system was to be connected to the warehouse software the company has used for years and which was supplied by a different supplier. After go-live, orders are transferred to the warehouse with errors. The implementation supplier says the old interface does not correspond to the documentation it was given. The company replies that a working connection formed part of the order. Who is right will be decided by the contract and by how the parties conducted themselves during the project.
First look at what result the contract promises
Where the supplier undertook to deliver a result, that is to say a deployed and working solution, the implementation contract bears the features of a contract for work. Under the law, work also includes the modification of a thing or a tangibly recorded result of another activity (Section 536(2) of the Commercial Code). Work is defective where its execution does not correspond to the result specified in the contract (Section 560(1)). If the contract describes the connection to the warehouse system as part of the result, faulty data transfer is a defect. If it expressly excluded the integration or does not mention it at all, it need not be a defect, and the dispute shifts to what the parties agreed elsewhere, for instance in the quotation or the analysis.
The supplier’s duty to warn
In an integration the supplier works with what the customer gives it: the interface documentation, the test environment, data exports and the specification of how the connection is to work. The law requires it to assess those inputs as a professional:
The contractor is obliged to warn the customer without undue delay of the unsuitable nature of things taken over from the customer, or of instructions given to it by the customer for the execution of the work, where the contractor could have identified that unsuitability by exercising professional care.
— Section 551(1) of Act No. 513/1991 Coll. (unofficial English translation)
Where unsuitable materials obstruct the proper execution of the work, the supplier must interrupt its work to the extent necessary until you replace them, change the instruction, or notify it in writing that you insist upon them. The completion date is extended by the period of interruption (Section 551(1)). If the supplier gave a warning and you insisted in writing on the use of the materials or instructions, it is not liable for the defects they cause (Sections 551(2) and 561). Nor is it liable where it could not have identified the unsuitability even by exercising professional care (Section 561). A supplier which failed to warn although it could have done so is liable for such defects (Section 551(3)).
The law speaks of things and instructions. In an integration, it is above all the specification of the connection and the materials you supply that fall into this category. Whether access to your own system or data exports are to be regarded as things within the meaning of that provision is not expressly resolved by the law, so it is safer to address them directly in the contract.
Where the interface is held by another supplier
Often the problem is that the old system is maintained by another firm, which will not open the interface, will not fix it or will not supply documentation. If the contract defined making the interface available as your cooperation, its absence is your delay as creditor (Section 370) and the implementation supplier is not in delay in so far as it cannot perform for that reason (Section 365(4)). Claims against the maintainer of the old system are then pursued under your contract with it.
Under a contract governed by Czech law, a similar rule applies: the contractor is to warn the customer of the unsuitable nature of a thing handed over or of an instruction where it could have identified the unsuitability by exercising the necessary care, and where it proceeds as the law prescribes, the customer has no rights arising from a defect resulting from that unsuitability (Section 2594 of Act No. 89/2012 Coll.).
What to obtain and what belongs in the contract
If a dispute is already under way, gather the integration specification, the interface documentation in the form in which you handed it over, and all communications showing whether and when the supplier warned of the problem. Assessing whether there is a defect and to whose account it falls is the subject of disputes over defective work.
On a new project, a dispute can be avoided by a list of interfaces specifying who is to supply the documentation and the test environment, who is responsible for the quality of the input data, and what happens if a third party does not cooperate. It should also include an acceptance test for the connection itself, not merely for the system. We set these rules in the implementation contract. If the system is already in use and the protocol has not been signed, see also the question on an unsigned acceptance protocol.
This answer provides general information on the law as at 12 September 2026. It does not constitute legal services or replace an assessment of an individual case. The details of your situation may differ. Book a consultation to discuss them.
