Every now and then, a contract appears that tells you exactly why disputes do not begin with the Notice of Dissatisfaction. They begin at the tender stage, when risk is moved, blurred, or disguised in a way that looks manageable on paper and becomes flammable on site.

More often than not, I worked with contracts significantly altered in the Employer's favour. However, they usually kept the original philosophy while minimising the Employer's risk — and you can identify the red flags at the outset and price them accordingly. But I also worked with contracts that are a hybrid — or better said, a Frankenstein of two different concepts, creating confusion. This is the story of one of those contracts. On its face, it carries the visual language of the FIDIC Red Book 1999: Employer's design, a Bill of Quantities, and the familiar assumption that design accuracy sits primarily on the Employer's side. But once you go through the Appendix and the Particular Conditions, the structure starts to mutate. The price is locked as a lump sum. Quantities in the BoQ are described as estimated only. The Contractor is expected to verify and absorb inconsistencies. The Engineer may look at the Contractor's post-award pricing breakdown, but is not bound by it when valuing Variations.

This is not quite Red Book, and not truly Yellow Book. It is a hybrid that can work only if everyone understands in advance how Variations, omissions and quantity shifts will actually be valued.

The first move is the amendment to Sub-Clause 14.1. The Contract Price is no longer treated like a classic remeasurement outcome. It becomes a lump sum price, subject only to adjustments expressly allowed under the Contract. At the same time, the BoQ remains in the documents, but its quantities are stated to be estimated only — not the actual and correct quantities of the Works the Contractor must execute.

That combination matters. In a normal Red Book logic, the BoQ is part of the commercial mechanism by which the final price is determined through actual measured quantities. In a Yellow Book logic, quantity risk largely sits with the Contractor because the Contractor controls design, and the Employer defines performance through the Employer's Requirements. Here, however, the contract tries to preserve the optics of one model while reallocating part of the commercial risk as though it were the other.

It gets more interesting still: free-issue materials and Variations are expressly to be measured and evaluated under Sub-Clauses 12.1 and 12.3. So the contract locks in the original price as a lump sum, but keeps a valuation mechanism in place when the scope changes. That is exactly where confusion begins.

Under the Particular Conditions, the Contractor must submit within 28 days after the Commencement Date a proposed breakdown of each item in the Bill of Quantities. On one level, that sounds reasonable. If the BoQ rates differ for similar work in different locations, the Contractor is being asked to show how that pricing was built. If quantities in the tender documents were inconsistent, this breakdown may explain how risk, preliminaries, overhead and profit were spread across the schedule.

But the clause then removes the fairness it appears to offer. The Engineer may take that breakdown into account when preparing Payment Certificates or evaluating Variations, but is not bound by it. In practical terms, the Employer gains visibility into where the tender documents were weak and how the Contractor priced the uncertainty, while the Contractor receives no assurance that the same logic will be respected later when a Variation or quantity dispute arises.

Another familiar move appears on projects of this type: the Employer or Engineer invokes "fit for purpose" as though it solves everything. It does not. Fit for purpose is often badly misunderstood, even on genuine design-build contracts. It certainly cannot be used as a slogan to wash away the contractual allocation of design responsibility where the Employer has expressly retained responsibility for the accuracy of the design documentation.

That matters even more where the Engineer starts valuing work by reference to notes on drawings, or by taking quantities from one discipline's design and applying them to another, rather than using the quantities or logic in the actual design documents. When the underlying design is already misaligned across disciplines, this approach turns a documentation problem into a valuation problem.

The result is predictable. Variations arrive in volume because the design is not coordinated. Yet once the scope changes, the parties discover they never agreed what baseline they are varying from: the BoQ, the drawings, the notes, the architectural package, the MEP package, or some post-award breakdown the Engineer is free to ignore.

The reason the tension is so severe is simple. The two books are built on different commercial assumptions.

Under the Red Book, the Employer carries the design risk. The Contractor prices the work through rates, and the final contract value is largely determined only when those rates are applied to the actual quantities executed. Under the Yellow Book, design responsibility and quantity risk are much more closely tied to the Contractor through the Employer's Requirements. Variations usually arise because the concept, performance requirement or methodology changes materially — not because the Employer's own design package was internally inconsistent from the start.

This contract tries to borrow elements from both. The Employer keeps much of the project-side design responsibility, but wants the Contractor to behave commercially as though it had accepted Yellow Book quantity risk. That is why it is properly called a Frankenstein contract. It is stitched together from parts that do not naturally belong to the same body.

There is no universal answer, because everything turns on the specification, the BoQ preambles and the exact drafting. But there are at least three practical approaches contractors should test early.

Most contractors do not press the issue during tender for one of three reasons. Sometimes they do not realise they can ask. Sometimes they still assume the quantities are broadly reliable because the form looks like Red Book. And sometimes they see the ambiguity as part of the strategy for winning the project in the first place.

I have seen both outcomes. Sometimes the risk pays off. Sometimes it does not. But once the project is live and the drawings begin to shift, the price of that silence becomes very real. If the contract does not clearly tell the parties how a hybrid valuation mechanism is meant to operate, each side will eventually construct the interpretation that best suits its own cash position.

That is the real lesson here. Disputes on mixed-form contracts are rarely created by one dramatic event. More often, they are the delayed consequence of one unanswered tender question.

• How will Variations be valued where the revised scope involves quantities that differ from the BoQ?
• Will the Engineer be bound by the Contractor's 28-day breakdown when evaluating Variations?
• Where a BoQ/drawing discrepancy is identified after contract award, what mechanism applies?
• If Sub-Clause 12.4 has been deleted — what is the Employer's position on overhead and profit on omitted work?
• How will cross-discipline design clashes be valued — from the BoQ, the drawings, or the architectural/MEP package?