A retail platform or intermediary can purchase products from multiple upstream manufacturers through different types of contracts. Traditional studies assume that the contracts and their terms are common knowledge; that is, each party of the supply chain can observe the contracts reached. In practice, however, this is not necessarily the case. Taking the result under contract observability as a benchmark, this paper investigates the contracting of the supply chain comprised of one retail platform and two upstream manufacturers when contracts are unobservable. Two representative contracts are considered: the traditional and succinct wholesale price contract and the nonlinear, more complicated, two-part tariff contract. We apply the framework of perfect Bayesian equilibrium and also use passive beliefs as an equilibrium refinement criterion to analyze the model. Our study shows that the classical result – that the more complicated contract can increase channel profitability – may no longer be valid when contracts are unobservable to competing manufacturers. On the contrary, the manufacturers and the whole supply chain can benefit from adopting wholesale price contracts rather than two-part tariff contracts. To examine the robustness of our findings, we consider a commission fee contract and another classic demand function in an extended analysis, and the results show that the main findings and key insights remain valid. The intuition behind these findings is that the unobservability of contracts affects the opportunistic behavior of competing manufacturers. The practical value of these findings is mainly reflected in the evidence that supply chain members must carefully consider the impact of contract (un)observability when agreeing on contracts.

零售平台或中间商可以通过不同类型的合同从多个上游制造商处购买产品。传统研究假设合同及其条款是常识；也就是说，供应链的每一方都可以遵守所达成的合同。然而，在实践中，情况并非一定如此。本文以合同可观察性下的结果为基准，研究了合同不可观察时由一个零售平台和两个上游制造商组成的供应链的承包。考虑了两个有代表性的合同：传统的、简洁的批发价格合同和非线性的、更复杂的两部分关税合同。我们应用完美贝叶斯均衡的框架，并使用被动信念作为均衡细化标准来分析模型。我们的研究表明，当合同对竞争制造商无法观察时，经典结果——更复杂的合同可以提高渠道盈利能力——可能不再有效。相反，制造商和整个供应链可以从采用批发价格合同而不是两部分关税合同中受益。为了检验我们发现的稳健性，我们在扩展分析中考虑了佣金合同和另一个经典需求函数，结果表明主要发现和关键见解仍然有效。这些发现背后的直觉是，合同的不可观察性会影响竞争制造商的机会主义行为。