When “Agreed” Is Not Agreement: Semantic Option Value in LLM-Agent Negotiation

Language-model agents can negotiate terms whose wording admits multiple future executions. Existing benchmarks measure agreement or utility but rarely test whether agents select semantic breadth because it preserves a valuable choice after uncertainty resolves. We formalize semantic option value as the expected-utility gain from interpreting a term after ob- serving the state rather than committing ex ante to its best fixed interpretation. We then introduce SOV-Neg, a wording-matched factorial benchmark that crosses an SOV-generating proposer-utility regime with interpretation control and revelation timing. In preregistered self-play, broad-clause agreement increases most when the proposer benefits from flexibility, controls interpretation, and accepts before state revelation. Transcript decomposition locates the response in proposer offers rather than responder offers, and the pattern replicates across benchmark tiers and remains stable under domain-exclusion analyses. The response is nevertheless model-dependent and sub-normative: agents under-realize the available option, obtain no reliable proposer-utility gain, and reduce agreement, responder utility, and social welfare. A utility analysis further shows that the regime changes the full proposer payoff geometry, so the causal estimand is the joint regime-by-protocol interaction rather than a pure effect of scalar option value. Finally, a clarity instruction reduces residual risk and a singleton-only interface removes semantic flexibility. Our interpretation commit-and-reveal protocol pre-serves broad wording but finalizes only matching executable interpretations. The results make contingent semantic breadth measurable while delineating the intervention and protocol guarantees precisely.