Source-linked AI summary
When Guardrails Look Effective: Construct Validity Failures in LLM Agent Commerce Evaluation
Peiying Zhu, Sidi Chang
TL;DR
LLM market simulations can produce economic-looking outcomes without instantiating the seller behavior or isolated intervention named in a claim. This paper audits those failures in a configurable hotel testbed using a construct-validity contract and forensic controls, finding scaffold-sensitive and unstable guardrail effects whose welfare interpretation depends on seller technology. The resulting evidence does not establish guardrail ineffectiveness; it identifies their apparent value as unidentified until incentive and protocol checks pass.
Problem
LLM market simulations may report prices, profits, surplus, and welfare without establishing that the intended economic constructs are instantiated or measured.
Method
The paper audits a multi-turn buyer–seller hotel testbed with scaffold controls, repeated generations, seller-incentive checks, scripted positive controls, and a four-part validity contract.
Results
The forensic case shows guardrail effects are scaffold-sensitive, generation-unstable, and dependent on seller technology, with the original interpretation Invalid under protocol isolation and the controlled study Inconclusive under unresolved incentive validity and stochastic stability.
Takeaways & Limitations
Policy claims from agentic market simulations should verify incentives, isolate protocol changes, nest replications correctly, and report complete welfare accounting before interpretation.
Takeaways & Limitations
The audit uses one instruction-tuned Qwen family, same-model self-play, synthetic hotel profiles, and scripted sellers that establish scorer responses rather than realistic seller behavior.
Abstract
from arXiv · showhide
Interactive simulations increasingly evaluate policies in markets populated by language-model agents. Their outputs can look economic---prices, profits, consumer surplus, and welfare---without instantiating the behavior named in the claim. We audit this risk in a multi-turn buyer--seller testbed for configurable hotel transactions. An initial implementation reported welfare gains from two marketplace guardrails of +87.4, +35.0, and +28.8 across a Qwen2.5 1.5B--14B ladder. It also gave guarded and unguarded agents different offer schemas and choice procedures. Holding the schema and buyer chooser fixed changes the paired contrasts to +7.2, -13.9, and +23.8. The four largest 14B single-generation effects averaged +229; after three generations per profile-condition, they averaged +37.6 (95% bootstrap interval [-34.2, 109.3]), while generation residuals account for 49.9% of variation in this post-hoc probe. A seller-incentive check is non-monotone: increasing profit pressure produces less profit than the default seller prompt. Scripted positive controls show why this matters. A profit-maximizing seller already attains first-best welfare, so guardrails mostly redistribute and reduce welfare; they create welfare only when the seller is explicitly programmed to force inefficient bundles. We contribute a construct-validity contract separating incentive validity, protocol isolation, stochastic stability, and welfare accounting, and returning INVALID or INCONCLUSIVE before substantive policy claims. In our case, the original estimate is INVALID under protocol isolation, while the controlled study remains INCONCLUSIVE under incentive validity and stochastic stability. The case does not show that guardrails are ineffective; it shows their apparent value is unidentified until the simulated agents and protocol pass these checks.
1 Introduction
This paper audits whether LLM marketplace simulations instantiate the economic constructs named in policy claims. A forensic study finds guardrail effects sensitive to scaffolding, stochastic generation, and seller incentives, motivating validity decisions before interpretation.
- Construct validity requires that an operationalization instantiate the role, incentive, information set, action space, and outcome mapping named in a claim.A fluent, internally consistent simulation can still fail to represent the strategic seller or welfare intervention it claims to evaluate.
- The original testbed changed policy and transaction-construction scaffolds together, including offer schemas and buyer choice rules.The controlled rerun used one schema and one chooser across guarded and unguarded cells.
- Single-generation treatment effects are unstable when replications are absent, and profile-level sign-flip testing found the selected effect compatible with label exchangeability (p = 0.50).Profiles, rather than generation rows, are the paired unit for inference.
- Seller roles require behavioral validation because stronger profit instructions need not produce the predicted profit-seeking response.Scripted positive controls further show that guardrail welfare effects depend on the assumed seller technology.
- The proposed contract permits substantive interpretation only after checks pass, classifying known construct violations as Invalid and insufficient precision or coverage as Inconclusive.A reusable nine-item checklist accompanies the contract.
- The study is bounded: it does not conclude that A2A guardrails fail, that Qwen is generally nonstrategic, or that hotel negotiation represents every market.Its conclusion concerns the evidentiary requirements for policy claims from LLM market simulations.
2 Related Work
Related work establishes LLM agents and interactive markets as evaluation settings while highlighting weaknesses in construct articulation, stochastic sampling, and aggregate metrics. This paper specializes that validity lens to multi-turn economic simulations.
- Prior work models LLMs as economic agents and studies interactive decision-making, bargaining, and multi-agent marketplaces.These environments support evaluation before deployment using assigned preferences, endowments, and market interactions.
- Equilibrium-referenced and pricing studies show that prompt parameters, model-provider identity, and seemingly innocuous wording can alter surplus division or collusive behavior.
- Construct-validity and measurement-modeling work asks whether operationalizations measure the intended theoretical objects, while reviews find these assumptions weakly articulated in LLM benchmarks.
- Prior evaluation research warns that prompt-equivalent evaluations vary, single-sample studies ignore stochastic generations, and costly agent evaluations often omit repeated runs and error bars.
- Trace-based work shows scalar task performance can remain stable while hidden-state discipline fails, motivating diagnostics beyond aggregate scores.The present setting applies this methodological connection to dialogue, treatment scaffolds, stochastic role enactment, and welfare accounting.
3 Setting and Evaluation Contract
The study uses configurable hotel transactions with hidden buyer preferences and ground-truth welfare accounting. Its validity contract separately tests incentives, protocol isolation, stochastic stability, and complete accounting before licensing policy conclusions.
- 3.1 Configurable transaction testbed: Each buyer has hidden component values, constraints, a willingness-to-pay cap, urgency, and an outside option; sellers offer mandatory bases and optional additions.Buyers accept the utility-maximizing feasible offer when it meets the assigned outside option.
- 3.1 Configurable transaction testbed: Welfare changes only when a transaction’s completion or composition changes, because price discrimination alone transfers surplus between buyer and seller.Welfare is reported as a fraction of each profile’s first-best welfare.
- 3.1 Configurable transaction testbed: The information rule blocks budget, WTP, urgency, and outside-option exchanges, while the conduct rule makes nonessential components declineable add-ons.Their factorial crossing yields None, Info, Conduct, and Both.
- 3.2 A validity contract before a policy claim: C1 requires measurable response to stated incentives, including predicted directional change under stronger profit incentives.Otherwise the seller role must be described behaviorally rather than strategically.
- 3.2 A validity contract before a policy claim: C2 requires shared schemas, decision rules, horizons, and parsing, with only intended information and conduct constraints varying.
- 3.2 A validity contract before a policy claim: C3 treats repeated generations as nested within profile-condition rather than independent buyers and requires effects to exceed generation noise.
- 3.2 A validity contract before a policy claim: C4 requires joint ground-truth reporting of completion, buyer surplus, seller profit, and welfare, without labeling transfers as welfare creation.Known C1 or C2 violations make causal interpretation Invalid; unresolved C1 or C3 evidence is Inconclusive.
- 3.2 A validity contract before a policy claim: Aggregate metrics cannot recover an unobserved construct when distinct traces share the same observed abstraction.The study therefore retains prompts, transcripts, parsed offers, profile types, and costs.
4 Audit Design
The audit combines controlled model evaluation, scaffold isolation, repeated-generation analysis, incentive checks, and scripted positive controls. Synthetic profiles and limited sampling constrain the scope and precision of the resulting evidence.
- 4.1 Models, profiles, and protocol: The evaluation uses 4-bit Qwen2.5-Instruct models at 1.5B, 3B, and 14B, with the same model instantiating buyer and seller across 30 held-out synthetic profiles.Conversations use two seller-question rounds, a structured offer, and temperature 0.2.
- 4.1 Models, profiles, and protocol: The 60-profile panel contains 43 negative outside-option utilities, which can make acceptance easier and motivates avoiding general market-level welfare claims.
- 4.2 Four audit experiments: The original implementation coupled treatment status to different offer schemas and buyer choice procedures; E1 reran all cells with one schema and chooser.After parsing, only the intended information and conduct booleans varied.
- 4.2 Four audit experiments: E2 repeated four post-hoc favorable 14B profiles three times per profile-condition, averaged replications before pairing, bootstrapped profiles, and decomposed variance.Because profiles were selected on original effects, E2 is exploratory rather than confirmatory.
- 4.2 Four audit experiments: E3 compares compliance-oriented, standard self-interested, and stronger profit-pressure seller prompts using profit, extraction questions, and matching behavior.The 18-dialogue smoke test checks monotonic movement of the intended construct but is not powered to rank prompts.
- 4.2 Four audit experiments: E4 crosses profit-maximizing and inefficient-bundling scripted sellers with all four guardrail cells to test whether metrics detect welfare improvement under explicit seller technologies.The inefficient seller forces up to two optional components whose values are below their costs into the base.
- 4.2 Four audit experiments: Bootstrap intervals resample 30 profiles, while the approximate 80%-power MDE is intended to expose low precision rather than certify prospective power.
5 Results
The audit finds that apparent welfare gains are highly sensitive to transaction scaffolding, stochastic replication, seller-incentive validity, and welfare accounting. The controlled evidence therefore supports measured audit findings but not a definitive claim that guardrails work or fail.
- Scaffold sensitivity: +7.2, −13.9, and +23.8 were the unified-schema-and-chooser contrasts for 1.5B, 3B, and 14B models.The effect shrank by 92% at 1.5B, reversed sign at 3B, and remained unresolved from zero at 14B.
- Stochastic stability: +229 was the mean of the four largest 14B single-generation effects, versus +37.6 after three generations per profile-condition.The repeated profile-bootstrap interval was [−34.2, 109.3], and generation residuals accounted for 49.9% of variation in the selected probe.
- Seller-incentive validity: 0.83 versus 0.17 rent-extraction questions accompanied 33.8 versus 57.5 seller profit under stronger versus standard profit pressure.The manipulation changed probing but did not establish a monotone, controllable profit-maximization construct.
- Positive controls: 169.1 mean welfare and 100% first-best attainment occurred for the profit-maximizing scripted seller, while both guardrails reduced welfare by 24.5.Under the inefficient-bundling stress seller, Both instead raised welfare by 18.7 and first-best attainment by 11.1 percentage points, showing opposite welfare signs under different seller technologies.
- Contract decision: The original headline is Invalid under protocol isolation, while the controlled estimand remains Inconclusive because incentive validity and generation stability are unresolved.Welfare accounting passes and distinguishes transfers from welfare creation, but neither “guardrails work” nor “guardrails fail” is licensed.
6 Implications for Agent Evaluation
Reliable agent evaluation requires validating role incentives, isolating transaction scaffolds, nesting stochastic replications correctly, and accounting for welfare creation separately from transfers. When these checks fail, evaluations should return Invalid or Inconclusive rather than force a policy verdict.
- Incentive validity: Seller roles should be validated through behavioral implications rather than inferred from prompt wording.Suggested checks include monotone responses to profit coefficients, revealed-preference consistency, or regret against a scripted optimum.
- Protocol isolation: A defensible policy ablation holds the offer schema and buyer chooser fixed before applying treatment flags.Scaffold changes should be reported as part of the intervention when they cannot be isolated.
- Stochastic stability: Repeated generations should be averaged within profile-condition, with paired effects and uncertainty estimated by resampling profiles.Treating generation rows as independent buyers narrows uncertainty without adding information.
- Abstention: Evaluation contracts should distinguish Invalid design violations from Inconclusive estimates whose precision or coverage is insufficient.Neither status should be translated into a substantive null.
- Implementation: The accompanying artifact provides a nine-item checklist, analysis script, generated tables, prompts, and run manifests.The artifact covers roles, schemas, trace retention, replication, accounting, and controls.
- Welfare accounting: Welfare reports should show completion, buyer surplus, seller profit, and welfare together because price changes can transfer surplus without creating it.A privacy or disclosure rule may improve buyer surplus while reducing seller profit one-for-one.
7 Limitations and Scope
The audit’s scope is limited by its model family, post-hoc replication sample, synthetic hotel environment, and scripted controls. These boundaries constrain generalization and prevent the reported probes from serving as broad estimates of seller behavior or population treatment effects.
- Model scope: All LLM results use one instruction-tuned model family, three sizes, 4-bit quantization, and self-play.Using the same model for buyer and seller may dampen adversarial behavior through shared priors or instruction-following style.
- Replication scope: The repeated-generation analysis covers four profiles selected after observing extreme effects, so it diagnoses instability but cannot estimate a population treatment effect.A powered confirmatory run would require pre-specified profiles and at least three generations per profile-condition.
- Environment scope: The synthetic hotel profiles contain 43 negative utilities among 60 profiles, and two question rounds may not afford the intended extraction dynamics.A null information effect could therefore be confounded by the opportunity structure.
- Behavioral realism: Scripted sellers are instruments rather than realistic behavioral models, demonstrating scorer responses under explicit technologies but not real-seller behavior.The contract is also a minimum gate, not a complete theory of external validity, platform equilibrium, or human welfare.
8 Conclusion
The case shows that precise-looking market outcomes can fail to identify the seller behavior or isolated intervention they appear to measure. Agentic evaluations should pass incentive, protocol, replication, and welfare checks before making policy claims, treating Invalid and Inconclusive as results.
- Conclusion: LLM market simulations can produce precise-looking prices and welfare without instantiating the claimed seller behavior or isolated policy intervention.The case study found scaffold sensitivity, generation instability, and non-monotone seller-incentive responses.
- Conclusion: Policy claims should verify incentives, isolate protocol changes, nest replications correctly, and report complete welfare accounting.When checks fail, Invalid and Inconclusive are evaluation outcomes rather than disclaimers.