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Incentivizing Quality Text Generation via Statistical Contracts
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Proposes cost-robust pay-for-performance contracts for LLM text generation under unknown inference costs, proving they correspond to minimax composite hypothesis tests, and evaluates them on code and MT-Bench benchmarks.
Problem
Pay-per-token pricing creates moral hazard: an LLM-serving agent can secretly use a cheaper model, and the principal does not know the agent's internal inference costs.
Approach
Define cost-robust contracts that incentivize the target action for any cost vector within a known spread bound, and show that optimal such contracts (min-budget and min-pay) are optimal minimax composite hypothesis tests scaled by a risk-dependent constant, generalizing a two-action result to arbitrary action counts and objectives.
Key results
- On MT-Bench, cost-robust monotone min-pay contract has expected payment 4.82 vs 4.19 cost-aware (+15%).
- On MT-Bench, cost-robust min-budget contract has budget 6.63 vs 6.16 cost-aware (+1.7%).
- Cost uncertainty raises optimal payment by about 14.7-14.9% on both MBPP and HumanEval code-generation contracts.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Add cost robustness to monotone min-pay contracts | Min-pay contract · MT-Bench | expected payment: 4.19 -> 4.82 (+15%) | Hurt |
| Add cost robustness to monotone min-budget contracts | Min-budget contract · MT-Bench | budget: 6.16 -> 6.63 (+1.7%) | Hurt |
| Add cost robustness to monotone min-variance contracts | Min-variance contract · MT-Bench | stdev(t): 1.71 -> 1.83 (+6.7%) | Hurt |
| Use cost-robust rather than cost-aware contracts | Binary-outcome code-generation contracts · MBPP | payment: +14.9% | Hurt |
| Use cost-robust rather than cost-aware contracts | Binary-outcome code-generation contracts · HumanEval | payment: +14.7% | Hurt |
| Use monotone contracts instead of threshold contracts | Cost-robust contracts · MT-Bench | budget | Helped |
Surprising findings
- Easier task (MBPP, higher pass rates) yields more expensive contracts, because the target model is harder to distinguish from alternatives when pass-rate gaps are small.
- Optimal monotone cost-robust contracts have an intuitive three-level structure: zero pay for lowest scores, base pay for intermediate, extra pay for highest.
Limitations
- Empirical outcome distributions come from benchmarks and do not reflect the richer real-world prompt distribution.
- Analysis assumes known cost-uncertainty bounds and a single target action; strictly intermediate targets need extra implementability assumptions.
- Cost estimates are order-of-magnitude, based on electricity energy consumption data.
Experimental setup
Datasets: MT-Bench MBPP HumanEval
Benchmarks: MT-Bench MBPP HumanEval
Metrics: expected payment (min-pay) budget (min-budget) stdev of payment (min-variance) pass@1
Baselines: cost-aware optimal contracts threshold contracts
Builds on: Saig et al. (NeurIPS 2023) two-action min-budget contract / hypothesis testing c Neyman-Pearson lemma Karlin-Rubin theorem LLM-as-a-judge (MT-Bench)
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