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When Shippers Become Algorithms: Candidate Exposure, Information Design, and the Concentration of LLM-Mediated Freight Markets
Takahiro Ezaki, Naoto Imura, Katsuhiro Nishinari
Intelligence
Status: succeeded | Model: Gemma-4-26B-A4B | Prompt: intel-v1 | Confidence: 91%
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Summary
This paper investigates the impact of delegating freight carrier selection to Large Language Model (LLM) agents on market concentration. Using agent-based simulations with 50 shipper agents (powered by GPT, Claude, and Gemini) and 20 carriers, the study finds that LLM agents converge on a single dominant carrier on day one, creating 'algorithmic monoculture.' The study identifies that platform information design, specifically disclosing remaining daily capacity, is an effective lever to reduce concentration and increase shipper surplus, whereas model diversity and list randomization are ineffective.
Entities (10)
Relation Signals (6)
LLM Agents → causes → Algorithmic Monoculture
confidence 95% · Agents converged at once: for a fixed sampled carrier population, the same carrier was the modal first choice of every model on day one... This is algorithmic monoculture materialized in a market.
LLM Agents → delegatetaskto → Carrier Selection
confidence 93% · Shippers are beginning to delegate carrier selection to large language model (LLM) agents.
Capacity Disclosure → reduces → Market Concentration
confidence 92% · disclosing each carrier's remaining daily capacity cut concentration by a third
Capacity Disclosure → increases → Shipper Surplus
confidence 90% · disclosing each carrier's remaining daily capacity ... doubled shipper surplus
Waterfall Tendering → ismechanismof → Digital Freight Matching
confidence 88% · The market implements the rules of digital freight matching: each load is offered down the shipper's ranked list of carriers (waterfall tendering)
List Exposure Size → influences → Market Concentration
confidence 85% · concentration rose steeply once lists exceeded about ten carriers
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Abstract
Abstract:Shippers are beginning to delegate carrier selection to large language model (LLM) agents. We ask what such delegation does to a freight matching market, and which platform design choices contain it. We carried out agent-based simulations in which fifty shipper agents, built on commercial LLMs from OpenAI (GPT), Anthropic (Claude), and Google (Gemini), procure truckload capacity for thirty days. The market implements the rules of digital freight matching: each load is offered down the shipper's ranked list of carriers (waterfall tendering), carriers have daily capacity limits, spot prices respond to congestion, and carrier ratings accumulate with transactions. We found three risks and one remedy that works. Agents converged at once: for a fixed sampled carrier population, the same carrier was the modal first choice of every model on day one, attracting up to 76% of requests. Because each agent picks from its own randomly drawn list of displayed candidates, the platform controls how many options each shipper sees; concentration rose steeply once lists exceeded about ten carriers, with the onset differing across models. Which carriers ended up dominant varied widely from one sampled market to another, and displaying true quality instead of estimated ratings changed neither the level nor this variability (by design, quality affects only what agents see, never delivery outcomes). Against these risks, disclosing each carrier's remaining daily capacity cut concentration by a third and doubled shipper surplus, while vendor diversification, list-order randomization, and popularity display showed no clearly detectable effect. Platform information design, ahead of model choice or model regulation, is the lever that works.
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- Source: https://arxiv.org/abs/2607.19967v1
- Canonical: https://arxiv.org/abs/2607.19967v1
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Highlights When Shippers Become Algorithms: Candidate Exposure, Information Design, and the Concentration of LLM-Mediated Freight Markets Takahiro Ezaki, Naoto Imura, Katsuhiro Nishinari • First simulation of freight procurement by populations of real LLM agents with endogenous capacity, pricing, and ratings. • Choices concentrate on one carrier on day one; capacity limits and congestion pricing then spread demand and give unrated entrants work. • Concentration rises steeply once each shipper’s randomly drawn candidate list exceeds about ten carriers; the onset depends on which LLM the shippers use. • Final market structure varies widely across sampled markets under every trust signal tested; displaying true quality does not reduce this variability. • Disclosing carriers’ remaining capacity lowers concentration and raises shipper surplus; diversification and list shuffling do not. arXiv:2607.19967v1 [physics.soc-ph] 22 Jul 2026 When Shippers Become Algorithms: Candidate Exposure, Information Design, and the Concentration of LLM-Mediated Freight Markets Takahiro Ezaki a,∗ , Naoto Imura a and Katsuhiro Nishinari a,b a Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8904, Japan b Department of Aeronautics and Astronautics, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan A R T I C L E I N F O Keywords: Digital freight matching Large language models Algorithmic monoculture Platform information design Market concentration Agent-based simulation A B S T R A C T Shippers are beginning to delegate carrier selection to large language model (LLM) agents. We ask what such delegation does to a freight matching market, and which platform design choices contain it. We carried out agent-based simulations in which fifty shipper agents, built on commercial LLMs from OpenAI (GPT), Anthropic (Claude), and Google (Gemini), procure truckload capacity for thirty days. The market implements the rules of digital freight matching: each load is offered down the shipper’s ranked list of carriers (waterfall tendering), carriers have daily capacity limits, spot prices respond to congestion, and carrier ratings accumulate with transactions. We found three risks and one remedy that works. Agents converged at once: for a fixed sampled carrier population, the same carrier was the modal first choice of every model on day one, attracting up to 76% of requests. Because each agent picks from its own randomly drawn list of displayed candidates, the platform controls how many options each shipper sees; concentration rose steeply once lists exceeded about ten carriers, with the onset differing across models. Which carriers ended up dominant varied widely from one sampled market to another, and displaying true quality instead of estimated ratings changed neither the level nor this variability (by design, quality affects only what agents see, never delivery outcomes). Against these risks, disclosing each carrier’s remaining daily capacity cut concentration by a third and doubled shipper surplus, while vendor diversification, list-order randomization, and popularity display showed no clearly detectable effect. Platform information design, ahead of model choice or model regulation, is the lever that works. 1. Introduction Freight procurement is becoming a task that firms delegate to machines. Large language models (LLMs) already sit inside supply chain workflows, where they analyze contracts, screen suppliers, and forecast demand (Wang, Jiang, Hong and Jiang, 2025; Song, Xie, Yang and Zhao, 2026; Nie, He, Mei, Qin, Li, Sun and Ma, 2025), and prototype multi-agent systems automate vendor scoring and selection end to end (Joshi, Singh, Parashar and Singh, 2026); the first commercial freight-procurement agents that automate carrier sourcing were announced in 2026 (project44, 2026). Digital freight matching (DFM) platforms such as Uber Freight and Full Truck Alliance have used algorithms to price loads and recommend carriers for a decade. The step now underway is different in kind. Shippers are handing the choice of carrier itself to autonomous agents that read a list of candidates and pick one. When thousands of shippers choose through a handful of foundation models, the matching process inherits whatever preferences those models share. They share a great deal. On the first simulated day of the experiments reported below, with the carrier population held fixed, GPT agents sent 34 to 38 of 50 requests to the same carrier out of twenty, and they did so under each of the three information conditions we study (a list showing quoted prices together with true reliability scores, with frozen ratings, or with live customer ratings); Claude and Gemini agents made that carrier their most frequent first choice as well. Which carrier plays this role depends on the sampled population, not on the model (Section 5.1). This is algorithmic monoculture (Kleinberg and Raghavan, 2021; Peng and Garg, 2024) materialized in a market. Independently deployed systems have been shown to hand the same individuals the same outcomes (Bommasani, Creel, Kumar, Jurafsky and Liang, 2022), and LLM populations converge on collective conventions (Ashery, Aiello ∗ Corresponding author. tkezaki@g.ecc.u-tokyo.ac.jp (T. Ezaki) ORCID(s): 0000-0003-4175-3028 (T. Ezaki); 0000-0002-1428-7407 (N. Imura); 0000-0003-3069-1757 (K. Nishinari) T. Ezaki et al.: Preprint submitted to ElsevierPage 1 of 17 When Shippers Become Algorithms and Baronchelli, 2025). In a matching market with capacity constraints, this creates an immediate operational problem: a carrier chosen by everyone cannot serve everyone’s demand, so most of that demand has to go somewhere else. Existing monoculture results are static. They study one-shot allocation and abstract from the feedback that markets supply. Carriers raise prices where demand piles up; capacity limits push displaced shippers onto second choices; ratings accumulate for whoever gets chosen and stay absent for whoever does not. Research on human markets describes how such feedback can go either way. Herding models show that observational and reputational signals can lock markets into inefficient, path-dependent outcomes (Banerjee, 1992; Bikhchandani, Hirshleifer and Welch, 1992; Smith and Sørensen, 2000), and the MusicLab experiment showed that social signals raise both inequality and unpredictability of success (Salganik, Dodds and Watts, 2006), with rating-system field experiments finding related herding effects (Muchnik, Aral and Taylor, 2013). Studies of platform reputation show that ratings inform but also inflate and lag (Tadelis, 2016; Filippas, Horton and Golden, 2022), and that entrants without a track record face cold-start exclusion (Pallais, 2014). No study yet follows LLM choosers, whose biases are documented and correlated, through a market whose prices, capacities, and ratings respond to their choices. Work on LLM agents in markets has shown that pricing agents collude without being instructed to (Fish, Gonczarowski and Shorrer, 2024) and has established LLMs as usable simulated economic agents (Filippas, Horton and Manning, 2024). The matching side, where monoculture should matter most, remains open. Freight is the right industry for this question, for two reasons. First, truckload procurement already has well-defined rules that make the problem concrete and measurable. Shippers keep ranked carrier lists (routing guides) and offer each load down the list (waterfall tendering); carriers reject loads when they run out of capacity; and spot prices carry a persistent premium over contract rates (Acocella and Caplice, 2023; Acocella, Caplice and Sheffi, 2020). Second, the tension our agents exhibit, loyalty to incumbent carriers versus opportunistic spot search, is the central empirical fact of this industry for human shippers (Harris and Nguyen, 2025b,a). Outcomes in this setting carry managerial meaning: service rates, freight rates, shipper surplus, and carrier revenue concentration, rather than abstract network statistics. Recent work quantifies the information value of freight platforms (Wu, Huang and Chen, 2024). We extend that agenda to the case where the platform’s users are themselves algorithms. We build a simulation platform in which real LLM agents procure freight inside a market with endogenous economics. Fifty shipper agents, powered by commercial models from OpenAI, Anthropic, and Google, choose among twenty carriers every day for thirty days. The market implements waterfall tendering over ranked preferences (each load goes to the agent’s first choice if that carrier has capacity left, otherwise to the second, then the third), a fixed daily capacity limit for each carrier, spot prices that adjust daily to excess-demand pressure, and a rating system in which a carrier’s displayed score aggregates the reviews of its completed shipments. A carrier that no one chooses accumulates no reviews. We manipulate three levers, each under the platform’s control: the exposure 퐿, the number of candidate carriers an agent can choose from (퐿 ∈ 3, 5, 10, 15, 20); the information structure of the trust signal (the true reliability disclosed directly, a rating frozen at its initial value so that it never updates, or the live customer rating); and disclosure interventions, including real-time capacity visibility, list-order randomization, and popularity display, plus a vendor-mix condition that splits shippers across the three LLMs. We report 226 experimental cells, five to ten independent runs per condition and about 190,000 individual carrier choices, and we log every prompt and response for audit. We found four things. First, the concentration onto a single favorite forms on day one under every information structure; the rules of the market then determine how much of it survives. Two mechanisms work against it. The crowded carrier’s price spikes, and price-sensitive agents respond by trying cheap unrated entrants, whose ratings converge to their true quality within thirty days. The cold-start trap that reputation theory predicts (entrants without ratings never get chosen, so they never earn ratings; Pallais, 2014) did not arise, because high prices at the crowded carrier keep pushing trials toward newcomers. Second, concentration rises sharply with exposure for two of the three vendors, with most of the rise between 퐿 = 10 and 퐿 = 15, while the third (Anthropic’s Claude) stays flat at every exposure. A deterministic scoring baseline, by contrast, concentrates from small 퐿 onward; a decomposition attributes the shape of the rise largely to candidate-set overlap and the vendor contrast to the models themselves. Third, the surviving market structure varies widely across independently sampled markets, and most of that variation traces to the drawn market rather than to the trust signal: replacing estimated ratings with disclosed true reliability changed neither the level nor the spread of final concentration. Fourth, of the four interventions, only capacity disclosure had a clearly detectable effect. Displaying remaining daily capacity next to each candidate cut concentration by a third and doubled shipper surplus; vendor diversification, list shuffling, and popularity display showed no clearly detectable T. Ezaki et al.: Preprint submitted to ElsevierPage 2 of 17 When Shippers Become Algorithms effect. Congestion management, which Arnosti, Johari and Kanoria (2021) implement by limiting applications and we implement through disclosure, is the effective lever in AI-mediated matching as well. The paper makes three contributions. We provide, to our knowledge, the first freight-specific market-level testbed that embeds populations of real LLM agents in a matching market combining waterfall tendering, binding capacity, congestion pricing, and endogenous ratings, with a decision-level audit trail; the platform extends to other two-sided logistics questions. We recast algorithmic monoculture as a contest between two feedback loops, rating accumulation that entrenches the early favorite and congestion pricing that works against it, which connects the monoculture literature (Kleinberg and Raghavan, 2021; Peng and Garg, 2024) to herding and market design (Banerjee, 1992; Roth, 2008) and explains why static analyses overstate lock-in. For practice, we give DFM platforms design guidance for the arrival of AI shippers: keep the exposure knob below the transition region, disclose capacity, and expect little from model diversity or list randomization. The remainder of the paper proceeds as follows. Section 2 positions our study in the literature. Section 3 describes the market institutions and the LLM agent architecture. Section 4 details the experimental design. Section 5 reports results, and Section 6 draws managerial and regulatory implications, discusses limitations, and outlines extensions. 2. Related work 2.1. Algorithmic monoculture Kleinberg and Raghavan (2021) show that when many decision makers adopt the same algorithm, welfare can fall even if that algorithm beats every alternative each firm holds, because errors correlate across adopters. Peng and Garg (2024) carry the argument into matching markets, where correlated evaluations distort both match quality and its distribution across participants. Bommasani et al. (2022) document the empirical signature, outcome homogenization: the same individuals receive the same decision from independently deployed models. Anwar, Schoenebeck and Dhillon (2024) find homogenization of consumption in long-run recommender simulations, and Ashery et al. (2025) show that populations of interacting LLMs converge on shared conventions without any central coordination. Ballestero, Hosseini, Khanna and Shorrer (2026) add experimental evidence on the strategic side: in coordination games, LLM subjects sustain their baseline similarity even when payoffs reward divergence, where human subjects differentiate. These studies hold the environment fixed while the algorithms decide. We let the environment answer back. Prices, capacities, and ratings in our market respond to what the agents do, and the imprint of monoculture then splits in two: the day-one consensus is as extreme as the static literature would predict, while its persistence is a property of the institutions, not of the model population. 2.2. Herding, social influence, and reputation Sequential-choice models show how public signals crowd out private information and lock markets onto arbitrary outcomes (Banerjee, 1992; Bikhchandani et al., 1992; Smith and Sørensen, 2000). The MusicLab experiment gave the mechanism its sharpest empirical form: social signals raised the inequality and, above all, the unpredictability of success (Salganik et al., 2006), and follow-up field experiments found the ratings version of the same asymmetry (Muchnik et al., 2013). The platform-reputation literature documents both the value and the pathologies of feedback systems: reputation shifts sales and survival (Cabral and Hortaçsu, 2010), ratings inflate and censor (Nosko and Tadelis, 2015; Filippas et al., 2022; Tadelis, 2016), and workers without a track record face measurable cold-start exclusion (Pallais, 2014). Our agents differ from the decision makers in herding models in a way that matters: LLM shippers share a prior rather than observe one another, so the cascade is complete at 푡 = 0. What our experiments add is the market’s reply. The unpredictability of outcomes that Salganik et al. (2006) induced with social signals appears in our market as a baseline property of the feedback institutions, present even when true quality is displayed, and the cold-start trap does not close on entrants because tender rejection, not subsidy, supplies them with trials. 2.3. Congestion and information design in matching platforms Roth (2008) lists congestion among the three failures a marketplace must manage. Arnosti et al. (2021) show that application congestion destroys welfare in matching platforms and that limiting or redirecting applications restores it; Guo, Haviv, Luo and Wang (2023) analyze queue disclosure as a signaling device; Kamenica and Gentzkow (2011) give the general framework in which a designer chooses what information to release. In freight, Wu et al. (2024) quantify the information value of a logistics platform, and Liu and Wahab (2025) design binding multi-period contracts to hedge the same friction our model generates endogenously, tender rejection under tight capacity. Our capacity disclosure T. Ezaki et al.: Preprint submitted to ElsevierPage 3 of 17 When Shippers Become Algorithms intervention belongs to this family: it is congestion management executed inside the prompt, and it addresses with information the same friction that Liu and Wahab (2025) address with contracts. 2.4. LLM agents in markets Filippas et al. (2024) establish LLMs as simulated economic agents; Park, O’Brien, Cai, Morris, Liang and Bernstein (2023) demonstrate believable multi-agent populations. On the market side, Fish et al. (2024) show that LLM pricing agents reach supracompetitive prices without being instructed to collude, and a fast-growing set of simulation studies examines governance, information asymmetries, and exploitation in LLM agent markets (Syrnikov, Pierucci, Galisai, Prandi, Bisconti, Giarrusso, Sorokoletova, Suriani and Nardi, 2026; Lei, Nguyen, Mehta, Li, Fu, Zheng, Chen, Liang, Torr and Yin, 2026; Erlei and Meub, 2026). Application work has moved procurement itself onto LLM rails, from supplier evaluation frameworks (Joshi et al., 2026) and domain-specialized supply chain models (Wang et al., 2025) to reviews of LLM adoption in supply chain management (Song et al., 2026), LLM-based demand prediction (Nie et al., 2025), and automated design of routing optimizers (Wang, Zhang, Guo and Wu, 2026). Two testbeds come closest to our aim. Liu, Shang and Jin (2026) build a programmable marketplace in which tool-using agents post jobs, negotiate, and accumulate reputation, and find that well-meant institutional interventions can degrade rather than improve market performance; Long, Simchi-Levi, Zhu, Su, Calmon and Calmon (2026) replace every player of a serial supply chain with generative agents and identify run-to-run decision instability, which they call agent bullwhip, as the central reliability risk. Neither market clears under binding capacity with congestion pricing and endogenous ratings, and neither asks the concentration question. The matching side, where correlated preferences meet capacity constraints, has lacked a testbed with endogenous economics. The simulation platform of this study provides one. 3. Market model and agent architecture Figure 1a summarizes the simulated market. Fifty shipper agents, each controlled either by a commercial LLM or by one of four reference algorithms described below, procure truckload service from twenty carriers over thirty days. Each day every shipper issues one load with a random origin–destination pair, a dimensionless distance index drawn uniformly from 10 to 100, and a weight drawn from 5, 10, 15, 20, 30 tons; the payment for a served load equals the carrier’s unit price times distance times tons, so all monetary quantities are expressed per ton-distance. The institutions follow truckload practice (Acocella and Caplice, 2023): the shipper agent returns a ranked list of three carriers, the platform tenders the load down that list, and the first carrier with remaining daily capacity serves it. If all three ranked carriers are full, the load goes unserved and the shipper incurs a delay cost of 0.3, in the same per-ton-distance units as prices. Within a day, loads are processed in a random order drawn each day, which also arbitrates who obtains scarce capacity. This waterfall corresponds to the routing-guide mechanics that shippers use in practice (Acocella et al., 2020). We cap the waterfall at three tenders because each additional tender costs the shipper coordination time and delay in practice, and routing guides rarely go deeper before a load is re-brokered; letting agents rank the full displayed list is a straightforward extension we leave for future work. Carriers. Each carrier 푗 has four static attributes drawn independently and uniformly from [0.3, 0.9]: an initial unit price, a true reliability 휌 푗 , a capacity weight, and a route specialty (Fig. 1b). Daily tonnage capacity is allocated in proportion to the capacity weight and calibrated so that aggregate capacity equals 1.3 times expected daily demand; individual carriers handle 27 to 70 tons per day. One carrier per sampled population has less daily capacity than the largest 30-ton load; it simply rejects such loads in the waterfall. Unit cost equals 0.6 times the initial price, which fixes each carrier’s margin. Prices. Carriers reprice daily in response to excess-demand pressure: 푝 푗,푡+1 = min ( max(̃푝 푗,푡 , 1.05푐 푗 ), 1.5 ) ,(1) ̃푝 푗,푡 = 푝 푗,푡 [ 1 + 훼 (휋 푗,푡 − 1) ], 휋 푗,푡 = 푞 srv 푗,푡 + 푞 rej 푗,푡 푄 푗 , where 푝 푗,푡 is carrier 푗’s unit price on day 푡, 푐 푗 its unit cost, 푄 푗 its daily tonnage capacity, 푞 srv 푗,푡 and 푞 rej 푗,푡 the tons it served and rejected that day, and 훼 = 0.10. In words: price moves by 10% of the gap between demand pressure 휋 푗,푡 and one, T. Ezaki et al.: Preprint submitted to ElsevierPage 4 of 17 When Shippers Become Algorithms LLM shipper agents N =50, three vendors Candidate list L carriers shown Price · Rating · Size · Specialty Ranked choices (top 3 preferences) Waterfall tendering Capacity consumed / tender rejected Spot price p j ∝ Excess-demand pressure Rating ( ̄r j , n j ) = Share of positive reviews reads daily update served loads → new reviews Rating loop (+) chosen → rated → chosen entrenches the consensus Price loop (−) crowded → expensive → avoided disperses demand, repairs information Platform levers: Exposure L · Trust signal (Truth / Static / Endogenous) · Capacity disclosure · List order next day t+1 (30 days) a 0.40.60.8 Initial unit price 0.4 0.5 0.6 0.7 0.8 0.9 True reliability (hidden) b Carrier 18 (cheap, reliable) Carrier population (bubble = capacity) 0 1 10 100 Initial track record n 0 (log) 0102030 Day 0.0 0.2 0.4 0.6 0.8 1.0 Concentration κ c Reference decision rules (L= 20) Deterministic scoring Random choice d Market 50 shippers × 20 carriers × 30 days Agents GPT / Claude / Gemini Lever 1 exposure L∈3, 5, 10, 15, 20 Lever 2 trust signal: truth / static / endogenous Lever 3 capacity disclosure, list order, popularity display, vendor mix Audit every prompt, response, and market state logged Experimental design Figure 1: A freight matching market with two feedback loops. (a) Daily loop. Shipper agents read a candidate list, return ranked choices, and the platform tenders each load down the list under capacity constraints. Ratings accumulate for chosen carriers (red loop) while congestion pricing penalizes crowded carriers (blue loop). The dashed box marks the levers we manipulate. (b) One sampled carrier population (seed 0). Bubble size gives daily tonnage capacity; color gives the initial number of rated shipments 푛 0 (log scale); dashed black circles mark carriers that start with no ratings. Carrier 18 combines a low price with high reliability. (c) Reference decision rules at퐿 = 20 (mean and seed range): the deterministic common-score rule concentrates strongly, random choice gives the floor. (d) Design summary; Table 1 gives the full condition-by-run accounting. bounded below by 1.05 times unit cost and above by an index of 1.5. A carrier that fills its book exactly holds its price; a carrier that turns demand away raises it. Because a rejected load continues down the routing guide, its tonnage can enter the pressure of more than one carrier on the same day; this is intended, since each rejection is a demand signal that carrier actually turned away, rather than double counting. Ratings. The true reliability 휌 푗 stays hidden. After each completed shipment the shipper leaves a review: a positive review with probability 휌 푗 , a neutral one otherwise. Real reviews are richer than this binary rule; we adopt it as a deliberate simplification that makes the displayed rating an unbiased estimate of 휌 푗 , so that any market pathology we find cannot be blamed on biased reviews. The platform displays the running mean ̄푟 푗 of these reviews together with the count 푛 푗 of reviewed shipments. Initial track records are set as 푛 0 = max(0, round(200푓 푗 ) − 10), where the fitness 푓 푗 is a Pareto draw (shape 1.5) rescaled to at most 0.95, so the distribution of 푛 0 is heavy-tailed: in the population of Fig. 1b, a few incumbents start with up to 180 reviewed shipments and four carriers start with none, displaying “no T. Ezaki et al.: Preprint submitted to ElsevierPage 5 of 17 When Shippers Become Algorithms customer ratings yet”. A carrier’s initial displayed rating is the mean of 푛 0 simulated reviews drawn with the same Bernoulli rule. Ratings update only when a carrier serves a load, so selection and information accumulate together. Reviews are distinct from the failed flag that appears in an agent’s transaction history, which marks loads that none of the three ranked carriers could serve. One design choice deserves emphasis: reliability affects outcomes only through the reviews it generates. Whether a load is served depends on capacity alone, and payments do not depend on 휌 푗 , so the trust signal is a pure coordination device rather than a predictor of delivery failures. The information ablation of Section 5.3 therefore measures the signal’s coordination role; a channel from휌 푗 to physical service failure is a deliberate omission we return to in the limitations. Shipper agents. Each shipper has a fixed procurement priority (cost, reliability, capacity, specialty, or balanced; ten shippers each) and is controlled by a decision rule. In the LLM conditions, the rule is a commercial model that reads a prompt and answers in JSON with three ranked carrier identifiers. The prompt shows the day’s load, a summary of the shipper’s last five shipments (unserved loads flagged as failed), and one line per candidate carrier: the quoted total price with its unit-price breakdown, the trust signal (which we manipulate below), the daily tonnage capacity, and the specialty score. Appendix A reproduces a complete prompt and a raw response. We use gpt-5.4-mini (OpenAI), claude- haiku-4-5 (Anthropic), and gemini-3.5-flash (Google) at temperature 0.7. GPT is the default vendor; Claude and Gemini replicate the exposure sweep, and one condition assigns the three vendors to shippers in near-equal shares (17/17/16 of the fifty). When fewer than twenty candidates are displayed (퐿 < 20), the 퐿 candidates are drawn uniformly at random from the active carriers for each load; candidate order is randomized per prompt unless stated otherwise. In the reference conditions, the rule is one of four simple algorithms: random, a uniform draw from the displayed list; preferential attachment (Barabási and Albert, 1999), which picks carriers with probability proportional to their discounted accumulated matches (the same 0.95-per-day discounted record used for 휅); conflicting attachment, a rule of our construction named after Leung and Weitz (2016), in which accumulated matches first attract and then repel; and a common-score rule that ranks all candidates by one fixed weighting of the displayed attributes (all parameters in Appendix B). The common-score rule is deterministic and identical across shippers, which makes it the closest analogue of a monoculture of literally identical classifiers; it serves as the main-text contrast together with the random floor (Fig. 1c). The two attachment rules stay near the random floor throughout and are reported in Appendix B. Outcome measures. Concentration 휅 푡 is computed on a running record of first choices that discounts the past: each first choice adds one unit to the chosen carrier’s total, and at the end of every day all totals are multiplied by 0.95 (relationship weights below 0.05 are dropped), giving the record an effective memory of about twenty days. 휅 푡 is the share of this discounted record held by the three largest carriers; on day one it coincides with the day’s first-choice share, and “final 휅” averages days 26–30. A realized version of the same index, computed on the loads carriers end up serving rather than on first choices, tracks the allocation after tender rejections. We also use the Herfindahl–Hirschman index (HHI) of daily first choices, the sum of squared choice shares across carriers, to measure how spread out each day’s demand is. Welfare and service outcomes are the service rate, the mean paid unit price, shipper surplus, and carrier revenue concentration. A served load is worth 1.2 per ton-distance to its shipper, so shipper surplus is 1.2 minus the paid unit price, averaged over loads, with unserved loads entering at the delay cost of −0.3. Appendix C shows that the results are unchanged under these alternative concentration indices. 4. Experimental design We manipulate three platform levers (Fig. 1d). The exposure sweep varies the number of displayed candidates 퐿 ∈ 3, 5, 10, 15, 20 for each vendor. The information ablation replaces the endogenous customer rating (the live score that updates with each review) with either the disclosed true reliability (truth) or a rating frozen at its initial value so that it never updates (static), holding everything else fixed; the comparison isolates what the rating dynamics contribute beyond the informativeness of the signal. The intervention comparison starts from the default condition (GPT, 퐿 = 20, endogenous ratings) and adds one intervention at a time: real-time capacity disclosure (remaining tons shown next to each candidate, updated within the day), fixed list order, popularity display (a discounted count of recent transactions shown as a popularity score), and the vendor mix that assigns GPT, Claude, and Gemini to shippers in near-equal shares. Every condition receives five independent runs that redraw the carrier population, the load stream, and the review outcomes, except the three trust-signal conditions, which receive ten runs each: their result is a statement about T. Ezaki et al.: Preprint submitted to ElsevierPage 6 of 17 When Shippers Become Algorithms FamilyConditionsRuns Cells Exposure sweep3 vendors × 5 exposures 575 Reference rules4 rules × 5 exposures5 100 Information ablationtruth, static (퐿=20)10 20 (endogenous)GPT 퐿=20 cells + 5 runs 105 Interventionscapacity, order, popularity 515 Vendor mixGPT/Claude/Gemini55 Price counterfactual훼 ∈ 0.05, 0.0236 Total226 Table 1 Experimental design. Runs counts independent repetitions per condition; each run redraws the carrier population, the load stream, and the review outcomes. The endogenous information condition reuses the five GPT 퐿 = 20 sweep cells and adds five more runs, so only the additional cells are counted in its row. dispersion across runs, and variance estimates from five runs would be too coarse to compare. For a given run index, the three information conditions share the carrier population, the load stream, and the candidate draws, so comparisons across them are paired; all other condition families redraw everything, and variation across their runs therefore mixes differences in the drawn market with stochastic history (we return to this in Section 5.3). Throughout, final 휅 per run is the unit of analysis: variance comparisons use Brown–Forsythe tests, location comparisons use exact two-sided Mann–Whitney tests (no ties occurred), paired Wilcoxon tests are used where conditions share seeds, and we report Holm–Bonferroni-corrected 푝-values alongside the raw ones within each family of comparisons. We report 226 cells (Table 1) and about 190,000 individual LLM decisions. The simulator logs every prompt, raw response, and market state transition; the full audit logs are released with the code and data. 5. Results 5.1. Choices concentrate on day one; the market spreads them out Figure 2a shows how first choices were distributed on the first day of the default condition. The single most- requested carrier drew 14 to 37 of 50 first choices depending on the run, and the three most-requested drew 62 to 96%; day-one 휅 averaged 0.79, far above anything the market later sustained. Which carrier plays the favorite depends on the drawn carrier population, not on the decision maker: each run redraws the population and elevated a different favorite, but for a fixed population every vendor and information condition produced the same modal first choice. Under the population of Fig. 1b, carrier 18, the cheapest of the highly reliable incumbents, topped the day-one tally in the default condition, under both other vendors, and under both alternative information structures, with 19 to 38 of 50 first choices. This initial concentration then declined steadily over the first week and settled at 휅 = 0.42 (range 0.35–0.45 across runs), roughly 2.4 times the random-choice floor of 0.18 (Fig. 2b). Figure 2c shows what happened to the day-one favorite itself: its excess demand drove its unit price to the cap within 13 days, and its first-choice share fell from 0.74 to about 0.12. Displaced and price-sensitive agents tried the cheap carriers that started with no or almost no reviews, and those carriers’ displayed scores converged to their true reliability within the month. For example, the strongest of these entrants (true reliability 0.90, displayed as 0.25 from four early reviews) served 81 loads and ended at a displayed rating of 0.84 (Fig. 2d). The cold-start exclusion that field evidence documents for human markets (Pallais, 2014) did not appear here, because congestion kept routing trials toward newcomers. Shippers paid for this partial self-correction: the mean paid price rose from 0.53 to 0.96 and shipper surplus halved (Fig. 2e). A counterfactual that slows the price response separates the two market forces behind the correction (Fig. 2f). At adjustment rates of 0.05 and 0.02 instead of 0.10, the favorite’s decline slowed (mean day-2 share 0.30 and 0.41 versus 0.23) but arrived at the same endpoint: by day 30 the day-one favorite held a share of 0.15–0.17 at every rate, and the initially unrated carriers still served 160 loads under the stickiest prices (196 at the base rate). Tender rejection under the capacity constraint did the dispersing and supplied the trials; congestion pricing set the pace. The cold-start trap stayed open even when prices barely moved. 5.2. Concentration rises sharply with exposure, and the rise depends on the vendor Figure 3a traces final concentration against the exposure 퐿. For GPT the curve stayed flat at 휅 = 0.26–0.31 up to 퐿 = 10 and then climbed steeply, reaching 0.40 at 퐿 = 15 and 0.43 at 퐿 = 20. Every run increased from 퐿 = 10 to T. Ezaki et al.: Preprint submitted to ElsevierPage 7 of 17 When Shippers Become Algorithms Carriers (rank-sorted per run) 0 10 20 30 Day-1 first choices a Top-1 votes across models & information structures: 19–38 / 50 (dots: individual runs) Day 1: choices pile onto a few carriers 0102030 Day 0.0 0.2 0.4 0.6 0.8 1.0 Concentration κ b Consensus erodes, then persists GPT, L=20 Random reference 0102030 Day 0.0 0.2 0.4 0.6 0.8 First-choice share c The day-1 favorite prices itself out First-choice share Unit price 0.6 0.8 1.0 1.2 1.4 Unit price (dashed) 0102030 Day 0.2 0.4 0.6 0.8 1.0 Displayed rating d Unrated entrants earn accurate ratings True reliability 0102030 Day 0.0 0.2 0.4 0.6 0.8 1.0 Index e Shippers pay for the correction Mean paid price Shipper surplus 0102030 Day 0.1 0.2 0.3 0.4 0.5 Day-1 favorite's share f Prices set the pace, capacity the endpoint Price adj. rate 0.10 (base) 0.05 0.02 (sticky) Figure 2: Choices concentrate on day one; capacity and prices spread them out (GPT,퐿 = 20, endogenous ratings). (a) Day-one first choices, carriers sorted by rank within each run: bars give the mean across five runs, dots individual runs. The top-ranked carrier draws 14–37 of 50 first choices per run. For a fixed carrier population, the same carrier tops every vendor and information condition (19–38 of 50). (b) Concentration 휅 over 30 days: thin lines are runs, the thick line the mean, gray the random-choice band. (c) The day-one favorite’s first-choice share (solid) and unit price (dashed) in the run of panel a: the price reaches the cap by day 13 and the share collapses. (d) Displayed ratings of the five initially unrated or underrated carriers converge to their true reliabilities (dotted), same run. (e) Mean paid price and shipper surplus (mean ± range across runs). (f) Share of the day-one favorite under three price-adjustment rates (means over three runs): stickier prices slow the decline without changing its endpoint. 퐿 = 15 (Fig. 3d). Gemini showed the same shape with a higher ceiling (0.51 at 퐿 = 20), and Claude never left the 0.25–0.31 band at any exposure. The deterministic common-score rule, in contrast, concentrated from small 퐿 onward (the two attachment rules stayed near the random floor; see Appendix B). Part of this rise is built into the display mechanics. The 퐿 displayed candidates are a fresh uniform draw per load, so raising 퐿 does two things at once: it makes different shippers’ lists overlap more, and it raises the probability that any given carrier, including the favorite, is on someone’s screen (퐿∕20). At small 퐿 the agents could not all pick the same carrier even if they wanted to. To separate these mechanics from agent behavior, we ask two questions of the data. First, do agents seize the favorite more eagerly when lists are long? No: conditional on the run’s day-one favorite actually being displayed, the probability that an agent picks it falls from 0.57 at 퐿 = 3 to 0.14 at 퐿 = 20. The rise in 휅 therefore comes from the favorite appearing on more screens, not from stronger attachment per encounter. Second, how much of the rise appears with no change in agent behavior and no market feedback at all? We freeze each run’s day-one choice frequencies and replay them, redrawing only the displayed subsets: this static benchmark yields daily top-three shares of 0.44∕0.57∕0.73∕0.78∕0.80 at 퐿 = 3∕5∕10∕15∕20. Candidate-set overlap alone thus reproduces a rise of the same shape, concentrated below 퐿 = 15, though at a level far above what the interacting market sustains. Together the two checks assign the roles: the display mechanics produce the shape of the rise, the market’s capacity and price feedback compress its level, and the LLM-specific content is the vendor contrast, since Claude faces identical display mechanics at every 퐿 and does not rise. Whether model-level mechanisms such as long-context retrieval degradation (Liu, Lin, Hewitt, Paranjape, Bevilacqua, Petroni and Liang, 2024) contribute on top of the mechanics would require designs that vary list length while holding display probability fixed, which we leave to future work. T. Ezaki et al.: Preprint submitted to ElsevierPage 8 of 17 When Shippers Become Algorithms Wider exposure cost shippers money. Surplus fell with 퐿 for all three vendors while the service rate stayed near 0.9 (Fig. 3b,c); the deterministic rule shows what a market breakdown would look like instead, with service collapsing to 0.23 at퐿 = 20. The daily trajectories (Fig. 3e) show where the transition operates: every exposure started concentrated, but only at 퐿≥ 15 did the day-one concentration survive the month. 35101520 Exposure L (log scale) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Final κ a Concentration vs. exposure 35101520 Exposure L (log scale) −0.2 0.0 0.2 0.4 Shipper surplus b Welfare mirrors concentration 35101520 Exposure L (log scale) 0.2 0.4 0.6 0.8 1.0 Service rate c LLMs keep the market serving L= 10 L= 15 0.30 0.35 0.40 Final κ d The jump, seed by seed (GPT) 0102030 Day 0.3 0.4 0.5 0.6 0.7 0.8 Concentration κ e Daily paths by exposure (GPT) L= 3 L= 5 L= 10 L= 15 L= 20 GPTClaudeGeminiDeterministicRandom Figure 3: Concentration responds to exposure with a vendor-dependent transition. (a) Final 휅 against exposure 퐿 (mean ± range across runs; bands are drawn where three or more runs are available; the shaded vertical band marks the transition zone). (b) Shipper surplus and (c) service rate against 퐿. (d) Final 휅 at 퐿 = 10 versus 퐿 = 15 for GPT; each dot is one run, lines connect runs. (e) Daily concentration for GPT by exposure (means across runs). 5.3. Information quality had no detectable effect on the level or the spread The second lever is the trust signal itself. The default is the endogenous rating, the live customer score that updates as reviews arrive. Holding the market fixed, we replaced it with either the true reliability (truth) or a rating frozen at its initial value (static), and asked whether the quality of the displayed information changes what the market converges to. The answer is that no effect is detectable. Mean final concentration barely moved across the three structures (0.46 under truth, 0.43 under static, 0.46 under endogenous; Fig. 4a; paired Wilcoxon 푝≥ 0.19, unpaired Mann–Whitney 푝≥ 0.21, for every pairwise comparison). Neither did the dispersion: the standard deviation of final 휅 was 0.10 under truth, 0.10 under static, and 0.08 under endogenous (Brown–Forsythe 푝≥ 0.75 for every pairwise comparison; no pairwise difference in means exceeds 0.04, and every bootstrap 95% confidence interval brackets zero, the widest being [−0.12, 0.05]). Under every structure, individual runs ranged from dispersed (휅 = 0.31–0.35) to locked onto an arbitrary favorite (휅 = 0.62–0.69), and the daily trajectories split into branches that never rejoined (Fig. 4b–d). What produces this wide dispersion? Because runs redraw the carrier population and the load stream, variation across them mixes differences in market fundamentals with stochastic history, and the shared seeds across the three information conditions let us bound the two parts. Final 휅 is strongly correlated across conditions run on the same population (푟 = 0.93 between truth and endogenous, 0.49–0.68 for the other pairs), and a variance decomposition attributes 76% of the total variation to which market was drawn, with the remaining 24% arising within a market across conditions and stochastic histories. The dispersion is therefore primarily a property of the sampled market: even with each carrier’s true reliability on display, which carriers the market elevated depended on the drawn population and on the early path of prices and displaced demand, and no trust-signal design we tested reduced either the level or the spread. Isolating true path dependence, in the sense of Salganik et al. (2006), would require repeated stochastic histories T. Ezaki et al.: Preprint submitted to ElsevierPage 9 of 17 When Shippers Become Algorithms on a fixed population, which our design does not include; we flag it as a direct extension. Two caveats apply. Failure to detect a difference is not equivalence: with ten runs per condition, level effects smaller than roughly 0.1 would go undetected. Reliability is also payoff-irrelevant here (Section 3), so this comparison of information structures isolates the coordination role of the trust signal rather than its value as a predictor of failures. For a platform, the practical reading is correspondingly narrow: in this market, neither keeping ratings live nor replacing them with verified true quality made the resulting market structure any less variable. truthstaticendogenous 0.3 0.4 0.5 0.6 0.7 Final κ ( L =20 ) a 0102030 Day 0.2 0.4 0.6 0.8 1.0 Concentration κ b Truth 0102030 Day 0.2 0.4 0.6 0.8 1.0 c Static 0102030 Day 0.2 0.4 0.6 0.8 1.0 d Endogenous Figure 4: Final concentration varies widely across sampled markets under every trust-signal structure (GPT, 퐿 = 20). (a) Final 휅 by trust-signal structure; bars give means across runs, dots individual runs (ten runs per structure). (b–d) Daily trajectories per structure, one line per run: under every structure the runs split into locked-in and dispersed branches. 5.4. Of the four interventions, only capacity disclosure has a clearly detectable effect The third lever is what else the platform displays. Starting from the default condition, we added one intervention at a time and compared final concentration against the no-intervention benchmark (Fig. 5a). Capacity disclosure lowered final 휅 to 0.29 (range 0.26–0.33), below every no-intervention run (Mann–Whitney 푝 < 0.001; the only effect that survives Holm–Bonferroni correction across the four intervention comparisons, adjusted 푝 = 0.003; difference in means −0.17, bootstrap 95% CI [−0.23, −0.12]). Fixed list order and popularity display straddled the benchmark with effects that changed sign from run to run (푝 = 0.44 and 푝 = 0.95; differences in means −0.04 and −0.02, with CIs [−0.11, 0.03] and [−0.09, 0.05] that bracket zero); the position bias documented in evaluation settings (Zheng, Chiang, Sheng, Zhuang, Wu, Zhuang, Lin, Li, Li, Xing, Zhang, Gonzalez and Stoica, 2023) did not survive contact with price and rating signals. The vendor mix ended slightly below the benchmark on average (final 휅 of 0.36–0.43 across runs, mean 0.41 against 0.46) without reaching significance (푝 = 0.075; adjusted 푝 = 0.23), consistent with the day-one result that the three vendors share the same favorite. With five runs the design cannot distinguish a null from a modest effect, but mixing models that agree clearly does not diversify demand the way capacity disclosure does. The mechanism of capacity disclosure operates upstream of the rating loop (Fig. 5b); the effect is attributable to the displayed information itself, not to the sequential execution the disclosure requires, since prompts carry no information about other shippers’ same-day decisions unless capacity is disclosed. With remaining capacity on display, the HHI of daily requests started at 0.11 on day one, against 0.31 without disclosure, so the pile-up that capacity and prices must undo later never formed. Service failures and their delay costs all but disappeared, which is where the surplus gain originated: in the concentration–surplus plane (Fig. 5c), capacity disclosure occupies its own corner, with shipper surplus of 0.41 against the baseline 0.19, while every other intervention clusters with the baseline. The gain is not a transfer from carriers: total surplus per ton-distance (shipper value minus carrier resource cost, with unserved loads at the delay cost, so that payments net out) rose from 0.63 under the benchmark to 0.76 under disclosure, because loads that would have failed were served. 5.5. The settled matching: who ends up with whom Concentration indices compress the outcome into one number. The audit trail also shows who ends up matched with whom. Figure 6 draws the final week of first choices (days 26–30) as a network from the five procurement priorities to the twenty carriers, for three runs that span the results above: the default-condition run of Fig. 2c–d, a locked-in run of the same condition, and a capacity-disclosure run. T. Ezaki et al.: Preprint submitted to ElsevierPage 10 of 17 When Shippers Become Algorithms None CapacityOrder Popular.Mix 0.0 0.2 0.4 0.6 Final κ a p<0.001 p=0.44 p=0.95 p=0.08 Concentration by intervention 0102030 Day 0.1 0.2 0.3 0.4 0.5 0.6 HHI of daily requests b Disclosure disperses demand upstream No intervention Capacity disclosure 0.30.40.50.6 Final κ 0.1 0.2 0.3 0.4 0.5 Shipper surplus c Concentration vs. shipper surplus No intervention Capacity disclosure Fixed list order Popularity display Vendor mix Figure 5: Capacity disclosure lowers concentration and raises shipper surplus; the other interventions show no clearly detectable effect (GPT, 퐿 = 20). (a) Final 휅 by intervention (bars: means; dots: runs). Brackets give uncorrected two-sided Mann–Whitney 푝-values against the no-intervention benchmark; only capacity disclosure remains significant after Holm–Bonferroni correction across the four comparisons (adjusted 푝 = 0.003). (b) HHI of daily first choices with and without capacity disclosure (mean ± range across runs). (c) Final 휅 against shipper surplus, one marker per run. Three points emerge. First, the concentration that survives in the default condition is structured by preference, not arbitrary (Fig. 6a). Cost-priority shippers spread over the cheap end of the price range (mean paid unit price 0.50), while reliability-priority shippers buy at the expensive top (mean 1.48), and the carriers they pick average a true reliability of 0.88 against 0.45 for the cost segment; one carrier receives no first choice at all. The 휅 = 0.44 at which this particular run settles reflects a few favorites per segment rather than one market-wide favorite. Second, the locked-in tail of Section 5.3 is a different matching structure under identical rules (Fig. 6b): the favorite absorbs 58–68% of the first choices of the reliability, capacity, and specialty segments (and none of the cost segment, which keeps buying cheap), while four carriers end the month without any first choice. Third, capacity disclosure changes the network, not just the index (Fig. 6c): demand spreads over nearly every carrier, the preference sorting at both ends of the price range persists, and reliability-priority shippers pay 1.09 instead of 1.48, because the pile-up that would have driven their favorites to the price cap never forms. a Cost Reliability Capacity Specialty Balanced Carriers by final unit price (cheap → expensive) Default run (κ=0.44) b Carriers by final unit price (cheap → expensive) Locked-in run (κ=0.63) c Carriers by final unit price (cheap → expensive) Capacity disclosure (κ=0.26) 0.0 0.2 0.4 0.6 0.8 1.0 Final displayed rating No demand in final week Figure 6: The settled matching between shipper priorities and carriers (first choices, days 26–30). Left nodes: the five procurement priorities (ten shippers each). Right nodes: the twenty carriers, ordered bottom to top by final unit price, sized by first choices received, colored by final displayed rating; open dashed circles mark carriers that receive no first choice in the final week. Edge width gives the number of first choices from a priority to a carrier. (a) The default-condition run of Fig. 2c–d. (b) A locked-in run of the same condition. (c) A capacity-disclosure run. T. Ezaki et al.: Preprint submitted to ElsevierPage 11 of 17 When Shippers Become Algorithms 6. Discussion and implications 6.1. Summary of results Our simulations gave a consistent answer to the opening question. When LLM agents take over carrier selection, their choices concentrate on a single carrier immediately: the pile-up appeared on day one in every experiment, at every exposure and under every trust signal, because the agents share the same preferences before the market has generated any history. What happened next depended on the market’s rules rather than on the models. The capacity limit spread the excess demand within days and gave unrated entrants work; congestion pricing set the speed of that spreading and passed its cost to shippers; and the surviving concentration varied widely from one sampled market to another under every trust signal we tested. Comparing the three levers by effect size: showing more candidates moved mean concentration from 0.26 to 0.43; replacing disclosed true quality with estimated ratings moved neither the mean nor the run-to-run dispersion; and disclosing capacity moved concentration by −0.15 while doubling shipper surplus. 6.2. Implications for platform design Four operational lessons follow for DFM platforms preparing for algorithmic shippers. First, the length of the candidate list is a market-structure decision, not a user-interface detail: below roughly ten displayed candidates the monoculture stays inert, and the transition zone sits within the range platforms use today. Second, capacity disclosure is the cheapest effective lever. The platform already holds the data, the intervention needs one line in the interface, and in our runs it cut concentration by a third and doubled shipper surplus by preventing the pile-up rather than repairing it. Third, interventions that sound protective under-deliver: mixing model vendors fails because the vendors share the same favorite, and randomizing or freezing list order washes out against price and rating signals. Fourth, do not expect better trust signals to stabilize the market. Neither keeping ratings live nor replacing them with disclosed true quality reduced the variability of the final market structure, and most of that variability traced to the drawn market itself rather than to the signal. The lesson is bounded by the model: quality here has no payoff consequences, so the signal’s only role is coordination; where quality predicts physical failures, better signals may still earn their keep through match quality even if they do not reduce concentration. A platform that wants a predictable market structure has to act on the mechanisms that create the lock-in, the congestion of early demand and its pricing, rather than on the accuracy of the scores it displays. The vendor comparison carries its own message. A shipper’s choice of foundation model is market-relevant: Claude populations never concentrated at any exposure while Gemini populations concentrated most, so the aggregate market outcome depends on a decision each firm makes privately for reasons of cost or convenience. 6.3. Implications for the monoculture debate Our experiments speak to both sides of the policy discussion around algorithmic homogeneity. On one side, they show that static audits of decision correlation overstate the danger: we found that in a market with binding capacity and responsive prices, the worst moment is the first one, after which the market itself undoes part of the concentration (at the shippers’ expense). On the other side, they show that the correction has limits: concentration settled at 2.4 times the random floor, the decline slowed when prices were sticky, and which carrier ends up dominant varies widely from one sampled market to another regardless of the trust signal on display. The practical remedy our experiments single out is transparency about capacity. This suggests disclosure obligations as a lighter-touch alternative to regulating the models themselves, complementing the antitrust concerns raised for LLM pricing agents (Fish et al., 2024). The dispersion result connects to a pattern now emerging across autonomous-agent operations: Long et al. (2026) document agent bullwhip, run-to-run decision instability, in serial supply chains, and our market exhibits its matching-market counterpart, a structure that varies widely from market to market under every trust signal. In both settings the instability survives remedies aimed at the signal (repeated sampling of the model there, better ratings here), which points attention toward the institutions the agents operate in rather than toward their outputs. 6.4. Relation to human freight markets The behaviors our agents display have documented human counterparts: carriers respond to shippers’ tendering consistency within a market cycle (Acocella et al., 2020), and long-term relationships coexist with spot opportunism (Harris and Nguyen, 2025b,a). LLM shippers reproduce the relationship channel through their transaction history and respond to spot prices, which makes the comparison meaningful, but we do not claim behavioral equivalence. A head-to-head benchmark against estimated human carrier-choice models is the natural next step, and the audit trail our platform produces makes that comparison feasible at the level of individual decisions. T. Ezaki et al.: Preprint submitted to ElsevierPage 12 of 17 When Shippers Become Algorithms 6.5. Limitations Five limitations bound the claims, and each points to a specific extension. First, the model is a stylized capacity- constrained procurement market inspired by freight rather than a calibrated freight market: fifty shippers, twenty carriers, a single stylized set of shipping routes, and stationary demand over thirty days; contract freight, differences across routes, and seasonality are absent, origins and destinations do not affect carrier suitability, specialty is a carrier scalar rather than a carrier–load match, and distance and tonnage scale every carrier’s quote by the same factor. The economic parameters were likewise fixed at one point of a larger space (aggregate capacity at 1.3× expected demand, price cap 1.5, delay cost 0.3, three-tender routing guides, thirty days), and only the price-adjustment rate received a robustness check; the capacity-disclosure effect in particular should strengthen as capacity tightens, and mapping these sensitivities is the immediate next use of the platform. These features change where demand piles up, not the fact that correlated preferences pile it up, so we expect the day-one result to survive them. Two quantitative questions do depend on scale and deserve dedicated sweeps: whether the sharp rise in concentration sits at an absolute list length (about ten candidates) or at a fraction of the carrier pool (half of twenty), and how 휅 and the HHI, which mechanically depend on the number of carriers, translate to larger markets. Second, carriers price mechanically and never act strategically: there is no carrier-side AI, no entry, and no exit. Strategic carriers would likely amplify our mechanism rather than dampen it, since a carrier that anticipates LLM preferences can court them, for instance with AI-written pitches that exploit the documented preference of LLMs for LLM-generated text (Laurito, Davis, Grietzer, Gavenčiak, Böhm and Kulveit, 2025); carrier exit would convert the welfare stakes from price premia into physical delivery failures (Elliott, Golub and Leduc, 2022). Both are worthwhile follow-ups. Third, quality is informational rather than payoff-relevant in our market: reviews are unbiased binary draws, reliability does not cause delivery failures, and so agents lose nothing by ignoring the trust signal. This isolates the signal’s coordination role, but it also means the null result of Section 5.3 should not be read as “information quality never matters”: where quality predicts physical failures, or where reviews inflate and censor as field studies document (Filippas et al., 2022; Nosko and Tadelis, 2015), the comparison could come out differently. Extensions with a payoff- relevant 휌 and biased review processes are the direct test; the same extensions would also let quality enter the surplus accounting. Fourth, results are snapshots of three specific commercial models at temperature 0.7 with one English prompt format. We did not vary prompt wording, and LLM choices are known to shift with phrasing; the fact that three independently trained models produced the same day-one favorite suggests the concentration result is not an artifact of our template, but a systematic prompt-sensitivity study, together with successor model versions, is needed before the vendor-specific numbers are treated as stable. Fifth, and related, our shipper populations are closer to replicated instances of one centrally designed agent than to independently engineered procurement systems: all fifty shippers share the same prompt template, decision format, temperature, and history structure, differing only in their stated priority. Real adopters would differ in contracts, service- level requirements, prompt engineering, tools, and organizational policy, and even firms using the same foundation model would implement different agent architectures. The day-one convergence is therefore best read as an upper bound, the worst case in which deployments share both the model and the task formulation, and the vendor-mix condition, which varies only the model, is a weak test of genuinely independent system design. How much heterogeneity in agent engineering dissolves the day-one consensus is an open question the platform can address directly. 7. Conclusion We placed populations of real LLM agents inside a freight matching market that answers back, with binding capacity, congestion pricing, and endogenous ratings, and measured what algorithmic procurement does to market structure. Demand concentrated on a single carrier on the first day in every condition we tested, so the interesting question is not whether AI shippers concentrate but what happens next, and that depends on the market’s rules. The capacity limit spread the excess demand, prices set the speed of the spreading, the surviving market structure varied widely from market to market under every trust signal, and the platform’s information choices, above all how many candidates to show and whether to reveal remaining capacity, governed the result more than the choice of model. For transportation research, this relocates the question. The arrival of AI shippers is not a forecast problem about what the models will prefer; their preference is uniform and immediate. It is a design problem about the market they will prefer it in. T. Ezaki et al.: Preprint submitted to ElsevierPage 13 of 17 When Shippers Become Algorithms Appendix A. Prompt and example response Each decision is one LLM call. The prompt below is reproduced verbatim from the audit log (default condition, day 1, one shipper; carrier lines abbreviated to three of twenty). The system instruction asks for three ranked carrier identifiers in JSON. You are a decision-making agent in charge of freight procurement at a shipper (cargo owner) company. From the list of candidate carriers, choose 3 carriers in order of preference, identified by their carrier ID (e.g. "carrier_07"). If your 1st choice is fully booked, your 2nd choice will be tendered, then your 3rd. Respond ONLY in the following JSON format (no other text before or after): "choices": ["<1st choice>", "<2nd choice>", "<3rd choice>"], "reason": "<your rationale in 1-2 sentences>" # Shipper ID: shipper_049 # Your priority: balanced (weigh all factors evenly) # Today’s request: loc_12 -> loc_07, distance 17, 15 tons, within 1 day # Summary of your past transactions: none # Candidate carriers (in random order) 1) carrier_10: quoted price 138 (unit price 0.54 x distance 17 x 15 tons), customer rating 0.42 (based on 74 rated shipments), company size: up to 27 tons/day, specialty 0.65 2) carrier_05: ... 3) carrier_01: ... Choose 3 carriers in order of preference (use their carrier IDs) and respond in JSON. The trust-signal line varies by condition: the truth condition shows reliability 0.42, the static condition shows the same customer-rating format frozen at its initial value, and the capacity-disclosure condition appends remaining capacity today: 34 tons to each line. Carriers without reviews display no customer ratings yet (new carrier). The five procurement priorities are inserted verbatim into the Your priority line: cost first (minimize freight cost), reliability first (avoid delivery problems), capacity first (prefer carriers that can handle volume), specialty first (prefer carriers whose expertise fits the route), and balanced (weigh all factors evenly); capacity-priority shippers act on the displayed company size (tons per day), since remaining capacity is hidden outside the disclosure condition. A typical raw response reads: "choices": ["carrier_18", "carrier_16", "carrier_19"], "reason": "carrier_18 offers the best overall value with a low quoted price, a high customer rating, and sufficient capacity for 15 tons." Appendix B. Reference decision rules The four reference rules replace the LLM call and receive the same candidate information. Random draws uniformly from the displayed list. Preferential attachment (Barabási and Albert, 1999) draws carrier 푗 with probability proportional to 1+푚 푗 , where 푚 푗 is the carrier’s accumulated matches discounted at 0.95 per day (the same discounted record that underlies 휅). Conflicting attachment draws with probability proportional to (1 + 푚 푗 ) exp(−0.05푚 푗 ), so accumulated matches attract and then repel; the rule is our construction, and the name borrows from the conflicting node-attachment mechanism of Leung and Weitz (2016). Common-score ranks candidates deterministically by one fixed linear score of the displayed attributes, −0.8 price+1.1 rating+0.8 capacity+1.0 specialty (attributes normalized to comparable scales); it sees the same observed rating the LLM agents see, never the true reliability (carriers without T. Ezaki et al.: Preprint submitted to ElsevierPage 14 of 17 When Shippers Become Algorithms reviews enter at the uninformative prior 0.5, and capacity enters as the normalized weight underlying the displayed tons per day). Figure A1 reports all four across the exposure sweep: the two attachment rules stay near the random floor at every exposure, which is why the main text uses the common-score rule and the random floor as its contrasts. 35101520 Exposure L (log scale) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Final κ a Concentration Random Preferential attachment Conflicting attachment Common-score (deterministic) 35101520 Exposure L (log scale) 0.2 0.4 0.6 0.8 1.0 Service rate b Service Figure A1: Reference decision rules across the exposure sweep (five runs each; mean ± range). (a) Final 휅. (b) Service rate. Appendix C. Alternative concentration measures The main text measures concentration on first choices. Three alternatives lead to the same conclusions: the realized version of 휅, computed on the loads carriers end up serving after tender rejections; the revenue share of the three highest-earning carriers; and the HHI of served loads. Figure A2 traces all three across the exposure sweep for GPT against the random floor. The realized measures compress the differences, because the capacity limit truncates how much demand any carrier can actually serve, but the ordering and the transition are unchanged. An attempted-tender version, computed on every tender the waterfall issues rather than on first choices alone, sits between the two: its final-week top-three share is 0.39 (range 0.27–0.55) across the default-condition runs, against 0.46 for first choices. 35101520 Exposure L (log scale) 0.18 0.20 0.22 0.24 0.26 0.28 0.30 Final realized κ a Realized allocation GPT Random 35101520 Exposure L (log scale) 0.20 0.25 0.30 0.35 0.40 0.45 Top-3 revenue share b Carrier revenue 35101520 Exposure L (log scale) 0.060 0.065 0.070 0.075 0.080 HHI of served loads c Served-load HHI Figure A2: Alternative concentration measures across the exposure sweep (GPT versus random; five runs each; mean ± range). (a) 휅 computed on realized allocations. (b) Revenue share of the top three carriers. (c) HHI of served loads. Declaration of generative AI and AI-assisted technologies in the writing process During the preparation of this work the authors used Claude (Anthropic) in order to improve the language and readability of the manuscript and to assist with the implementation of the simulation and analysis code. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication. T. Ezaki et al.: Preprint submitted to ElsevierPage 15 of 17 When Shippers Become Algorithms Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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