## Overview Leboncoin, a major French classifieds platform serving 30 million monthly visitors, implemented a production-scale generative AI solution to address a persistent user experience challenge known as "blank page syndrome." This case study illustrates the deployment of large language models in a consumer-facing application where the AI acts as a writing assistant rather than a replacement for human creativity. The implementation showcases thoughtful LLMOps practices including extensive prompt engineering, iterative user testing, cost management considerations, and careful integration of legal compliance into the development process. The business problem was clear and measurable: sellers struggled to write effective ad descriptions, which created a negative cycle of poor-performing listings leading to frustrated users who were less likely to post future ads. This directly impacted both the quality and quantity of ads on the platform. Rather than providing simple templates or writing tips, Leboncoin chose to leverage generative AI to fundamentally transform the ad creation experience while maintaining the authentic, human character that defines their marketplace. ## Technical Architecture and Infrastructure The production system is built on **Claude Haiku**, accessed through **AWS Bedrock** infrastructure. This architectural choice is significant from an LLMOps perspective as it demonstrates the use of managed AI services rather than self-hosted models, which offers several operational advantages including simplified scaling, reduced infrastructure management overhead, and access to enterprise-grade security and compliance features. AWS Bedrock provides a unified API for accessing foundation models while handling the underlying infrastructure complexity. The system operates as a **multimodal solution**, processing multiple input types to generate descriptions. The AI considers uploaded photos, item titles, and structured item details (category-specific attributes) to generate contextually relevant descriptions. This multimodal approach is critical for the use case, as visual information about the item's condition and characteristics can significantly inform the quality of generated text. The architecture must handle image preprocessing, feature extraction, and coordinated processing of visual and textual inputs before generating the final description. From a production deployment perspective, the feature is integrated directly into the ad creation flow across multiple categories including consumer goods and vehicles, with real estate being the next planned expansion. This category-by-category rollout strategy represents a pragmatic LLMOps approach that allows for domain-specific optimization and risk mitigation rather than attempting a platform-wide deployment simultaneously. ## Prompt Engineering as Core Development Activity One of the most revealing insights from this case study is how **prompt engineering became the central focus of the development and iteration process**. The product manager explicitly notes that "what's particularly noteworthy is how much of our discovery process ultimately centered around prompt creation. The prompts became the critical interface between user needs and generative AI capabilities." This observation highlights a fundamental shift in product development for LLM-powered features: the primary engineering artifact isn't traditional code but rather the carefully crafted prompts that guide model behavior. The team invested significant iteration cycles in refining prompts to achieve the right balance between structure and flexibility. Initial attempts produced descriptions that were too verbose and formal, reading "more like product catalogs than leboncoin's ads." This mismatch between model output and platform expectations required multiple rounds of prompt refinement. The team had to encode platform-specific knowledge into the prompts, including: - **Tone calibration**: Achieving a conversational yet informative style that matches how real users communicate on the platform - **Length constraints**: Generating concise descriptions rather than exhaustive product specifications - **Keyword optimization**: Ensuring generated descriptions include search-relevant terms that improve item discoverability - **Platform conventions**: Capturing the unique "language" of leboncoin ads that users recognize and trust This iterative prompt development process was tightly coupled with user testing, creating a feedback loop where real user reactions directly informed prompt modifications. The case study notes "each iteration brought us closer to that perfect leboncoin tone," suggesting a systematic approach to evaluating and refining prompt effectiveness based on qualitative user feedback. ## Evaluation and Testing Methodology The team employed a **user-centric evaluation approach** that prioritized qualitative assessment alongside any automated metrics. Multiple rounds of user interviews and testing sessions were conducted, with test participants providing feedback on different versions of AI-generated outputs. This human-in-the-loop evaluation methodology is particularly appropriate for a consumer-facing feature where subjective qualities like "authenticity" and "natural tone" are critical success factors that automated metrics struggle to capture. Key evaluation dimensions included: - **Tone and style appropriateness**: Does the generated text match platform conventions? - **Completeness**: Are essential details included without being overwhelming? - **Keyword effectiveness**: Will the description help the item appear in relevant searches? - **Authenticity**: Does it sound like a real person wrote it, or like generic AI-generated content? The product manager's background context emphasizes that "our users became our co-creators, helping us fine-tune not just the length and style of descriptions, but also the essential keywords that make ads more discoverable." This collaborative approach to evaluation represents a best practice in LLMOps where domain experts and end users contribute specialized knowledge that technical teams may lack. Notably, the case study indicates that "the traditional discovery process remains essential, but with AI products, we found ourselves cycling through iterations much faster." This acceleration of iteration cycles is both an advantage and a challenge in LLMOps—while rapid experimentation is possible, it requires disciplined evaluation frameworks to ensure changes represent genuine improvements rather than just differences. ## Production Constraints and Cost Management The team made a **deliberate architectural decision to limit users to one AI-generated description per ad**. This constraint reflects sophisticated thinking about LLMOps operational considerations beyond pure technical capability. The rationale encompasses multiple dimensions: - **Cost control**: The case explicitly acknowledges that "AI is expensive," and unlimited regeneration would create unpredictable and potentially unsustainable infrastructure costs - **Environmental considerations**: The team notes that "AI models are notorious energy consumers," demonstrating awareness of the carbon footprint implications of generative AI at scale - **User experience design**: Constraining generation encourages users to "think carefully about their ads from the start" rather than treating the AI as an unlimited content vending machine This single-generation constraint represents a thoughtful LLMOps tradeoff where business sustainability, environmental responsibility, and user behavior design align. It's a reminder that production LLM systems must balance capability with practical operational constraints. The team essentially implemented a form of **rate limiting at the feature level**, baking resource management into the product design rather than treating it as a pure infrastructure concern. From a technical implementation perspective, this constraint likely involves session tracking and state management to prevent repeated generations for the same ad draft, though the case study doesn't detail the specific enforcement mechanisms. ## Human-in-the-Loop Design and Control A critical LLMOps principle demonstrated in this implementation is the positioning of AI as an **assistive co-pilot rather than autonomous agent**. The generated description serves as a starting point that users can accept as-is, edit and refine, or completely replace. This design acknowledges several important considerations for production AI systems: - **Trust and adoption**: Users are more likely to adopt AI features when they maintain agency and control - **Error mitigation**: Even well-tuned LLMs can produce inappropriate or incorrect content; human review provides a safety layer - **Customization and personalization**: Sellers may have specific details or selling points the AI cannot infer from photos and basic details - **Platform authenticity**: Maintaining the "human touch" is explicitly called out as important to preserving what makes the classifieds platform unique This architectural choice reflects a mature understanding that production LLM systems often work best in collaboration with humans rather than attempting to replace human judgment entirely. The interface design must accommodate this workflow, providing easy editing capabilities while making the AI contribution valuable enough that users don't simply delete everything and start over. ## Legal and Compliance Integration The case study highlights an **innovative approach to legal compliance** where legal team members were embedded in the design process from day one rather than serving as gate-keepers at the end. This collaborative model addressed several critical questions early in development: - **Data handling with AI partners**: How is user data (photos, text, item details) shared with the AI provider? What data residency and privacy implications exist? - **Transparency and disclosure**: How and where should users be informed that AI is generating their descriptions? - **Liability and content responsibility**: Who is responsible for AI-generated content that may be inaccurate or problematic? This proactive legal integration represents a best practice for LLMOps, particularly in consumer-facing applications operating under European data protection frameworks like GDPR. The case notes this approach resulted in "no last-minute redesigns, no painful compromises," suggesting that early legal involvement actually accelerated time-to-market by preventing late-stage blockers. From an LLMOps governance perspective, this demonstrates the importance of establishing compliance frameworks before deployment rather than retrofitting them afterward. The team likely implemented mechanisms for data handling transparency, user consent flows, and audit trails that satisfy regulatory requirements while maintaining a seamless user experience. ## Business Impact and Metrics The production system has demonstrated measurable business impact with **ads using the AI-generated description feature showing a 20% increase in both inquiries and completed transactions**. This metric is particularly meaningful because it measures actual business outcomes (successful transactions) rather than just engagement metrics or AI performance scores. The 20% improvement suggests the AI is genuinely creating better-performing ads, likely through: - More complete and informative descriptions that answer buyer questions - Better keyword inclusion that improves search visibility - More professional presentation that builds buyer confidence - Reduced friction in the ad creation process leading to more listings overall These results validate the product hypothesis that better descriptions drive marketplace performance. However, it's worth noting that the case study doesn't provide detailed statistical methodology, control group definitions, or confidence intervals, so we should view these figures as indicative rather than rigorously controlled experimental results. The team also reports strong qualitative feedback, with "this is exactly what I needed" becoming a common refrain in testing sessions. This combination of quantitative business metrics and qualitative user satisfaction suggests a successful production deployment. The feature's success was recognized externally when Leboncoin received the grand prize for innovation at the Grand Prix Favor'i e-commerce in March 2025, organized by FEVAD (the French e-commerce and distance selling federation). ## Scaling and Category Expansion Strategy The team's approach to **category-by-category expansion** demonstrates pragmatic LLMOps scaling strategy. After initial deployment in consumer goods and mobility (vehicles), they're now expanding to real estate—but explicitly not just rolling out the existing solution as-is. The product manager asks: "Why not just roll out what we already have? Well… selling an apartment is quite different from selling a Just Dance game." This recognition that different categories require domain-specific optimization is critical for production LLM systems. Real estate ads require different information architecture: - Standard property attributes (square footage, number of rooms, location) - Neighborhood context and local amenities - Lifestyle and community aspects - Potentially integration with external data sources for points of interest Each category essentially requires its own prompt engineering effort, evaluation methodology, and potentially different model configurations. This category-specific approach allows for: - **Risk management**: Problems in one category don't cascade across the platform - **Optimization**: Domain-specific fine-tuning and prompt engineering for maximum relevance - **Learning**: Insights from one deployment inform subsequent category launches - **Resource allocation**: Staggered rollout allows for focused engineering attention The case study notes "the discovery process will be faster this time around" for real estate, suggesting the team has developed reusable frameworks and methodologies even though the specific prompts and evaluation criteria must be adapted for each domain. ## Critical Analysis and Considerations While the case study presents a largely positive narrative, a balanced assessment should consider several factors: **Limited technical transparency**: The case provides minimal detail about the actual technical implementation—prompt structures, image processing pipelines, response time requirements, fallback mechanisms, or monitoring approaches. This makes it difficult to assess the sophistication of the LLMOps practices beyond what's explicitly described. **Claimed results without methodology**: The 20% improvement figure is impressive but lacks context about statistical significance, sample size, control group definition, or potential confounding factors. Was this a randomized experiment or an observational comparison? How were transactions attributed to the AI feature versus other factors? **Cost sustainability questions**: While the team acknowledges AI is expensive and implements single-generation limits, the case doesn't address whether the current architecture is economically sustainable at scale or what unit economics look like. As adoption grows, will infrastructure costs become prohibitive? **Model dependency risks**: Relying on Claude Haiku via AWS Bedrock creates vendor dependency. What happens if pricing changes significantly, if model behavior shifts with updates, or if the service experiences outages? The case doesn't discuss model versioning strategies or fallback mechanisms. **Content quality monitoring**: How does the system detect and prevent problematic generations at scale? What monitoring and alerting exist for quality degradation? The human-in-the-loop design provides a safety layer, but are there automated quality checks before content reaches users? **Multimodal complexity**: Processing images alongside text adds complexity. How robust is the system to various image qualities, angles, or types? What happens when images are ambiguous or misleading? Despite these questions, the case study demonstrates several LLMOps strengths: thoughtful prompt engineering as a core discipline, user-centric evaluation methodology, deliberate operational constraints, human-centered design that maintains user agency, and proactive legal integration. The measurable business impact and external recognition suggest a genuinely successful production deployment that balances technical capability with practical considerations.