Collective behavior forecasting usually relies on historical outcome traces: past sales, past votes, or prior campaign performance. Such history-side records are useful when comparable cases exist, but they are brittle for new products, emerging political actors, or settings where historical patterns fail to generalize. We propose a complementary response-side approach: instead of only extrapolating from past aggregate outcomes, we prompt large language models as third-party domain analysts to infer collective outcomes from structured context and behavioral signals. We introduce EBI, an Exogenous–Behavioral–Inference framework that represents each task through external context, behavioral signals, and the target aggregate outcome. Across three real-world domains — health supplements, elections, and crowdfunding — we preliminarily evaluate whether response-side inference is competitive with history-side baselines, which are restricted to historical outcome data. We present the EBI system, a web-based tool designed for users with limited access to historical data, survey infrastructure, or analyst resources, allowing them to explore domain-specific EBI predictions, run live inference across multiple models, and ablate exogenous and behavioral features interactively to understand prediction drivers.