Collaborative search enables users to address complex information needs jointly. Advances in large language models (LLMs) enable autonomous agents to support collaborative information seeking within shared communication spaces. However, little is known about how these agents shape search experiences, behaviors, and outcomes compared with conventional search tools. We conduct a user study with 80 students working in 40 pairs. Each pair completes four collaborative search tasks under four scenarios: QAAgent, which answers queries using an LLM's parametric knowledge; CoSearchAgent, which uses multi-user conversational context to reformulate queries, retrieve web information, and generate source-grounded answers, while asking clarifying questions when needed; an in-chat search plugin that returns result lists; and manual Google Search. After each task, participants complete several questionnaires assessing their search experience and independently write a report; after all tasks, they participate in individual interviews. Results show that LLM-based agents reduce perceived workload, support collaboration, and reshape communication and evidence-use practices. They also encourage more independent paraphrasing, albeit at the cost of report richness. To the best of our knowledge, this is the first systematic empirical comparison of LLM-based agents and conventional tools for collaborative search across search experience, user behavior, and collaborative outcomes.

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