Large Language Models (LLMs) in agentic workflows combine multi-step reasoning, heterogeneous tool use, and collaboration across multiple specialized agents. Existing LLM serving engines optimize individual calls in isolation, while multi-agent frameworks focus on orchestration without system-level performance planning. As a result, repeated prompts, overlapping contexts, and fragmented CPU-GPU execution create substantial redundancy and poor hardware utilization, especially in batch analytics scenarios. We introduce Halo, a system that brings batch query processing and optimization into agentic LLM workflows. Halo represents each workflow as a structured query plan DAG and constructs a consolidated graph for batched queries that exposes shared computation. Guided by a cost model that jointly considers heterogeneous resource constraints, prefill and decode costs, cache reuse, and GPU placement, Halo performs plan-level optimization to minimize redundant execution. The Processor integrates adaptive batching, KV-cache sharing and migration, along with fine-grained CPU-GPU pipelining to maximize holistic hardware efficiency. Evaluation across six benchmarks shows that Halo achieves up to 3.6x speedup for batch inference and 2.6x throughput improvement under online serving, scaling to workloads of thousands of queries and complex graphs. These gains are achieved without compromising output quality. By unifying query optimization with heterogeneous LLM serving, Halo enables efficient agentic workflows in data analytics and decision-making applications.