Deep-search agents are commonly trained via repeated interaction with real-world search APIs and browser environments. While realistic, this paradigm is expensive, difficult to reproduce, and sensitive to external drift. Existing simulator-based alternatives reduce dependence on live retrieval, but they largely inherit task distributions from pre-existing QA datasets, limiting their alignment with the behaviors required for deep search. We introduce SynSearch, a framework for training deep-search agents entirely within a synthetic search–browse environment. Our core idea is to jointly design the environment and the training-task distribution, enabling the synthesis of environment-aligned tasks and reinforcement learning of compact language models in multi-turn search settings. By grounding task validity in executable environment trajectories rather than surface-level plausibility alone, SynSearch unifies task generation, trajectory verification, and policy optimization within a low-marginal-cost and reproducible framework. Experiments on multi-hop QA and deep-search benchmarks show that SynSearch consistently improves both answer accuracy and search behavior, with especially large gains on deep-search tasks. Moreover, SynSearch offers low marginal cost, reproducibility, controllability, and strong performance under real-search evaluation, highlighting synthetic environments as an effective substrate for training deep-search agents.

Open-web deep research is limited not only by how long an agent searches, but also by how it organizes evidence into independent, verifiable branches. Existing systems often scale single-path ReAct trajectories or long-context search, which can deepen one hypothesis while leaving alternative entities, constraints, or falsifying evidence underexplored. We propose P-Search, a parallel plan–act–verify framework that first builds a grounded background state, then decomposes the task into independently executable evidence branches, runs branch-specific search agents in parallel, and iteratively verifies unresolved constraints before final synthesis. For training, we avoid end-to-end credit assignment over a full multi-stage rollout. Instead, P-Search-React learns reusable grounded search-report behavior from raw-question search instances and is reused for branch execution and targeted supplementary search, while P-Search-Plan learns schema-constrained routing, decomposition, and verification decisions. We supervise this structure with star-shaped subgraph data and role-specific GRPO rewards. Experiments show that instantiating P-Search with open-source backbones such as GLM-5 and Tongyi-DR-30B improves their reported baselines on most open-web benchmarks, while the trained 8B P-Search-React + P-Search-Plan system reaches 59.2% on GAIA and 70.0% on Xbench-DeepSearch. Ablations confirm the complementary roles of Plan, ReAct, and Verify, as well as the transferability of the trained planner across ReAct backbones.