IdleSpec: Exploiting Idle Time via Speculative Planning for LLM Agents
Daewon Choi, Kyunghyun Park, Woomin Song, Saket Dingliwal, Sai Muralidhar Jayanthi, Jinwoo Shin, Aram Galstyan
Abstract
Large language model (LLM)-based agents solve complex tasks by leveraging multi-step reasoning with iterative tool calls and environment interactions, which incur idle time while waiting for observations. Despite the prevalence of idle time in most agentic scenarios, existing works treat it as an unavoidable overhead or propose restricted solutions that overlook varying computational budgets across different tool calls and future observation uncertainty, thereby leading to suboptimal utilization of idle time. In this paper, we introduce IdleSpec, a scalable and generic inference approach that leverages idle-time computation to improve agent performance while minimizing latency overhead. Specifically, IdleSpec iteratively generates plan candidates during idle periods and, once observations become available, aggregates them to guide the next reasoning step. For effective plan generation under observation uncertainty, IdleSpec samples between complementary drafting strategies (i.e., progressive and recovery) from a learned distribution that is updated via posterior feedback. Our experiments demonstrate that IdleSpec significantly improves agent performance in various agentic scenarios by effectively utilizing idle time. In particular, on the GAIA and FRAMES, IdleSpec achieves 55.6% average accuracy with Gemini-2.5-Flash, surpassing the vanilla baseline without idle-time usage by 5.1%. Furthermore, for MLE-Bench, which involves substantial delay from code executions, IdleSpec achieves performance gains of up to 9.1% on the Any Medal rate, highlighting its generalizability to long-horizon tasks.
AI Impact Assessments
(1 models)Scientific Impact Assessment: IdleSpec
1. Core Contribution
IdleSpec addresses a genuine and previously underexploited phenomenon in LLM agent workflows: the idle time agents spend waiting for tool execution, environment responses, and sub-agent completions. The key insight is that this waiting period—which dominates total execution time (81–93% across benchmarks)—can be productively used for speculative planning rather than being wasted.
The framework introduces a two-phase approach: (a) iteratively drafting plan candidates during idle periods using two complementary strategies (progressive and recovery), and (b) aggregating these candidates once observations arrive to guide subsequent reasoning. The strategy selection mechanism uses Thompson sampling with Beta-distributed posterior updates based on a binary forecast signal, providing an adaptive balance between exploitation (progressive planning) and exploration (recovery planning).
The distinction from prior work, particularly Sleep-Time Compute (Lin et al., 2025), is meaningful: IdleSpec accounts for heterogeneous idle-time durations across tool calls, handles observation uncertainty through dual-mode speculation, and treats generated plans as soft references rather than hard constraints.
2. Methodological Rigor
Strengths in experimental design:
Weaknesses:
3. Potential Impact
Practical impact: The framework is immediately applicable to any ReAct-style agent workflow, as demonstrated across two different agent frameworks (OAgents and SmolAgents). The gains are particularly notable for harder tasks (GAIA Level 3: +6.5% with Gemini-2.5-Flash) and execution-heavy environments (MLE-Bench: +9.1% medal rate), suggesting high value in production agentic systems.
Conceptual impact: The paper reframes idle time from a systems optimization problem (reducing latency) to a performance optimization problem (improving accuracy), which is a useful conceptual shift. The idea that speculative computation during tool execution can improve reasoning quality—not just speed—opens a new dimension for test-time compute scaling.
Limitations in impact scope: The approach assumes sequential tool calling; highly parallelized multi-tool workflows would have less idle time to exploit. The additional token cost during idle windows could be significant at scale.
4. Timeliness & Relevance
This work is highly timely. LLM agents are rapidly being deployed in production (coding assistants, research agents, customer support), and tool-call latency is a recognized bottleneck. The emergence of "test-time compute" as a scaling paradigm (including Sleep-Time Compute, which this paper directly extends and improves upon) makes this a natural and well-positioned contribution. The observation that idle time constitutes 81-93% of total execution time across diverse benchmarks establishes clear practical motivation.
5. Strengths & Limitations
Key Strengths:
Notable Limitations:
6. Overall Assessment
IdleSpec makes a solid, well-motivated contribution to the emerging area of compute-efficient LLM agent execution. The insight about exploiting idle time for quality improvement (rather than just latency reduction) is valuable, and the dual-mode speculative planning with adaptive selection is a clean technical contribution. The experimental validation is broad, though statistical rigor could be strengthened with more seeds and larger evaluation sets. The framework's simplicity and compatibility with existing methods enhance its practical adoptability.
Generated May 22, 2026
Comparison History (16)
Paper 2 (IdleSpec) likely has higher impact due to a more novel, general algorithmic contribution: exploiting idle time with speculative planning under observation uncertainty, applicable across many LLM-agent settings. It shows measurable gains on multiple established benchmarks (GAIA, FRAMES, MLE-Bench), suggesting methodological rigor and broad, timely relevance to agent latency/performance tradeoffs in real deployments. Paper 1 provides an important domain-specific benchmark and taxonomy for finance spreadsheets, with clear real-world relevance, but its impact is narrower (evaluation-focused, finance-centric) and less broadly transferable than IdleSpec’s inference strategy.
Paper 2 likely has higher impact: it introduces a general, system-level inference strategy (idle-time speculative planning) applicable across many LLM-agent settings with tool latency (web, code, robotics, assistants), offering immediate practical deployment value and broad cross-field relevance. The method is timely given agent tool-use latency and shows measurable gains on widely used benchmarks (GAIA, FRAMES, MLE-Bench). Paper 1 is novel and useful, but its impact is narrower (text-to-image prompting evaluation) and depends on adoption of a specific benchmark/judge setup, with more domain-specific applicability.
Paper 2 is more novel and broadly impactful: it introduces an adaptive “embedding by elicitation” representation for Bayesian optimization of variable-length text under aggregate-only feedback, a common real-world constraint in deployed AI systems. This creates a general framework that can transfer to many optimization problems over natural-language artifacts (system prompts, policies, rubrics), bridging LLMs, BO, and representation learning. Paper 1 is useful and timely but is a more incremental systems-level inference improvement (idle-time speculative planning) with narrower conceptual reach. Both seem empirically validated, but Paper 2’s framing and applicability suggest higher impact.
Paper 1 introduces a highly novel conceptual framework by bridging Bayesian optimization with LLMs, utilizing them as dynamic semantic representation builders rather than just text generators. This methodological innovation for optimizing discrete, variable-length text under sample-constrained aggregate feedback addresses a major challenge in AI alignment and deployment. While Paper 2 offers a practical systems-level optimization (speculative planning) for agents, Paper 1's core algorithmic contribution has broader implications for combining traditional probabilistic ML with modern LLMs.
IdleSpec introduces a novel, broadly applicable inference-time optimization that exploits idle time during LLM agent tool calls—a pervasive but underexplored inefficiency. Its generic, scalable approach with learned drafting strategies applies across diverse agentic scenarios (web browsing, coding, QA), offering broader impact potential. Spreadsheet-RL, while practically useful, addresses a narrower domain (spreadsheet automation) with a more incremental contribution (applying RL fine-tuning to a specific task type). IdleSpec's methodological innovation in speculative planning under uncertainty has wider implications for the growing field of LLM agents.
IdleSpec addresses a fundamental efficiency problem in LLM agent inference—idle time during tool calls—with a novel speculative planning approach that is broadly applicable across agentic scenarios. It demonstrates significant performance improvements (5-9%) on established benchmarks (GAIA, FRAMES, MLE-Bench) with rigorous experimental evaluation. Paper 1 (HarnessAPI) is primarily an engineering contribution reducing boilerplate code for API/MCP tool deployment, which, while practical, has narrower scientific novelty and impact. IdleSpec's methodology—learned drafting strategy distributions with posterior feedback—introduces genuinely new ideas with broader implications for LLM agent systems.
While Paper 1 offers a valuable efficiency and performance optimization for general LLM agents, Paper 2 addresses a fundamental bottleneck in AI for science (bridging discrete text and topological/continuous scientific data). Its direct applications in drug design and chemical synthesis offer profound potential for real-world scientific discovery and transformation across scientific domains, giving it a higher potential for broad scientific impact.
SciCore-Mol addresses a fundamental challenge in scientific AI—bridging the gap between LLMs and molecular/chemical data—with a novel modular architecture that integrates topology-aware perception, diffusion-based generation, and reaction reasoning. It has broader scientific impact spanning drug design, chemical synthesis, and scientific discovery. While IdleSpec is a clever systems optimization for reducing LLM agent latency through speculative planning during idle time, it represents an incremental efficiency improvement rather than enabling fundamentally new capabilities. SciCore-Mol's cross-disciplinary relevance and potential to accelerate molecular science gives it higher long-term impact.
Paper 1 addresses a fundamental and ubiquitous bottleneck in LLM agents—idle time during tool execution—with a generic speculative planning approach. Its methodology is broadly applicable across the rapidly expanding field of autonomous agents, promising high cross-domain scientific impact. In contrast, while Paper 2 demonstrates impressive industrial scale and solves a critical problem in livestreaming recommendation, its scientific contributions are more domain-specific, limiting its breadth of impact compared to the foundational LLM inference improvements in Paper 1.
IdleSpec introduces a novel, generalizable inference-time optimization that exploits idle time during LLM agent tool calls—a broadly applicable technique with clear practical benefits (5-9% accuracy gains with minimal latency overhead). Its methodological contribution (progressive/recovery drafting strategies with learned distributions) is technically rigorous and applicable across diverse agentic scenarios. Paper 1, while providing interesting empirical analysis of LLM providers in a Risk game setting, is more narrowly scoped as a benchmark evaluation study with less transferable methodological contributions. IdleSpec's approach can be adopted widely across LLM agent frameworks, giving it broader potential impact.
Paper 2 addresses a fundamental inefficiency (idle time during tool execution) ubiquitous in LLM-based agentic workflows, offering broad applicability across numerous domains. While Paper 1 presents an innovative approach to collaborative driving, its impact is largely confined to autonomous vehicles. The generalizability of IdleSpec to various complex, long-horizon tasks and its significant performance gains on standard AI agent benchmarks give it a higher potential for widespread scientific and practical impact in the rapidly growing field of foundation model agents.
Paper 2 (LCGuard) likely has higher scientific impact: it addresses a timely, broadly relevant safety/privacy risk introduced by latent KV-cache communication—an emerging paradigm for efficient multi-agent LLMs. The approach formalizes leakage via adversarial reconstruction and proposes a general, model-agnostic mitigation framework that can be adopted across systems, affecting both ML security and multi-agent learning. Paper 1 (IdleSpec) is novel and practically useful for latency/throughput, but its impact is narrower (agent inference optimization) and more incremental relative to existing speculative/planning techniques.
Paper 1 proposes a novel algorithmic approach to optimize LLM agent efficiency by utilizing idle time, backed by strong empirical gains on standard benchmarks. This concrete, measurable improvement in inference methodology is highly relevant to current AI research and likely to drive immediate citations and follow-up work, whereas Paper 2 offers a conceptual architectural framework that, while valuable for engineering, may have less direct scientific impact.
Paper 2 (IdleSpec) likely has higher impact due to broad, immediately deployable applicability across LLM agent systems where tool/IO latency is common (code execution, web, APIs). Its speculative planning framework is generic, improves accuracy while controlling latency, and is timely for agentic workflows and efficiency. The methodological contribution (idle-time utilization with adaptive drafting and posterior feedback) can transfer across domains and models. Paper 1 is novel for ToM benchmarking/data synthesis, but its impact is narrower (social reasoning benchmarks) and may be more sensitive to dataset/design choices.
Paper 2 identifies a fundamental bias in LLMs-as-judges, a widely used paradigm across AI research and industry. Its extensive, rigorous evaluation across 11 models provides critical insights into context-induced biases and negativity asymmetry. While Paper 1 offers a valuable system optimization for agent latency, Paper 2's findings have broader, immediate implications for the reliability of AI evaluation pipelines and general LLM behavior.
IdleSpec addresses a fundamental efficiency problem applicable to all LLM-based agents across diverse scenarios, offering a generic and scalable approach that exploits idle time during tool calls. Its broader applicability across agentic AI (reasoning, coding, retrieval) gives it wider impact potential. While Paper 1 makes solid contributions to ToM reasoning in persuasive dialogue with a novel dataset and framework, it targets a narrower application domain. Paper 2's infrastructure-level innovation—speculative planning during idle time—is more likely to be widely adopted and influence future agent system design across the field.