What Happens Before Decoding? Prefill Determines GUI Grounding in VLMs
Existing training-free approaches for GUI grounding often rely on multiple inference runs, but each forward pass still independently interprets the instruction and visual layout. This paper reveals a two-stage paradigm: the prefill stage determines candidate UI elements, while decoding refines coordinates. Errors in prefill are uncorrectable during decoding. The authors propose Re-Prefill, a training-free method that introduces an attention-guided second prefill stage to refine target selection. Experiments across four VLMs and five benchmarks show consistent improvements, with gains of up to 4.3% on ScreenSpot-Pro.
[2605.12549] What Happens Before Decoding? Prefill Determines GUI Grounding in VLMs
[Submitted on 10 May 2026]
Title:What Happens Before Decoding? Prefill Determines GUI Grounding in VLMs
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Abstract:Existing training-free approaches for GUI grounding often rely on multiple inference runs, such as iterative cropping or candidate aggregation, to identify target elements. Despite this additional computation, each forward pass still independently interprets the instruction and parses the visual layout, without enabling progressive interaction among visual tokens. In this paper, we study what happens during GUI grounding in Vision-Language Models (VLMs) and identify a previously overlooked bottleneck. We show that grounding follows a two-stage paradigm: the prefill stage determines candidate UI elements, while the decoding stage subsequently refines the final coordinates. This asymmetry establishes prefill as the critical step, as errors in candidate selection cannot be effectively corrected during decoding. Based on this observation, we propose Re-Prefill, a training-free method that revisits inference by introducing an attention-guided second prefill stage to refine target selection. Specifically, visual tokens that consistently receive high attention from the query position, i.e., the final token, across layers are extracted as a preliminary target hypothesis and appended to the input, together with the instruction hidden states, enabling the model to deeply re-think its decision before coordinate generation. Experiments across four VLMs and five benchmarks, including ScreenSpot-Pro, ScreenSpot-V2, OSWorld-G, UI-Vision, and MMBench-GUI, demonstrate consistent improvements without additional training, with gains of up to 4.3% on ScreenSpot-Pro. Code will be available at this https URL.
Subjects:
Computer Vision and Pattern Recognition (cs.CV)
Cite as: arXiv:2605.12549 [cs.CV]
(or arXiv:2605.12549v1 [cs.CV] for this version)
https://doi.org/10.48550/arXiv.2605.12549
arXiv-issued DOI via DataCite (pending registration)
Submission history
From: Jiaping Lin [view email] [v1] Sun, 10 May 2026 07:04:07 UTC (2,089 KB)
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