StreamPro: From Reactive Perception to Proactive Decision-Making in Streaming Video
Pith reviewed 2026-05-20 23:29 UTC · model grok-4.3
The pith
A new benchmark and two-stage training let video models decide when to respond under partial observations.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
StreamPro-Bench tests three perspectives including Proactive Agency to measure early reliable decisions under partial observations, and the StreamPro framework uses CB-Stream Loss in supervised fine-tuning followed by Group Relative Policy Optimization with multi-grained rewards to jointly optimize correctness and timing, reaching 41.5 on the new benchmark versus a prior best of 10.4 while scoring 78.9 on StreamingBench-RTVU.
What carries the argument
The two-stage training framework that applies CB-Stream Loss to address supervision imbalance in supervised fine-tuning then Group Relative Policy Optimization with turn-level and trajectory-level rewards.
Load-bearing premise
The new benchmark accurately captures a model's ability to make early yet reliable decisions under partial observations, and the training framework effectively balances response correctness and timing without introducing biases from the imbalance in silence and response signals.
What would settle it
A direct test in which models must output responses at fixed early fractions of video clips containing only partial evidence, checking whether accuracy remains high on StreamPro-Bench but drops sharply on real-time streaming tasks.
Figures
read the original abstract
Proactive streaming video understanding requires models to continuously process video streams and decide when to respond, rather than merely what to respond. This naturally introduces a decision-making problem under partial observations, where models must balance early prediction against sufficient evidence. However, existing benchmarks largely follow a "see-then-answer" paradigm, where responses are triggered only after explicit evidence appears, effectively reducing proactive reasoning to delayed perception. As a result, they fail to evaluate a model's ability to make timely and reliable decisions under incomplete observations. Moreover, training proactive models is inherently challenging due to the extreme imbalance between silence and response signals in streaming trajectories, as well as the need to jointly optimize response correctness and timing. To address these challenges, we introduce StreamPro-Bench, a new benchmark that evaluates streaming models from three complementary perspectives: Perception Understanding, Temporal Reasoning, and Proactive Agency, where the last measures a model's ability to make early yet reliable decisions under partial observations. We further propose StreamPro, a two-stage training framework for proactive learning. First, we introduce CB-Stream Loss to mitigate the severe supervision imbalance during supervised fine-tuning (SFT). Then, we apply Group Relative Policy Optimization (GRPO) with a multi-grained reward design that involves both turn-level and trajectory-level rewards. Experiments show that StreamPro significantly improves proactive performance. On StreamPro-Bench, it achieves 41.5, substantially outperforming the previous best (10.4), while also maintaining strong performance on real-time streaming benchmarks, achieving 78.9 on StreamingBench-RTVU.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims that existing streaming video benchmarks follow a reactive 'see-then-answer' paradigm and fail to evaluate proactive decision-making under partial observations. To address this, it introduces StreamPro-Bench, which evaluates models on Perception Understanding, Temporal Reasoning, and Proactive Agency (the latter measuring early yet reliable decisions). It further proposes StreamPro, a two-stage framework: CB-Stream Loss during SFT to handle severe silence/response imbalance, followed by GRPO with multi-grained (turn-level and trajectory-level) rewards to jointly optimize correctness and timing. Experiments report StreamPro scoring 41.5 on StreamPro-Bench (vs. prior best of 10.4) while achieving 78.9 on StreamingBench-RTVU.
Significance. If the benchmark's Proactive Agency metric is shown to be independently and reproducibly defined, the work would meaningfully advance proactive streaming understanding by providing both a new evaluation axis and a training recipe that balances earliness against reliability. The empirical margin on the new benchmark and retention of performance on existing real-time benchmarks would constitute a concrete step beyond reactive perception models.
major comments (1)
- [StreamPro-Bench description] StreamPro-Bench section (description of Proactive Agency metric): The paper does not specify how reference response timestamps are generated (human annotation protocol, automated event detection, or otherwise). Without an explicit, reproducible definition of ground-truth timing under partial observations, the 41.5 vs. 10.4 gap cannot be unambiguously interpreted as evidence that the CB-Stream Loss + multi-grained GRPO framework 'effectively balances response correctness and timing.' This is load-bearing because the central contribution is the joint introduction of benchmark and method.
minor comments (2)
- [Experiments] The abstract and experimental section would benefit from a brief statement of the number of videos, average stream length, and annotation protocol used to construct StreamPro-Bench so readers can assess its scale and independence from the training trajectories.
- [Training framework] Notation for the multi-grained reward components (turn-level vs. trajectory-level) should be introduced with explicit equations or pseudocode to clarify how they are combined during GRPO.
Simulated Author's Rebuttal
We thank the referee for the constructive feedback on our manuscript. The point regarding the reproducibility of the Proactive Agency metric in StreamPro-Bench is well-taken and directly impacts the interpretability of our central results. We address it in detail below and will incorporate the requested clarifications in the revised version.
read point-by-point responses
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Referee: StreamPro-Bench section (description of Proactive Agency metric): The paper does not specify how reference response timestamps are generated (human annotation protocol, automated event detection, or otherwise). Without an explicit, reproducible definition of ground-truth timing under partial observations, the 41.5 vs. 10.4 gap cannot be unambiguously interpreted as evidence that the CB-Stream Loss + multi-grained GRPO framework 'effectively balances response correctness and timing.' This is load-bearing because the central contribution is the joint introduction of benchmark and method.
Authors: We agree that an explicit, reproducible protocol for generating reference response timestamps is necessary to support claims about the balance between earliness and reliability. The current manuscript provides only a high-level description of the Proactive Agency metric. In the revision we will add a dedicated subsection (likely 3.2.3 or equivalent) that details the human annotation protocol: (1) the exact guidelines provided to annotators for identifying the earliest timestamp at which a reliable proactive decision can be made under partial observations, (2) the number of annotators per video and the aggregation method (e.g., majority vote or consensus), (3) inter-annotator agreement statistics (Cohen’s kappa or equivalent), and (4) quality-control procedures such as pilot studies and adjudication of disagreements. These additions will make the ground-truth timing definition fully reproducible and allow readers to assess whether the reported 41.5 score genuinely reflects improved timing-aware decision making rather than benchmark artifacts. revision: yes
Circularity Check
No significant circularity; empirical results on independent benchmarks
full rationale
The paper introduces StreamPro-Bench and the StreamPro training framework (CB-Stream Loss + GRPO with multi-grained rewards) as separate contributions, then reports empirical scores (41.5 on StreamPro-Bench, 78.9 on StreamingBench-RTVU) against prior baselines. No equations, fitted parameters renamed as predictions, or self-citation chains appear in the abstract or described derivation. The benchmark and method are presented as addressing distinct challenges (partial-observation timing and supervision imbalance), with performance claims resting on external evaluation rather than reducing to inputs by construction. This is the common honest case of a self-contained empirical paper.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The extreme imbalance between silence and response signals in streaming trajectories requires special handling in training.
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Is the visual information observed so far sufficient to answer the Query without guessing? • If NO→replyWait. • If YES→proceed to Step 2
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Does the inferred answer differ from the last answer you provided? • If NO→replyWait. • If YES (First Trigger)→reply with the actual answer. • If YES (Answer Update due to new evidence)→reply with the updated answer. Output Constraint: • Do not output anything other thanWaitor the actual content of the answer. 29 Rubric Generation Prompt You are designing...
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