Progress Probes P²: Lightweight Runtime Monitoring for Vision-Language-Action Models

Progress Probes trains linear probes on activations from the open-source π0.5 vision-language-action model, using successful demonstrations to estimate task progress. At runtime, the monitor flags executions whose estimated progress lags or deviates from the expected trajectory. Across simulated and real-world manipulation tasks, the method was competitive with or outperformed VLM-based monitors and methods trained with failure data, without requiring an external foundation model.