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Prefix executability checks whether an agent’s steps remain possible along the way

Key Takeaways

  • A tool-use evaluation protocol exposes invalid intermediate actions that a successful final answer can conceal.
  • ### A completed task can hide a broken sequence
  • The [Prefix Executability study](https://arxiv.org/abs/2610.40444) argues that final task success is an incomplete measure of a tool-using agent.
  • An agent may eventually reach the requested state after making invalid calls, using unavailable identifiers or recovering from an earlier failure.
  • The proposed protocol asks whether every growing prefix of the emitted tool-call sequence remains executable.

A completed task can hide a broken sequence

The Prefix Executability study argues that final task success is an incomplete measure of a tool-using agent. An agent may eventually reach the requested state after making invalid calls, using unavailable identifiers or recovering from an earlier failure.
The proposed protocol asks whether every growing prefix of the emitted tool-call sequence remains executable. It checks tool schemas, state preconditions and data dependencies. This allows alternative valid routes instead of requiring the agent to reproduce one reference sequence.

Measure where reliability first breaks

The authors introduce a family of related measurements. Prefix Executability Curves describe survival as a trajectory grows, First-Error Depth records the first failure and Trajectory Executability Rate measures fully executable runs. Recovery Rate helps distinguish uninterrupted execution from success achieved after a break.
The evaluation uses stratified samples from BFCL-v3 multi-turn and ToolSandbox, together with complete Airline and Retail base splits of τ³-bench. The study reports that final-success rankings and prefix-level rankings can diverge. In particular, conditioning execution on an unchecked plan can improve a final-success point estimate while reducing trajectory executability.

Validation must be assessed as an intervention

The paper also reports that explicit pre-execution plan checking can improve prefix survival while retaining competitive terminal success. That is a measured intervention in the evaluated settings, not a guarantee that every validator will repair every planning failure.
For a team testing an operational agent, keep the original tool trace and the relevant state with each result. Compare how often a task succeeds with how often it succeeds through valid intermediate actions. An implementation that quietly repairs a broken identifier may finish the job, but its recovery behavior still matters when diagnosing reliability or deciding which validation step deserves attention.

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