Constructive Specification for Plug-and-Play Learnware Agents

Large language models are increasingly deployed at scale as API-accessible, tool-augmented agents, forming a heterogeneous, fast-evolving agent ecosystem. A central challenge is query-level identification: selecting the most suitable agent per query from candidates provided as black-box services, where costly input-output evaluation makes exhaustive profiling and router retraining impractical at scale. The learnware paradigm offers a unified perspective on this challenge by advocating capability specifications and reducing identification to specification matching, avoiding pool-dependent retraining and exhaustive supervision. We operationalize it with constructive specification, which builds hierarchical capability representations from limited profiling over diverse benchmarks, using an optimism-guided profiler that prioritizes informative regions and prunes low-utility areas with theoretical guarantees. At serving time, we enrich query context with system-maintained benchmarks and map queries into the same specification space for multi-granularity similarity matching, enabling plug-and-play identification without accessing agent internals or training any additional selector. Experiments show that our approach, selecting among lightweight agents, outperforms contenders and matches or surpasses much larger models on several tasks.