Working paper / 2026
Conversational AI LLM Bench
Abstract
We define conversational AI LLMs as the language models at the center of cascaded voice pipelines, between speech recognition and speech synthesis, that return a first token within one second: a median time to first token of 1,000 ms or less. The Conversational AI LLM Bench (CALLM) benchmarks this category. Each model operates every agent of a multi-agent front desk in healthcare, legal, and customer support, with production-length prompts, executable tools, and per-call database state. A simulated caller pursues a locked task over a simulated phone line, so the model receives only what a speech recognizer would have transcribed, errors included. A deterministic verifier scores tool calls, handoffs, and final database state; pass@1 and pass5 measure single-attempt capability and five-attempt reliability.
Clone the repository, point a model at an industry pack, and reproduce the evaluation environment.
Retell AI's own LLM for phone agents, run through Retell's agent playground API with the agent handbooks off.
| Scope | pass^5 | pass@1 | E / M / H | Tools | Handoff | Final DB | TTFT | Cost/hr |
|---|---|---|---|---|---|---|---|---|
| Cross-industry | 60.6% | 80.0% | 96 / 80 / 72 | 80.4% | 96.1% | 93.9% | 352ms | N/A |
| Healthcare | 61.7% | 77.7% | 100 / 83 / 62 | 77.7% | 95.3% | 94.3% | 389ms | N/A |
| Legal | 50.0% | 84.3% | 87 / 88 / 80 | 84.3% | 99.3% | 91.7% | 342ms | N/A |
| Customer support | 70.0% | 78.0% | 100 / 70 / 75 | 79.3% | 93.7% | 95.7% | 323ms | N/A |
E / M / H is pass@1 on easy, medium, and hard tasks. Gate columns are the share of conversations that cleared each check.