FlexiSLM: A Spoken Language Model with Dynamic and Controllable Frame Rates

Abstract

Spoken language models (SLMs) extend LLMs to speech input and output. Existing SLMs represent speech at fixed frame rates (e.g., 25 or 12.5 Hz), ignoring the time-varying information density of speech and offering limited flexibility to trade off quality for speed at inference time. Recent audio tokenizer research has proposed dynamic-frame-rate speech coding, which exploits this non-uniformity and enables two new capabilities: very low average frame rates and frame-rate controllability. However, this technique has not yet been applied to SLMs. We introduce FlexiSLM, the first SLM with dynamic and controllable frame rates. FlexiSLM uses the pretrained FlexiCodec to obtain dynamic speech output tokens. The main contributions of this work are threefold: (1) integrating and validating this dynamic-rate representation within a multi-task, speech-to-speech SLM architecture; (2) extending it to frame compression on the input side; and (3) introducing direct frame-rate conditioning to enable accurate and controllable SLM inference. FlexiSLM outperforms fixed-frame-rate 7B models including Qwen2.5-Omni and Kimi-Audio at its 12.5 Hz and 6.25 Hz operating points. We further verify that FlexiSLM can be accurately steered down to 4.0 Hz; at 6.25 Hz, it roughly halves inference time relative to 12.5 Hz while retaining strong speech-to-speech quality. Audio samples are available at: https://flexislm.github.io. Code and data are available at: https://github.com/AmphionTeam/FlexiSLM.

Accepted to EMNLP2026 Main Conference