Vllm v0.31 Enables Native Constrained Decoding for Tool-Calling
Something you can actually use or run today.
On October 20, 2026, UbiCloud reported that vLLM v0.31 added native support for constrained decoding, replacing the need for custom engine forks to maintain tool-call schema accuracy.
Reliable tool-calling is the bedrock of agentic workflows; native support in vLLM simplifies the stack for high-performance agent hosting.
This is a standard 'infrastructure maturity' update. It removes a major pain point for developers who were previously forced to maintain fragile custom logic for JSON output.
Latency benchmarks comparing native vLLM constrained decoding to external libraries like Outlines.
- Eliminates the need for maintaining custom vLLM forks for schema enforcement.
- Guarantees 100% schema adherence for structured JSON and function calling.
- Improves reliability for agentic workflows interacting with external tools.
vLLM v0.31+ provides native support for constrained decoding to improve output format reliability.
The original claim that vLLM released a specific 'fix' for tool-call accuracy was a mischaracterization of the new native constrained decoding feature.