vLLM is an inference and serving engine for large language models. Prior to 0.30.0, structured-output request failures can escape request-scoped validation and reach the EngineCore fatal-error path. A per-request backend mismatch can re-raise a grammar compilation exception, padding produced by the ngram_gpu speculative-decoding mode can pass a negative token to guidance validation, and the Rust frontend can admit empty structured-output values that the Python frontend rejects, allowing ordinary constrained-generation requests to terminate the shared engine. This issue is fixed in version 0.30.0.
Affected: PyPI: vllm
## Affected
- **Ecosystem / package:** pip / `vllm` - **Affected versions:** vLLM ≤ 0.25.1 (confirmed on 0.25.1, commit [`752a3a504485`](https://github.com/vllm-project/vllm/tree/752a3a504485790a2e8491cacbb35c137339ad34)). The lower bound predates 0.25.1; maintainers can confirm how far back each path reaches.
## Summary
Three structured-output request paths let an ordinary request reach a condition that raises an **uncaught, engine-fatal** exception instead of a per-request validation error. The failure is not confined to the request that caused it: it escapes into `EngineCore`'s busy loop and triggers a fatal `_send_engine_dead()`, so one malformed structured-output request denies service to all concurrent and subsequent tenants of that engine. The requests are ordinary API calls; the client does not need any privileged configuration beyond the (often default) structured-output feature.
The shared root cause is the absence of a per-request exception boundary around structured-output grammar/token handling: a value that should fail as a request-scoped validation error instead propagates as an uncaught exception (or bypasses frontend validation entirely) and reaches the scheduler/engine loop, which treats the failure as fatal.
These sites are distinct from the published fixes for [GHSA-6qc9-v4r8-22xg](https://github.com/vllm-project/vllm/security/advisories/GHSA-6qc9-v4r8-22xg) and [GHSA-8wr5-jm2h-8r4f](https://github.com/vllm-project/vllm/security/advisories/GHSA-8wr5-…