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[ CVE-2026-107286 · Published 8 Oct 2026 ]

Pydantic AI: Concurrency-limited models can keep their slot when a streamed request ends early

CVSS 7.5 EPSS <1%Fix available

[ Description ]Official description

Pydantic AI is a Python agent framework for building applications and workflows with Generative AI. From 2.10.0 until 2.53.0, streamed requests made through ConcurrencyLimitedModel or limit_model_concurrency can retain shared concurrency slots because anyio.CapacityLimiter associates an acquired slot with the borrowing task while streaming cleanup can run in a different task. Early stream termination, cancellation, consumer exceptions, or complete stream_text() consumption with debounce_by=0.1 can therefore leave capacity occupied, eventually preventing later requests that share the long-lived limiter from proceeding and causing a denial of service. Agent-level max_concurrency and non-streaming model requests are not affected. This issue is fixed in version 2.53.0.

[ Fixes ]Fixes and patches

  • pydantic-ai 2.53.0Fixed version · GitHub Advisories · pip package pydantic-ai: upgrade to 2.53.0 or later (affected >= 2.10.0, < 2.53.0)
  • pydantic-ai-slim 2.53.0Fixed version · GitHub Advisories · pip package pydantic-ai-slim: upgrade to 2.53.0 or later (affected >= 2.10.0, < 2.53.0)
  • Patch on github.comVendor patch or bulletin · NVD · Reference tagged 'Patch' by NVD
  • Patch on github.comVendor patch or bulletin · NVD · Reference tagged 'Patch' by NVD
  • Patch on github.comVendor patch or bulletin · NVD · Reference tagged 'Patch' by NVD

[ Packages ]Affected open-source packages

[ References ]References