{"type":"post","id":"pydantic-ai-v2-64-0-adds-a-modelfallback-2b5ea5","slug":"pydantic-ai-v2-64-0-adds-a-modelfallback-2b5ea5","title":"Pydantic AI v2.64.0 adds a ModelFallback capability, a first_event_timeout setting and moves json-repair to an optional dependency","description":"Pydantic AI released v2.64.0 on October 30, adding a constructible ModelFallback capability, per-attempt request preparation, and a timeout that can let a stalled stream fall back to another model.","retrieval_nugget":"Pydantic AI released v2.64.0 on October 30, adding a constructible ModelFallback capability, per-attempt request preparation, and a timeout that can let a stalled stream fall back to another model. The release gives agent builders more explicit runtime controls, while adding dependency and compatibility changes worth testing before an upgrade.","published_at":"2026-10-31","updated_at":"2026-10-31","record_date":"2026-10-31","date_kind":"published_at","topics":["harness-engineering","agent-runtime"],"entities":["Pydantic AI","Pydantic","ModelFallback","BoxdSandbox","ArtifactScanner","Azure AI Voice Live","ElevenLabs Agents"],"related_patterns":["harness-architecture-outlives-model-choice"],"novelty_label":"new_fact","what_changed":"Pydantic AI’s v2.64.0 release adds ModelFallback, a first_event_timeout setting, and makes json-repair optional, beyond the prior archive coverage of v2.63.0.","prior_refs":["memory:eb3d0cd2-c2fa-811c-a87e-82bbf404c8de","memory:32895520-ce74-8397-a122-b099582dc531"],"source_url":"https://github.com/pydantic/pydantic-ai/releases/tag/v2.64.0","source_urls":["https://github.com/pydantic/pydantic-ai/releases/tag/v2.64.0"],"schema_version":"newruntime-agent-readable-v0.2","stable_id":"post:pydantic-ai-v2-64-0-adds-a-modelfallback-2b5ea5","status":"published","visuals":[],"editor":{"role":"editor","name":"Andrey Reshetnikov","profile":"https://newruntime.com/owner-profile.md","profile_json":"https://newruntime.com/owner-profile.json","same_as":["https://www.linkedin.com/in/reshetnikov1/","https://t.me/lapetuse"]},"routes":{"html":"https://newruntime.com/posts/pydantic-ai-v2-64-0-adds-a-modelfallback-2b5ea5/","markdown":"https://newruntime.com/posts/pydantic-ai-v2-64-0-adds-a-modelfallback-2b5ea5.md","json":"https://newruntime.com/posts/pydantic-ai-v2-64-0-adds-a-modelfallback-2b5ea5.json"},"next_reads":[{"type":"topic","path":"/topics/harness-engineering/","url":"https://newruntime.com/topics/harness-engineering/","title":"Harness Engineering","reason":"Explore the harness-engineering topic hub.","media_type":"text/html","markdown_url":"https://newruntime.com/topics/harness-engineering.md","json_url":"https://newruntime.com/topics/harness-engineering.json"},{"type":"topic","path":"/topics/agent-runtime/","url":"https://newruntime.com/topics/agent-runtime/","title":"Agent Runtime","reason":"Explore the agent-runtime topic hub.","media_type":"text/html","markdown_url":"https://newruntime.com/topics/agent-runtime.md","json_url":"https://newruntime.com/topics/agent-runtime.json"},{"type":"collection","path":"/posts/","url":"https://newruntime.com/posts/","title":"Field Notes","reason":"Open the latest editorial analysis.","media_type":"text/html","json_url":"https://newruntime.com/posts.json"},{"type":"collection","path":"/source-ledger.json","url":"https://newruntime.com/source-ledger.json","title":"Source Ledger","reason":"Inspect the public source evidence graph.","media_type":"application/json"}]}