Anthropic Removes Model Version Pinning, Breaking Client Applications
Anthropic's Model Version Policy Change
Anthropic has sent emails to users announcing the deprecation of claude-sonnet-4-5-20250929, forcing all users to migrate to claude-sonnet-4-6. According to their documentation, "claude-sonnet-4-6" always refers to the newest version of Claude Sonnet 4.6, and they no longer list specific version numbers on their model page.
Breaking Change for Client Applications
This change means client applications built using Sonnet will unpredictably break at random times when the model version changes. There's no way to pin to a specific version anymore - checking the model listing in their API playground and documentation confirms this limitation.
When users contacted Anthropic customer support, they encountered an AI chatbot called "Fin" (outsourced from Intercom) that provided contradictory information. The chatbot stated: "if I use the version 'claude-sonnet-4-6', it will always refer to the latest model version, so I should instead pin to the specific version, 'claude-sonnet-4-6'" - which are identical strings.
The user reports that Anthropic's AI chatbot doesn't comprehend that these strings are the same, and there are no actual human support representatives available to address this issue.
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