Anthropic has officially deprecated Claude Sonnet 4.5, setting a hard retirement date of November 30, 2026. Developers using the specific snapshot ID claude-sonnet-4-5-20250929 have exactly 61 days to migrate before requests fail outright with no grace period. This move follows the company's established lifecycle policy, where deprecated models remain functional but unsupported until their scheduled termination.

The Trap of Dated Model IDs

The core risk lies in hardcoded snapshot IDs versus rolling aliases. While aliases like claude-sonnet-5-5 automatically track the latest version, dated IDs like claude-sonnet-4-5-20250929 are frozen in time. If your environment variables or configuration files still point to this specific string, your application will break on December 1 unless you proactively update the reference. This pattern is particularly dangerous for solo builders who set up scripts once and neglect them for months.

Migration Path and Breaking Changes

The recommended replacement is claude-sonnet-5-5, which offers identical pricing and a matching 1M token context window. However, a simple find-and-replace is insufficient. At least one breaking change introduced in Sonnet 5.5 alters output shape without throwing an error, meaning silent failures are possible in production. Developers must test against real inputs rather than relying on single-prompt validation to ensure behavioral consistency.

Audit Your API Usage Immediately

Anthropic provides a free Usage export in the Claude Console that breaks down API calls by key and model. This CSV allows you to verify if any active keys are still hitting the retiring ID. It is critical to note that this deprecation schedule applies strictly to Anthropic-operated platforms, including the Claude API, AWS Claude Platform, and Microsoft Foundry. Users accessing Sonnet 4.5 via Amazon Bedrock or Google Cloud must check those providers' independent retirement schedules, as they do not automatically align with Anthropic's dates.

Key Takeaways

  • Retirement Date: November 30, 2026, for the snapshot ID claude-sonnet-4-5-20250929.
  • Replacement: Migrate to claude-sonnet-5-5, which maintains the same price and context window.
  • Action Item: Use the Claude Console Usage export to identify hardcoded dated IDs in your stack.
  • Scope Warning: Bedrock and Google Cloud users must verify separate retirement timelines.
  • Testing Requirement: Validate output shapes manually, as some changes may not trigger explicit errors.

The Bottom Line

Sixty-one days sounds like ample runway until you are debugging a broken script on November 29. Run the audit now; the cost of migration is negligible today, but the cost of downtime in December is not.