Claude Sonnet 4.5 End-of-Life Schedule: Retirement Dates & Migration Alternatives

Executive summary: Anthropic has set the official retirement deadline for Claude Sonnet 4.5 on November 30, 2026. After this sunset date, all API requests to claude-sonnet-4-5-20250929 will fail. Upgrading to the recommended replacement, Claude Sonnet 5.5, requires code adjustments for adaptive thinking parameters, tokenizer changes, and removed assistant prefill.

It usually starts with an routine notification email. The subject line states a model your production systems rely on is being retired. You skim it, intend to handle it during next sprint, and move forward. Then one morning, every production request fails silently, stalling support bots, summarization pipelines, or code agents.

Claude Sonnet 4.5 is now in its sunset cycle. Anthropic has published firm dates, and the deprecation countdown has begun. Below is a comprehensive migration guide detailing the official schedule, breaking changes, and replacement models.

Official Sunset Schedule for Claude Sonnet 4.5

According to Anthropic's official model deprecation schedule:

Key Logistics to Consider:

1. Platform Variations: Anthropic's retirement date directly impacts Anthropic-hosted endpoints (the Claude API, Claude Platform on AWS, and Microsoft Foundry). Cloud hyperscalers like Amazon Bedrock and Google Cloud Vertex AI maintain independent deprecation timetables.
2. Standard 60-Day Notice: Anthropic adheres to a minimum 60-day deprecation policy for GA models. Teams must audit their infrastructure promptly to prevent service disruptions.
3. Auditing Active Callers: Developers should export API usage logs from the Claude Console and perform repository-wide searches for claude-sonnet-4-5 across staging, serverless lambdas, and background cron jobs.

Pricing and Cost Dynamics: Sonnet 4.5 vs. Sonnet 5.5

On paper, list prices show a significant per-token reduction:

Tokenizer Expansion Factor

While list rates are approximately 33% cheaper, Claude Sonnet 5.5 employs an enhanced tokenizer. For identical code and English prose, the new tokenizer produces approximately 30% more tokens. Factoring in this token density difference:

• Output at $10/MTok with 30% more tokens effectively balances to ~$13 per legacy-equivalent million tokens.
• Real-world bill reductions hover around 12% to 15% rather than a full 33%, depending on formatting and thinking depth.

Additionally, thinking tokens generated during reasoning passes are billed at standard output rates, and high-resolution image inputs incur higher token costs.

Replacement Options: Sonnet 5.5 vs. Sonnet 5 vs. Haiku 4.5

Recommendation: Migrate mission-critical coding agents and complex reasoning pipelines to Sonnet 5.5. High-volume, low-complexity classification tasks can be routed to Haiku 4.5 for maximum margin efficiency.

Step-by-Step API Migration Code Example

import anthropic

client = anthropic.Anthropic()

# Updated Sonnet 5.5 invocation
response = client.messages.create(
    model="claude-sonnet-5-5",
    max_tokens=4096,
    messages=[
        {"role": "user", "content": "Analyze the stack trace and generate a fix patch."}
    ],
    output_config={"effort": "medium"}  # Replaces legacy budget_tokens
)

# Parse content blocks safely
for block in response.content:
    if block.type == "text":
        print(block.text)

Breaking API Changes to Address:

• Remove Sampling Parameters: Non-default temperature, top_p, or top_k values will throw a 400 Bad Request error on Sonnet 5.5.
• Adopt Effort Setting: Replace thinking: {"budget_tokens": ...} with output_config: {"effort": "high"} or "medium".
• Eliminate Assistant Prefill: Prefilling partial assistant answers in the message history is rejected on 5.5 endpoints.
• Tool Choice Formatting: Use tool_choice: {"type": "auto"} with strict parameter schemas.

Frequently Asked Questions

When does Claude Sonnet 4.5 officially stop working?

The Claude API retires Claude Sonnet 4.5 on November 30, 2026. Subsequent calls will return API errors.

Is Claude Sonnet 5.5 a drop-in parameter swap?

No. While swapping the model ID is simple, legacy thinking budgets, temperature overrides, and assistant message prefills must be removed to avoid 400 validation failures.

Can Claude Code CLI assist with migration?

Yes. Developers using the Claude Code terminal harness can run /claude-api migrate this project to claude-sonnet-5-5 to automatically refactor SDK calls.

Conclusion

Migrating away from Claude Sonnet 4.5 before November 30, 2026 ensures continuous uptime and unlocks the superior reasoning and coding performance of Sonnet 5.5. Engineering teams should audit their codebases now, conduct regression benchmarks, and deploy updated client SDKs well ahead of the deadline.