TokenMix Research Lab · 2026-04-25

Claude Sonnet 4 vs 4.5 vs 4.6 2026: API Migration Guide

Last Updated: 2026-04-30
Author: TokenMix Research Lab
Data checked: 2026-04-30

Do not migrate from Claude Sonnet 4 to Sonnet 4.5 by default. Sonnet 4 is deprecated, Sonnet 4.5 is active, and Sonnet 4.6 is the right default target for most teams.

Anthropic's model deprecations page lists claude-sonnet-4-20250514 as deprecated on April 14, 2026 with API retirement scheduled for June 15, 2026. It lists claude-sonnet-4-5-20250929 as active with retirement not sooner than September 29, 2026, and claude-sonnet-4-6 as active with retirement not sooner than February 17, 2027. That changes the old "Sonnet 4 vs 4.5" decision. The real 2026 question is whether to skip directly to 4.6.

Table of Contents

• Quick Verdict
• Current Model Status
• Sonnet 4 vs 4.5 vs 4.6 Comparison
• Pricing And Cost Math
• Migration Paths
• Migration Risk Matrix
• When To Use 4.5 Temporarily
• Version Pinning Rules
• TokenMix.ai A/B Test Pattern
• What Not To Do
• Final Recommendation
• FAQ
• Related Articles
• Sources

Quick Verdict

If you are still on claude-sonnet-4-20250514, migrate now. Use Sonnet 4.6 unless regression tests prove 4.5 is safer for your exact workflow.

Current Model Status

This matters for SEO and operations. A guide that says "migrate from Sonnet 4 to 4.5" is no longer wrong in capability terms, but it is incomplete in lifecycle terms. 4.6 gives the cleaner forward path.

Sonnet 4 vs 4.5 vs 4.6 Comparison

Sonnet 4.5 was a good upgrade when it launched. Sonnet 4.6 changed the default recommendation. Same price band, newer model, longer deprecation runway.

Pricing And Cost Math

The price decision is simple: Sonnet 4.5 and Sonnet 4.6 are both in the same starting API price band: $3 per million input tokens and $15 per million output tokens. Sonnet 4 should not be priced as a future option because it is deprecated.

Assume 100 million input tokens and 30 million output tokens per month.

The important economics are not 4.5 vs 4.6 headline price. They are output length, task success, fallback rate, latency, and how much traffic should not be on Sonnet at all.

Migration Paths

Migration checklist:

Minimal code change:

from anthropic import Anthropic

client = Anthropic(api_key="your-anthropic-api-key")

message = client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=800,
    messages=[{"role": "user", "content": "Run this migrated prompt."}],
)

print(message.content[0].text)

Migration Risk Matrix

The migration risk depends on what your app does with the model output. Open-ended chat can tolerate style drift. Strict parsers and agent loops need more validation.

This is why "newer model" is not the whole migration plan. Use 4.6 as the target, but validate the failure mode that actually matters to the product.

When To Use 4.5 Temporarily

4.5 is not the default target, but it can be useful as a bridge.

Set an expiration date for this choice. 4.5 has a shorter listed runway than 4.6.

Version Pinning Rules

Date-stamped model IDs are useful, but they are not a reason to avoid migration.

Pinning helps you know what happened. It does not make a deprecated model safe forever.

TokenMix.ai A/B Test Pattern

TokenMix.ai is useful when you want to compare versions without wiring every provider path separately.

Example:

from openai import OpenAI

client = OpenAI(
    api_key="your-tokenmix-key",
    base_url="https://api.tokenmix.ai/v1",
)

for model in ["claude-sonnet-4-5", "claude-sonnet-4-6", "claude-haiku-4-5"]:
    response = client.chat.completions.create(
        model=model,
        messages=[{"role": "user", "content": "Run this migration test prompt."}],
    )
    print(model, response.choices[0].message.content[:200])

Check the live model catalog before running a version-specific test.

What Not To Do

Final Recommendation

If you still use Claude Sonnet 4, migrate to Sonnet 4.6 now. Use 4.5 only as a temporary bridge for regression-sensitive workflows. Then add routing so not every task pays Sonnet prices.

FAQ

Is Claude Sonnet 4 deprecated?

Yes. Anthropic lists claude-sonnet-4-20250514 as deprecated on April 14, 2026, with retirement scheduled for June 15, 2026.

Is Claude Sonnet 4.5 deprecated?

No. It is active, with retirement not sooner than September 29, 2026. But it is not the best default for new work now that Sonnet 4.6 is active.

Should I migrate from Sonnet 4 to 4.5 or 4.6?

Migrate to 4.6 by default. Use 4.5 only if regression tests show 4.6 creates unacceptable behavior for your workflow.

Is Sonnet 4.6 more expensive than 4.5?

No at the headline API price band. Both are listed at $3 per million input tokens and $15 per million output tokens. Effective cost can still differ if output length changes.

Why not use Opus 4.7 for everything?

Opus 4.7 is more expensive. Use it for hard tasks where Sonnet fails or where the higher success rate pays for itself.

Can I keep using a date-stamped Sonnet 4 model?

Only until retirement. Date-stamped IDs help reproducibility, but they do not override deprecation schedules.

How do I test migration quality?

Use real prompts, compare task success, measure formatting errors, log token counts, and inspect failures. Do not rely only on benchmark headlines.

Can TokenMix.ai help with Sonnet migration?

Yes. TokenMix.ai can run version and provider comparisons through one OpenAI-compatible gateway, then route tasks to Sonnet, Haiku, Opus, or non-Claude models by cost and quality.

Related Articles

• Claude 3.7 Sonnet Pricing 2026: Retired, Upgrade To 4.6
• Claude Sonnet 4.5 Free Access 2026: API Test vs 4.6 Safely
• Claude Sonnet 4.6 Free Trial 2026: 5 Safe API Test Paths
• Claude API Pricing 2026: Opus, Sonnet, Haiku Costs Compared
• Claude Sonnet vs Opus 2026: Pricing, Quality, Routing Guide
• Claude Haiku vs Sonnet 2026: Cost, Quality, Routing Rules
• AI API Gateway 2026: 7 LLM Routing and Fallback Options

Sources

• Claude model deprecations
• Claude Sonnet 4.5 launch
• Claude Sonnet 4.6 launch
• Claude API pricing