Migrating to Claude Sonnet 4.6: Complete API Guide

TL;DR

Migrating from Sonnet 4.5 to 4.6 is straightforward—update the model ID and optionally adopt new features. Key changes: Adaptive Thinking replaces extended thinking, context compaction is automatic, and 1M context is available in beta. No breaking changes for basic usage.

Quick Migration

For most applications, migration is a single line change:

# Before (Sonnet 4.5)
response = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    ...
)

# After (Sonnet 4.6)
response = client.messages.create(
    model="claude-sonnet-4-6-20260217",
    ...
)

Model Identifiers

Model	Identifier

Sonnet 4.6 (Latest)

claude-sonnet-4-6-20260217

Sonnet 4.6 (Alias)

claude-sonnet-4-6

Sonnet 4.5 (Previous)

claude-sonnet-4-5-20250929

New Features to Adopt

1. Adaptive Thinking

Replace binary extended thinking with granular effort control:

# Old (Sonnet 4.5)
response = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    thinking={"type": "enabled", "budget_tokens": 10000},
    ...
)

# New (Sonnet 4.6)
response = client.messages.create(
    model="claude-sonnet-4-6-20260217",
    thinking={"type": "enabled", "effort": "high"},  # or "low", "medium", "auto"
    ...
)

2. Context Compaction

No code changes needed—compaction is automatic for long conversations. Your application gains unlimited conversation length for free.

3. 1M Context Window

Enable beta access for large-context requests:

# Large context request (beta)
response = client.messages.create(
    model="claude-sonnet-4-6-20260217",
    max_tokens=8192,
    betas=["max-tokens-1m"],  # Enable 1M context
    messages=[{"role": "user", "content": massive_document}]
)

API Parameter Changes

Parameter	4.5 Behavior	4.6 Behavior

thinking.budget_tokens

Required for thinking

Deprecated, use effort

thinking.effort

N/A

New: low/medium/high/auto

max_tokens

Max 8192

Max 16384 (beta)

context limit

200K tokens

1M tokens (beta)

Backwards Compatibility

Sonnet 4.6 maintains backwards compatibility:

• All existing parameters work unchanged
• budget_tokens still functions but is deprecated
• Response format is identical
• Tool use, vision, and streaming work the same

Pricing Changes

Tier	4.5 Price	4.6 Price

Standard (0-200K)

$3/$15

$3/$15

Long Context (200K+)

N/A

$6/$30 (new)

If using >200K tokens, expect 2x pricing on the portion exceeding 200K.

Performance Improvements

Sonnet 4.6 delivers better results without code changes:

Metric	4.5	4.6	Improvement

SWE-bench

77.2%

79.6%

+2.4%

OSWorld

61.4%

72.5%

+11.1%

Math

62%

89%

+27%

Testing Migration

Recommended testing approach:

import anthropic

def test_migration():
    client = anthropic.Anthropic()

    # Test basic functionality
    response = client.messages.create(
        model="claude-sonnet-4-6-20260217",
        max_tokens=1024,
        messages=[{"role": "user", "content": "Hello, confirm you're Sonnet 4.6"}]
    )
    assert "4.6" in response.content[0].text or response.model == "claude-sonnet-4-6-20260217"

    # Test adaptive thinking
    response = client.messages.create(
        model="claude-sonnet-4-6-20260217",
        max_tokens=2048,
        thinking={"type": "enabled", "effort": "medium"},
        messages=[{"role": "user", "content": "Solve: What is 15% of 240?"}]
    )
    assert any(block.type == "thinking" for block in response.content)

    # Test tool use (unchanged)
    # Test vision (unchanged)
    # Test streaming (unchanged)

    print("Migration tests passed!")

Rollback Plan

If issues arise, rollback is simple:

# Rollback to Sonnet 4.5
MODEL = os.getenv("CLAUDE_MODEL", "claude-sonnet-4-5-20250929")

response = client.messages.create(
    model=MODEL,
    ...
)

Sonnet 4.5 remains available and supported.

Migration Checklist

• [ ] Update model identifier to claude-sonnet-4-6-20260217
• [ ] Replace budget_tokens with effort parameter
• [ ] Test core functionality in staging
• [ ] Monitor costs for long-context requests
• [ ] Update error handling for new response fields
• [ ] Consider adopting 1M context for applicable use cases
• [ ] Deploy with rollback capability

Conclusion

Sonnet 4.6 migration is low-risk with significant upside. The model delivers better performance across all benchmarks while maintaining API compatibility. Most teams can migrate with a simple model ID change and progressively adopt new features.