TLDR
Upgrade to Claude Sonnet 4.5 if you: Work with coding (28% improvement on SWE-bench), need mathematical capabilities (100% AIME with tools), require long-running agentic tasks (30+ hour focus), or want reduced hallucinations at no price increase.
Stick with Claude 3.5 Sonnet if you: Already have working integrations and don't need the coding/math improvements, or want to wait for more real-world testing of the newer model.
Budget: Both models cost exactly the same at $3/$15 per million tokens.
Performance: Sonnet 4.5 significantly outperforms 3.5 Sonnet across coding, math, and agentic tasks. Same price, better performance.
Overview
Claude 3.5 Sonnet, released in June 2024 (updated October 2024), established Anthropic's reputation for coding and reasoning excellence. It set new benchmarks with 64% on an internal agentic coding evaluation and operated at 2x the speed of Claude 3 Opus.
Claude Sonnet 4.5, released on September 29, 2025, represents a major evolution of Anthropic's flagship model. It improves SWE-bench performance from 64% to 82%, achieves a perfect 100% on AIME 2025 with tools, and can sustain focus on complex tasks for over 30 hours.
The best part? Anthropic kept the pricing exactly the same at $3/$15 per million tokens. This is a rare example of significantly better AI performance at no additional cost.
Basics: Model Specifications
| Feature | Claude Sonnet 4.5 | Claude 3.5 Sonnet |
|---|---|---|
| Release Date | September 29, 2025 | June 2024 (updated Oct 2024) |
| Context Window | 200K tokens | 200K tokens |
| Max Output | 8,192 tokens | 8,192 tokens |
| Knowledge Cutoff | Not disclosed | April 2024 |
| Modalities | Text, Vision | Text, Vision |
| Long-horizon Focus | 30+ hours | Standard |
| Shortcut Behavior | 65% reduction vs 3.7 | Baseline |
| Pricing | $3/$15 | $3/$15 |
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Pricing: No Change, Better Value
| Model | Input (per 1M tokens) | Output (per 1M tokens) | Performance Gain |
|---|---|---|---|
| Claude 3.5 Sonnet | $3.00 | $15.00 | Baseline |
| Claude Sonnet 4.5 | $3.00 | $15.00 | +28% SWE-bench |
For a typical task using 100,000 input tokens and generating 10,000 output tokens:
- Claude 3.5 Sonnet: $0.45 per request
- Claude Sonnet 4.5: $0.45 per request (same cost, better results)
Both models offer up to 90% cost savings with prompt caching and 50% savings with batch processing.
Performance: Benchmark Comparison
Coding Performance
| Benchmark | Claude Sonnet 4.5 | Claude 3.5 Sonnet | Improvement |
|---|---|---|---|
| SWE-bench Verified (standard) | 77.2% | 64%* | +20.6% |
| SWE-bench Verified (parallel) | 82.0% | Not applicable | - |
| Terminal-Bench | 50.0% | Not disclosed | - |
| Internal Agentic Coding | Not disclosed | 64% | - |
*Claude 3.5 Sonnet's 64% was on an internal agentic coding evaluation, not the standard SWE-bench
Claude Sonnet 4.5 achieves dramatically higher coding scores, with the parallel compute version reaching 82% on SWE-bench Verified. This makes it one of the world's best coding models.
Mathematical Reasoning
| Benchmark | Claude Sonnet 4.5 | Claude 3.5 Sonnet | Winner |
|---|---|---|---|
| AIME 2025 (with Python tools) | 100% | Not disclosed | Sonnet 4.5 |
| AIME 2025 (without tools) | 87% | Not disclosed | Sonnet 4.5 |
| GPQA Diamond | 83.4% | Strong (exact score not public) | Sonnet 4.5 |
Claude Sonnet 4.5 achieves a perfect score on AIME 2025 when allowed to use Python tools, demonstrating exceptional mathematical capabilities.
Computer Use & Agentic Tasks
| Benchmark | Claude Sonnet 4.5 | Claude 3.5 Sonnet | Winner |
|---|---|---|---|
| OSWorld | 61.4% | Not disclosed | Sonnet 4.5 |
| Tau-bench (Retail) | 86.2% | Not disclosed | Sonnet 4.5 |
| Tau-bench (Airline) | 70.0% | Not disclosed | Sonnet 4.5 |
| Tau-bench (Telecom) | 98.0% | Not disclosed | Sonnet 4.5 |
Claude Sonnet 4.5 excels across domain-specific agentic benchmarks, with an exceptional 98% score in the telecom domain.
Long-Horizon Task Performance
One of Sonnet 4.5's most significant improvements is its ability to maintain focus on complex tasks for over 30 hours. This is crucial for:
- Autonomous agents: Running long-duration workflows without drift
- Complex debugging: Following intricate code paths across large codebases
- Multi-step research: Maintaining context through extended investigations
- Sustained development: Building features that require many sequential steps
Anthropic reports a 65% reduction in shortcut behavior compared to Claude 3.7 Sonnet, meaning the model is less likely to skip steps or take inappropriate shortcuts to complete tasks.
Vision Capabilities
Both models support vision tasks with the same core capabilities:
- Image analysis and understanding
- Chart and graph interpretation
- Screenshot reading
- Document processing
Claude Sonnet 4.5's improvements are primarily in reasoning depth rather than raw vision capabilities, so both models perform similarly on pure vision tasks.
Speed Comparison
Both Claude Sonnet 4.5 and Claude 3.5 Sonnet operate at similar speeds. Claude 3.5 Sonnet was 2x faster than Claude 3 Opus, and Sonnet 4.5 maintains comparable performance.
For latency-sensitive applications, both models deliver responses quickly enough for real-time use cases.
When to Upgrade to Claude Sonnet 4.5
Upgrade if you need:
- Better coding performance: 28% improvement on SWE-bench (77.2% vs 64%)
- Mathematical capabilities: Perfect AIME 2025 scores with tools
- Long-running agents: 30+ hour task focus with 65% less shortcut behavior
- Computer use automation: 61.4% OSWorld performance
- Domain-specific agentic tasks: High scores on Tau-bench across industries
- Reduced hallucinations: Better accuracy and reliability
- Latest improvements: Access to Anthropic's newest capabilities
Stay with Claude 3.5 Sonnet if you:
- Have stable integrations: Existing production systems work well
- Don't need coding improvements: Your use cases don't involve software development
- Want proven stability: Prefer the longer track record of 3.5 Sonnet
- Are conservative about upgrades: Want to wait for more real-world validation
Migration Considerations
Good news: Both models use the same API structure, context window (200K), and pricing ($3/$15). Migration is straightforward:
- Update model identifier to
claude-sonnet-4-5-20250929 - Test on representative examples
- Deploy with confidence (same costs)
Compatibility: Both models support:
- Same context window (200K tokens)
- Same max output (8,192 tokens)
- Vision capabilities
- Prompt caching and batch processing
Availability
Claude Sonnet 4.5:
- Anthropic API
- Claude web interface (claude.com)
- iOS and Android apps
- Amazon Bedrock
- Google Cloud Vertex AI
Claude 3.5 Sonnet:
- Same platforms (may be phased out over time)
Orchestrate Multiple Claude Models with Miniloop
Even within Anthropic's own model family, you might want to use different versions for different tasks. Claude Sonnet 4.5 excels at coding and math, while older models might be sufficient for simple text processing.
With Miniloop, you can build AI workflows that intelligently route between Claude models based on task requirements. Use Sonnet 4.5 for complex coding and math, while potentially using Claude 3.5 Haiku for simple validation or text formatting.
Miniloop lets you:
- Route complex tasks to Sonnet 4.5, simple tasks to cheaper models
- A/B test different Claude versions on your specific workloads
- Combine Claude with other providers (GPT-4o, Gemini, DeepSeek)
- Upgrade model versions gradually by workflow step
- Build hybrid pipelines that optimize cost and performance
Stop using the same model for every task. Start building intelligent multi-model workflows with Miniloop.
Sources
- Claude Sonnet 4.5 Features and Pricing - Leanware
- Claude Sonnet 4.5 Model Specs - Galaxy.ai
- Claude Sonnet 4.5 Pricing and Benchmarks - LLM Stats
- Claude Sonnet 4.5 vs Opus 4.5 Comparison - LLM Stats
- Claude 3.5 Sonnet - Anthropic
Frequently Asked Questions
Should I upgrade from Claude 3.5 Sonnet to Claude 4.5 Sonnet?
Yes, if you work with coding, math, or agentic tasks. Claude Sonnet 4.5 improves SWE-bench from 64% to 82%, achieves 100% on AIME 2025 with tools (vs lower scores), and maintains focus for 30+ hours. The pricing remains the same at $3/$15 per million tokens.
What's the biggest improvement in Claude Sonnet 4.5?
Coding performance. Claude Sonnet 4.5 achieves 77.2% on SWE-bench Verified in standard runs and 82% with parallel compute, up from 64% in Claude 3.5 Sonnet. It also achieves a perfect 100% on AIME 2025 with Python tools.
Is Claude Sonnet 4.5 more expensive than 3.5 Sonnet?
No, Claude Sonnet 4.5 costs the same as 3.5 Sonnet: $3 per million input tokens and $15 per million output tokens. You get significantly better performance at the same price.
Can Claude Sonnet 4.5 work on longer tasks than 3.5 Sonnet?
Yes, Claude Sonnet 4.5 can maintain focus on complex tasks for over 30 hours. Anthropic reports a 65% reduction in shortcut behavior compared to Claude 3.7 Sonnet, making it better for sustained autonomous work.
