Claude 3.5 Sonnet has been superseded by the Claude 4.x family. Move to Claude Sonnet 4.6 or Claude Opus 4.6 for new projects.
Why It's Now on Hold
Claude 3.5 Sonnet was a standout model when it launched in mid-2024 — strong coding benchmarks, 200K context, excellent instruction-following. It earned its Adopt ring.
Since then, Anthropic has shipped two generations of successors:
- Claude Sonnet 4.6 (Feb 2026) — direct successor, significantly better at coding tasks, same price tier, 200K context
- Claude Opus 4.6 (Feb 2026) — top-of-line model for complex autonomous tasks, 1M context, stronger reasoning
Claude 3.5 Sonnet's API was shut down on October 28, 2025 (Vertex AI: deprecated August 2025, shut down February 2026). All requests now return errors. This isn't a soft deprecation — the model is unavailable.
Migration Path
| If you were using 3.5 Sonnet for... | Switch to |
|---|---|
| General coding tasks | Claude Sonnet 4.6 |
| Complex, multi-step reasoning | Claude Opus 4.6 |
| Cost-sensitive batch work | Claude Haiku 4.5 |
Key Characteristics
| Property | Value |
|---|---|
| Status | API shut down (Oct 28, 2025) |
| Context window | 200,000 tokens |
| Successor | Claude Sonnet 4.6 |
| Provider | Anthropic |
| API model ID | claude-3-5-sonnet-20241022 |
Claude 3.5 Sonnet by Anthropic is one of the top-performing models for software engineering tasks. Released in mid-2024, it quickly became a favourite for coding because of its strong instruction-following, long context window (200K tokens), and ability to reason through multi-step problems.
Why It Matters for Engineers
Claude 3.5 Sonnet consistently scores at or near the top on coding benchmarks like SWE-bench, which tests a model's ability to resolve real GitHub issues. In practice, this translates to:
- Writing complete, working functions and modules from a description
- Explaining complex code accurately
- Debugging by reading error messages and stack traces
- Refactoring code while preserving intent
How to Access It
You can use Claude 3.5 Sonnet through:
- The Claude.ai web interface (free and paid plans)
- The Anthropic API for programmatic access
- Third-party tools built on top of it (Cursor, Claude Code, Cline, etc.)
Key Characteristics
| Property | Value |
|---|---|
| Context window | 200,000 tokens (~150,000 words) |
| Strengths | Code generation, instruction following, long-context reasoning |
| Provider | Anthropic |
| API model ID | claude-3-5-sonnet-20241022 |
Newcomer Tip
If you're just getting started, Claude.ai's free plan gives you access to this model. Try pasting in a function you don't understand and asking "Explain this code line by line." The quality of the explanation is a good first-pass test for any coding model.