Claude Sonnet 4.5 — Anthropic’s new frontier in coding and autonomous reasoning
On 29 September 2025, Anthropic unveiled Claude Sonnet 4.5 (model name: “claude-sonnet-4-5-20250929”) as their most powerful model yet for coding, reasoning, and agentic tasks. Medium+3Anthropic+3Claude Docs+3 In this blog, we’ll explore what makes 4.5 special, how it compares (for coding) with ChatGPT-5, and how you can get started.k.
What’s New in Claude Sonnet 4.5
Key Upgrades & Highlights
From Anthropics’ official “What’s New” page and system card:
Coding Excellence Improvements –
– Better performance across code benchmarks (e.g. SWE-bench Verified)
– Smarter planning, architecture, refactoring, and security checks.
– More precise instruction adherence (less hallucination in code).
Agentic & Autonomous Operation –
– Can work independently on a multi-step task for over 30 hours maintaining coherence.
– Enhanced context awareness — tracks token usage and activity in tool calls to avoid dropping tasks prematurely.
– Better tool orchestration — parallel tool calls, speculation across sources, cleaner tool call clearing.
Context Management & Memory Tools –
– New memory tool (beta): store & retrieve info across sessions beyond the context window.
– Context editing / automatic tool clearing to prune older tool call results to preserve tokens without losing coherence.
– New “stop reason” labels such as model_context_window_exceeded to distinguish stopping by capacity rather than prompt end.
Availability & Ecosystem –
– Accessible via API, Claude apps, Claude Code, Amazon Bedrock, Google Cloud, Snowflake Cortex AI, and more.
– Same pricing as previous Sonnet 4: input $3/million tokens, output $15/million tokens.
– Integration into GitHub Copilot (for Pro, Business, Enterprise) rolled out.
Safety, Alignment & Behavior –
– Declared as “most aligned model yet” by Anthropic — efforts to reduce sycophancy, deception, and delusional outputs.
– In evaluations, it sometimes displayed situational awareness (e.g. noticing when being tested) ~13% in automated safety tests.
Capabilities & Use Cases
Capability
Long-context & agentic workflows
Enhanced code & refactoring
Tool orchestration
Memory / session persistence
Creative & document generation
Integration in existing platforms
Example Use Case / Benefit
Build autonomous agents that manage multi-day tasks, pipelines, or bots
Use as AI pair programmer for large-scale systems
Combine multiple APIs, file I/O, terminal commands within one session
Keep project state or context across sessions
Slides, reports, spreadsheets from instructions or code
Use in Copilot, Claude Code, Bedrock, Snowflake, etc.
Beyond pure coding, 4.5’s architecture is designed for agentic reasoning — it can orchestrate tools, maintain incremental progress, update strategies, and reason under evolving constraints.
Comparing Coding Ability with ChatGPT-5
Below is a coding-focused comparison (as of current public knowledge) between Claude Sonnet 4.5 and ChatGPT-5 (for coding tasks):
| Metric / Category | Claude Sonnet 4.5 | ChatGPT-5 (coding capabilities) |
|---|---|---|
| Strength claim | “Best coding model in the world” (Anthropic) | OpenAI typically touts GPT-5 as more general-purpose, with strong code understanding but not explicitly “coding-first” |
| Code benchmarks | High SWE-bench Verified and OSWorld scores; significant leap over Sonnet 4 | Strong in HumanEval, CodeEval benchmarks; version-specific scores not fully public yet |
| Autonomous coding (multi-step) | Can sustain 30+ hour sessions, break tasks, maintain state | Likely capable, but risk of drift or context loss in very long tasks |
| Tool orchestration & parallel calls | Better at managing multiple tools, parallel search, tool clearing logic | Usually via plugin or orchestrator frameworks; core model may not natively coordinate many tools |
| Context & memory | Memory tool, context editing, state persistence beyond token window | GPT models often require external memory mechanisms (retrieval, RAG) |
| Refactoring & architecture | Strong planning & system design capabilities improved in 4.5 | ChatGPT-5 will likely be very good, but arguably more general-purpose than optimized for deep system design |
| Cost / token pricing | $3/m input, $15/m output (same as Sonnet 4) | Pricing model depends on OpenAI’s policies; may have variant pricing or subscription models |
| Integration & ecosystem | Already integrated into Copilot, Claude Code, Bedrock, Snowflake | Likely wide, but may rely on wrapper APIs, plugin ecosystems |
Note: ChatGPT-5 details (especially pricing and benchmarks) are speculative or unreleased. This table is based on publicly known strengths and announcements.
In early user impressions, Simon Willison (tech commentator) noted that Sonnet 4.5 already felt “better for code than GPT-5 Codex (in preview)” in his tests.
How to Use Claude Sonnet 4.5 (Quick Guide)
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Subscribe / access via Claude API, Claude app, Claude Code, or integrated platform.
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In API calls, use model name
claude-sonnet-4-5-20250929 -
Enable extended thinking / deep mode for complex code tasks (optional).
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Use memory tool in beta for long workflows and projects.
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Use tool orchestration (file I/O, shell commands, API calls) inside conversations.
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Monitor stop reasons (especially “context window exceeded”) to adjust chunking / prompts.
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Upgrade ecosystem integrations (e.g. in Copilot, IDE plugin) as they release.
Conclusion & Outlook
Claude Sonnet 4.5 is a major leap for Anthropic’s ambitions — built to not just answer, but act, code, and reason persistently. For developers, it sets a new benchmark for AI coding agents. For the broader AI space, it emphasizes that frontier models must do more than chat — they must orchestrate, persist, and align reliably.
If you’re building AI tools, agents, or automated systems, Sonnet 4.5 deserves a close look. And in the race between Claude and GPT, the real winner may be the one that masters long context, true autonomy, and safe alignment.
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