Claude Sonnet 4.5 is the latest flagship large language model (LLM) from Anthropic, introduced in late 2025 as a major upgrade to the Claude family.
In simple terms, Claude Sonnet 4.5 is an AI assistant designed to excel at complex coding tasks, building autonomous agents, and reasoning over very long contexts.
Anthropic touts it as “the best coding model in the world” and the strongest model for creating sophisticated agents. It also brings substantial improvements in reasoning, math, and overall intelligence compared to its predecessors.
This article breaks down what’s new in Claude Sonnet 4.5, its key strengths, technical specs, and how it stacks up against top competitors like OpenAI’s ChatGPT-4 Turbo and Google’s Gemini 1.5 Pro.
What’s New in Claude Sonnet 4.5 (vs. Claude Sonnet 4)
Claude Sonnet 4.5 builds on the Claude 4 series with multiple significant enhancements over the previous Claude Sonnet 4 model:
- Superior Coding Performance: Sonnet 4.5 achieves state-of-the-art results on software coding benchmarks (e.g. SWE-bench Verified). It demonstrates better planning and system design, improved security-aware coding (finding vulnerabilities), and stricter adherence to specifications when writing code. Anthropic reports that experts see “dramatically better” domain-specific knowledge in fields like software development, with Sonnet 4.5 delivering higher accuracy on complex coding tasks than older models. In practice, developers have observed it maintaining focus on multi-step coding projects for over 30 hours continuously without losing track – a testament to its extended concentration and “extended thinking” capabilities.
- Stronger Autonomous Agents: Claude 4.5 is purpose-built for agentic AI use-cases, meaning it can work independently for extended periods while using tools and making decisions. Compared to Sonnet 4, it introduces extended autonomous operation – the model can tackle goals for hours on end with steady, incremental progress and factual progress updates. It has new context awareness to monitor its own token usage and avoid losing track of objectives in long sessions. Tool use is more efficient and parallelized: Sonnet 4.5 can call multiple tools or API functions in parallel (for example, performing several web searches or reading multiple files simultaneously) to gather information faster. This, combined with better state management, means Claude can serve as a robust AI agent handling complex workflows (for coding assistants, research agents, etc.) with minimal human intervention. In short, it’s the best model Anthropic has ever made for building complex AI agents that operate autonomously.
- Long-Context Reasoning: One of Claude 4.5’s standout features is its very large context window. It supports a 200,000-token context by default – roughly the length of 150,000 words or hundreds of pages of text – allowing it to ingest and reason about very large documents or collections of data in one go. Moreover, Anthropic offers a limited beta version with an expanded 1 million token context window, pushing the boundaries of long-context AI. This long memory means Claude 4.5 can maintain coherence and recall details across lengthy conversations or files. In fact, on the OSWorld benchmark (which tests real-world computer tool use and long-horizon tasks), Sonnet 4.5 now leads with a score of 61.4% – a huge jump from Sonnet 4’s 42.2% just a few months prior. The model’s improved long-context handling, along with a new Memory tool that lets it store/retrieve information beyond the context window, enables more effective reasoning over extended sessions and large data sets.
- Refined Communication & Alignment: Alongside its raw capabilities, Claude 4.5 has a more polished interaction style and safer behavior. It communicates in a concise, direct manner and focuses on factual updates when running long workflows (avoiding unnecessary verbosity unless asked). Under the hood, Anthropic also notes that Claude Sonnet 4.5 is their “most aligned” frontier model to date. That means it has improved guardrails and follows user instructions accurately while avoiding harmful or off-topic outputs to a greater degree than earlier Claude models. This makes it a reliable assistant for high-stakes domains like law, medicine, finance, and other expert fields, where early testers have noticed “dramatically better” reasoning and domain knowledge. In creative tasks, Sonnet 4.5 is also no slouch – it can generate presentations, visual designs, and narrative content with a polished, on-point style that matches or exceeds Claude Opus 4.1 (Anthropic’s other flagship) in those areas. Essentially, Claude 4.5 combines creativity with precision, making it versatile for both technical and imaginative applications.
In summary, Claude Sonnet 4.5 advances the Claude series on all fronts: it’s more intelligent, more capable of handling long and complex tasks, better at using external tools/computers, and safer and more user-friendly in its responses.
Anthropic recommends upgrading to Sonnet 4.5 for all Claude users, as it is a drop-in improvement over Sonnet 4 in nearly every aspect.
Technical Specs, Pricing, and Availability
To fully understand Claude Sonnet 4.5, let’s look at its key specifications and how you can access it:
- Context Window: 200K tokens by default, with an optional 1M-token context in beta for select users. This huge context length (equivalent to ~800k words) is one of the largest in the industry, enabling the model to handle very long inputs or conversations. (For comparison, 200k tokens is about 5× the context of GPT-4’s 32k model, and the 1M beta context is unprecedented outside of Google’s experimental models.)
- Multimodal Input: Claude Sonnet 4.5 supports text and image inputs, and produces text outputs. This means you can give it an image (e.g. a diagram or a screenshot) alongside your prompt and it can analyze or discuss the image. This multimodal capability, combined with its coding skills, even allows it to interpret diagrams or extract data from charts within images as part of solving problems.
- API Model Name: The model identifier for developers is
claude-sonnet-4-5(with version/date suffix). For example, via the Claude API you would use the model"claude-sonnet-4-5-20250929"to call Sonnet 4.5. It’s also offered on third-party AI platforms: on AWS Bedrock asanthropic.claude-sonnet-4-5-20250929-v1:0and on Google Cloud Vertex AI asclaude-sonnet-4-5@20250929. In other words, developers can readily integrate Claude 4.5 into their apps through Anthropic’s API or major cloud AI services. - Pricing: Unchanged from Claude 4. Claude Sonnet 4.5 uses the same pricing as the previous generation, which is $3 per million tokens for input and $15 per million tokens for output. This rate is significantly cheaper than many competing high-end models. For instance, OpenAI’s GPT-4 Turbo (128k) costs about $10 per million input tokens and $30 per million output tokens, so Claude 4.5 offers a considerably lower cost-per-token for both prompting and generation. In practice, $3/million tokens works out to just $0.003 per 1,000 tokens – an economical rate that makes large-context and code-intensive uses much more affordable. Pricing is the same whether you use Claude 4.5 via API or the Claude.ai interface (billing depends on your subscription plan in the latter case).
- Where to Access: Everywhere. Anthropic made Claude Sonnet 4.5 available across all its channels upon launch. If you’re a general user, you can chat with Claude 4.5 on the official Claude.ai website (the Claude web app) – it’s typically the default model for Pro users on the Max plan. There are also Claude mobile apps for iOS and Android which give you the same Claude assistant on your phone, with seamless sync of conversations across devices (e.g. start on web, continue on mobile). The mobile apps support voice input and even let Claude interface with your phone’s tools (with permission) for things like reading your location or setting reminders. For developers, the Claude API and SDKs provide direct access to Sonnet 4.5 in your own software – just use the model name as mentioned. Additionally, Claude 4.5 is integrated into Claude Code, Anthropic’s coding-optimized interface (including a VS Code extension and a new terminal environment), where it can write, execute, and edit code with special tooling. Finally, Anthropic has also rolled out Claude integrations in platforms like Slack (so teams can query Claude right within chat) and a Claude for Chrome browser extension for using Claude as a browsing assistant. In summary, whether you’re using the Claude app, the developer API, or third-party enterprise services, Claude Sonnet 4.5 is readily accessible as Anthropic’s top-tier model.
Claude Sonnet 4.5 vs. ChatGPT-4 Turbo vs. Google Gemini 1.5 Pro
Claude 4.5 enters a competitive field of cutting-edge AI models. Here’s a brief comparison of how it stacks up against two other leading models in late 2025 – OpenAI’s ChatGPT-4 Turbo and Google DeepMind’s Gemini 1.5 Pro:
- OpenAI ChatGPT-4 Turbo: This is OpenAI’s enhanced GPT-4 model, released in late 2023 as a faster, more context-rich successor to GPT-4. GPT-4 Turbo introduced a 128,000-token context window (about 300 pages of text), multimodal input with image understanding, and major cost reductions (roughly 3× cheaper on input tokens and 2× on outputs compared to the original GPT-4). In practice, ChatGPT-4 Turbo is a very capable general-purpose model, excellent at a wide range of tasks and widely integrated (it powers ChatGPT’s latest version, including the Vision and voice features for users, and is available via OpenAI’s API). However, in specialized areas like coding and agent autonomy, Claude Sonnet 4.5 has an edge. Anthropic optimized Claude 4.5 extensively for coding workflows and tool use – for example, developers note Claude’s ability to directly execute code, use version control, and manage long coding sessions with fewer resets. Claude 4.5’s context window (200k) is also larger than GPT-4 Turbo’s 128k, allowing it to handle bigger inputs in a single prompt. Additionally, from a pricing standpoint, Claude’s token costs are lower (OpenAI’s 128k GPT-4 costs about $10/million in, $30/million out, versus Claude’s $3/$15). That said, GPT-4 Turbo remains extremely strong in general knowledge, language understanding, and creative writing, and many users might continue to use both models for different strengths. But for those building complex coding agents or needing marathon AI sessions, Sonnet 4.5 offers a compelling advantage in focus and tooling – Anthropic even calls it the best model for “using computers” (e.g. controlling browsers, terminals, etc.).
- Google Gemini 1.5 Pro: Gemini 1.5 Pro is Google DeepMind’s state-of-the-art model (announced early 2024) that pushed the envelope especially in long-context and multimodal abilities. It’s a multimodal model that can handle text, images, and even audio/video inputs, and notably introduced an experimental 1 million token context window in private preview. In fact, Gemini 1.5 Pro supports up to 2 million tokens context in production on Google Cloud, the largest of any widely available model. This allows it to parse truly massive documents or hours of media. Gemini also demonstrated near-perfect recall in long documents (finding a “needle in a haystack” of a million tokens with 99.7% accuracy). In terms of raw performance, Gemini 1.5 Pro was reported to reach quality on par with Google’s previous flagship (Gemini 1.0 Ultra) while being more efficient. When comparing Claude 4.5 to Gemini: Gemini’s strengths are its multimodal prowess and ultra-long context, which currently exceed Claude’s (e.g. Gemini can analyze a lengthy video or 10+ hours of audio in one go, which Claude cannot do as it’s limited to text/images). However, Gemini 1.5 Pro was initially available only to select developers via Google’s AI Studio/Vertex AI, so it’s not as openly accessible. Moreover, for interactive coding assistance and agent tasks, Claude 4.5’s finely-tuned autonomy and coding tools integration make it very effective. Claude’s 1M token beta context narrows the gap, and its focus on iterative problem-solving over long sessions aligns with use cases like pair programming or research analysis. In summary, Gemini 1.5 Pro is a formidable model with unprecedented scale (great for giant datasets and multimodal analysis), while Claude 4.5 offers a more accessible, specialized solution for coding and agent applications with excellent long-text handling (200k+ tokens) and a strong safety alignment.
Bottom line: Claude Sonnet 4.5 stands out as an AI model tailored for real work – especially coding projects, complex analytical tasks, and autonomous agent scenarios.
Its improvements over Claude 4 make it more reliable and powerful in extended use sessions, and its large context window and tool-using abilities unlock new possibilities (like AI agents that write and execute code or browse the web on your behalf).
With availability through Claude’s web and mobile apps, as well as an API at reasonable pricing, Sonnet 4.5 is both cutting-edge and user-friendly.
While competitors like ChatGPT-4 Turbo and Gemini 1.5 Pro each have their own advantages, Anthropic’s Claude 4.5 firmly secures its place among the top-tier AI models of 2025, offering a compelling blend of intelligence, context length, and coding/agent expertise for anyone from casual users to developers and enterprises.