GPT-5.1 vs Claude Sonnet 4.5 — Which one leads the frontier in 2025?

OpenAI’s GPT-5.1 is an incremental but product-focused update that introduces two usage-flavored variants (Instant and Thinking), extended prompt caching, and new developer tools; Anthropic’s Claude Sonnet 4.5 is a targeted upgrade oriented at coding, agentic workflows, and long-running, tool-heavy tasks. Both push agentic capabilities and improved safety, but they take different trade-offs in pricing, ergonomics and how they expose “thinking” versus “doing.”

What is GPT-5.1 and what are its headline features?

GPT-5.1 is an OpenAI update (released November 2025) to the GPT-5 line. OpenAI markets 5.1 as an upgrade that improves conversational warmth and usability, and it introduces two delivery variants: GPT-5.1 Instant (warmer, more conversational, lower-latency) and GPT-5.1 Thinking (longer, deeper reasoning when needed). The update also expands ChatGPT’s personality presets and introduces finer developer controls such as a reasoning_effort knob (including a new 'none' setting for latency-sensitive workloads).

GPT-5.1 — notable engineering and developer features

• Adaptive/variable reasoning: GPT-5.1 dynamically varies how many tokens it “spends thinking” based on task difficulty; simple queries return faster with far fewer reasoning tokens while complex queries get more internal deliberation. OpenAI reports substantial speedups on the easier half of representative ChatGPT tasks.
• Two modes (Instant / Thinking): Auto-routing and developer control let product experiences prefer low latency or deeper reasoning.
• New developer tools: apply_patch for editing code reliably and a shell tool to run shell commands from the model pipeline (improves agentic workflows and programmatic automation).
• Steerability / personalities: Expanded presets (Professional, Friendly, Candid, Quirky, etc.) and settings that let the model change tone and persona.
• Multimodal support & tool integrations: GPT-5.1 as having multimodal intelligence (text, images, and richer web/tool integrations), as well as built-in tool-calling and web search for developers.

Reported developer/benchmarks improvements

OpenAI and early partners report that GPT-5.1 outperforms GPT-5 on a variety of code and reasoning suites, and runs 2–3× faster than GPT-5 in some tool-heavy contexts while using fewer tokens for many tasks. Representative benchmark numbers published show gains on SWE-bench and GPQA variants (detailed below).

What is Claude Sonnet 4.5 and what are its headline features?

Claude Sonnet 4.5 (released Sept 29, 2025) is Anthropic’s Sonnet-class frontier model. Anthropic positions Sonnet 4.5 as its most capable model for coding, agentic tasks, and “using computers” — meaning it’s explicitly optimized for actions such as editing files, running code, interacting with web pages, spreadsheets and long, multi-step agentic workflows. Anthropic emphasizes alignment improvements (reduced sycophancy, deception, etc.) alongside greater long-horizon persistence.

Claude Sonnet 4.5 — standout engineering & product features

• Agentic endurance / long-running tasks: Sonnet 4.5 can sustain continuous autonomous work for over 30 hours on realistic engineering tasks — a big jump from earlier Opus models that managed hours rather than days. This is central to the “agents that build software” pitch.
• Best-in-class coding & ‘computer use’: Sonnet 4.5 shows top performance on software engineering benchmarks (SWE-bench high scores) and adds product features like improved Claude Code with checkpoints, integrated file creation (spreadsheets, slides), and code execution features.
• Alignment and safety: Anthropic reports Sonnet 4.5 is their “most aligned frontier model,” with training procedures and internal safety classifiers aimed at reducing problematic behaviors and preventing misuse (ASL-3 classification for sensitive categories is referenced ).
• Multimodal and document understanding: Claude supports text and image input, improved extraction from image-heavy documents (Box’s early tests show image extraction accuracy gains), and APIs via Anthropic, AWS Bedrock and Vertex AI. Audio/video support is less emphasized publicly than OpenAI’s broader multimodal claims, though Anthropic continues to extend modalities.

How do their architectures and capabilities differ?

Architecture and inference style (high level)

• OpenAI / GPT-5.1: Built as a hybrid reasoning system that adapts reasoning effort per request. OpenAI describes the model as able to trade off latency, token consumption and reliability via reasoning_effort. GPT-5.1 integrates tightly into OpenAI platform features (ChatGPT UI, API, web search, tool-calling) and introduces specialized tools for developer workflows (apply_patch, shell). That indicates a design that optimizes both interactive UX and programmatic agents.
• Anthropic / Claude Sonnet 4.5: Engineered as an agent-centric model with explicit emphasis on “computer use” and long-running stateful workflows. Sonnet’s endurance (30 hours) and features like checkpoints and code execution suggest architecture and training that favor persistent context management, robust tool orchestration, and strong code editing ability. Anthropic’s safety-first engineering (e.g., classifiers, alignment tuning) is baked into model behavior.

Tooling, agent orchestration and environment control

• GPT-5.1 provides first-class developer controls for reasoning/latency tradeoffs and new tools to edit code and run shell commands; plus improved “thinking” budgets, target coding and agent workflows. OpenAI’s product ecosystem (ChatGPT, a new Atlas browser agent mode, Microsoft partnership) makes it a strong integrator for tool-heavy applications.
• Claude Sonnet 4.5 is explicitly billed as best-in-class for coding and agent construction; optimized to operate tools and control environments—its Claude Agent SDK and Claude Code improvements (checkpoints, file creation, code execution) reflect a focus on reliable multi-step automation and safe persistence.

Context window, memory and session handling

• GPT family (OpenAI): GPT-5/5.1 supports a 400K token context window—specifically 272K input tokens and 128K output tokens; combined input/output and cached context handling that can push effective session length higher. GPT-5.1 adds extended prompt caching (up to 24 hours) to improve follow-up performance.
• Claude Sonnet 4.5 (Anthropic): Claude Sonnet 4.5 uses a context window of 200,000 lexical units (expandable to 1 million lexical units for specific applications) to process input and maintain the dialogue state within this limit, butSonnet 4.5 can sustain extended autonomous runs(up to 3 hours) and better maintain internal state across files/sessions.

Safety & alignment approaches

Both companies continue to bake alignment into training and deployment. Anthropic leans heavily on constitutional and red-teaming frameworks and calls out reduction in sycophancy or deceptive behavior in Sonnet 4.5; OpenAI emphasizes instruction-following, reduced hallucination and configurable personality/preset controls in 5.1.

Bottom line: GPT-5.1 optimizes product ergonomics and developer flow; Sonnet 4.5 optimizes for agentic reliability, coding quality and sustained tool usage. The underlying architectures are proprietary and similar in the high-level Transformer + instruction-tuning sense, but design choices and integrations differ.

Public benchmarks compare

note: benchmark methodologies vary; “tool-enabled” vs “no-tools” results differ

Benchmark snapshots (representative numbers)

Real-world qualitative results

• Task-specific metrics (agentic / long-horizon): Sonnet 4.5 highlights very large gains for long-running agentic tasks (ability to sustain multi-hour or day-scale workflows). Anthropic and reporters cite Sonnet sustaining ~30 hours of autonomous operation; GPT-5.1 emphasizes faster small-task latency and token efficiency for conversational and tool-calling tasks. These are different axes (endurance vs interactive latency).
• Coding & code editing: Sonnet claims zero-error rates on certain internal edit benchmarks that previously had ~9% error; GPT-5.1 reports improvements and new tools (apply_patch)Both vendors focused heavily on coding reliability this cycle.
• Mode differences: Many benchmark numbers depend on whether tool access (execution environment, python tool) was allowed during evaluation. Performance with tools can be dramatically different. OpenAI/GPT-5.1 explicitly documents “reasoning_effort” settings that change behavior; Anthropic documents hybrid modes (near-instant vs extended thinking) for its Sonnet/Haiku/Opus families.

Practical takeaway: If your workload is heavy on structured, testable code and autonomous agent execution, Sonnet 4.5 shows measurable advantages. If you need broad general-purpose chat and fast developer iterability, GPT-5.1 focuses on that product space .

How do their multimodal capabilities compare?

GPT-5.1: broad multimodality + tool integrations

OpenAI’s GPT-5 family (and GPT-5.1) supports text + vision + audio + video inputs in ChatGPT workflows, and it continues to expand audio and browse/agent features in ChatGPT products (e.g., Atlas browser + agent mode). GPT-5.1’s design intentionally blends multimodal understanding with tool calling (web search, function calls), which is ideal for interactive assistants that must combine vision, text and external knowledge.

Claude Sonnet 4.5: mature vision + document extraction; agents for “computer use”

Sonnet 4.5 supports text and image inputs and performs strongly on image-heavy document extraction (Box reported ~80% accuracy vs 67% for the prior Sonnet). Sonnet 4.5’s unique angle is how those multimodal inputs are used inside long agentic sessions (for example, examining screenshots, running commands, generating code, and iterating).

Practical differences

• If your workflow needs immediate, broad audio/video understanding plus web browsing and multimodal chat → GPT-5.1’s product positioning and integrations (ChatGPT Atlas/browser agent, web search) make it a powerful choice.
• If your workflow is heavy on code, document automation, and long agentic sessions that interact with files and UIs → Claude Sonnet 4.5 is tailored for those “computer use” workloads and currently advertises stronger long-horizon, tool-orchestration endurance.

How much do GPT-5.1 API and Claude Sonnet 4.5 API cost?

Interpretation: At list price GPT-5.1 is materially cheaper per token for input and output than Sonnet 4.5 (roughly ~2–3× cheaper on output by list price), but real cost depends on caching, batching, and how many tokens the model uses (OpenAI claims GPT-5.1 uses fewer tokens on many simple queries).

CometAPI provides access to both GPT-5.1 API and Claude Sonnet 4.5 API, and the API price is 20% of the official price. You can use both models on CometAPI without changing your vendor.

Cost selection guidance

• If raw per-token list cost is the primary factor, GPT-5.1 is cheaper on list rates. If your workload is token-efficient (few tokens per call) and latency sensitive, GPT-5.1’s reasoning_effort options can further reduce bills by spending fewer internal tokens on easy queries.
• If your workload requires running extended agentic sessions that do lots of internal state changes, file edits, or long horizon processes that are hard to cache, Claude Sonnet 4.5 may provide better task completion value despite higher per-token list prices because it is optimized for lengthy multi-step work and developer productivity gains

Which model should you choose for specific use cases?

Use case: interactive chatbot, customer support, high concurrency, low latency

Recommendation: GPT-5.1.
Why: GPT-5.1 Instant’s lower latency, token efficiency on simple tasks and steerability (personality presets) make it a strong fit for high-volume chatbots and customer experiences where per-request latency and cost matter. OpenAI’s reasoning_effort='none' option is specifically designed for latency-sensitive workloads.

Use case: developer productivity, code editing, long agentic automation (CI, infra, long workflows)

Recommendation: Claude Sonnet 4.5.
Why: Sonnet’s explicit engineering for “computer use,” checkpoints in Claude Code, and demonstrated long-running autonomous operation (~30 hours) make it favorable for sustained engineering tasks and agentic automation that must keep context for many steps and hours.

Use case: multimodal document extraction / image-heavy workflows

Recommendation: Both are competitive — choose based on environment.
Why: Both vendors support multimodal workflows. Sonnet has demonstrated meaningful gains in extracting structured data from images/documents; GPT-5.1 emphasizes broader multimodal + tool integrations and web browsing. If your workflow includes web search + multimodal chat, GPT-5.1 may be easier; if it’s heavy file automation and spreadsheet manipulation, Sonnet may be superior.

Conclusion — “Which is better?”

There is no single answer. Claude Sonnet 4.5 looks like the practical leader when your primary need is autonomous, long-running, code-centric work (agents that use files, execute, test and iterate). GPT-5.1 is the more productized, conversationally polished upgrade of the GPT family with developer ergonomics (extended caching, new tools), making it ideal for broad conversational assistants, rapid developer workflows . For any production decision, run a short, representative pilot and cost model it end-to-end — the architectures are both strong, but the right choice depends on whether you prioritize agentic tooling+reliability (Sonnet) or conversational UX + ecosystem integrations (GPT-5.1).

Regarding the question—— GPT-5.1 vs Claude Sonnet 4.5: which is better— if you want to find the answer yourself, then visit  GPT-5.1 API and Claude Sonnet 4.5 API through CometAPI, the latest model version is always updated with the official website. To begin, explore the model’s capabilities in the Playground and consult the API guide for detailed instructions. Before accessing, please make sure you have logged in to CometAPI and obtained the API key. CometAPI offer a price far lower than the official price to help you integrate.

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