Best Foundation Models for Muse

Claude Sonnet 5.5 is a general-purpose AI model from Anthropic built for everyday professional tasks, agentic coding, knowledge work, and fast iteration. It is the second model in the Claude 5.5 family and is intended to complement Claude Opus 5.5 by offering lower-cost performance on more clearly defined workloads. Anthropic reports that Sonnet 5.5 generates output more than 30% faster than Sonnet 5 and usually completes tasks with fewer tokens. Pricing remains $2 per million input tokens and $10 per million output tokens, with cache reads priced at $0.20 per million tokens. On Terminal-Bench 4.0, the model scores 70.6%, compared with 10.3% for Sonnet 5, while also posting sizable improvements on FrontierCode, CursorBench, and other coding benchmarks. Its knowledge-work results are also substantially stronger, including a GDPval-AA score of 1844 and an AA-Briefcase score of 1811, both close to Claude Opus 5.5. Anthropic says early testers found the model faster, more collaborative, more concise, and particularly strong at design-oriented work such as polished interfaces and presentation decks. The model supports adjustable effort levels so users can trade off speed and cost against more extended reasoning and checking. Claude Sonnet 5.5 is available through Claude apps, Claude Code, the Claude Platform, and major cloud providers including AWS, Google Cloud, and Microsoft Azure.

GPT-6 Astra is an advanced general-purpose AI model from OpenAI designed for demanding agentic, technical, scientific, and professional workflows. The model combines reasoning with computer-use capabilities that let it interact with applications, websites, development environments, productivity software, and specialized tools. It can perform activities such as online research, CRM updates, form completion, calendar organization, data analysis, software testing, troubleshooting, and frontend quality assurance. Astra is also trained for professional artifact creation, including documents, presentations, spreadsheets, analyses, websites, applications, and visual designs that follow provided templates and business conventions. Its software engineering capabilities support complex coding, codebase understanding, debugging, database migration, verification, and extended autonomous development tasks. OpenAI has also introduced a Codex context system for Astra that can maintain notes across context windows and search earlier conversation and tool history instead of relying exclusively on repeated summarization. Scientific capabilities span areas such as mathematics, biology, chemistry, health, data analysis, and the use of specialized research software. The model includes strengthened alignment and cybersecurity safeguards intended to help it remain within authorized task boundaries while supporting legitimate activities such as secure code review and vulnerability remediation. GPT-6 Astra is available across supported ChatGPT plans and developer platforms, including the OpenAI API and Amazon Web Services infrastructure.

Claude Fable 5.1 is a frontier AI model from Anthropic built for demanding coding, research, knowledge work, and autonomous multi-step workflows. It delivers higher performance than Claude Fable 5 across benchmarks covering scientific research, terminal-based coding, business automation, computer use, multidisciplinary reasoning, and agentic software development. The model is particularly suited to long-running tasks where it must investigate problems, use tools, maintain context, verify intermediate work, and continue operating with limited supervision. Early evaluations highlighted improvements in areas such as root-cause analysis, code review, browser automation, financial research, document drafting, slide creation, and complex engineering workflows. Fable 5.1 also introduces lower cache-read pricing, which Anthropic says can reduce costs by roughly 25% for typical workloads and considerably more for context-heavy agentic tasks. Enterprise customers can use new privacy-oriented safeguard options that are designed to support zero-data-retention-style deployments while still maintaining misuse protections. Cybersecurity safeguards have also been refined to intervene less often on legitimate defensive work while continuing to restrict higher-risk activities such as exploit generation. Anthropic offers the model through Claude Code, Claude Cowork, Claude.ai, the Claude API, and supported cloud platforms. Claude Fable 5.1 is intended for developers, researchers, enterprises, and professional teams that need strong reasoning and coding performance without relying on Anthropic’s most restricted-access model tier.

Claude Opus 5.5 is a frontier AI model from Anthropic built for coding, research, business analysis, computer use, and complex agentic workflows. The model is particularly suited to long-running software engineering tasks such as codebase migrations, audits, debugging, optimization, and large-scale refactoring. Anthropic also positions Opus 5.5 for professional knowledge work including financial analysis, legal workflows, research reports, spreadsheets, presentations, and business automation. Compared with Claude Opus 5, the model is designed to use fewer tokens, generate responses more quickly, and reduce typical workload costs. Its communication style has also been refined to make outputs clearer, easier to scan, and more consistent with user-defined writing requirements. Opus 5.5 includes stronger defenses against prompt injection and is less likely to take irreversible or out-of-bounds actions during autonomous tasks. Enterprise safety features include action classification, an auditable open-source sandbox, code review capabilities, preserved thinking protections, and configurable safeguards for sensitive domains. The model supports zero data retention and is available with additional verification programs for vetted life sciences and cybersecurity organizations. Claude Opus 5.5 is accessible through Anthropic’s applications and API as well as AWS, Google Cloud, and Microsoft Azure.

GPT-6 Luna is a lightweight, cost-efficient model in OpenAI’s GPT-6 family built for coding, professional tasks, computer use, and high-volume AI applications. The model incorporates advances from the same generation as GPT-6 Astra while emphasizing lower inference costs and greater efficiency for everyday workloads. API pricing is $0.10 per million input tokens and $0.50 per million output tokens, making Luna suitable for applications that process large volumes of requests. In professional work, GPT-6 Luna can execute multi-step workflows involving business applications, tools, and structured tasks across functions such as sales, marketing, operations, support, finance, and HR. For software engineering, the model can work on real codebases, perform extended development tasks, and operate within coding agents such as Codex. Its computer-use capabilities allow AI agents to interact with software interfaces and carry out longer workflows that require repeated actions and decisions. OpenAI also reports substantial factuality improvements over GPT-5.6 Luna, with higher reasoning settings enabling stronger performance on difficult factual questions. GPT-6 prompt caching provides higher cache-hit rates and discounted cached input, helping persistent agents and long conversations reuse context more efficiently. GPT-6 Luna is available through ChatGPT Work, Codex, the OpenAI API, and the ChatGPT desktop app for eligible users.

GPT-6 Sol is a frontier AI model from OpenAI built for professional knowledge work, software development, business automation, computer use, and agentic applications. It brings many of the advances introduced with GPT-6 Astra to a model optimized for greater cost efficiency and higher-volume use. GPT-6 Sol can apply configurable reasoning effort to complex tasks, giving developers and users control over how much computation is used for different workloads. Its coding capabilities support long-horizon software engineering, codebase modification, debugging, testing, and agent-driven development in tools such as Codex. For professional workflows, the model can work across applications and tools to complete multi-step tasks in areas such as sales, marketing, operations, finance, support, and HR. OpenAI also reports improved factuality compared with GPT-5.6 Sol, reducing errors on its internal evaluation of difficult real-world conversations. Computer-use capabilities allow Sol-powered agents to navigate software interfaces and perform extended workflows that require multiple actions and decisions. Enhanced prompt caching provides higher cache-hit rates and discounts on reused input context, making the model better suited to persistent agents and long conversations. GPT-6 Sol is available to developers through the OpenAI API as gpt-6-sol and is also offered in ChatGPT Work and Codex.

Claude Opus 5 is Anthropic’s new state-of-the-art Opus model for coding, knowledge work, automation, science, and everyday AI assistance. The model is designed to provide near-frontier intelligence at a lower cost than Claude Fable 5 while keeping the same base pricing as Opus 4.8. Claude Opus 5 performs strongly on software engineering benchmarks, business task automation, computer use, novel problem solving, visual generation, and scientific research tasks. Users can adjust effort settings to trade off intelligence, speed, and token usage depending on the task. The model is also better at verifying its work, iterating carefully, building test harnesses, debugging root causes, and solving multi-step engineering problems. Anthropic highlights improvements in life sciences, including structural biology, organic chemistry, bioinformatics, and protein-related tasks. Claude Opus 5 includes alignment and safety safeguards designed to support beneficial work while blocking higher-risk cybersecurity and biology misuse. It is available through Claude.ai, Claude Max, Claude Pro, Claude Code, Claude Cowork, and the Claude API under the model name claude-opus-5. By combining stronger reasoning, coding ability, scientific capability, configurable effort, Fast mode, and enterprise-ready deployment options, Claude Opus 5 gives users a powerful model for demanding daily work.

GPT-5.6 Terra is OpenAI’s balanced GPT-5.6 model for users who need strong performance across everyday work, development tasks, enterprise workflows, and technical analysis. The model is part of the GPT-5.6 family alongside Sol and Luna, with Terra positioned as the middle tier for capable, cost-efficient use. Terra is described as having competitive performance to GPT-5.5 while being 2x cheaper, making it useful for teams that want advanced capability without always using the flagship model. It supports coding workflows, agentic tasks, cybersecurity-related defensive work, biology workflows, knowledge work, and tool-assisted automation. In benchmark previews, Terra appears alongside Sol and Luna in evaluations for coding, biology, ExploitBench, and ExploitGym. The model benefits from the GPT-5.6 safeguard stack, which includes model-level refusals for prohibited cyber assistance, real-time cyber and biology misuse classifiers, and account-level risk review. These safeguards are designed to preserve access to legitimate work such as code review, debugging, vulnerability research, patch development, security education, and defensive testing. GPT-5.6 Terra is planned for availability through the API, Codex, and broader OpenAI products after the limited preview period. GPT-5.6 Terra helps teams get a balanced model for high-quality AI work when they need strong reasoning and automation at a lower cost than Sol.

Claude Mythos 5 is a frontier AI model from Anthropic created for highly trusted users working on advanced cybersecurity, infrastructure protection, and scientific research. It is based on the same core model as Claude Fable 5, but certain safeguards are lifted for approved partners operating under restricted access programs. The model offers exceptional performance across software engineering, cybersecurity analysis, autonomous development workflows, scientific reasoning, visual understanding, and long-context tasks. In cybersecurity, Claude Mythos 5 is positioned for cyberdefenders and critical infrastructure providers who need advanced AI support for securing complex systems. In life sciences, the model has demonstrated strong capabilities in drug design, protein research, molecular biology, and genomics. Claude Mythos 5 can perform long-running research and technical workflows with minimal high-level human input. Anthropic designed the model for controlled deployment because its advanced capabilities could create misuse risks if broadly available without safeguards. Access is initially limited to Project Glasswing partners, with broader trusted access programs planned for cybersecurity and select biology researchers. Claude Mythos 5 helps approved organizations apply powerful AI to high-impact technical and scientific challenges while operating within a stricter governance model.

Claude Haiku 5.5 is an Anthropic AI model optimized for high-volume applications where latency, efficiency, and reasoning capability are important. Anthropic positions the model for workloads including classification, routing, information extraction, and subagent tasks. Adaptive thinking is enabled by default, allowing Claude Haiku 5.5 to determine when reasoning is useful and how much reasoning to apply to a request. Developers can configure the effort parameter to adjust the balance between response quality, processing speed, and cost. The model has a 1 million token context window, compared with 200,000 tokens for Claude Haiku 4.5, enabling it to process substantially larger prompts and datasets. Its maximum output length has also increased from 64,000 to 128,000 tokens, with thinking tokens counting toward the configured output limit. Claude Haiku 5.5 includes browser use capabilities through the Claude API and Google Cloud, expanding its usefulness for AI agents and workflows involving web interaction. The model uses the newer tokenizer shared with Claude 4.7 and later models, causing identical text to consume approximately 30% more tokens than on Claude Haiku 4.5, although the exact difference varies by content. Claude Haiku 5.5 is suited to developers and organizations building scalable AI applications, routing systems, extraction pipelines, subagent architectures, and other workloads that benefit from a combination of fast responses and configurable reasoning.

GPT-6.1 Sol is an upgraded OpenAI model that combines advanced intelligence with lower operating costs for coding, professional knowledge work, computer use, scientific research, and autonomous agents. OpenAI positions it as offering near-GPT-6 Astra intelligence at one-fifth of Astra's standard input and output token prices. The model delivers substantial improvements over GPT-6 Sol in software engineering, complex document understanding, business automation, and long-horizon computer-use workflows. On DeepSWE v1.1, GPT-6.1 Sol matches GPT-6 Astra at approximately one-fifth of the cost and surpasses GPT-6 Sol's highest score by 6.4 percentage points at lower reasoning effort. On AutomationBench, it scores 4.8 percentage points higher than GPT-6 Sol at the same reasoning setting and 2.2 points above Opus 5.5 at medium reasoning effort. GPT-6.1 Sol also improves computer use, coming within 2.1 percentage points of GPT-6 Astra on the OSWorld 2.0 offline set at maximum reasoning effort while costing roughly one-seventh as much per task. For scientific workflows, the model more than doubles GPT-6 Sol's Terminal-Bench Science 0.1 score at maximum effort while reducing average task cost by more than half. Factuality has also improved, with the share of responses containing a factual error at low reasoning effort falling from 11.4% with GPT-6 Sol to 7.7% with GPT-6.1 Sol on OpenAI's difficult error-focused evaluation. Developers can access GPT-6.1 Sol through the OpenAI API for $2 per million input tokens, $0.10 per million cached input tokens, and $10 per million output tokens, while eligible users can access it through ChatGPT Work and Codex.

GPT-5.4 mini is an advanced AI model designed to provide a balance between high performance, speed, and cost efficiency. It is built to handle a wide range of tasks, including coding, reasoning, tool usage, and multimodal understanding. Compared to earlier versions, GPT-5.4 mini delivers significantly improved performance while operating at faster speeds. The model is particularly effective in environments where low latency is essential, such as real-time coding assistants and interactive applications. It supports capabilities like function calling, tool integration, and image-based reasoning, making it highly versatile. GPT-5.4 mini is also well-suited for subagent architectures, where it can efficiently process smaller tasks within larger AI systems. Developers can use it to automate workflows, analyze data, and build responsive AI-driven applications. Its strong performance across benchmarks shows that it approaches the capabilities of larger models in many scenarios. At the same time, it maintains a lower cost, making it ideal for high-volume usage. Overall, GPT-5.4 mini provides a powerful and scalable solution for modern AI development.

Claude Fable 5.5 is an anticipated but currently unannounced model in Anthropic's Claude family, and Anthropic has not confirmed that a model with this name will be released. As of September 30, 2026, Claude Fable 5.1 remains the latest officially documented Fable model. Fable represents Anthropic's highest-end model tier for demanding reasoning and long-horizon agentic work, while the newer Opus 5.5 and Sonnet 5.5 occupy lower-cost positions in the Claude lineup. Anthropic's current documentation gives Fable 5.1 a 1-million-token context window and maximum output length of 128,000 tokens. It supports text and image inputs with text output and uses adaptive thinking that remains active throughout model operation. Fable 5.1 defaults to high reasoning effort and is listed as having a June 2026 reliable knowledge cutoff and training-data cutoff. API pricing is $10 per million input tokens and $50 per million output tokens, while prompt-cache reads cost $0.25 per million tokens and Batch API processing receives a 50% input and output discount. Anthropic's official documentation currently provides model identifiers for Fable 5.1 across the Claude API, Amazon Bedrock, Google Cloud, Microsoft Foundry, and Claude Platform on AWS. No equivalent model identifier, specifications, benchmark results, pricing, availability information, or release schedule has been published for Claude Fable 5.5.

OpenAI Bel is an unconfirmed artificial intelligence model reportedly associated with OpenAI's research into large-scale foundation models. The name has appeared in unofficial discussions describing a possible new generation of pretrained AI systems. Reports suggest that Bel may represent a successor to an earlier internal training effort referred to as Doug. Some accounts describe the project as a potential foundation for future models related to the Astra family and later GPT generations. Unverified claims place its parameter count above 10 trillion, although OpenAI has not disclosed any supporting architectural information. The model's training methodology, supported modalities, context window, and inference capabilities remain unknown. No official benchmark evaluations have established its performance in reasoning, coding, mathematics, or other AI tasks. OpenAI has not announced public access, API availability, pricing, or a release schedule for a model named Bel. The available information therefore characterizes Bel as a rumored research project rather than an established commercial AI product.