Claude Opus 4.5 & Grok 4.1 fast Series: In-Depth Review | Performance, Usage, and Comparison of the Latest AI Models from Anthropic / xAI

mitsumonoAI implemented an update in November 2025, which included the addition of these new AI models.

• Claude Opus 4.5
• Claude Opus 4.5 Thinking
• Grok 4.1-fast-reasoning
• Grok 4.1-fast-non-reasoning

Claude Opus 4.5: The Pinnacle of Expressive Power and Contextual Understanding That Transcends Human Ability

What is Claude Opus 4.5?

Claude Opus 4.5 is the top-tier model in the Claude series, announced by Anthropic in November 2025.

It further refines the "human-like natural writing" and "long-text comprehension" praised in the previous Opus 3.5 generation, boasting performance that surpasses competing models, particularly in subtle nuance, emotional expression, and complete memory retention within ultra-long contexts.

Key Characteristics of Claude Opus 4.5

• Literary and Sophisticated Text Generation
  Mechanical stiffness is completely eliminated, allowing for emotional text that resonates with the target audience and copywriting that perfectly mimics a company's brand voice.
• "Perfect Memory" in Ultra-Long Context
  Despite having a context window equivalent to several books, it remembers even minor details within it without omission, generating consistent responses.
• Enhanced Coding Quality
  Its ability to generate "beautiful code"—considering maintainability and readability, not just writing code—has dramatically improved.
• Enhanced Safety (Constitutional AI)
  Based on Anthropic's philosophy, the safety design to prevent bias and harmful output has been further strengthened, increasing reliability for corporate use.

Suitable Applications for Claude Opus 4.5

• Creative Writing
  Content creation where word choice is crucial, such as novels, scripts, marketing copy, and press releases.
• Advanced Multi-Lingual Translation and Localization
  Natural translations that feel seamless, taking into account cultural background and idioms, rather than mere language conversion.
• Editing and Proofreading of Long Documents
  Checking consistency in papers or contracts, unifying tone and style, and refining expressions.

Specific Use Case Example for Claude Opus 4.5

Example: Creating a Brand Story for the Rebranding of a Long-Established Inn (Ryokan)

Evaluation of the Generated Result

Claude Opus 4.5 generated a brand story that beautifully harmonized the gravitas of the long-established inn with modern values, using sophisticated metaphors like "polished by time, the greatest artisan."

Specifically, centering on the concept of "the luxury of doing nothing," it instantly draws the reader into its world by depicting scenes such as the light through the shoji screens and the murmuring of the valley stream, appealing to the five senses. It completely lacks any unnatural AI tone, achieving a level of completion that could be posted to a website without revision, as if a professional copywriter had carefully refined it.

Claude Opus 4.5 Thinking: The Optimal Solution Guided by a Transparent Thought Process

What is Claude Opus 4.5 Thinking?

Claude Opus 4.5 Thinking is a model specialized in explicitly carrying out a "thought process (Chain of Thought)" before generating an answer, in addition to the expressive power of Opus 4.5.

It is ideal for tasks where you want to prevent the "black box" effect, as it allows you to verify step-by-step how the AI thought, which options it considered, and why it reached its conclusion when faced with complex problems.

Key Characteristics of Claude Opus 4.5 Thinking

• Visualized Thought Process
  It displays a "thought block" (or handles the process internally) before the final answer, allowing users to verify the logical steps taken.
• Complex Logical Construction and Self-Verification
  The AI itself verifies a hypothesis once it is generated and corrects any errors before outputting the final answer, resulting in an extremely high accuracy rate for difficult tasks.
• Advanced Coding and Debugging
  It not only speculates on the cause of an error but also traces the entire code dependency chain within its thought process to propose fundamental corrections.

Suitable Applications for Claude Opus 4.5 Thinking

• Complex System Design and Architecture Selection
  Scenarios requiring the selection of technology based on evidence, comparing and examining merits and drawbacks from multiple perspectives.
• Identification of Legal and Ethical Risks
  Tasks that involve logically and comprehensively listing potential risks in contracts or business plans.
• Identification and Correction of Difficult Bugs
  Debugging work that systematically eliminates possible causes one by one for low-reproducibility bugs.

Specific Use Case Example for Claude Opus 4.5 Thinking

Example: Drafting a Roadmap for Large-Scale Legacy System Migration

Evaluation of the Generated Result

Instead of simply listing schedules, Claude Opus 4.5 Thinking first presented the reasoning for adopting the "Strangler Fig pattern" along with a trade-off evaluation, clarifying the logical foundation of the plan.

Furthermore, after thoroughly identifying multifaceted challenges such as the "character code conversion risk" specific to COBOL and "organizational skill gaps," it derived a realistic five-year roadmap to overcome them. The resulting plan is highly sophisticated, rivaling that of a consultant, having deeply considered both technical feasibility and organizational issues.

Grok 4.1 Series: The World's Fastest Intelligence and Master of Real-Time Information

What is the Grok 4.1 Series?

It is the latest model developed by xAI, trained on "Colossus," one of the world's largest GPU clusters.

Its key features are real-time information access from X (formerly Twitter) and overwhelming inference speed. Users can select between "Reasoning" (logic-focused) and "Non-reasoning" (speed-focused) depending on the application.

Grok 4.1-fast-reasoning: The Fusion of Real-Time Information and Logic

Characteristics and Applications

It searches and analyzes vast numbers of posts on X and the latest news in real time, logically integrating this information to provide answers.

It excels in situations requiring a deep understanding of "what is happening in the world right now," such as stock price prediction, election analysis, and situational awareness during disasters.

Specific Use Case Example: Analyzing New Product Reaction Based on SNS Trends

Evaluation of the Generated Result

Grok 4.1-fast-reasoning skillfully integrated real-time sentiment, such as "frustration over reselling" on X, with concrete sales performance data, specifically the "10.36 million units" figure from Nintendo's financial reports.

Going beyond a simple summary of SNS reactions, it instantly generated a multifaceted market report—stating that "despite price dissatisfaction, demand is explosive"—by cross-analyzing the data with reliable primary sources on the web. This resulted in extremely persuasive insights that merge facts and trends, something impossible for other models relying only on static training data.

Grok 4.1-fast-non-reasoning: The "Instantaneous Power" That Leaps Over the Thinking Barrier

Characteristics and Applications

By omitting the deep reasoning process, it generates text faster than a human can type in a chat.

It is ideal for scenarios requiring speed and "quick wit," such as witty conversations, rapid brainstorming, simple data processing, and instantaneous multi-lingual translation.

Specific Use Case Example: Generating Ideas for a Meeting Icebreaker

Evaluation of the Generated Result

The overwhelming speed with which the list of 20 items appeared just moments after the instruction was impressive. Witty, relatable ideas that would make any IT engineer laugh, such as the "Server Down Scream Contest" and the "Blue Screen Dance," were instantly proposed. This demonstrates that Grok's unique humor and instantaneous power—setting it apart from the exemplary answers of general AIs—make it a powerful tool for changing the atmosphere of a room.

Model Comparison: Claude 4.5 vs Thinking vs Grok 4.1 Series

Key Points for Model Selection

✅ For human-like expressiveness and consistently high-quality text: Claude Opus 4.5
It is unmatched in situations where word choice is critical, such as writing official corporate documents, marketing copy, or narratives.

✅ If you need to solve complex problems and require transparency of the rationale: Claude Opus 4.5 Thinking
This model, which explicitly shows its thought process, is the best choice for system development, management decisions, and legal checks where "why that answer was reached" is crucial.

✅ If you want to deeply analyze "current" information and make predictions: Grok 4.1-fast-reasoning
It is indispensable for marketing and investment decisions that require tracking constantly changing market trends and making logical forecasts.

✅ If speed is your top priority, or if you want unique ideas: Grok 4.1-fast-non-reasoning
It dramatically improves the speed of your work, serving as a partner for brainstorming outside the box or as a tool requiring zero-latency response time.

Summary: Accelerate Your Business with the Latest AI Models

The Claude 4.5 series, which pursues Anthropic's philosophy of "depth," and the Grok 4.1 series, which pursues xAI's philosophy of "speed and freshness."

Each model possesses distinctly different strengths.

Claude behaves like an excellent writer or a seasoned engineer, while Grok functions like a sharp researcher who instantly grasps global information or a fast-paced partner.

At mitsumonoAI, by selectively using these models according to the task, we strongly support our customers' businesses in terms of both quality and speed. We encourage you to experience the amazing performance of the latest models.