We’ve done the testing to give you the benefits of the latest OpenAI models, without breaking your custom knowledge + AI stack. Enjoy two curated modes — Optimal and Smart.
13% higher accuracy compared to standard models
10% lower hallucination rates for better clarity
Faster, more adaptive responses across all use cases
Zero technical effort required to access frontier AI
Optimal and Smart tuned for balanced performance and maximum reasoning capability.
More accurate, more stable outputs.
No retraining or reconfiguration required.
“There are countless AI models and endless settings — CustomGPT.ai makes the right choices for you so our agents just work.”
GPT-5.1 is available on all plans.
Premium and Enterprise plans allow for additional customization:
Premium: Access to GPT-5.1 Optimal when using Complex Query Breakdown
Enterprise: Full access to GPT-5.1 Optimal and GPT-5.1 Smart
Learn more about GPT-5.1 in our docs
GPT-5.1 is OpenAI’s latest flagship model released in November 2025, delivering significant improvements in instruction-following, adaptive reasoning, and response speed. For CustomGPT.ai users, this means your AI agents become instantly smarter – they understand your questions more accurately, follow complex instructions more reliably, and respond faster to everyday queries while maintaining our industry-leading accuracy standards of 13% higher accuracy and 10% lower hallucination rates.
OpenAI’s API offers dozens of configuration options—model variants, reasoning effort levels, verbosity settings, and system prompt requirements—that most business users aren’t equipped to optimize. CustomGPT.ai has spent thousands of engineering hours testing every combination, identifying the optimal configurations, and packaging them into two simple choices (Optimal and Smart) with pre-optimized system prompts and anti-hallucination measures that maintain our benchmark-leading quality standards.
CustomGPT.ai achieved 13% higher accuracy and 10% lower hallucination rates compared to OpenAI’s Assistant API V2 in independent benchmarking validated by Tonic.ai. We implement anti-hallucination measures at every stage of our pipeline – from query preprocessing and prompt engineering to response validation – and provide citation sources for every answer. With GPT-5.1, we’ve maintained these industry-leading standards while delivering the newest model’s performance improvements.
Access GPT-5.1 variants through your agent’s Intelligence settings in the model dropdown menu. You’ll see GPT-5.1 Optimal, and GPT-5.1 Smart listed prominently at the top. For new agents created with Complex Reasoning capability, GPT-5.1 Optimal is automatically pre-selected as the default.
No reconfiguration required. Your existing agents continue working exactly as before with their current model selections. If you want to upgrade an agent to GPT-5.1, simply open the Intelligence settings, select your preferred GPT-5.1 variant from the dropdown, and save – the transition is instant with no retraining, no data re-ingestion, and no downtime.
Select GPT-5.1 Optimal (our recommended default) for general business applications requiring balanced performance. Pick GPT-5.1 Smart when handling complex analytical tasks, detailed technical questions, or scenarios where accuracy matters more than response time. Not sure? Start with Optimal – it handles 95% of business use cases excellently.
GPT-5.1 Optimal applies moderate thinking with modest latency – perfect for most business applications requiring accuracy and reasonable speed. GPT-5.1 Smart engages deeper reasoning with some latency—best for complex problem-solving, multi-step analysis, and situations demanding maximum accuracy.
Yes, GPT-5.1 variants work seamlessly with all existing capability modes. Your capability mode determines how we retrieve and rank information from your knowledge base, while your model selection determines how that information gets processed and presented. Both GPT-5.1 variants are available across Optimal Choice, Highest Relevance, and Complex Reasoning capabilities.
GPT-5.1 brings three major improvements: better instruction-following (your agent more reliably does exactly what you ask), adaptive reasoning (automatically applying more thinking to complex questions while responding quickly to simple ones), and a warmer conversational tone (responses feel more natural and engaging). Combined with CustomGPT.ai’s optimized prompting, you get frontier AI performance with enterprise-grade reliability.
No additional cost – GPT-5.1 is included in your existing CustomGPT.ai subscription at the same price. GPT-5.1 Optimal and Smart adaptively adjust thinking time based on question complexity, meaning faster responses for straightforward questions and more thorough answers only when needed.
We’ve extensively tested GPT-5.1 across thousands of queries using our proprietary anti-hallucination framework that achieved 13% higher accuracy in independent benchmarking. Each GPT-5.1 variant comes with carefully engineered system prompts, context boundary enforcement, citation requirements, and response validation—all optimized specifically for GPT-5.1’s architecture to maintain our industry-leading quality standards.
GPT-5.1 availability follows your current plan’s model access settings. Enterprise plans have full access to all GPT-5.1 variants. Premium plans can access GPT-5.1 Optimal when using Complex Query Breakdown agentic capability.
Yes, GPT-5.1 models are fully supported through the CustomGPT.ai API.
OpenAI reports GPT-5.1 achieved 94% accuracy on AIME 2025 math benchmarks and significant improvements on coding evaluations. CustomGPT.ai enhances these base capabilities with optimized system prompts and anti-hallucination measures that delivered 13% higher accuracy than standard OpenAI implementations in independent testing, ensuring your business-critical applications get reliable, trustworthy responses.
GPT-5.1 represents the current frontier in business AI, offering superior instruction-following, adaptive reasoning, and improved conversational quality. When delivered through CustomGPT.ai’s optimized platform with proven anti-hallucination measures and no-code simplicity, GPT-5.1 provides the ideal combination of cutting-edge performance and enterprise reliability for business applications ranging from customer support to internal knowledge management.
CustomGPT.ai’s approach combines four key strategies: RAG (Retrieval-Augmented Generation) to ground responses in your actual content, optimized system prompts that enforce context boundaries, response validation that checks for consistency, and citation requirements that enable verification. Our platform achieved 10% lower hallucination rates than standard implementations in independent benchmarking, and GPT-5.1 support maintains these industry-leading standards.
Yes—CustomGPT.ai specifically designed its GPT-5.1 implementation for non-technical business users. No coding, no prompt engineering, no configuration complexity required. Simply select your preferred variant (Optimal, or Smart) from a dropdown menu, and CustomGPT.ai handles all technical optimization, system prompting, and anti-hallucination measures automatically. Over 6,000 organizations including small businesses and enterprises use CustomGPT.ai without technical expertise.
GPT-5.1 excels at customer support (handling inquiries with improved instruction-following and natural conversation), internal knowledge management (helping employees find information quickly with adaptive reasoning), lead qualification (engaging prospects with contextually appropriate responses), content assistance (generating marketing materials with better creativity and accuracy), and technical documentation (explaining complex topics with clearer, less jargon-heavy language).
Your agents get smarter instantly — with Optimal and Smart modes built for real business performance.
