Which AI Models Are Best for Legal Work? Claude Sonnet 5, Claude Fable 5, and GPT-5.6 Are Now Live in Lito
AI Summary
Which AI models are best for legal work? The newest frontier models from Anthropic and OpenAI, including Claude Sonnet 5, Claude Fable 5, and GPT-5.6 Sol and Terra, are now generally available in Lito, Litera's award-winning Legal AI agent, for chat and custom skills. This post covers what each model does best, when to use flexible Generative AI (GenAI) versus deterministic legal-specific AI, how firms can govern and enable these models securely, and why time saved alone isn't the right measure of Legal AI value.
TL;DR
- Claude Sonnet 5, Claude Fable 5, and GPT-5.6 Sol and Terra are now live in Lito for chat, custom skills, and multi-document review templates
- The most important model choice isn't between frontier models; it's between flexible GenAI and deterministic, rules-based accuracy on high-stakes work. Lito has both, giving legal professionals the right AI for the job
- The real measure of Legal AI isn't efficiency alone; it's Return on AI (RoAI): Efficiency Growth, Relationship Growth, and Business Growth together
Table of Contents
- Why Choosing the Right AI Model for Legal Work Is Getting Harder
- What Are Claude Sonnet 5, Claude Fable 5, and GPT-5.6?
- Which AI Model Should Law Firms Use for Legal Work?
- How Does Lito Use Claude Sonnet 5, Claude Fable 5, and GPT-5.6?
- What Other Models Are Available in Lito?
- What Does This Mean for Your Firm's Return on AI?
- How Do Law Firms Get These Models in Lito?
- Frequently Asked Questions
Why Choosing the Right AI Model for Legal Work Is Getting Harder
Artificial Intelligence (AI) now moves faster than any firm's technology roadmap. New frontier models land in days, not quarters, and clients expect their firms to keep pace without ever compromising accuracy. But that speed creates a quieter problem: with a new model arriving seemingly every week, which one should you actually trust with a redline, a due diligence review, or a client-facing summary?
On high-stakes work, "mostly right" is a risk no lawyer can sign off on. And when every model lives in a different tab, behind a different login, the firm ends up paying for AI that sits unused.
Twelve-month technology roadmaps are obsolete. Firms don't need a prediction about where AI is headed; they need a partner that evolves in real time, putting the latest and most capable models into lawyers' hands as they become available, inside the tools they already work in, governed by the same enterprise-grade security they already trust.
That's what Litera delivers. Now, three of the most capable frontier models available are live in Lito, Litera's award-winning Legal AI agent: Anthropic's Claude Sonnet 5 and Claude Fable 5, and OpenAI's GPT-5.6 Sol and Terra.
All three are generally available to Lito customers for chat and for the custom skills and multi-document review templates lawyers build in Lito Studio. They ship toggled off, so administrators keep full control of the rollout. Availability in the model selector can vary by your organization's configuration, region, and entitlements.
What Are Claude Sonnet 5, Claude Fable 5, and GPT-5.6?
Claude Sonnet 5, Claude Fable 5, and GPT-5.6 are the newest frontier models from Anthropic and OpenAI, each built for a different kind of work.
Claude Fable 5 is Anthropic's most capable model to date, state-of-the-art across knowledge work, complex reasoning, and analysis. It's especially strong at document-heavy work: it reads diagrams, charts, and tables nested inside long PDFs, and sustains reasoning across long, multi-stage tasks. That makes it well suited to the deepest, most complex matters, where the analysis has to hold up across hundreds of pages.
Claude Sonnet 5 pairs near-flagship intelligence with the speed and lower cost that high-volume work demands. It's built for agentic, multi-step tasks and sustained focus on long documents, with lower rates of hallucination than the model before it. That makes it a strong fit for the everyday drafting, summarizing, and review that fill a lawyer's day.
GPT-5.6 Sol and Terra are OpenAI's newest lineup, ranging from flagship models built for the hardest reasoning to faster, lighter options for everyday use. GPT-5.6 brings OpenAI's latest advances in coding, reasoning, and a hardened security posture to the same governed Lito experience.
Which AI Model Should Law Firms Use for Legal Work?
The right model depends on the job. And the most important choice isn't between one frontier model and another. It's between flexible GenAI and deterministic precision.
GenAI models like Claude Sonnet 5, Claude Fable 5, and GPT-5.6 are built for open-ended work: first-draft language, research, summaries, and exploratory analysis. They're powerful, but even the most capable probabilistic model can't guarantee precision on the work a firm can't get wrong. That's why Lito pairs these frontier models with deterministic, rules-based engines shaped over 30 years of legal expertise. On the redline, the comparison, and the high-stakes review where a probable answer carries too much risk, Lito surfaces deterministic accuracy, powered by a proprietary redline algorithm that's more accurate than general-purpose LLMs. In practice, that combination cuts a redline from roughly five hours to 30 minutes.
That combination is what no AI-first startup can shortcut. Frontier intelligence handles the flexible work. Rules-based engines refined over three decades of legal expertise stand behind output where accuracy is non-negotiable. Speed and accuracy stop being a trade-off and lawyers get to choose the right AI for the task at hand.
How Does Lito Use Claude Sonnet 5, Claude Fable 5, and GPT-5.6?
Lito uses all three models across its chat and Studio experience. In chat, lawyers select the model that fits the task from a model dropdown Litera has tested and curated for quality, reliability, and performance, then work against their documents in plain language. The choice persists for the chat session, so the experience stays consistent from start to finish.
In Lito Studio, the no-code workflow builder, lawyers build for themselves: custom AI skills and multi-document review templates, tuned to the firm's own matters and playbooks. Every skill is saved with the model it was built on, so a skill built on Claude Fable 5 always runs on Claude Fable 5. The result is repeatable, predictable, and consistent across the team. No new platforms, no new logins, no behavior change required.
What Other Models Are Available in Lito?
Lito gives firms a curated choice of leading models rather than locking them into one. Alongside Claude Sonnet 5, Claude Fable 5, and GPT-5.6 Sol and Terra, Lito supports other frontier models for chat, custom skills, and multi-document review templates. Firms can match the model to the work and never fall behind the pace of AI.
This multi-model approach reflects a simple reality: no single model is best at everything. The firm that can move seamlessly between models, governed under one security and compliance framework, is the firm that stays ahead.
What Does This Mean for Your Firm's Return on AI?
A faster model, on its own, only saves time. And in a billable-hour model, efficiency without growth is a dangerous proposition. When AI compresses legal work, billable hours contract. The math only works if the time AI saves converts into expanded matters and new client relationships.
That's why the real measure of Legal AI isn't efficiency alone. It's RoAI: Efficiency Growth (time saved on high-volume work), Relationship Growth (the deeper client trust that comes from more accurate, consistent output), and Business Growth (the new and existing matters that open up when lawyers have capacity for the work that actually grows a practice). Most platforms deliver one part of that equation. Litera is built to deliver all three, by unifying the practice and business of law on a single trusted platform.
These newest models don't just make Lito faster. They give lawyers more capacity for the deeper client relationships and strategic work that fuel sustainable growth. That's the difference between AI that saves time and AI that raises the bar on what a firm can achieve.
How Do Law Firms Get These Models in Lito?
Claude Sonnet 5, Claude Fable 5, and GPT-5.6 Sol and Terra are available to Lito customers inside the tools lawyers already use, within the same enterprise-grade security and governance built for legal and certified to ISO 27001 and SOC 2 Type 2. They ship toggled off, so administrators retain full control of the rollout.
Getting started is straightforward:
- Administrators enable the models in the admin center
- Lawyers select a model from the dropdown in Lito chat or build custom skills and templates on it in Lito Studio
- Availability in the selector can vary by your organization's configuration, region, and entitlements
No new infrastructure to provision. No new platforms to learn. The models work inside the tools your team already trusts, with the governance your firm already relies on.
Frequently Asked Questions
Which AI models are available in Lito?
Lito supports a curated set of leading frontier models, including Anthropic's Claude Sonnet 5 and Claude Fable 5, OpenAI's GPT-5.6 Sol and Terra, and other Claude and GPT models, for chat, custom skills, and multi-document review templates.
What is the difference between Claude Sonnet 5 and Claude Fable 5?
Claude Sonnet 5 balances near-flagship intelligence with speed and lower cost, making it a strong fit for high-volume, everyday work. Claude Fable 5 is Anthropic's most capable model, built for the deepest, most complex analysis, including document-heavy work with diagrams, charts, and tables.
Can you trust GenAI to redline a contract?
On high-stakes work like redlining, deterministic accuracy matters more than a probable answer. Lito pairs frontier GenAI models with rules-based engines shaped over 30 years, using a proprietary redline algorithm that's more accurate than general-purpose LLMs, so lawyers get precision where the stakes are highest.
How do firms enable the new models in Lito?
The models ship toggled off. A firm's administrators enable them in the admin center, after which lawyers select a model from the dropdown in Lito chat or build custom skills on it in Lito Studio. Availability can vary by configuration, region, and entitlements.
Are the new models secure enough for legal work?
The models run inside the same enterprise-grade security and governance framework Lito customers already use, certified to ISO 27001 and SOC 2 Type 2, with administrators controlling the rollout.
What is RoAI, and why isn't time saved the right measure?
RoAI stands for Return on AI. It measures AI value across three components: Efficiency Growth (time saved), Relationship Growth (deeper client trust), and Business Growth (existing and new revenue). A faster model alone only saves time. Litera is built to deliver across all three, converting the hours AI frees up into capacity for the deeper relationships and new matters that drive sustainable growth.
How does a firm choose the right AI model for each task?
The most important choice isn't between frontier models. It's between flexible GenAI for open-ended work like drafting and research, and deterministic, rules-based AI for high-stakes work like redlining and contract comparison. Lito makes both available on the same platform, so lawyers use the right tool for each task without switching between products.
See how the latest models work inside the workflows your team already trusts.
