Claude Sonnet 5 vs Fable 5: Which AI Model Should You Use in Lito?
Summary
The Legal Knowledge Engineering team at Litera evaluated both Claude Sonnet 5 and Claude Fable 5 on legal tasks inside Lito, the award-winning Legal AI agent from Litera. All are former practicing lawyers across multiple jurisdictions and practice areas. Sonnet 5 is the faster, more responsive choice for pace-sensitive work like checking provisions and working through documents interactively. Fable 5 is the depth model for complex analysis, multi-document synthesis, and drafting where prose quality and doctrinal precision carry the most weight. Ultimately, the right model depends on what you're working on.
TL;DR
- Sonnet 5 is the faster choice for iterative legal work; Fable 5 goes deeper on complex analysis, multi-document synthesis, and high-stakes drafting
- Both models were evaluated by the Legal Knowledge Engineering team at Litera on legal tasks inside Lito
- Verification remains standard practice with either model, and the right choice shifts by task
In This Guide
- How Does Claude Sonnet 5 Handle Legal Work?
- How Does Claude Fable 5 Handle Legal Work?
- Should You Use Sonnet 5 or Fable 5 for Legal Tasks?
- What Lawyers Should Know Before Using Any AI Model
- How Does Litera Evaluate and Deploy Trusted Legal AI?
- Frequently Asked Questions
Choosing a Legal AI model is a task-level decision, and the best evaluation comes from lawyers doing the work. That decision comes up the moment you open a complex agreement or sit down to draft a client memo, weighing a faster first pass against a stronger one.
The Legal Knowledge Engineering team at Litera, former practicing lawyers across multiple jurisdictions and practice areas, evaluated Claude Sonnet 5 and Claude Fable 5 inside Lito. Here's what they found.
How Does Claude Sonnet 5 Handle Legal Work?
Claude Sonnet 5 is Anthropic's general-purpose model in the Claude 5 generation, built for speed and intelligence in equal measure. Sonnet 5 completed responses noticeably faster than Fable 5 in Lito, while remaining accurate and dependable across legal tasks. If you're working interactively through documents, checking provisions and following up in real time, Sonnet 5 is the right model to reach for.
The LKE team found Sonnet 5 to be thorough across most tasks, sometimes more thorough than the question required. On straightforward queries, it occasionally added background context, adjacent provisions, and implications that weren't asked for. When you're exploring an issue or working through an open-ended problem, that extra depth can surface angles you perhaps hadn't yet considered. With a specific, outcome-focused question, the additional material adds length without adding value.
Sonnet 5 occasionally answered a closely related question alongside the one asked, or covered most elements of a multi-part query while quietly dropping one. Both were the exception rather than the rule, and a quick read-through against the original ask is enough to catch either.
How Does Claude Fable 5 Handle Legal Work?
Claude Fable 5 is Anthropic's most capable widely released model, designed for demanding reasoning and complex, sustained analysis. Anthropic notes it reasons more deeply than its other models by design. In the LKE evaluation, that came through clearly. Fable 5 was the most consistently reliable model the team has evaluated in Lito to date.
On accuracy, Fable 5 stood out. It was rarely wrong on the legal point and regularly surfaced details other models missed: an incomplete sentence buried in an agreement, insurer rating thresholds, secondary provisions that materially change an economic analysis. On open legal questions, it was appropriately measured, avoiding premature definitive conclusions where the answer genuinely depends on jurisdiction or facts not in the document.
Fable 5 also produced the most lawyer-like prose of any model the LKE team has tested. For client emails, memos, and drafting tasks, the output was closest to final form. Evaluators found that Fable 5 regularly appended brief drafting notes, flagging assumptions made and items to confirm. When you're reviewing the output, that instinct to surface uncertainty rather than bury it makes the work easier to finalize.
The trade-off is time. Fable 5 takes noticeably longer to complete responses than Sonnet 5, often considerably longer on complex analysis. On interactive document work where turnaround matters, Sonnet 5 is the better fit. When the question is complex, and the output will be used with minimal editing, the wait is worth it. Fable 5 also runs long on simpler questions and tightens quickly with a brief style instruction.
Should You Use Sonnet 5 or Fable 5 for Legal Tasks?
Both models perform well on legal tasks inside Lito. The decision comes down to which trade-off fits the work in front of you. Sonnet 5 moves faster and handles iterative work well; the occasional over-generation on simple questions is easy to manage. Fable 5 goes deeper and produces stronger prose, and the speed trade-off is predictable.
The table below maps the LKE team's findings to common legal tasks in Lito.
| Task | Reach for Sonnet 5 | Reach for Fable 5 |
|---|---|---|
| Checking a specific provision | ✓ | |
| Interactive Q&A through a document | ✓ | |
| Rapid iterative sessions | ✓ | |
| Complex multi-document synthesis | ✓ | |
| High-stakes doctrinal analysis | ✓ | |
| Drafting client emails and memos | ✓ | |
| Near-final output with minimal editing | ✓ | |
| Open-ended contract review | Workable | ✓ Preferred |
| Exploratory, open-ended ideation | ✓ | |
| Multi-part analysis where completeness is critical | ✓ |
Across a high-volume day of checking provisions and following up on previous answers, Sonnet 5 is the stronger fit. When the output matters more than the turnaround — whether that's a complex synthesis across multiple documents, a piece of drafting that needs to withstand review, or an analysis where dropping a single element of a multi-part question would be a problem — Fable 5 earns its wait time.
What Lawyers Should Know Before Using Any AI Model
Some behaviors aren't specific to either model. They come with the territory of working with current frontier AI models like Sonnet 5 and Fable 5 on legal tasks and knowing them going in makes them easy to manage.
- Jurisdictional assumptions surface when a prompt doesn't specify. If your question has a jurisdictional dimension, name it.
- Verify outputs on nuanced points of law, including checking that assertions are grounded in the cited source. This is standard practice regardless of which model you use.
- Confident errors on fine doctrinal questions can occur even with current frontier models. Taking a minute to read the output against your original ask catches most issues.
The LKE team's evaluation confirms both models perform well on legal tasks. These are the professional habits that belong alongside any AI output, on any task.
How Does Litera Evaluate and Deploy Trusted Legal AI?
The findings in this post come from the Legal Knowledge Engineering team at Litera, former practicing lawyers across multiple jurisdictions and practice areas, whose ongoing role is to evaluate AI on the legal tasks Lito users bring to it every day. Generic benchmarks rarely reflect the jurisdictionally specific, fact-dependent, high-stakes work lawyers bring to Lito every day.
Litera has spent more than 30 years building legal technology that lawyers rely on. That depth of practice-area knowledge is embedded in how the LKE team evaluates models, what they test for, and which behaviors they flag as manageable versus material. The bar is whether a model performs when a senior associate is working through a 200-page agreement at 11 PM.
Both Claude Sonnet 5 and Claude Fable 5 are available in Lito now, evaluated against that standard.
Frequently Asked Questions
What is Claude Sonnet 5 and is it accurate enough for legal work?
Claude Sonnet 5 is Anthropic's general-purpose model in the Claude 5 generation, evaluated by the Legal Knowledge Engineering team at Litera on legal tasks inside Lito. It's accurate and dependable, with occasional over-generation on simple questions and rare misses on multi-part queries. For iterative legal work inside Lito, Sonnet 5 is the right model to reach for.
What is Claude Fable 5 and how does it compare to Sonnet 5 for legal tasks?
Claude Fable 5 is Anthropic's most capable widely released model, designed for demanding reasoning and complex, sustained analysis. In the LKE evaluation, it was the most consistently reliable model tested in Lito to date, with stronger accuracy on nuanced legal points, more lawyer-like prose, and output closest to final form on drafting tasks. For high-stakes legal work where accuracy is the priority, Fable 5 is the stronger choice.
Which Claude model should I use for contract review and drafting in Lito?
Fable 5 is the stronger fit for drafting and near-final output. The LKE team found its prose to be the most lawyer-like of any model evaluated, and it regularly flagged assumptions and items to confirm. Sonnet 5 handles interactive clause-level review and quick provision checks well, with faster turnaround.
How do Claude Sonnet 5 and Fable 5 compare for legal contract review?
Sonnet 5 handles clause-level review and quick provision checks well. On deep contract analysis, multi-document synthesis, or review where completeness across a complex agreement is critical, Fable 5 is the stronger fit. The LKE team found Fable 5 regularly surfaced details other models missed on complex agreements.
Which model is faster, and when does speed matter more than depth?
Sonnet 5 completes responses noticeably faster than Fable 5, often considerably faster on complex analysis. Working interactively through a document, checking provisions, or running multiple follow-up prompts are where that speed shows. Complex questions, high-stakes analysis, and work that will be used with minimal editing are where Fable 5 is the stronger fit.
Do I need to verify outputs from Claude Sonnet 5 and Claude Fable 5?
Yes, for both. Jurisdictional assumptions, nuanced doctrinal points, and occasional confident errors are frontier-model realities regardless of which model you're using. The LKE team treats verification as standard professional practice. Reading the output against your original ask and spot-checking that cited sources support the assertions made covers most of what needs verifying.
How does Litera decide which AI models to make available in Lito?
The Legal Knowledge Engineering team at Litera, former practicing lawyers across multiple jurisdictions and practice areas, evaluates models on legal tasks before deployment. The standard is accuracy on legal work, behavior on high-stakes analysis, and usability for lawyers in their daily workflows. That process is how Litera determines the best Legal AI for lawyers working inside Lito, drawing on more than 30 years of legal-specific expertise.
Trusted Legal AI, Evaluated by Lawyers
Both Claude Sonnet 5 and Claude Fable 5 are available in Lito now, evaluated by lawyers who understand the work you do. Let's talk about what that means for your team.
