What Opus 4.5 is and why lawyers are paying attention
Anthropic announced Claude Opus 4.5 in late November 2025 and positioned it as its most powerful frontier model to date, supported by updated release notes from Anthropic's help center.[1] Anthropic's launch materials emphasize performance improvements across difficult tasks, including long-horizon work and improved robustness against prompt injection attacks.[2]
Even though many public examples focus on engineering, the underlying capabilities translate well to legal workflows because complex legal work resembles long-horizon problem solving: interpret intent, maintain constraints, reconcile definitions, and produce consistent outputs across long documents.
Capabilities that map cleanly to legal work
1) Long-horizon drafting and consistency across large documents
Legal drafting often fails not at the sentence level, but at the document system level: definitions drift, cross-references break, representations conflict with covenants, and schedules diverge from the body. Anthropic's launch materials highlight long-horizon performance and token efficiency, which matters when you are iterating through multiple revisions and keeping context stable.[2]
Practical uses:
- Contract consistency checks: defined terms, section references, notice provisions alignment.
- Litigation packs: chronology + exhibit list + witness outline generated from the same record.
- Template adaptation: tailoring a known-good pleading to a new fact pattern while preserving required elements.
2) Spreadsheet-adjacent work (damages support, settlement modeling, fee analytics)
Anthropic's Opus 4.5 announcement includes claims about improved Excel automation and financial modeling performance on internal evaluations.[2] For firms, this matters in litigation finance contexts, damages computations, and internal profitability analysis.
3) Better resistance to prompt injection (still not a free pass)
Anthropic states Opus 4.5 made substantial progress in robustness against prompt injection attacks, and describes it as harder to trick than other frontier models in their benchmarking, with references to external evaluation partners.[2] That is directly relevant to legal document workflows, where models may be exposed to adversarial text embedded in documents.
Still, "more robust" does not mean "safe to run privileged documents through any interface." Prompt injection resistance is one layer. You still need data access controls, retention clarity, and strict output review.
Risks and ethics: the legal-specific failure modes
Risk 1: Confidentiality and privileged material
ABA guidance highlights confidentiality obligations under Model Rule 1.6 and instructs lawyers to be cognizant of the duty to keep confidential information relating to representation unless consent or an exception applies.[3] This applies regardless of which vendor you use: you must understand what is stored, where, for how long, and who can access it.
Risk 2: "Polished wrong" output and hidden assumptions
Better models tend to be more persuasive writers. In law, persuasiveness without correctness is dangerous. Your integration should force the model to expose assumptions and to label uncertainty. That means standardized output formats: issue list, assumptions, what evidence is missing, and what must be verified.
Risk 3: Over-delegation of judgment
ABA guidance emphasizes competence and understanding the benefits and risks of technology used in representation.[3] Even if Opus 4.5 is excellent at drafting, it is not the lawyer. You remain responsible for the final work product.
How to integrate Opus 4.5 safely in a firm
1) Start with "document engineering" workflows
The safest early wins are internal drafting systems and quality checks: clause libraries, checklist generation, internal knowledge-base summarization, and style normalization.
2) Establish "no raw client data" boundaries until governance is complete
Many firms begin with de-identified scenarios or public documents. Then they move toward confidential use only after confirming enterprise terms, retention controls, and staff training.
3) Implement review gates and auditability
- Require attorney review before any AI-assisted draft goes to a client or court.
- Maintain an internal record of prompts used for high-risk deliverables.
- Use a citation verification checklist for any legal authority mentioned.
Bottom line
Claude Opus 4.5 is framed by Anthropic as a step forward in long-horizon task performance and safety robustness, including improved prompt injection resistance, which is specifically relevant to document workflows common in law.[2] The winning approach is the same as with any frontier model: treat it like a powerful junior collaborator, then design the surrounding system for confidentiality, verification, and responsible supervision aligned to ABA duties.[3]
References
- Anthropic Support, "Release Notes: Claude Opus 4.5 launch" (Nov 24, 2025).
- Anthropic, "Introducing Claude Opus 4.5" (Nov 2025).
- American Bar Association, "ABA issues first ethics guidance on a lawyer's use of AI tools" (Jul 29, 2024).