Claude for Lawyers: Comparing Two Contract Versions
6 min read
The second draft arrives with a note that says "minor edits." You know better. Somewhere in forty pages, a defined term has been narrowed, a cap has moved, and a notice period has changed from thirty days to fifteen. A comparison tool will show you every changed character. What it will not do is tell you which changes matter, group them by topic or explain how one interacts with another.
That is where Claude can help: as a reader that sorts and describes changes, with you checking the result against the text. This article sets out a method and its limits. It is a drafting and review aid for professionals, not legal advice, and the duties come first.
Duties before prompts
Using an assistant on a contract raises questions that sit above any technique. They are for you and your organization to answer, and they come first.
- Confidentiality. A client's draft is confidential. Check what your engagement terms, your firm's policy and your professional rules allow before putting any of it into an assistant. If in doubt, ask.
- Settings. If you do use one, know what it retains and uses. Claude Privacy Settings: A Real Walkthrough covers the controls for consumer plans. Firm and enterprise accounts have separate terms.
- Redaction. Replace party names, addresses and figures with placeholders where you can. The analysis of what changed rarely needs real names.
- Responsibility. Whatever Claude produces, the review is yours. Treat it as a first pass by a junior colleague who is fast and sometimes wrong.
No legal advice, no final review
An assistant's summary of a contract is not a legal opinion. It can miss a change, misread a clause or invent a section that is not there. Use it to organize your review, then read the actual text of every change that matters.
What an assistant is good and bad at here
Reasonable uses
- Grouping changes by topic: payment, liability, term, termination
- Describing in plain words what a changed clause now does
- Spotting where a change in one place affects another clause
- Drafting a cover note that lists the changes for the client
Where it can fail
- Missing a small change in a long document, such as one word in a definition
- Describing a clause from general knowledge, not from your text
- Treating similar clauses as the same when a qualifier changed
- Tidying the meaning of a clause that is genuinely ambiguous
That last failure is subtle and important. Contracts are sometimes ambiguous on purpose or by accident. A summary that makes them sound clear has removed information.
Set it up so it works from the text
Anthropic's prompting guidance gives two tips that matter for this task. Put long documents first and your question after them, and when you use several documents, label each one so Claude can tell them apart. Ask for quotes, so each statement can be traced.
Document A is the version we sent on 3 September. Document B is the counterparty's revised version.
Document A: [paste]
Document B: [paste]
Using only the text of these two documents, list every substantive change between A and B, in the order the clauses appear. For each change give: the clause number, the exact words in A, the exact words in B, and a one-sentence description of the effect, stated neutrally. Do not rely on general knowledge of how such clauses usually read. If you are not sure whether something has changed, say so rather than guessing.
”The quotes are the safeguard. When both versions are quoted, you can find each change in seconds and see whether Claude read it correctly.
Group and rank, as a second step
Once the list exists, ask for structure. Keep this separate from the first step so that the list stays tied to the text.
Group the changes above into: changes that look cosmetic, changes that shift money or risk between the parties, changes to rights of termination or exit, and changes to defined terms. Within each group, order by how much you think I should look at it first, and say why in one line. Mark clearly that this ordering is your judgment, not a legal conclusion.
”Ask what it might have missed
A useful instruction in any review is to ask the reviewer to say where they are weakest.
What kinds of changes would be easiest for you to miss in a comparison like this? Given these two documents, which sections should I check manually regardless of what you reported? Also list any clause that appears in A and not in B, or the reverse.
”Deleted and added clauses are a common blind spot, because it is easier to compare two versions of the same paragraph than to notice a missing one.
Verify against a true comparison
A document comparison tool is more reliable than an assistant for finding every textual difference, because it does not interpret. A good workflow uses both: the tool to guarantee nothing is missed, the assistant to explain and organize what the tool found.
- 1
Run a redline in your usual tool
This is the complete record of what changed.
- 2
Give Claude the two versions and ask for the change list with quotes
As in the prompt above.
- 3
Reconcile the two lists
Anything in the redline that Claude did not list is a miss. Anything Claude listed that is not in the redline is an error.
- 4
Read the clauses that matter in full
The summary points you to them. It does not replace reading them.
Reconciling the two catches both kinds of mistake, and it takes a few minutes.
Use it for what comes after
When you know what changed, Claude can help you write: a plain-language cover note for the client, a list of issues to raise with the other side, or a table of positions for a call. Keep those documents factual, and have them reviewed before they leave.
Other assistants have similar legal workflows, each with its own limits: Gemini for Lawyers: Summarizing a Contract Without Missing the Catch and Copilot for Lawyers: What It Can and Can't Review are worth reading alongside this one. For the habits behind a trustworthy prompt, see The Prompt Framework That Fixes a Vague Claude Answer.
Official sources
- Prompting best practices | Claude Developer Docs
- Is my data used for model training? | Claude Privacy Center
Checked on October 24, 2026. This article is a review method, not legal advice; follow your professional and organizational rules. Features, plans and names change often, so the vendor's own pages are the final word.