Codex and Claude Code in Legal Work: Getting Started with Word-Based Practice
Hello, I'm Noriaki Asato, Representative Attorney at LegalAgent.
Depending on the matter, at our firm (LegalAgent) we deliver many requests within one business day, and in some cases on the same day. In this column I describe the day-to-day work procedures that support this kind of rapid turnaround.
I have tried a variety of tools in pursuit of more efficient work, and in my recent practice Codex has become a very natural fit. This time, I will write about how Codex can be used in a law firm, going into how I actually use it. The overall picture of where generative AI can be used in an attorney's work and where an attorney's judgment is required is laid out in What Is an AI Lawyer? How to Divide Work Between Attorneys and AI in Corporate Legal Practice in the Age of Generative AI.
Choosing a Tool and Its Characteristics as a Work Environment
Codex may generally give the impression of being a tool for software developers to write code. Indeed, it is widely known for uses such as writing programs, fixing bugs and running tests.
However, when I actually used it in legal practice, I found that Codex can easily serve as a main tool for everyday legal work as well. Claude Code is also widely discussed for business use, and it too is an excellent tool, but at present I feel that Codex suits my daily work better.
A major reason is the screen layout and ease of operation. My impression when I started using them was that in Claude the entry points for chat and various features were separate, and there were times when I was unsure where to begin my work (screen designs are updated from time to time, so this is only my impression at the time). Someone used to development might be able to switch between them, but for a non-engineer it can be hard to see "where to do what right now."
Codex, by contrast, has a clear entry point for work. Simply by giving instructions from the same screen, I can request everything from reading a specified folder to revising Word documents, legal research and multiple tasks in parallel. In legal practice, where non-engineers use the tool every day, being able to start work without hesitation is a real help.
Another reason it suited my practice was the feel of GPT-5.5, which I was using when I wrote this column. Claude Opus 4.7 is also an excellent model, but for drafting contract documents in Japanese and examining fine points, I found GPT-5.5 at that time easier to use in terms of the clarity of its writing and the granularity of its analysis. The unnatural Japanese expressions that stood out in earlier models had settled down, and there were more expressions that could be used as they were in legal practice. In addition, by selecting Fast mode I could proceed with work at a practical speed. This is an impression based on my personal experience at the time, and it does not mean that any particular model is always all-purpose, but under the conditions of use at that time, I felt it made it easier to keep going with work involving large volumes of statutory provisions and materials. Usage limits vary depending on the plan, the period and the volume of work.
When using generative AI in legal work, many people probably imagine a procedure of pasting statutory text into a browser chat, asking a question, and manually putting the returned text back into Word. That alone provides some help, but actual legal work is not complete just by reading an answer. The work is only finished once judgments and tasks such as which part of the contract to revise, how to keep the tracked changes, how to separate comments for the counterparty from internal comments, and how far to push in negotiation in light of past similar cases and the business relationship have all been layered together.
The advantage of Codex is that it does not merely answer questions; it can look directly at folders in the local environment, duplicate the necessary files and edit them while comparing the differences. Because it works by entering the workspace itself, it greatly reduces going back and forth between screens.
Folder Access and Word Integration Through a Custom-Built MCP
In earlier uses of generative AI, attorneys and firm staff had to select the target files by hand and upload them from a browser screen. Having a person search folders and attach not only the current contract but also past executed versions and related minutes was a considerable burden in practice. This was because, to decide what to have the AI read, a person had to do preprocessing: finding the latest version while recalling the history of the matter.
Once you set up an environment in which you can instruct Codex on the scope of its access, the shape of this preprocessing changes. For example, I give instructions such as the following.
- Searching for past similar matters
- Checking contracts previously concluded with the same counterparty
- Creating and duplicating the working files for the current task
By specifying the necessary scope within the folders to which access has been granted, Codex itself searches for candidate past cases and materials. Of course, having permission does not mean we let it scan all of the firm's materials without limit. Based on our duty of confidentiality to clients and the information management rules for each matter, we narrow the scope and have it search within that.
A law firm's storage accumulates past contracts, firm templates, Word files with comments, versions returned by counterparties, minutes, research materials and so on. Instead of having a person find and attach these every time, having Codex refer to folders within the necessary scope greatly shortens the time spent preparing files in advance.
However, there were also difficulties in using Codex as it is for contract review. While Codex excels at editing text files and processing across multiple files, its standard features alone made it difficult to accurately keep tracked changes while preserving Word formatting, and to make revisions with minimal differences without disturbing clause numbering, table structure or the position of comments.
So at our firm we built our own dedicated MCP that allows Codex to perform editing operations while Word remains open. By incorporating this MCP, revisions to the text, insertion of tracked changes and addition of balloon comments can be reflected directly in the Word file the attorney is viewing on screen.
This integrated environment has greatly improved how the work feels. Because revisions can be recorded as Word tracked changes through the custom MCP, the attorney can give additional instructions immediately while looking at the screen, such as "This wording is too strong as a way of putting it to the counterparty, so please soften it" or "Please replace this clause with the alternative." Multiple files can also be prepared in parallel.
We also built our own MCP that automatically creates comparison versions of Word files. In practice, there is frequent need to create old-and-new comparison tables and comparison histories, but selecting the target files and running the compare function by hand becomes more cumbersome as the number of files increases. If I give instructions to Codex through the MCP, it determines which files should be compared and generates the comparison version, which greatly reduces the effort of checking.
Specific Use Cases in Legal Practice
Reviewing a Service Agreement
Take as an example a review of a service agreement on the service provider's side. In the traditional procedure, you would save the received file in the designated location, look for past similar matters and templates, and open Word while recalling the past negotiation history and the relationship with the counterparty. Revising the provisions by hand, adding comments while taking care not to disrupt the tracked changes, and checking the cross-references between clause numbers one by one takes a corresponding amount of time.
When using Codex, the first instruction looks something like this: "I want to respond to a review request on the service provider's side. Please refer to our communications with the client, past similar matters and the appropriate template, make the necessary revisions, and proceed with the review in line with the checklist that has been set up." On receiving this instruction, Codex checks the specified folders, grasps the differences from past cases and templates, and then prepares a revision policy, tracked changes in Word, and a draft list of points to confirm.
The attorney carefully checks the resulting draft. Because the burden of preparing the first draft and searching for materials is reduced, the attorney can concentrate on the core analysis, such as the business background, the balance of power in the transaction and judging where a settlement with the counterparty might lie.
Reviewing Materials in Legal Due Diligence
It is also useful when handling large volumes of materials in legal due diligence (legal DD). Enormous amounts of disclosed materials must be reviewed, such as the target company's commercial registry records, shareholder register, investment agreements and various internal rules.
In the traditional approach, the person in charge opened the materials one by one and compiled points to confirm and questionnaires by hand. In addition, manually uploading files one at a time to a cloud service actually takes more effort in situations where new materials are added every day.
If access to the target folder is set up, you can skip attaching individual files and move quickly to creating a list of the disclosed materials in the folder, identifying missing materials and checking the important parts of each document. While referring to past investigation reports and matter templates, you can proceed all the way through organizing findings, drafting questions and preparing a draft report. Because our firm also has an MCP that edits Excel directly, creating and updating the question sheet can also be done from Codex.
Note that even in an operation that does not involve manually attaching files, it cannot be said that no external transmission occurs at all. Even when referring to a local folder, the data needed for processing may be sent to the model provider's servers. The premise is to confirm what is sent, the terms of use, the settings for opting out of use for training, and the data retention period, and then to operate within an appropriate scope consistent with confidentiality toward clients.
Everyday Legal Consultations and Legal Research
The same applies to consultations from business divisions, such as "Is it all right to go ahead with this initiative?" or "Is this wording appropriate in light of advertising-related laws?" An attorney does not simply cite the statute but makes a judgment based on the company's past answers, the history of matters previously referred for management decisions, and consistency with the terms of use.
By instructing Codex to search for related past answer memos and similar consultations and to prepare a draft answer, you can improve both the quality of answers and the speed of response. In addition, at our firm we have set up a system in which practitioner books, statutory texts, public guidelines and other materials accessible within the firm are converted to text data, to the extent legally permitted, so that they can be searched. A distinctive feature is that it eliminates the fragmented workflow of a typical database search, where you "search on an external site, copy the results and paste them back into your document," and allows you to proceed consistently from research to preparing a draft answer in the same work environment.
The Areas of Judgment Attorneys Handle and Changes in Operations
Introducing Codex does not mean the attorney's role is lost. Rather, I think the more these tools are used, the greater the weight of the attorney's final judgment becomes.
AI quickly handles tasks such as finding past contract examples, identifying differences between clauses, and putting draft tracked changes and comments into Word. However, judgments such as how much risk the company should tolerate, which clauses to concede on in view of the future relationship with the counterparty, and which issues should be referred to management as business decisions must be made by the attorney. Rather than a tool that makes final judgments in place of the attorney, Codex is something that handles preparatory work and the first drafts of deliverables so that the attorney can concentrate on sophisticated legal judgment.
This way of working also brings changes to a law firm's day-to-day operations. Traditionally, the common flow was for junior attorneys and staff to spend time searching for past materials and preparing first drafts, which senior attorneys then reworked. Of course, that also played a role in practical training, but there is no need to keep devoting large amounts of time to routine file searches and mechanical comparison of provisions. By shifting to a model in which Codex organizes the materials and prepares the first draft, and the attorney examines its soundness and decides on the revision policy, we can build a sustainable working structure that makes use of AI without relying solely on securing more staff. This is precisely the AI Native Law Firm that our firm aims to be.
Through collaboration between AI agents and attorneys, our firm will continue building a practice that raises the quality and speed of contract review, legal consultations and M&A work and contributes to our clients' growth.
About Our Legal Outsourcing
On the premise of the AI-enabled way of working introduced in this column, we provide ongoing support for contract review and in-house legal work. For details of our support, please see the page below.
Frequently asked questions
How does Codex help attorneys in legal practice?
It is useful as a primary tool that takes on preparatory work and first drafts in day-to-day legal work. This is because Codex has a clear entry point for work and can directly check designated folders in the local environment, creating working files and comparing differences while searching for past examples and related materials. It likely becomes a working environment for quickly preparing starting drafts of contract revisions and statutory research while cutting out switching between screens.
What arrangements are needed to review Word contracts using Codex?
It requires setting up a dedicated integration environment. This is because Codex's standard functions alone make it difficult to revise a document while preserving Word formatting and leaving tracked changes, without breaking clause numbering, table structures or comment positions. At LegalAgent, we built our own MCP that allows editing operations, insertion of tracked changes and addition of balloon comments with Word left open, as well as an MCP that automatically generates comparison versions, and we use them in practice.
Does introducing Codex make final judgments by attorneys unnecessary?
In my view, final judgments by attorneys do not become unnecessary; rather, they carry more weight. AI quickly handles searching past contract examples, extracting differences between clauses and preparing proposed revisions in Word, but it cannot replace judgments such as how much risk the company should accept, or on which clauses to make concessions in light of the relationship with the counterparty. The likely model is one in which Codex takes on the preparatory work and attorneys concentrate on sophisticated legal judgments.
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