Mastering Multi-File Tasks: A Guide to Gemini Code Assist Agent Mode

By Tobias Theel

Introduction

Software development often involves tasks that aren’t contained within a single function or file. Whether you’re refactoring a service layer, implementing a new API endpoint that requires changes from the database to the router, or updating dependencies across a monorepo, context is everything.

Standard AI chat assistants are great for snippets, but Gemini Code Assist’s Agent Mode is designed for the big picture. It doesn’t just suggest code; it plans and executes changes across your entire workspace.

In this guide, we’ll walk through how to effectively use the Agent mode to handle complex engineering tasks.

What is Agent Mode?

Agent mode allows Gemini to act as an autonomous collaborator. Instead of you manually copying and pasting suggestions, the agent:

  1. Analyzes your entire codebase to understand dependencies.
  2. Proposes a plan consisting of multiple steps.
  3. Executes changes by directly modifying files in your IDE.
  4. Iterates based on your feedback or compiler errors.

Step-by-Step: Working with the Agent

1. Identify a Complex Task

Don’t use the Agent for simple logic (standard Chat is faster for that). Use it for tasks like:

  • “Add a new ‘Status’ field to the User entity and update all associated DTOs and mappers.”
  • “Refactor the authentication logic to use JWT instead of session cookies.”
  • “Migrate this project from using fmt.Print to a structured logging library like uber-go/zap.”

2. Enter Agent Mode

In your IDE (VS Code or JetBrains), open the Gemini Code Assist panel. Depending on your version, you can usually trigger the agentic behavior by selecting the Agent toggle or using a specific command like @agent.

3. Provide a Clear Objective

The prompt is the most important part. Be specific about your goal and any constraints.

Example Prompt:

“@agent Implement a global error handling middleware for our Gin-based API. Log all errors to the console using our existing logger and return a standardized JSON response structure. Update the main router to use this middleware.”

4. Review the Proposed Plan

Before touching your code, Gemini will present a plan. It might look like this:

  1. Create internal/middleware/error_handler.go.
  2. Define ErrorResponse struct in internal/models/response.go.
  3. Modify cmd/api/main.go to register the middleware.

Pro Tip: Read this plan carefully. If the agent missed a step, tell it: “Actually, make sure the error handler also captures panics.”

5. Execute and Review Diffs

Once you approve the plan, the Agent starts working. It will show you a “Diff View” for every file it modifies.

  • Green: Added code.
  • Red: Removed code.

You can review each change individually, accepting or rejecting them as you go. This ensures you remain the “pilot” while the AI acts as the “co-pilot.”

6. Verification and Iteration

After the Agent finishes, run your tests or build the project. If something breaks, you don’t have to fix it manually. Just tell the agent the error message:

“The build failed in main.go because the logger wasn’t initialized in the middleware package. Can you fix that?”

The Agent will analyze the error and propose a follow-up fix.

Best Practices for Agent Success

  • Keep your workspace clean: The Agent works best when your git state is clean so you can easily see what it changed.
  • Context is King: Open the files most relevant to your task before starting. While the Agent can search your project, having the key files active helps it prioritize context.
  • Incremental Steps: For massive changes, break them down into smaller Agent sessions.

Conclusion

The Agent mode in Gemini Code Assist marks a shift from “AI as a search engine” to “AI as an implementation partner.” By trusting it with the boilerplate and the “plumbing” of your cross-file changes, you free up your mental energy for architectural decisions and complex problem-solving.

Give it a try on your next refactor!

Have you tried the Agent mode yet? What’s the most complex task it has solved for you? Let me know in the comments!

Gemini Agent Mode in action

Learn more about Gemini Code Assist here

comments powered by Disqus