Aider AI Pair Programmer: Fixing Real Bugs in an Existing Codebase

You have a bug report sitting in your backlog. You know the general area of the codebase where it lives, but tracing the call chain and writing the fix is going to eat an hour you don't have. This is exactly the scenario where Aider earns its keep — it reads your actual files, reasons through the logic, and writes the patch directly to disk.

Aider is not an autocomplete widget bolted onto an editor. It's a terminal tool that speaks to a large language model, understands your repo's git history, and makes multi-file edits that it then commits on your behalf. The workflow feels closer to pair programming than to prompt engineering.

What you'll learn

• How to install and configure Aider against an existing project
• How to point Aider at a specific bug and give it enough context to reason correctly
• How to review, accept, or reject its edits before they hit your repo
• How to handle multi-file bugs and keep the context window focused
• Common pitfalls when using Aider on a production codebase

Prerequisites

You'll need Python 3.10 or later, an OpenAI API key (or an Anthropic key if you prefer Claude models), and a project that already lives in a git repository. Aider leans heavily on git: it commits each change so you can git diff or git revert at any point. If your project isn't in git yet, run git init && git add -A && git commit -m "initial" before you start.

Installing Aider

The simplest path is a global pip install into an isolated environment. Using pipx keeps it off your project's dependency list:

pipx install aider-chat

If you'd rather use pip directly:

pip install aider-chat

Once installed, export your API key. Aider reads it from the environment:

export OPENAI_API_KEY="sk-..."

To use Claude instead, export ANTHROPIC_API_KEY and pass --model claude-3-5-sonnet-20241022 (or whichever model you want) when you launch Aider. The GPT-4o and Claude Sonnet tiers both handle multi-file edits reliably; the cheaper models work for small isolated fixes but tend to lose track of context in larger files.

Launching Aider Against Your Project

Navigate to your project root — the directory that contains .git — and run:

aider

Aider starts an interactive session and announces which model it's using. You'll see a prompt like:

> 

From here you can add files to the context window with /add, remove them with /drop, and type natural-language instructions. Aider does not automatically read your entire codebase — you tell it which files matter for this task. This is intentional: focused context produces better edits than a firehose of code the model has to ignore.

Adding Context: Which Files to Include

Think about the bug before you type anything. Identify the file (or files) most likely involved. For a Python Django project where a view is returning the wrong HTTP status code, you'd start with the view file and maybe the serializer:

/add myapp/views.py myapp/serializers.py

Aider confirms what's in context and shows the token count. If you're close to the model's context limit, it will warn you. At that point, drop files that aren't directly relevant. You can always add them back later if the investigation leads there.

Adding your test file for the affected module is almost always worth it. Aider can read the existing test patterns and write new assertions that match your style rather than inventing its own conventions.

Describing the Bug Clearly

This is where most people underdeliver.