Using Claude Code to Understand Unfamiliar Code
Claude is a surprisingly useful tool for reading code you didn't write. Here's how to use it to get up to speed faster.
Most Claude Code discussion focuses on writing code. But reading and understanding code you didn't write is just as important — especially when onboarding to a new codebase, debugging something unfamiliar, or reviewing a large PR.
Claude is good at this, and the workflow is a bit different than for writing.
Ask for the mental model first
Before diving into specifics, I ask Claude to explain the overall structure: "Read this module and give me the mental model I need to work with it. What does it do, what are the main concepts, and what should I understand about how it's organized?"
This takes 30 seconds and saves the confusion of trying to understand line-by-line without the big picture.
Trace a specific flow
For understanding how something actually works: "Trace the execution path when a user logs in. Start from the login endpoint and follow the code through to where the session is created." Claude walks through it step by step, referencing the actual code.
This is faster than reading through multiple files manually and is better at following cross-file calls.
Explain specific patterns
When I encounter a pattern I don't recognize: "This code is doing X. What pattern is this and why would someone use it?" Claude identifies the pattern, explains the rationale, and flags whether it's idiomatic or unusual.
This is useful for unfamiliar frameworks, legacy code with non-obvious approaches, or codebases with strong opinions about architecture.
Find the dangerous parts
For code review of a large PR or legacy module: "Read this code and tell me: what are the parts most likely to have bugs, and what would I need to understand before safely modifying this?" Claude surfaces the parts with complex state, tricky dependencies, or implicit assumptions.
The limit: Claude doesn't run the code
Claude's understanding is static — it reads the code but can't observe runtime behavior. For questions about what actually happens at runtime (which code path executes given this input, what this query actually returns), reading the code only gets you partway. You still need logs, debuggers, and tests to validate what you read.
Use Claude for the structural understanding. Use running the code for behavioral verification.