Code Review Using Claude Code
Code review is the process of reading code to find bugs, security problems, performance issues, and style violations before that code goes live. Traditionally, a senior developer does this manually. With Claude Code, you get a fast, detailed review any time — even at midnight, even when your team is offline.
What Claude Code Looks for in a Review
Claude Code reviews code across several dimensions at once. A human reviewer might focus on one area at a time. Claude checks all of them in a single pass.
┌──────────────────────────────────────────┐ │ CODE REVIEW DIMENSIONS │ │ │ │ 🐛 Bugs — Logic errors, │ │ edge cases missed │ │ │ │ 🔒 Security — Injections, exposed │ │ secrets, auth gaps │ │ │ │ ⚡ Performance — Slow loops, N+1 │ │ queries, memory leaks │ │ │ │ 📖 Readability — Confusing names, │ │ missing comments │ │ │ │ 🏗 Structure — Responsibilities │ │ mixed, poor separation │ └──────────────────────────────────────────┘
How to Ask Claude Code to Review Your Code
The simplest approach is to paste the code and ask for a review. But a focused prompt gives you a much more useful result.
Basic Review Prompt
"Review this code and list any bugs, security issues, or performance problems. Be specific about line numbers." [paste your code here]
Focused Review Prompt
"This is a user authentication function in Node.js. Focus on: security vulnerabilities and input validation. Ignore style issues for now." [paste your code here]
Telling Claude what to focus on saves time. If you ask for a general review of a 300-line file, you get a long response that covers everything. If you ask for security only, you get a sharp, actionable list.
Types of Code Review You Can Do
Bug Hunt Review
Ask Claude to find logic errors, off-by-one mistakes, and unchecked edge cases. This works well before you submit a pull request.
Prompt: "Find any bugs in this function. Pay special attention to edge cases like empty input, null values, and negative numbers."
Security Review
Ask Claude to check for vulnerabilities that could be exploited. Common ones include SQL injection, cross-site scripting, missing authentication checks, and hardcoded credentials.
Prompt: "Review this Express route handler for security issues. I want to know about SQL injection, missing auth checks, and any exposed sensitive data."
Performance Review
Ask Claude to find slow code. Common targets are nested loops, repeated database queries inside loops, and large data structures loaded into memory unnecessarily.
Prompt: "This function runs on every API request and feels slow. Find any performance bottlenecks and suggest faster alternatives."
Readability Review
Ask Claude to flag confusing variable names, missing comments, and functions that try to do too many things at once.
Prompt: "Review this code for readability. Should new developers on the team be able to understand this quickly? What would you rename or split apart?"
A Real Code Review Workflow
Here is a step-by-step process you can repeat for every feature you build.
STEP 1: Write your code as normal
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STEP 2: Self-review — read it once yourself
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STEP 3: Paste into Claude Code with a focused prompt
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STEP 4: Read Claude's findings — understand each one
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STEP 5: Fix the real issues, skip the minor style notes
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STEP 6: Ask Claude to re-check the fixed version
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STEP 7: Commit to version control
Reading and Acting on Claude's Review Output
Claude returns a structured list of findings. Each finding includes the problem, where it is, and why it matters. Here is how to read that output:
Example Claude Review Output
Issue 1 — HIGH severity — Security Line 24: The `userId` parameter is inserted directly into the SQL string. This allows SQL injection attacks. Fix: Use a parameterized query instead. Issue 2 — MEDIUM severity — Bug Line 41: If `data` is null, the `.map()` call on line 41 will throw an error. Add a null check before the map. Issue 3 — LOW severity — Readability Line 55: The variable `x` doesn't describe what it holds. Rename it to `filteredUsers` for clarity.
Priority Order for Fixes
🔴 HIGH — Fix immediately. Security and data bugs. 🟡 MEDIUM — Fix before shipping. Logic errors. 🟢 LOW — Fix when convenient. Style and naming.
Do not skip HIGH severity findings. They represent real risk. LOW severity findings are optional, especially under deadline pressure.
Comparing Two Versions of Code
Claude Code can compare your original code with a revised version and tell you which one is better and why. This is useful when you are unsure whether your refactor actually improved anything.
Prompt: "Here are two versions of the same function. Tell me which is better for performance and why. Version A: [paste original] Version B: [paste refactored version]"
Reviewing Code You Did Not Write
Claude Code works well for reviewing open-source code, third-party libraries, or code written by a colleague. Paste the code and ask Claude to explain it and flag anything suspicious.
Prompt: "Explain what this code does and flag any parts that look risky or hard to maintain."
This is especially useful when you inherit a codebase and need to understand it quickly before making changes.
Setting Review Standards for Your Project
Add your review standards to your CLAUDE.md file so Claude applies them automatically in every session.
CLAUDE.md — Review Standards: ────────────────────────────── ## Code Review Rules - All user inputs must be validated before use - No console.log statements in production code - Functions must be under 40 lines - Every API route needs authentication middleware - Use parameterized queries — never string concatenation in SQL
With these rules in place, you can simply say "review this file" and Claude applies your team's standards automatically.
Limitations to Know
Claude Code does not run your code. It reads it statically, the same way a human reader does. This means:
- It cannot catch bugs that only appear with specific runtime data
- It cannot detect race conditions that only happen under load
- It may miss issues that depend on external system behavior
Use Claude's review alongside — not instead of — actual test runs and automated linters like ESLint or RuboCop.
Key Points
- Claude Code reviews for bugs, security, performance, readability, and structure in a single pass
- Focused prompts (security only, performance only) give sharper results than open-ended ones
- Fix HIGH severity findings before anything else — they carry real risk
- Use Claude to review code you did not write, including inherited or open-source code
- Store your review standards in
CLAUDE.mdto apply them automatically every session - Claude reviews code statically — pair it with actual tests for complete coverage