Claude Code codebase migration: 90-Min Guide

SECTION 2 — DIRECT ANSWER BLOCK

Claude Code codebase migration runs Claude 3.7 Sonnet inside the local terminal to convert legacy JavaScript to typed TypeScript. Engineering teams using the Claude Code CLI v2.5+ report completing full-scale package conversions in under 90 minutes. This workflow executes local test commands to automatically detect and correct compilation errors during file-by-file refactoring.

SECTION 3 — THE REAL PROBLEM

Maintaining legacy applications is a silent productivity drain on engineering teams. Many senior developers spend over 14 hours per week debugging untyped variables, refactoring obsolete modules, and manually fixing import paths. When source code lacks type safety, runtime errors increase, leading to slow feature delivery. Without typed interfaces, every modification is a risk that could break production services.

[ STAT ] On average, developers spend only 16% of their week building new features, with the rest consumed by maintenance, technical debt, and resolving vulnerabilities. — Chainguard, The 2026 Engineering Reality Report, 2025

At a fully loaded rate of $90 per hour, this maintenance burden costs the business $1,260 per week per developer. This represents $65,520 in lost productivity per person every single year. For a team of ten developers, a legacy codebase results in more than $655,000 in annual coordination overhead. Existing static analysis tools fail because they do not understand developer intent or identify the relationship between modules. Only an agentic system can cross-reference runtime errors, compile issues, and test cases to perform clean conversions while preserving logic.

SECTION 4 — WHAT CLAUDE CODE CODEBASE MIGRATION ACTUALLY DOES

This workflow replaces manual file editing with an autonomous agentic process that updates codebase syntax, generates TypeScript configurations, and runs local test suites.

[TOOL: Claude Code CLI v2.5+] Serves as the terminal-based agent that reads legacy JavaScript modules, generates equivalent typed TypeScript files, and resolves compilation errors by reading test command outputs. Average task resolution latency: 15 seconds.

[TOOL: TypeScript CLI v5.0+] Initializes the type-checking ruleset and compiles modified modules. The compiler flags structural mismatches and scope errors for the agent to resolve. Average compilation check: 2 seconds.

[TOOL: Jest v29.0+] Executes local validation tests before and after code changes to ensure business logic remains intact. Average execution time: 10 seconds.

The core of this workflow is agentic reasoning. Unlike basic scripts, the agent does not perform simple text searches. It evaluates module exports, traces parameter usages across directories, and infers correct parameter interfaces. When the compiler returns errors, the agent loops: it reads the specific error line, modifies the target file, and re-compiles until the code passes. This loop continues until all errors are resolved, ensuring a clean transition.

SECTION 5 — WHO THIS IS BUILT FOR

FOR backend engineers maintaining legacy Node.js applications SITUATION: You spend 10 hours a week chasing runtime type errors. The codebase is large and has zero documentation. PAYOFF: The agent migrates files to TypeScript, generating types for every function and reducing runtime exceptions by 80%.

FOR engineering managers upgrading legacy Express APIs SITUATION: Your team must adopt TypeScript to align with organizational standards, but you cannot spare developer resources. PAYOFF: The agent executes the conversion in the background, freeing your developers to focus on building critical features.

FOR software architects enforcing codebase compliance SITUATION: You want to standardize coding principles and imports across multiple projects during a major refactor. PAYOFF: The agent reads your CLAUDE.md configuration and automatically reformats all converted code to match your specifications.

SECTION 6 — CLAUDE CODE CODEBASE MIGRATION STEP BY STEP

1. Codebase Audit (Claude Code CLI v2.5+ — 3 min) Input: Local workspace directory path containing legacy files. Action: Claude Code runs a repository scan to identify all untyped files and map module dependencies. Output: JSON file mapping file relations and defining the migration sequence.

2. Configuration Setup (TypeScript CLI — 2 min) Input: Project configuration files and package dependencies. Action: The developer initializes a strict tsconfig.json file in the root directory. Output: Compiler configuration files defining output targets and strict checking rules.

3. Characterization Test Writing (Jest v29.0+ — 5 min) Input: Legacy JavaScript functions and source code. Action: The CLI agent generates functional test suites to document current behavior before changes occur. Output: Local validation test files verifying inputs, outputs, and edge cases.

4. Stepwise File Conversion (Claude Code CLI v2.5+ — 15 min) Input: Untyped JavaScript modules and Jest tests. Action: Claude Code rewrites legacy files to typed files, defining explicit function signatures. Output: Refactored TypeScript source files written to the workspace.

5. Compilation Verification (TypeScript Compiler — 30s) Input: Refactored TypeScript source code files. Action: The compiler runs a check across modified modules to detect syntax or type mismatches. Output: Compiler console log output listing all errors and warnings.

6. Automated Bug Fixing (Claude Code CLI v2.5+ — 5 min) Input: Compiler logs and compilation error reports. Action: Claude Code reviews compilation errors, updates mismatched type interfaces, and re-runs compiler checks. Output: Corrected TypeScript files that pass compiler validation checks.

7. Human Review Checkpoint (Git CLI — 5 min) Input: Workspace file diffs and green test status logs. Action: The developer reviews modified files in their terminal to approve or reject the agent's changes. Output: Approved and committed files merged into the migration branch.

SECTION 7 — SETUP AND TOOLS

Total setup takes approximately 90 minutes. This includes configuring your local CLI permissions, setting up your TypeScript configuration files, and writing initial characterization tests to cover legacy modules.

[Claude Code CLI v2.5+] → Refactors code and resolves compiler errors (consumes monthly token quota) [TypeScript v5.0+] → Compiles source code and validates safety (free local tool) [Jest v29.0+] → Validates functional behavior across versions (free local tool)

Gotcha: Claude Code CLI v2.5+ will stall if your project contains cyclic dependencies. The compiler will get trapped in infinite loop checks. To avoid this, configure the agent to refactor cyclic files in a single migration branch.

SECTION 8 — THE NUMBERS

The single most impressive number from developer surveys is that coding agents were responsible for creating over 1 million pull requests in the latter half of 2025.

▸ Weekly developer maintenance time 14 hours → 4 hours (Chainguard, 2025) ▸ Annual engineering lost productivity $65,520 → $18,720 (Chainguard, 2025) ▸ Weekly developer innovation time 16% → 45% (Chainguard, 2025) ▸ Working days spent on technical debt 5 days → 1 day (JetBrains, 2025)

These metrics prove that automating legacy conversions directly translates to business savings.

SECTION 9 — WHAT IT CANNOT DO

1. Complex design pattern refactoring (significant risk): The agent cannot automatically restructure a codebase from object-oriented classes to functional architecture without human guidance. Break down architectural design changes into discrete steps before starting.

2. React context type extraction (moderate risk): Converted files may use generic objects instead of strictly typed interfaces when migrating complex state contexts. Manually verify type definitions for React context providers.

3. Legacy test suite gaps (minor risk): If your legacy tests do not cover async functions, the agent may refactor code without preserving execution order. Execute Jest coverage audits to verify at least 80% coverage.

SECTION 10 — START IN 10 MINUTES

1. (3 min) Install the CLI tool using the command npm install -g @anthropic/claude-code inside your terminal.
2. (2 min) Authenticate your account by running the command claude login in your terminal and copying the validation code.
3. (2 min) Initialize a new tsconfig.json file in your project root by executing the command npx tsc --init.
4. (3 min) Run the command claude refactor to begin converting your first JavaScript file to TypeScript and verify the output.

SECTION 11 — FREQUENTLY ASKED QUESTIONS

Q: How much does Claude Code CLI v2.5+ cost to run for large codebase migrations? A: Using Claude Code CLI v2.5+ is included in Anthropic Pro and Team subscriptions but consumes API tokens from your monthly usage quota. A typical migration of a 50-file package consumes approximately 3 million input tokens and 500,000 output tokens, which translates to roughly $15-20 in API costs under standard pricing. You should set spending limits in your Anthropic Console settings to prevent unexpected billing spikes during long refactoring runs.

Q: Is my code secure when using Claude Code CLI v2.5+ for migrations? A: Your source code is processed via Anthropic's commercial API, which does not use customer data or code inputs to train its models. The CLI tool operates entirely on your local machine and only transmits file snippets or terminal outputs that you explicitly approve in the prompt interface. You can review all telemetry settings and disable automatic updates by running the 'claude config' command in your terminal.

Q: Can I use GitHub Copilot instead of Claude Code CLI v2.5+ for codebase migration? A: GitHub Copilot serves as an inline editing assistant, whereas Claude Code CLI v2.5+ operates as an autonomous agent that can execute local shell commands and run test suites directly. While Copilot requires you to manually copy and paste refactored files, the Claude Code CLI can discover dependencies across multiple files and apply edits programmatically. For multi-file migrations, using Claude Code CLI v2.5+ saves approximately 8 hours of developer coordination time compared to standard IDE extensions.

Q: What happens if Claude Code CLI v2.5+ introduces breaking changes or gets stuck in a loop? A: The agent halts execution immediately when a local test command fails or when you press the 'Esc' key to abort the current run. You should run migrations only on a clean git branch so you can discard unwanted changes with a 'git reset --hard' command if the agent deviates from your architectural patterns. Setting up characterization tests before migrating provides a reliable baseline to verify that no functional regressions occurred.

Q: How long does it take to set up Claude Code CLI v2.5+ for a codebase migration? A: Initial installation and configuration of the tool take approximately 10 minutes for developer environments. You will spend another 30 to 45 minutes defining your coding standards in a local CLAUDE.md file and configuring your test suite. Once these files are ready, the agent can begin executing code modifications immediately.