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Build a Code Migration Tool with Claude

Why build a migration tool with Claude?

Code migrations are where teams lose time: renaming APIs, updating package versions, replacing deprecated patterns, and fixing edge cases across hundreds of files. Claude is a strong fit for this work because it can understand context, preserve intent, and propose safe edits instead of blind search-and-replace. In this walkthrough, you’ll build a practical migration tool from scratch that scans a repository, plans changes, applies transformations, and outputs a reviewable diff.

To keep costs low while still using native official-quality models, this tutorial uses 59API as the model gateway. It provides cheap pay-as-you-go access to Claude models like Opus, Sonnet, Haiku, and Fable, with an OpenAI SDK-compatible endpoint at https://api.59api.com. That means you can wire it into existing tooling quickly, including projects already using Claude Code, Codex, or OpenAI clients.

What the tool will do

We’re building a command-line migration assistant with four stages:

• Discover: walk the repo and collect source files.
• Analyze: ask Claude to identify migration targets and risky files.
• Transform: generate patch-style edits per file.
• Review: write a diff plus a summary so a human can verify changes.

This keeps the workflow safe. Claude suggests changes, but your tool controls when to write files.

Step 1: Set up the project

Initialize a Node.js project and install the SDK plus a few utility packages:

npm init -y

npm install openai fast-glob dotenv

Create a .env file:

OPENAI_API_KEY=your_59api_key

OPENAI_BASE_URL=https://api.59api.com/v1

59API is useful here because you do not need to rewrite your client logic. Its OpenAI-compatible interface lets you switch endpoints while keeping the same application flow, and the pricing model is friendly for iterative development where you may run many small prompts during testing.

Step 2: Scan the codebase

Use fast-glob to collect relevant files. Start narrow: only include source files you intend to migrate.

Example targets:

• JavaScript/TypeScript: src/**/*.{js,ts,tsx}
• Python: app/**/*.py
• Configuration: *.json, *.yml

Read each file, but keep prompts compact. Send Claude the minimum useful context: file path, surrounding code, and a short migration goal such as “replace deprecated SDK methods with the new client interface.”

Step 3: Ask Claude for a migration plan

Before editing anything, have Claude produce a structured plan. This reduces accidental breakage and makes your tool more deterministic. Ask for JSON output with fields like files_to_change, risk_level, transformation_rules, and manual_review_notes.

Example prompt idea: “Review this repository snapshot and identify the files and patterns that must change to migrate from API version A to version B. Return only valid JSON.”

This is where a high-quality model matters. Claude’s stronger reasoning helps it understand cross-file dependencies, while 59API keeps the per-run cost low enough that you can analyze larger repos without burning budget.

Step 4: Generate file-level patches

For each file in the plan, send Claude the current contents and ask for a unified diff or a replacement block. Prefer diff output because it is easier to review and apply safely.

• Input: file path, original content, migration rule, and constraints.
• Output: exact patch, plus a short explanation.

Your tool should validate that the patch only touches the intended file and that the result parses. For JavaScript and TypeScript, run a formatter and type check after applying changes.

Step 5: Add guardrails

A migration tool should fail safely. Add checks for:

• Backup creation: keep original files before writing.
• Dry-run mode: show diffs without changing disk.
• Prompt budget: cap file size and chunk long files.
• Verification: run tests or at least syntax validation.

Because 59API charges pay-as-you-go, you can run these guarded iterations economically while improving prompts and patch quality. That is especially helpful when you are tuning file-chunking logic or comparing Claude Sonnet versus a faster, cheaper model for simple refactors.

Step 6: Write the production workflow

When the core works, turn it into a reliable CLI:

• Command: migrate --from v1 --to v2 --dry-run
• Logging: print progress per file and per stage.
• Config: support ignore patterns and migration presets.
• Output: write migration-report.json and a git diff summary.

Finally, document how developers can point the tool at different AI models without changing code. That flexibility is a major advantage of an API relay like 59API: you can choose the model that fits the task, keep one integration path, and avoid vendor lock-in.

From prototype to team tool

Once the tool is stable, integrate it into CI as a review helper rather than an automatic writer. Let it open migration pull requests, attach summaries, and flag files that need human inspection. That gives teams a repeatable path for large-scale upgrades without turning the AI into a black box.

If you want cheap, official-quality Claude access with OpenAI SDK compatibility and a referral rebate on top, it is worth signing up for 59API before you scale your migration experiments. It keeps the economics sane while you build something genuinely useful.

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