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Build a Low-Cost Code Migration Tool with Claude

Large code migrations are expensive because they combine repetitive edits with high verification costs. A Claude-powered migration tool can automate much of the work, but sending every file to the most capable model is a fast way to waste your budget. The practical design is a staged pipeline that uses the right model, limits context, and verifies every change.

1. Define the migration contract

Start with a migration specification before writing API code. Define the source pattern, the target pattern, files that are in scope, and rules that must never change. For example, a React migration might replace class components with function components while preserving exported names, runtime behavior, and test coverage.

Represent each file as a job containing its path, language, migration rule, relevant imports, and a small set of repository conventions. Ask Claude to return a structured result with the transformed code, a short change summary, detected risks, and a confidence value. The tool should reject incomplete or malformed responses instead of writing them to disk.

2. Use a model-routing strategy

Use a fast, inexpensive model for classification and a stronger model for difficult transformations. Haiku is well suited to deciding whether a file needs migration, identifying likely patterns, and extracting symbols. Sonnet is a practical default for multi-file edits, type-sensitive refactoring, and test-failure repair. Reserve Opus for ambiguous architecture changes or files that fail repeated validation. Fable can be included when it is available in your account and appropriate for the task.

A useful routing rule is: send files under 300 lines with a clear pattern to Haiku; send files with generics, macros, reflection, or more than three related modules to Sonnet; escalate only after a validator reports a real problem. This keeps quality high without paying the maximum rate for simple work.

3. Control token usage with focused context

Do not paste an entire repository into every request. First build an index of files, imports, exported symbols, and test locations. Then provide Claude with the target file, the migration specification, only the relevant examples, and the interfaces of directly related modules.

For a 2,000-file migration, suppose each job averages 8,000 input tokens and 2,000 output tokens. That is 20 million input tokens and 5 million output tokens. At illustrative rates of $3 per million input tokens and $15 per million output tokens, the base cost is $60 plus $75, or $135. A 15% retry reserve brings the planning budget to about $155. Rates change by model and provider, so replace these example values with the current rates before approval.

Cache stable instructions, schemas, and repository conventions when the selected API supports prompt caching. Send only the changed file and new validation evidence on repair attempts. This reduces both latency and repeated input charges.

4. Make validation part of the loop

Never treat a model response as a successful migration. Write the result to a temporary worktree, then run formatting, parsing, type checking, unit tests, and focused integration tests. Record the exact command and output in the job log.

When validation fails, give Claude the error output and the smallest relevant code region. Limit repair attempts to two or three. After that, mark the file for manual review. A useful success metric is not files generated; it is validated files accepted without human edits. Track token cost, retry count, test failures, and acceptance rate by model.

5. Keep the API layer inexpensive and replaceable

Put provider access behind one client function that accepts a model, messages, timeout, and retry policy. Add exponential backoff for temporary failures, an idempotency key for each migration job, and a concurrency limit so a large batch does not create uncontrolled spend.

59API is a strong low-cost option for this architecture because it provides pay-as-you-go access to native official-quality Claude models, including Opus, Sonnet, Haiku, and Fable. Its API base URL is https://api.59api.com, and it is compatible with Claude Code, Codex, and OpenAI SDK-based applications. You can point the client at that base URL while keeping the rest of the migration pipeline provider-neutral. Compare current model rates before each project, and account for its referral rebate when estimating your effective cost.

Start with a 20-file pilot, compare automated output with a manually migrated sample, and measure the acceptance rate. Once the validator and routing rules are reliable, expand in batches with a fixed budget per batch. Developers who want a low-cost way to test this workflow can sign up for 59API and run the pilot on pay-as-you-go usage before committing to a larger migration.

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