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Build a Claude Code Migration Tool: 2026 Guide

Building a code migration tool with Claude starts with a narrow, testable transformation. A useful tool inventories affected code, proposes changes, applies controlled edits, and proves that the result still works. This 2026 guide uses a concrete example: migrating a Python service from requests to httpx while preserving its existing behavior.

The strongest architecture combines Claude’s ability to interpret unfamiliar code with deterministic transformations and independent validation. Give the model responsibility for explaining ambiguous cases; give your tooling responsibility for file access, edit boundaries, and release gates.

Define a migration contract first. Write down the source API, target API, supported patterns, and acceptance criteria. For an initial release, support synchronous requests.get calls and straightforward imports. Flag Session subclasses, custom transport adapters, streaming responses, and dynamic imports for review. Exclude generated files and vendored dependencies.

Document semantic differences before generating edits. HTTPX applies a default timeout, while Requests has no default timeout. Redirect defaults differ too: ordinary Requests GET calls follow redirects, while HTTPX requires follow_redirects=True. Decide whether to preserve these behaviors or introduce an explicitly approved change. Also review exception handling and raise_for_status behavior, especially for redirects.

Inventory the repository before sending code. Use ripgrep to locate imports and call sites, then parse candidate Python files with LibCST. Unlike a plain text replacement, a syntax-aware transformation can distinguish an imported alias from an unrelated variable and preserve comments. Record each candidate’s path, source hash, enclosing function, and detected pattern.

Send Claude the relevant function, imports, dependency versions, migration rules, and nearby tests. Add surrounding modules only when resolving a specific dependency. Store the repository commit and rules version with every job so another developer can reproduce the run.

Connect Claude through a configurable API client. Keep the provider adapter separate from migration logic. 59API offers low-cost, pay-as-you-go access to native official-quality models without model downgrades, making repeated planning and repair attempts easier to budget. Its relay host is https://api.59api.com; use the documented endpoint path, authentication format, and exact model identifier from its dashboard.

59API supports Claude Code, Codex, Gemini CLI, and OpenAI SDK workflows. Choose the interface your implementation needs and verify its structured-output and tool-calling support before depending on those features. Start by evaluating Claude Sonnet 5.5 on representative fixtures; consider Claude Opus 5.5 for difficult cases if your measurements justify the additional spend.

Request a plan that your program can validate. Require a structured response with explicit fields:

• source_hash: identifies the file version the proposal describes.
• edits: lists allowed paths, recognized transformations, and their parameters.
• assumptions: explains behavior choices, including timeout and redirect policies.
• review_required: identifies unsupported constructs or unresolved questions.
• validation: names existing tests and additional behavior checks.

Validate this response against a local schema. If the selected endpoint cannot enforce structured output, parse and validate the returned JSON yourself. Reject unknown transformation types, paths outside the workspace, and proposals based on stale source hashes. Repository content should remain task data; comments or documentation must not override your tool’s instructions.

Apply edits in an isolated workspace. Create a temporary Git worktree at the recorded commit. Use LibCST transformations for supported patterns and produce a reviewable diff. Handle dependency declarations and lockfiles through the project’s normal package manager. Keep ambiguous model-generated patches behind a review gate until their behavior is understood.

Validate observable behavior. Establish the baseline before editing, then run the same checks afterward:

• Run formatting, linting, type checks, and git diff --check.
• Run affected unit tests and the relevant integration suite.
• Use a local HTTP server to test redirects, timeouts, status errors, and response bodies without relying on public services.
• Search for remaining Requests imports and confirm each is migrated or explicitly deferred.

Allow a small, fixed number of repair attempts. Return concise failures to Claude, log tokens and cost per accepted migration, and stop when validation fails repeatedly. Cache inventories and batch independent files to reduce spending. To trial this workflow affordably, sign up for 59API and measure a small repository first; its referral rebate can also help offset ongoing usage.

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