How to Use Magic.dev on Large Repositories

A 100-million-token context window is only worth paying for if your repository is actually large. On a weekend side project it is dead weight. On a codebase where nobody remembers every service, it changes which questions are answerable.

The realistic use case is onboarding, impact analysis, and the kind of refactor where grep returns four hundred hits and you need to know which twelve matter.

What Counts as a Large Repository Here?

If your codebase spans more than one deployable service, or exceeds a few hundred thousand lines, you are in the target range. Under about 50,000 lines, a standard assistant with a 200K window handles nearly everything.

How Do You Ask Questions That Exploit the Context?

• Ask about invariants: “What guarantees does the billing module assume about user IDs?”
• Ask for impact: “If I change this response shape, which clients break?”
• Ask for history: “Which modules touched this table in the last year?”
• Ask for gaps: “Which services have no tests for the auth path?”

Notice that none of these name a file. Naming files is what you do with a small-context tool.

Can It Actually Refactor Across 100M Tokens?

It can plan and execute multi-file changes, but the context window is the input, not the output. A refactor still produces a diff you review. The advantage is that the plan accounts for call sites a smaller model would miss entirely.

How Do You Validate Its Answers?

Treat long-context output as a map, not a fact sheet. Ask it to cite the file path for any specific claim, then open that file. When a model reads a hundred thousand files, it can blend details from two similar modules without noticing.

What Does It Cost to Run on a Big Repo?

Indexing a large repository takes real time and compute the first run, and you should expect a slow initial period. Pricing is negotiated rather than public, so budget for a team licence rather than a per-seat hobby plan.

When a Smaller Tool Wins

Fast iteration inside one file, quick script edits, or anything where you already know the answer and want typing help. Magic.dev is overhead for that. Keep a lightweight editor agent alongside it.

Quick Summary

• Repository-scale context only pays off above roughly 50,000 lines.
• Ask cross-service questions instead of file-specific ones.
• The context window shapes the plan, not the final diff.
• Make it cite file paths when you verify claims.
• Pair it with a lighter agent for single-file work.