Claude Code got the architecture wrong (so we ran a controlled experiment to find out why)

If you have used Claude Code on a large codebase, you have probably felt this. The output compiles. The tests pass. But something feels off. The API surface is parallel to something that already exists. The approach is a workaround dressed up as an implementation. A senior engineer on your team would have done it differently. The instinct is to blame the model. The actual problem is something else entirely. Coding agents explore large codebases through trial and error Claude Code, Cursor, and every other coding agent navigates your codebase the same way: grep, glob, read files, repeat. On a small codebase this works well enough. On a codebase with millions of lines across thousands of files, this process produces a systematically incomplete picture. The agent makes the best architectural decision it can with what it found. When extension points exist but the agent never found them, it creates a parallel implementation instead of extending what already exists. When conventions exist but