What hand-written context looks like
A hand-written CLAUDE.md is written by someone who knows the project. For our benchmark, we wrote context files for three repos: Cal.com (42 lines), Hono (39 lines), and Pydantic (39 lines). They include the stack, key directories, naming conventions, build commands, and domain-specific rules an agent needs to follow.
The Cal.com handwritten file, for example, specifies: use useLocale() for all user-facing strings, translation keys go in a specific JSON file, path aliases map @calcom/ to packages/, and app integrations live in a specific directory structure. These are the things an agent can't discover by reading a few files.
What sourcebook generates
sourcebook produces a ~70-line brief for the same Cal.com repo. It includes build commands (extracted from package.json/turbo.json), critical constraints (testing patterns, git history anomalies, circular dependencies), core modules ranked by import count, dominant coding conventions, environment variables, and active development areas from git history.
The key difference: a human writes what they know matters. sourcebook writes what the data shows matters. The human captures domain rules (always use the i18n system). sourcebook captures structural facts (these 5 files are imported by everything else, these files change together).
Head-to-head benchmark
Same tasks, same model (claude-sonnet-4, pinned), same harness. Format: time / files / patch lines