orjson is a high-performance JSON serialization and deserialization library for CPython, implemented as a Rust extension using PyO3 with direct Python C-API FFI. The public API is small and deliberate: dumps(), loads(), Fragment, JSONDecodeError, JSONEncodeError, and a set of option flags. Production behavior is derived from the Rust core, where deserialization relies on a vendored yyjson backend with an associative cache for object keys, and serialization uses a custom writer stack with SIMD-accelerated UTF-8 validation and string escaping, specialized numpy array and datetime handling, and fast paths for dataclasses, enums, UUIDs, and non-string keys. Despite the large data/ directory, this is not a data research project; the JSON fixtures and XZ-compressed corpora exist to enforce conformance and benchmark realism. The architecture is a classic C-extension pattern: src/lib.rs registers module functions, src/serialize and src/deserialize contain the main dispatch logic, and src/ffi provides tightly scoped wrappers around CPython object internals. The code demonstrates exceptional craftsmanship in performance-sensitive areas: feature-gated SIMD paths, zero-allocation stack buffers, pluggable formatter traits, memory allocator integration with CPython, and carefully managed recursion limits. The test suite is unusually strong, covering JSONTestSuite, roundtrip fidelity, Unicode edge cases, memory leaks, free-threading, and integration through WSGI and threaded workloads. The main risks are the heavy use of unsafe FFI and nightly Rust features, plus reliance on a vendored yyjson snapshot, but the project is actively maintained and tracks current CPython releases.