glyphsLib is a Python library that provides bidirectional conversion between Glyphs.app source files (.glyphs) and the UFO (Unified Font Object) format with Designspace documents. It serves as the core bridge in font production pipelines, enabling tools like fontmake to compile variable and static OpenType fonts from Glyphs sources. The library handles the full round-trip of font metadata, glyph outlines, components, anchors, hints, guidelines, kerning, OpenType features, instances, axes, and custom parameters, supporting both Glyphs v2 and v3 file formats. Architecturally, glyphsLib employs a builder pattern with UFOBuilder and GlyphsBuilder classes that orchestrate conversion through specialized sub-modules for each font data domain (paths, components, anchors, features, etc.). Key entry points include the high-level functions `to_ufos()`, `to_designspace()`, and `to_glyphs()` in the `glyphsLib.builder` package, as well as CLI tools `glyphs2ufo` and `ufo2glyphs`. The library integrates deeply with the fontTools ecosystem, using fontTools.designspaceLib for axis definitions and ufo2ft for feature writing and filters. External dependencies include fonttools, ufoLib2, defcon, openstep-plist, and xmldiff. The library also bundles GlyphData XML files for glyph classification and production name mapping. Notable features include smart component decomposition, bracket/brace layer handling, anchor propagation, token expansion in feature code, automatic STAT table generation, and variable feature conversion from Glyphs-specific conditional syntax to feaLib variation blocks. The codebase is well-tested with extensive round-trip tests and regression tests against real font families.