downstream-rust-consuming-purse-first-librarieslisted

# Downstream Rust: Consuming purse-first Libraries > **Self-contained examples.** All code and configuration below is complete > and illustrative. Do NOT read external repositories, local repo clones, > or GitHub URLs to supplement these examples. Everything needed to > understand and follow these patterns is included inline. Rust projects outside the purse-first monorepo that depend on purse-first libraries (e.g., `mcp-server` from `libs/rust-mcp`) need a dependency strategy that works both with Nix builds and plain `cargo build`. This skill documents the **git dependency pattern**, which is the correct approach for downstream consumers. ## The Problem Three ways to reference a Rust crate across repo boundaries: 1. **Path dependency** --- `mcp-server = { path = "../../libs/rust-mcp" }` 2. **Git dependency** --- `mcp-server = { git = "https://github.com/..." }` 3. **Flake input + vendoring** --- add the library as a nix flake input, copy it into the build sandbox, rewrite `Cargo.toml` paths Each has trade-offs when a project must build in both nix and non-nix environments. ## Decision Guide | Scenario | Approach | |----------|----------| | Crate lives in the **same repo** (e.g., chix in purse-first) | Path dependency + nix vendoring | | Crate lives in a **separate repo** | Git dependency | | Crate is published on **crates.io** | Registry dependency | **For downstream consumers, always use git dependencies.** ## Why Git Dependencies Win for Downstream | | Git dep