Ferritin MCP Server

🩸 Ferritin

A terminal-based Rust documentation viewer

Ferritin reads rustdoc JSON output to display Rust documentation directly in your terminal. It supports local workspace crates, standard library documentation, and crates from crates.io, with search functionality and modern terminal features including mouse support, syntax highlighting, and clickable links.

Features

• Interactive documentation browser with keyboard and mouse navigation
• One-shot CLI for quick documentation lookups in the terminal
• Search across crates or within specific crates
• Works seamlessly across three contexts:
  • Local workspace crates (requires nightly toolchain)
  • Standard library (requires rust-docs-json rustup component)
  • Crates from crates.io (fetched from docs.rs on demand)
• Modern terminal UI with features like italics, unicode box drawing, syntax highlighting, OSC8 hyperlinks, cursor changes in terminals that support it, and mouse hover previews
• Fast navigation with history and breadcrumb trail

Installation

Prebuilt binary (recommended)

Prebuilt binaries skip compilation and are available for x86_64/aarch64 macOS, x86_64 Linux, and x86_64 Windows.

If you have cargo-binstall, it reads the release metadata and installs the right binary for your platform:

cargo binstall ferritin

Otherwise, the installer scripts detect your platform, download the matching binary, and place it in ~/.cargo/bin:

macOS and Linux:

curl --proto '=https' --tlsv1.2 -LsSf https://github.com/jbr/ferritin/releases/download/ferritin-latest/ferritin-installer.sh | sh

Windows (PowerShell):

powershell -c "irm https://github.com/jbr/ferritin/releases/download/ferritin-latest/ferritin-installer.ps1 | iex"

Both of the above always install the most recent release. You can also pick a specific version or download archives directly from the releases page.

From source

To build from source (or for a platform without a prebuilt binary):

cargo install ferritin

Optional: Standard library documentation

To view documentation for std, core, and alloc:

rustup component add rust-docs-json

For local workspace documentation

Local crate documentation requires a nightly toolchain:

rustup toolchain install nightly

Note: There is a relationship between ferritin's version and the nightly toolchain version. Ferritin supports a range of recent rustdoc JSON format versions, and if your nightly generates a newer format, ferritin will make a best effort at parsing it.

Usage

Interactive mode

Launch the interactive browser:

ferritin -i

Once running, use h or ? to see available keybindings. Basic navigation:

• g - go to an item by path (e.g., std::vec::Vec)
• s - search (Tab to toggle between current crate and all crates)
• l - list available crates
• Arrow keys, j/k, or mouse to navigate
• Click on links to follow them

CLI mode

View documentation for a specific item:

ferritin get std::vec::Vec
ferritin get serde::Serialize

Search for items:

ferritin search "hash map"
ferritin search --crate tokio "spawn"

List available crates in your workspace:

ferritin list

Output format

Ferritin auto-detects how to render: ANSI colors on a terminal, plain text when piped, and a compact markdown-flavored format under a coding agent. Override it with --format <tty|plain|agent|json>. --format json emits a structured representation — of the resolved item for get, or of the results for search/list — for scripting and programmatic consumers:

ferritin get std::vec::Vec --format json

(The JSON shape is still settling as the library API evolves; a published JSON Schema is planned.)

Caching and Storage

Ferritin keeps its caches under a single directory — $FERRITIN_HOME if set, else $XDG_CACHE_HOME/ferritin, else ~/.cache/ferritin. Everything in it is reconstructible (fetched or derived), so it is always safe to delete:

• Crates.io documentation: raw rustdoc JSON at docs/{crate}/{version}/{format-version}.json, with derived binary caches beside it (an rkyv archive for fast partial loads, and a .index file generated lazily on first search)
• crates.io metadata: the crate-names artifacts (crate-names/) used for version resolution and typeahead, and per-crate version lookups (crates-io-versions/)
• Standard library search indices: written to {rustc sysroot}/share/doc/rust/json/ if available
• Local workspace documentation: built into the workspace's own target/doc/, not the shared cache

Earlier versions cached under $CARGO_HOME/rustdoc-json; a cache found there is migrated automatically on first run.

Environment Variables

ferritin serve (the opt-in serve/acme features; not present in distributed binaries) is configured 12-factor style:

Development-only knobs (not part of the stable interface): FERRITIN_TEST_MODE (normalized snapshot-test output), FERRITIN_REINDEX (rebuild search indexes in memory, bypassing the disk cache), and the search-scoring probes FERRITIN_NAME_WEIGHT, FERRITIN_ANCESTOR_WEIGHTS, and FERRITIN_NAME_PREFIX_COUNT.

Current Status

Ferritin is at version 0.x and actively used by the author as a primary documentation interface. It's ready for general use, though the output format should be considered unstable and may change between versions.

If you're scripting against ferritin's output, be aware that the text format may change. Pin to a specific version or be prepared to update your scripts.

Platform Support

Ferritin is developed primarily on Unix-like systems and is also tested on Windows, where both the CLI and interactive TUI work (prebuilt x86_64 Windows binaries are provided). If you encounter issues on any platform, please open an issue or pull request.

Related Projects

Ferritin was originally developed to support the rustdoc-mcp MCP server, which provides Rust documentation access for Claude Code and other MCP clients.

License

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