fcp-rust

MCP server for Rust codebases via rust-analyzer.

What It Does

fcp-rust lets LLMs navigate, inspect, and refactor Rust code through intent-level commands. Instead of reading raw source files, the LLM sends operations like find Config @kind:struct or rename old_name new_name @file:main.rs and fcp-rust routes them through rust-analyzer's LSP for accurate, project-aware results. Built on the FCP framework.

Written in Rust using rmcp for MCP transport and rust-analyzer as the language intelligence backend.

Quick Example

rust_session('open /path/to/my-project')

rust_query('find Config @kind:struct')
rust_query('refs Config @file:main.rs')
rust_query('diagnose')

rust([
  'rename old_handler handle_request @file:routes.rs',
  'generate Display @struct:Config',
])

rust_query('map')

Available MCP Tools

Tool	Purpose
rust(ops)	Batch mutations — rename, extract, inline, generate, import
rust_query(q)	Inspect the codebase — find, def, refs, symbols, diagnose, map
rust_session(action)	Lifecycle — open, status, close
rust_help()	Full reference card
enrich(path)	Extract diagnostics and symbols from a single .rs file (no session required)

Verb Reference — Navigation

Verb	Syntax
find	find QUERY [kind:KIND] [@selectors...]
def	def SYMBOL [@selectors...]
refs	refs SYMBOL [@selectors...]
symbols	symbols PATH [kind:KIND]
impl	impl SYMBOL [@selectors...]

Verb Reference — Inspection

Verb	Syntax
diagnose	diagnose [PATH] [@all]
inspect	inspect SYMBOL [@selectors...]
callers	callers SYMBOL [@selectors...]
callees	callees SYMBOL [@selectors...]
map	map
unused	unused [@file:PATH]

Verb Reference — Mutation

Verb	Syntax
rename	rename SYMBOL NEW_NAME [@selectors...]
extract	extract FUNC_NAME @file:PATH @lines:N-M
inline	inline SYMBOL [@selectors...]
generate	generate TRAIT @struct:NAME
import	import SYMBOL @file:PATH @line:N

Verb Reference — Session

Verb	Syntax
open	open PATH
status	status
close	close

Selectors

@file:PATH          Filter by file path substring
@struct:NAME        Filter by containing struct
@trait:NAME         Filter by containing trait
@kind:KIND          Filter by symbol kind (function, struct, enum, etc.)
@module:NAME        Filter by module name
@line:N             Filter by exact line number
@lines:N-M          Line range (for extract)

Installation

Build from source

cargo install --git https://github.com/os-tack/fcp-rust

MCP Client Configuration

{
  "mcpServers": {
    "fcp-rust": {
      "command": "fcp-rust"
    }
  }
}

Architecture

src/main.rs                    MCP server — 4 tools, stdio transport
    │
src/mcp/                       MCP integration
  ├── server.rs                RustServer (rmcp tool handlers)
  └── params.rs                Request/response types
    │
src/domain/                    Domain layer
  ├── verbs.rs                 Verb registration (query + mutation + session)
  ├── model.rs                 RustModel (workspace state, LSP client, symbol index)
  ├── query.rs                 Query handlers (find, def, refs, diagnose, map, etc.)
  ├── mutation.rs              Mutation handlers (rename, extract, inline, generate)
  └── format.rs                Output formatting
    │
src/resolver/                  Symbol resolution
  ├── selectors.rs             @file, @struct, @trait, @kind, @module, @line, @lines
  ├── index.rs                 In-memory SymbolIndex
  ├── pipeline.rs              Multi-tier resolution (index → LSP → tree-walk)
  └── fuzzy.rs                 Fuzzy matching
    │
src/lsp/                       rust-analyzer integration
  ├── client.rs                JSON-RPC client
  ├── transport.rs             stdio transport
  ├── types.rs                 LSP type definitions
  ├── workspace_edit.rs        Apply edits to disk
  └── lifecycle.rs             Server lifecycle management
    │
src/fcpcore/                   FCP framework (Rust port)
  ├── tokenizer.rs             DSL tokenizer
  ├── parsed_op.rs             Operation parser
  ├── verb_registry.rs         Verb spec registry + reference card
  ├── event_log.rs             Event sourcing (undo/redo)
  ├── session.rs               Session lifecycle
  └── formatter.rs             Response formatter

Development

cargo test          # Run all tests
cargo build         # Build binary
cargo clippy        # Run lints

License

MIT