Architecture

SharpLsp is built on a three-tier architecture that separates concerns between fast syntax operations and rich semantic analysis.

graph LR
    Editor([Editor / IDE])

    subgraph T1[Tier 1 — Rust LSP Host]
        direction TB
        LSP[LSP Server\nJSON-RPC over stdio]
        Router[Request Router]
        VFS[Virtual File System]
        TS[tree-sitter\nC# and F# grammars]
        Salsa[salsa cache]
        LSP --> Router
        LSP --> VFS
        VFS --> TS
        TS --> Salsa
    end

    IPC[[IPC\nMessagePack\nnamed pipes / Unix sockets]]

    subgraph T2[Tier 2 — C# Sidecar]
        direction TB
        Roslyn[Roslyn\nMSBuildWorkspace]
        Decompiler[ICSharpCode.Decompiler]
        CSharpFeatures[completions\ndiagnostics\nrefactoring]
        Roslyn --> Decompiler
        Roslyn --> CSharpFeatures
    end

    subgraph T3[Tier 3 — F# Sidecar]
        direction TB
        FCS[FSharpChecker]
        ProjInfo[Ionide.ProjInfo]
        Fantomas[Fantomas]
        FSharpLint[FSharpLint]
        FCS --> ProjInfo
        FCS --> Fantomas
        FCS --> FSharpLint
    end

    Editor -- LSP 3.17 --> LSP
    Router -- syntax under 5ms --> Salsa
    Router -- semantic under 200ms --> IPC
    IPC <--> Roslyn
    IPC <--> FCS

Tier 1 — Rust LSP Host

The host process handles:

• LSP protocol: JSON-RPC over stdio, targeting LSP 3.17 behavior
• Virtual File System (VFS): In-memory file state with change tracking
• tree-sitter parsing: Incremental C# parsing at sub-millisecond speeds (F# grammar integration is pending; F# syntax features route to the sidecar)
• salsa cache: Incremental computation — only reprocess what changed
• Request routing: Fast syntax requests stay in Rust, semantic requests go to sidecars

Tier 2 — C# Sidecar (Roslyn)

A long-running .NET 10 process providing:

• MSBuildWorkspace for solution/project loading
• Roslyn-backed C# features: completions, diagnostics, code actions, refactoring
• ICSharpCode.Decompiler for go-to-decompiled-source
• MessagePack serialization over named pipes / Unix domain sockets

Tier 3 — F# Sidecar (FCS)

A separate .NET 10 process for F# support:

• FSharpChecker for type checking and analysis
• Ionide.ProjInfo for project system integration
• FSharpLint for additional diagnostics

IPC Protocol

Communication between the Rust host and .NET sidecars uses:

• MessagePack binary serialization (compact, fast)
• Named pipes (Windows) or Unix domain sockets (Linux, macOS)
• 4-byte little-endian length prefix framing
• Target: <500us round-trip overhead

Request Routing