Support shared LSP server instances across sessions

[mingley]

Problem

The rust-analyzer-lsp plugin (and any LSP plugin using the declarative lspServers config) spawns a new LSP server process per Claude Code session. For rust-analyzer, each instance independently loads and indexes the entire project, consuming 500MB–2GB+ of memory. Running multiple Claude Code sessions against the same project means multiple redundant instances of the same server, all holding duplicate indexes in memory.

Current Architecture

The LSP Manager:

1. Reads lspServers config from the plugin manifest
2. Spawns a child process per session via stdio
3. Manages a 1:1 client-server lifecycle tied to the session

Stdio transport is inherently single-client — there's no mechanism to discover or reuse an existing server process.

Proposed Solution

Add an option for LSP server definitions to share a single server process across sessions working on the same project. Two possible approaches:

Option A: Socket-based transport with process reuse

Allow lspServers config to specify a socket transport:

"lspServers": {
  "rust-analyzer": {
    "command": "rust-analyzer",
    "transport": "socket",
    "socketPath": "/tmp/claude-lsp-rust-analyzer-{workspaceHash}.sock",
    "shared": true,
    "extensionToLanguage": { ".rs": "rust" }
  }
}

The LSP Manager would:

1. Check if a server is already running (via PID file or socket probe)
2. If running, connect as an additional client
3. If not running, spawn the server and register the PID/socket for future sessions
4. Reference-count connections — shut down the server when the last session disconnects

Option B: Managed daemon with multiplexing proxy

For LSP servers that only support stdio (like rust-analyzer), the LSP Manager could:

1. Spawn a lightweight proxy that owns the stdio connection to the server
2. Accept multiple client connections over a Unix socket
3. Multiplex requests and demultiplex responses, maintaining per-client state for open documents

Minimum viable version

Even without full multiplexing, the simplest improvement would be:

• Detect when another session already has an LSP server running for the same workspace root
• Reuse that server's index/cache directory so startup is near-instant even if a new process is spawned
• rust-analyzer supports --log-file and cache directories that could be shared

Impact

• Memory savings scale linearly with concurrent sessions (easily 1–4GB reclaimed for typical Rust projects)
• Faster session startup — no re-indexing when a warm server already exists
• Applies to all LSP plugins, not just rust-analyzer

[github-actions]

Found 2 possible duplicate issues:

1. [BUG] Claude Code Extension Spawning New rust-analyzer Processes For Every New Chat #26752
2. [FEATURE] Support gopls daemon mode to share single instance across multiple Claude Code sessions #19517

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