HSP — Harness Server Protocol for Agent-First LSP

A standalone MCP server that bridges the Language Server Protocol into Claude Code, exposing LSP-backed operations as typed MCP tools. HSP is the harness layer around language servers: it keeps LSP protocol details inside the server while exposing graph-oriented operators for agents. Claude Code's built-in LSP() tool covers ~9 methods and is often buggy — this bridge covers the protocol surface for any language server while evolving toward a smaller graph-operator interface: find semantic nodes, inspect nodes, expand graph edges, stage mutations, and verify.

Tools

The target public surface is documented in docs/tool-surface.md. Implemented graph operators include:

Tool	Purpose
lsp_grep	Text search plus semantic binding; groups identifier hits by symbol identity.
lsp_symbols_at	Expands all semantic symbols on a line, including function args, with last-graph navigation.
lsp_symbol	Inspects one semantic node from a graph index, file:Lx, or file_path plus symbol/line.
lsp_goto	Resolves definition/declaration/type/implementation destinations through one command.
lsp_refs	Expands references for a semantic node or graph index.
lsp_outline	Shows compact file/workspace breadcrumbs from document symbols.
lsp_calls	Walks incoming/outgoing call edges from a graph node via direction=in|out|both.
lsp_types	Walks super/sub type hierarchy edges from a graph node via direction=super|sub|both.
lsp_session	Inspects, adds, warms, and restarts workspaces/LSP sessions via action=status|add|warm|restart.
lsp_diagnostics	Reports diagnostics as the main verifier surface.
lsp_fix	Lists code actions on a semantic target with line diagnostics; stages edit-backed actions for lsp_confirm.
lsp_rename	Previews and stages semantic renames before lsp_confirm.
lsp_move	Previews file moves (single or batched) and import/update edits before lsp_confirm.
lsp_confirm	Commits the currently staged edit transaction.
lsp_log	Appends agent-bus events, notes, timed questions, replies, and workspace weather through the broker.

The remaining protocol-shaped tools are transitional. The cut direction is one-way: as each workflow tool lands (lsp_outline, lsp_calls, lsp_fix, lsp_session, lsp_move), the corresponding raw LSP command wrapper is removed from the public registry — no aliases. Formatting is deliberately not exposed to agents; use editor/save hooks, pre-commit hooks, CI, or a direct formatter run instead. See docs/tool-surface.md for the full raw → workflow cut map.

File arguments may be full paths, relative paths, or unique basenames. For example, lsp_outline(file_path="NodesWindow.cs") resolves the file under active workspaces; if the basename is not unique, the tool returns the matching paths and asks for a more specific path.

Plugin Install Shape

HSP now ships as one plugin with a broker-owned router for Python, C#, and Rust language routes. Builtin routing is the default unless an explicit legacy LSP_SERVERS/LSP_COMMAND chain is configured; each request forwards the target URI/root to the broker, and the broker chooses a route from the file extension or workspace markers. That keeps each route's LSP chain, method cache, warmup state, and broker session separate while letting a broker restart pick up route-table changes.

Built-in routes:

Route	LSP chain	Selection signals
Python	ty server;basedpyright-langserver --stdio	.py, .pyi, pyproject.toml, setup.py, setup.cfg
C#	csharp-ls	.cs, *.sln, *.csproj, Directory.Build.props, global.json
Rust	rust-analyzer	.rs, Cargo.toml, rust-project.json

Set HSP_ROUTE=python, HSP_ROUTE=csharp, or HSP_ROUTE=rust to force a route for workspace-level operations where no file URI is available. Explicit LSP_SERVERS or legacy LSP_COMMAND still wins and keeps the old single-chain mode, so the split plugin repos continue to work while users migrate to the unified hsp plugin.

Legacy Split Plugins using hsp

• hsp-cs — C# via csharp-ls.
• hsp-py — Python via ty (Astral), with basedpyright fallback for call hierarchy and willRenameFiles.

Want to add a new language? Add a builtin route in hsp.router plus the plugin manifest's native lspServers entry. The old "one repo per LSP" shape still works, but the preferred interface is a single HSP plugin with routing inside the runtime.

How the model calls the tools

Semantic targets, not raw protocol calls. Tools accept graph indices, bare Lxx, file:Lx, unique basenames, or file_path plus symbol/line:

lsp_symbol(file_path="src/app.py", symbol="OmfiApp")
lsp_goto(file_path="src/app.py", symbol="workflow", line=476, mode="all")
lsp_refs(target="[0]")           # graph index from the previous lsp_grep/lsp_symbols_at
lsp_symbols_at("L78")            # bare Lxx — resolves against the last printed graph
lsp_symbols_at("HistoryUI.cs:L78")  # basename + line, no full path required

Sample lists shown by lsp_grep (samples L57,L694,...) are non-exhaustive — a trailing ... means more refs exist; unfold with lsp_refs([N]) or raise max_hits. The full count is always reported as refs N.

Batching. Multiple symbols in one file, multiple files in one call:

lsp_diagnostics(file_path="a.py,b.py,c.py")
lsp_diagnostics(pattern="src/**/*.py")

Output format. Line-number-anchored text, no JSON envelopes. Each response is prefixed with [server method] so the model sees which LSP handled the request:

[ty textDocument/hover]
<class 'OmfiApp'>
Standalone ComfyUI frontend built on AppKit.

For LSP Plugin Authors

hsp mcp is the MCP server; your plugin bundles it. Users install one plugin (yours), get both the native lspServers integration (for hooks/diagnostics) and the graph-oriented MCP tool set.

1. Declare the MCP server in plugin.json

{
  "name": "ty-lsp",
  "version": "1.0.0",
  "lspServers": {
    "ty": { "command": "ty", "args": ["server"] }
  },
  "mcpServers": {
    "ty-lsp-extended": {
      "command": "uvx",
      "args": ["hsp", "mcp"],
      "env": {
        "LSP_SERVERS": "ty server;basedpyright-langserver --stdio"
      }
    }
  }
}

2. (Optional) Wire the redirect hook

Claude's built-in LSP() tool is incomplete and occasionally silent-fails (e.g. returning 0 results when the server supports the operation). Ship a PreToolUse hook that denies LSP() with a redirect message listing the MCP alternatives:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "LSP",
        "hooks": [
          {
            "type": "command",
            "command": "hsp-redirect-hook"
          }
        ]
      }
    ]
  }
}

The published HSP Claude plugin already bundles these hooks. Plugin authors copy the block only when building a new downstream plugin; users should not hand-install hook config. Hook declarations live in hooks/claude.json and use the clean hsp hook stdin <kind> adapter shape. Ambient bus hooks are enabled by default when the plugin is installed; set HSP_HOOKS=0 only when you need the adapter to drain its JSON payload and exit without recording an event.

3. Configuration via env vars

Set in the env block of your mcpServers entry:

Variable	Required	Description
HSP_ROUTER	No	Builtin routing is the default when no explicit LSP_SERVERS/LSP_COMMAND is set. Set 0/false/off only to force legacy single-chain mode.
HSP_ROUTE	No	Force one builtin route (python, csharp, or rust) when router mode is enabled and a request has no target file URI.
HSP_HOOKS	No	Controls bundled ambient bus hooks. Hooks are on by default; set 0/false/off to make hsp hook stdin <kind> drain stdin and exit without recording an event.
LSP_SERVERS	Required only for custom/legacy plugin configs	;-separated chain in priority order. Each entry is <command> <args...>. First = primary. Example: ty server;basedpyright-langserver --stdio;pyright-langserver --stdio
LSP_ROOT	No	Workspace root path (defaults to cwd)
LSP_PREFER	No	Per-method server override: method1=command,method2=command. Skips the cold-call probe and routes directly. Example: workspace/willRenameFiles=basedpyright-langserver,textDocument/callHierarchy=basedpyright-langserver
LSP_REPLACE	No	Post-filter command substitution: old=new,old=new. Applied to LSP_SERVERS entries and LSP_PREFER targets so a user can swap a binary without rewriting the whole config. Example: basedpyright-langserver=pylance-language-server replaces basedpyright everywhere the plugin mentions it.
LSP_TOOLS	No	Which tools to register. all = every public tool. Comma list = explicit opt-in. Default = all public tools.
LSP_EXCLUDE	No	Comma-separated tools to exclude from the enabled set. (Legacy name: LSP_DISABLED_TOOLS — still accepted.)
HSP_BROKER	No	Broker mode: auto (default) shares one warm LSP chain across agents and falls back to direct mode if the broker is unreachable; on requires the broker; off restores one LSP chain per MCP process.
HSP_BROKER_SOCKET	No	Override the user-scoped Unix socket. Useful for isolated tests or separate broker pools.
HSP_BROKER_LOG	No	Override the broker log path. Default: $XDG_STATE_HOME/hsp/broker.log or ~/.local/state/hsp/broker.log.
HSP_BROKER_IDLE_TTL_SECONDS	No	Idle broker session TTL. Default 14400 seconds. Set 0 to disable automatic idle eviction.
LSP_DEVTOOLS	No	Set 1/true/on to expose the live broker to python-devtools for runtime introspection. Registers broker, bus, registry, and lsp under app id hsp-broker by default.
LSP_DEVTOOLS_APP_ID	No	Override the devtools app id. Default: hsp-broker.
LSP_DEVTOOLS_READONLY	No	Devtools readonly mode. Default: enabled, so agents can inspect broker state without mutation tools.
HSP_PROBE_CAPABILITIES	No	Opt into startup capability probing (1/true/on). Default off so MCP startup never launches heavy language servers before the initialize handshake. Runtime fallback still handles unsupported methods.
LSP_PROJECT_MARKERS	No	Comma-separated filenames or glob markers that mark a project root (e.g. pyproject.toml,setup.py,*.csproj,.git). When a file outside the current workspace folders is accessed, the bridge walks up looking for these markers and adds the found root to the LSP's workspace via workspace/didChangeWorkspaceFolders. Routes contribute their language's markers. Default: .git.
LSP_WARMUP_PATTERNS	No	Comma-separated glob patterns (e.g. *.py,*.pyi for Python, *.rs for Rust). When a workspace folder is added (initial spawn or via auto-detection), the bridge bulk-emits textDocument/didOpen for matching files so the LSP eagerly indexes them. Prevents the "cold index" failure mode where willRenameFiles returns 0 edits because nothing has been indexed yet. No warmup if unset.
LSP_WARMUP_MAX_FILES	No	Cap on how many files to warm per workspace folder. Default 500.

Legacy format (still accepted when LSP_SERVERS is unset): LSP_COMMAND/LSP_ARGS for primary, LSP_FALLBACK_COMMAND/LSP_FALLBACK_ARGS for first fallback, LSP_FALLBACK_2_COMMAND/LSP_FALLBACK_2_ARGS for subsequent fallbacks. Prefer LSP_SERVERS for new configs.

Chain behavior: per-method. On -32601 the next server in the chain is tried; the first success is cached for that method. All subsequent calls skip to the cached server. LSP_PREFER lets you pre-seed that cache to avoid the first-call cost when you already know which server handles a method best.

Standalone / CLI Usage

uv tool install hsp     # or: pip install hsp

LSP_COMMAND=ty LSP_ARGS=server hsp mcp
LSP_COMMAND=rust-analyzer hsp mcp
LSP_COMMAND=gopls LSP_ARGS=serve hsp mcp

The MCP server speaks stdio through hsp mcp — useful for testing or for non-plugin MCP clients. Bare hsp is reserved for the workgroup status/debug surface, so an accidental terminal run shows the current bus/broker shape instead of blocking on stdio.

Architecture

Claude Code
    ↕ MCP (stdio)
hsp mcp
    ↕ JSONL / Unix socket  [broker mode, default]
hsp-broker
    ↕ JSON-RPC / LSP (stdio)
┌─── Primary LSP (ty, rust-analyzer, ...)
└─── Fallback LSP  (basedpyright, pyright, ...)  [lazy-spawned]

• Broker mode is default when an LSP chain or builtin router is configured. Multiple agents reuse the same broker-owned LSP chain for the same root/config hash, reducing CPU and keeping route selection, method routing, diagnostics, and future alias memory aligned.
• The broker owns the agent-bus slice: lsp_log appends workspace events to tmp/hsp-bus.jsonl, opens timed questions, records replies, settles closed windows, and renders compact weather. Tickets, build gates, and opt-in edit denial are documented in docs/agent-bus.md; harness support and open teamwork tickets live in docs/harness-capability-matrix.md.
• With LSP_DEVTOOLS=1, the broker starts python-devtools and registers live broker, bus, registry, and lsp objects so agents can attach via the python-devtools MCP bridge and inspect daemon state directly.
• Primary and fallback are both lazy-spawned — no LSP processes start until the first semantic tool call that needs them.
• Method-level negative capability cache avoids repeated primary round-trips for operations the primary doesn't implement.
• Document sync reads from disk on each tool call (no in-memory tracking of user edits — the files on disk are the source of truth).
• lsp_session(action="status") reports broker PID, socket, log path, live sessions, client PIDs, open documents, cached method routes, and request counts. action="restart" stops the matching broker session so the next request respawns it; action="stop" stops without respawn.

Context

Built to address claude-code#40282 — Claude Code's native LSP tool is missing operations and buggy for some that it does implement. This bridge will be progressively phased out as Claude Code's built-in implementation matures.

License

MIT