Language Server Protocol (LSP)

Hermes runs full language servers — pyright, gopls, rust-analyzer, typescript-language-server, clangd, and ~20 more — as background subprocesses and feeds their semantic diagnostics into the post-write lint check used by write_file and patch. When the agent edits a file, it sees exactly the errors that edit introduced — not just syntax errors, but type errors, undefined names, missing imports, and project-wide semantic issues the language server detects.

This is the same architecture top-tier coding agents use. Hermes ships it self-contained: no editor host required, no plugins to install, no separate daemon to manage.

When LSP runs

LSP is gated on git workspace detection. When the agent's working directory (or the file being edited) is inside a git repository, LSP runs against that workspace. When neither is in a git repo, LSP stays dormant — useful for messaging gateways where the cwd is the user's home directory and there's no project to diagnose.

The check is layered: in-process syntax check first (microseconds), then LSP diagnostics second when syntax is clean. A flaky or missing language server can never break a write — every LSP failure path falls back silently to the syntax-only result.

Concretely, on every successful write_file or patch:

1. Hermes captures a baseline of current diagnostics for the file.
2. Performs the write.
3. Re-queries the language server, filters out diagnostics that were already in the baseline, and surfaces only the new ones.

The agent sees output like:

{
  "bytes_written": 42,
  "dirs_created": false,
  "lint": {"status": "ok", "output": ""},
  "lsp_diagnostics": "LSP diagnostics introduced by this edit:\n<diagnostics file=\"/path/to/foo.py\">\nERROR [42:5] Cannot find name 'foo' [reportUndefinedVariable] (Pyright)\nERROR [50:1] Argument of type \"str\" is not assignable to \"int\" [reportArgumentType] (Pyright)\n</diagnostics>"
}

The lint field carries the syntax-check result (microsecond in-process parse via ast.parse, json.loads, etc.); the lsp_diagnostics field carries the semantic diagnostics from the real language server. Two channels, independent signals — the agent sees a syntax-clean file with semantic problems as lint: ok plus a populated lsp_diagnostics.

Supported languages

Language	Server	Auto-install
Python	pyright-langserver	npm
TypeScript / JavaScript / JSX / TSX	typescript-language-server	npm
Vue	@vue/language-server	npm
Svelte	svelte-language-server	npm
Astro	@astrojs/language-server	npm
Go	gopls	go install
Rust	rust-analyzer	manual (rustup)
C / C++	clangd	manual (LLVM)
Bash / Zsh	bash-language-server	npm
YAML	yaml-language-server	npm
Lua	lua-language-server	manual (GitHub releases)
PHP	intelephense	npm
OCaml	ocaml-lsp	manual (opam)
Dockerfile	dockerfile-language-server-nodejs	npm
Terraform	terraform-ls	manual
Dart	dart language-server	manual (dart sdk)
Haskell	haskell-language-server	manual (ghcup)
Julia	julia + LanguageServer.jl	manual
Clojure	clojure-lsp	manual
Nix	nixd	manual
Zig	zls	manual
Gleam	gleam lsp	manual (gleam install)
Elixir	elixir-ls	manual
Prisma	prisma language-server	manual
Kotlin	kotlin-language-server	manual
Java	jdtls	manual

For "manual" entries, install the server through whatever toolchain manager makes sense for that language (rustup, ghcup, opam, brew, …). Hermes auto-detects the binary on PATH or in <HERMES_HOME>/lsp/bin/.

A few servers are installed alongside a peer dependency that npm won't auto-pull. The current case is typescript-language-server, which requires the typescript SDK importable from the same node_modules tree — Hermes installs both packages together when you run hermes lsp install typescript or auto-install fires on first use.

CLI

hermes lsp status          # service state + per-server install status
hermes lsp list            # registry, optionally --installed-only
hermes lsp install <id>    # eagerly install one server
hermes lsp install-all     # try every server with a known recipe
hermes lsp restart         # tear down running clients
hermes lsp which <id>      # print resolved binary path

hermes lsp status is the best starting point — it shows which languages will get semantic diagnostics today and which need a binary installed.

Configuration

The defaults work for typical setups; nothing to set if the binaries are on PATH.

# config.yaml
lsp:
  # Master toggle. Disabling skips the entire subsystem — no servers
  # spawn, no background event loop runs.
  enabled: true

  # How long to wait for diagnostics after each write.
  wait_mode: document      # "document" or "full"
  wait_timeout: 5.0

  # How to handle missing server binaries.
  #   auto    — install via npm/pip/go install into <HERMES_HOME>/lsp/bin
  #   manual  — only use binaries already on PATH
  install_strategy: auto

  # Per-server overrides (all optional).
  servers:
    pyright:
      disabled: false
      command: ["/abs/path/to/pyright-langserver", "--stdio"]
      env: { PYRIGHT_LOG_LEVEL: "info" }
      initialization_options:
        python:
          analysis:
            typeCheckingMode: "strict"
    typescript:
      disabled: true       # skip TS even when its extensions match

Per-server keys

• disabled: true — skip this server entirely even when its extensions match a file.
• command: [bin, ...args] — pin a custom binary path. Bypasses auto-install.
• env: {KEY: value} — extra env vars passed to the spawned process.
• initialization_options: {...} — merged into the LSP initializationOptions payload sent in the initialize handshake. Server-specific; consult the language server's docs.

Installation locations

When install_strategy: auto, Hermes installs binaries into <HERMES_HOME>/lsp/bin/. NPM packages land in <HERMES_HOME>/lsp/node_modules/ with bin symlinks one level up. Go binaries come from go install with GOBIN pointed at the staging dir.

Nothing is ever installed to /usr/local/, ~/.local/, or any other shared location — the staging dir is fully Hermes-owned and is removed when you reset the profile.

Performance characteristics

LSP servers are lazy-spawned on first use. Editing a Python file in a project that's never seen .py traffic spawns pyright; the spawn takes 1-3 seconds for most servers (rust-analyzer can take 10+ on a cold project). Subsequent edits in the same workspace re-use the running server.

The LSP layer adds a few milliseconds to clean writes when no diagnostics are emitted. When diagnostics are emitted, the wait budget is wait_timeout seconds — typically the server responds in tens of milliseconds for pyright/tsserver and a few seconds for rust-analyzer mid-indexing.

Servers are kept alive for the life of the Hermes process. There's no idle-timeout reaper — the cost of restarting the server's index on every write would be far higher than holding the daemon.

Disabling

Set lsp.enabled: false in config.yaml to disable the entire subsystem. The post-write check falls back to the in-process syntax check (ast.parse for Python, json.loads for JSON, etc.) which ships unchanged from earlier versions.

To disable a single language without disabling the whole layer:

lsp:
  servers:
    rust-analyzer:
      disabled: true

Troubleshooting

hermes lsp status shows a server as "missing"

The binary isn't on PATH and isn't in <HERMES_HOME>/lsp/bin/. Run hermes lsp install <server_id> to attempt an auto-install, or install the binary manually through the language's normal toolchain.

Backend warnings section in hermes lsp status

Some servers ship as thin wrappers around an external CLI for actual diagnostics — they spawn cleanly and accept requests but never emit errors when the sidecar binary is missing. The most common case is bash-language-server, which delegates diagnostics to shellcheck. When hermes lsp status shows a Backend warnings section, install the named tool through your OS package manager:

apt install shellcheck      # Debian / Ubuntu
brew install shellcheck     # macOS
scoop install shellcheck    # Windows

The same warning is logged once at server spawn time in ~/.hermes/logs/agent.log.

Server starts but never returns diagnostics

Check ~/.hermes/logs/agent.log for [agent.lsp.client] entries — both stderr from the language server and protocol errors land there. Some servers (rust-analyzer especially) need to finish a project-wide index before they emit per-file diagnostics; the first edit after server start may complete with no diagnostics, with subsequent edits picking them up.

Server crashed

A crashed server is added to the broken-set and won't be retried for the rest of the session. Run hermes lsp restart to clear the set; the next edit re-spawns.

Editing a file outside any git repo

By design, LSP only runs inside a git repository. If the project isn't yet initialized, run git init to enable LSP diagnostics. Otherwise the in-process syntax-only fallback applies.