err-mcp

err-mcp

A local MCP server for the err app — a local-first structured editor whose vault lives in the browser (OPFS).

err-mcp holds no vault data. It is a relay: MCP clients talk JSON-RPC to this process over stdin/stdout, and the process forwards each call over a loopback WebSocket to the open err web app, which executes it in its own engine. Nothing is reimplemented here and nothing is stored here.

MCP client  ──stdio──▶  err-mcp  ──ws://127.0.0.1:8787/app/ws──▶  err web app (vault)

Install

With Go 1.25 or newer — this is the path an agent can run unattended:

go install github.com/1l0/err-mcp@latest

Prebuilt binaries for macOS, Linux and Windows (arm64/amd64) are attached to every release. The release script verifies SHA256SUMS:

scripts/install.sh                 # latest release into ~/.local/bin
PREFIX=/usr/local scripts/install.sh
VERSION=v0.10.1 scripts/install.sh

Configure your MCP client

err-mcp speaks MCP over stdin/stdout, so the client launches it. For OpenCode:

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "servers": {
      "err": {
        "type": "local",
        "command": ["err-mcp"],
        "timeout": { "startup": 60000, "execution": 120000 }
      }
    }
  }
}

Then open the err web app, click the MCP button in the header, enable Local MCP and confirm the port (default 8787). The app pairs automatically. Stop any other err-mcp first: the launched process owns the bridge port.

Check the connection with curl -s http://127.0.0.1:8787/healthz or call err_status from the MCP client.

Flags

Security model

• The MCP side has no token, OAuth or remote endpoint: the client owns the process and its stdio.
• The app bridge is token-authenticated and bound to loopback by default. The token is generated on first start and stored at os.UserConfigDir()/err/mcp-token (~/Library/Application Support/err/ on macOS, ~/.config/err/ on Linux), directory 0700, file 0600.
• GET /pair hands the token to the browser only for the paired origin, which is recorded in os.UserConfigDir()/err/mcp-origin.
• Tools are tiered: the default toolset cannot rekey the vault, import segments, run GC or disclose the vault key.
• --audit-log records one JSONL line per call; stderr always gets the log.

Tools

The catalog is generated from the app's protocol descriptor, which is embedded at build time (see Development), so it always matches the app's RPC surface. err_status reports the tool count, the toolset and the catalog_fingerprint the app also computes; a mismatch means the app and the server were built from different protocol versions.

The default safe tier covers reading and editing Items, the parent/child graph, tree views and undo/redo. Three projection tools keep results small: list_titles, outline and search_items.

Bulk import is the create_tree tool: it creates a nested tree of Items in one call and one undo step. An agent that can read local files can build the arguments itself.

Development

go vet ./...
go test ./...
go build .

Layout: main.go is the command; internal/mcp is the server (catalog, tool handlers, app bridge, audit, tokens, embedded descriptor).

internal/mcp/domain.fds is a compiled FileDescriptorSet of the app's RPC protocol, embedded with go:embed. It is committed on purpose: that is what lets anyone run go install with no code generation step and no dependency on generated Go packages. It is produced by the app's protocol tooling and never edited by hand. TestCatalogFingerprint pins the expected descriptor, so a stale .fds fails the build instead of silently advertising the wrong schema.

Releasing

Tags track the err app version — v0.10.0 is the first standalone release — so comparing err-mcp --version with the app's app_version stays meaningful. Release builds stamp the tag with -ldflags; a binary installed with go install ...@v0.10.0 reports the module version Go recorded, and a build from a checkout reports its pseudo-version.

Pushing a v* tag runs .github/workflows/release.yml: it vets, tests, builds all six platform targets, packages them with LICENSE and THIRD_PARTY_LICENSES.txt, writes SHA256SUMS, publishes the GitHub release, and then verifies go install github.com/1l0/err-mcp@<tag> on a clean runner. Branch pushes only run ci.yml.

macOS tarballs are unsigned; sign and notarize them separately if you publish them to a Homebrew tap (packaging/err-mcp.rb is a template). See docs/RELEASING.md.

License

MIT — see LICENSE.