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Skein

MCP Server

The Skein CLI ships an MCP (Model Context Protocol) server that gives coding agents on-demand access to the language spec and the compiler’s structured diagnostics — without stuffing the whole spec into the agent’s base prompt.

Terminal window
skein mcp

The server speaks JSON-RPC 2.0 over stdio (newline-delimited messages) and needs no network access: the language spec is embedded in the binary at build time.

Registering

Section titled “Registering”

Claude Code

Section titled “Claude Code”
Terminal window
claude mcp add skein -- skein mcp

Or add it to .mcp.json in your project root to share it with your team:

{
  "mcpServers": {
    "skein": {
      "command": "skein",
      "args": ["mcp"]
    }
  }
}

Cursor

Section titled “Cursor”

Add to .cursor/mcp.json (project) or ~/.cursor/mcp.json (global):

{
  "mcpServers": {
    "skein": {
      "command": "skein",
      "args": ["mcp"]
    }
  }
}

Other MCP-capable tools (Codex, Windsurf, Zed, …) follow the same pattern: configure a stdio server with command skein and arguments ["mcp"].

Tools

Section titled “Tools”

skein_spec_lookup

Section titled “skein_spec_lookup”

Fetches a named section of the language specification. Pass a section number or a title fragment:

{ "section": "6.4" }
{ "section": "agents" }

If nothing matches, the result lists every available section title, so an agent can self-correct in one round trip.

Section titled “skein_docs_search”

Case-insensitive search across the spec corpus. Returns each matching section title with the matching lines:

{ "query": "idempotent" }

skein_compile_check

Section titled “skein_compile_check”

Compiles a .skein file — or every file under a project’s src/ when given a directory (checking src/ and test/, matching skein test’s discovery) — and returns the compiler’s structured JSON diagnostics directly. Warnings are included; ok reflects errors only:

{ "path": "src/main.skein" }
{
  "ok": false,
  "files_checked": 1,
  "errors": [
    {
      "code": "E0012",
      "severity": "error",
      "message": "Missing capability declaration for http.out",
      "location": { "file": "src/main.skein", "line": 3, "col": 13 },
      "fix_hint": "Add the capability declaration to the module",
      "fix_code": "capability http.out(\"example.com\")",
      "span": { "start": { "line": 2, "col": 3 }, "end": { "line": 2, "col": 3 } },
      "edit_kind": "insert_line"
    }
  ],
  "warnings": [
    {
      "code": "W0002",
      "severity": "warning",
      "message": "Unused capability 'store.table' — declared but never exercised",
      "location": { "file": "src/main.skein", "line": 4, "col": 3 },
      "fix_hint": "Remove this capability declaration if it is no longer needed",
      "fix_code": ""
    }
  ]
}

Skein’s compiler errors carry fix_hint and fix_code precisely so agents can apply fixes mechanically — MCP delivers them without shelling out and parsing CLI output. When the fix is an exact edit, span (1-based, end-exclusive) and edit_kind say where and how to apply it without any per-error-code logic: replace the spanned text with fix_code, insert_before/insert_after it at the span’s edges, insert_line it as a new line indented to the span’s start column, or delete_line the spanned lines. A null edit_kind marks fix_code as an illustrative template rather than a verbatim edit.

Section titled “Related”
  • skein new scaffolds an AGENTS.md primer (and a CLAUDE.md pointer) into every project; skein agents refreshes it after a toolchain upgrade.
  • Published agent docs: llms.txt, llms-small.txt, llms-full.txt.