compare-cli MCP server

compare-cli-mcp

MCP (Model Context Protocol) server wrapping compare-cli for agent pipelines. Three tools, stdio transport, JSON-first responses with optional human report.

Design contract: ../docs/mcp.md. This package ships the implementation; behavior and schemas track the design version.

Part of the contract-ops suite.

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Install

compare-cli-mcp has compare-cli as a peer dependency, so install both:

npm install -g compare-cli@^0.3.0 compare-cli-mcp@^0.1.1

Then run:

compare-mcp           # spawns the MCP server on stdio

Wire into an MCP client

Claude Desktop / Claude Code

Add to your MCP-client config (e.g. ~/Library/Application Support/Claude/claude_desktop_config.json on macOS):

{
  "mcpServers": {
    "compare-cli": {
      "command": "compare-mcp"
    }
  }
}

With base-directory lockdown (recommended for unattended use)

{
  "mcpServers": {
    "compare-cli": {
      "command": "compare-mcp",
      "env": {
        "COMPARE_MCP_BASE_DIR": "/path/to/contract/documents"
      }
    }
  }
}

Every path argument the agent supplies is then resolved against COMPARE_MCP_BASE_DIR and rejected if it escapes the directory (symlinks collapsed). Without the env var the server reads any file the process can read — see the security note in ../docs/mcp.md §3.2.

Tools

Tool	Purpose
compare_files	Two-document comparison. Each side accepts path, content_base64, or content_text.
compare_with_negotiation	Base is read from nda-review-cli's negotiation.json. Returns base.negotiation_resolution so the agent can audit which signal produced the agreed text.
compare_demo	Zero-arg synthetic comparison against bundled fixtures. Deterministically returns exit_class: "substantive". Use to confirm the server is wired up.

Tool descriptions, input schemas, and output schemas are advertised on tools/list. The schemas are stable across v1.x of this package; any change is fenced by the snapshot golden in tests/fixtures/tools-list.json.

Output shape

Every successful tool call returns:

• structuredContent — the exact compare --json shape (top-level ok, exit_class, exit_code, base, candidate, summary, differences, warnings). Stable across compare-cli v1.x per its AGENTS.md. This is the contract the agent reads.
• content[0] — the same JSON serialized as a text block, for MCP clients that don't yet read structuredContent.
• content[1] (optional, when caller passes include_human_report: true) — the human-readable report.

compare_with_negotiation adds structuredContent.base.negotiation_resolution ∈ {"status", "per_round_agreed", "clause_status"} indicating which of the three resolution tiers produced the agreed base text.

Errors vs drift

Substantive drift is a successful tool call. The agent asked what's different and the server answered. Routing on exit_class is the contract — error envelopes (isError: true) are reserved for I/O / argument failures plus one explicit precondition exception:

Code	Trigger
INPUT_NOT_FOUND	Path not found / unreadable.
INPUT_MALFORMED	Malformed .docx / .pdf.
PDF_NO_TEXT_LAYER	Scanned PDF; pdfjs extracted zero characters.
INVALID_ARGS	Schema-validation failure (missing args, oneOf violation, etc.).
PATH_OUTSIDE_BASE_DIR	COMPARE_MCP_BASE_DIR is set and the path escapes it.
NO_AGREED_ROUND	compare_with_negotiation precondition: no agreed round found. The CLI returns exit 2 here; the MCP server promotes it to an error because no comparison ran.
NOT_SIGNED_OFF	require_signoffs: true and signoffs.a / signoffs.b is missing.
INTERNAL_ERROR	Unexpected; surfaces underlying message so the agent isn't blind.

Codes are stable across v1.x of this package.

Running the test suite

# From the repo root:
npm install              # workspaces resolve compare-cli + compare-cli-mcp
cd mcp && npm test       # 22 tests, ~100 ms

The conformance + snapshot suite is in tests/test_protocol.mjs. The tools/list golden lives at tests/fixtures/tools-list.json. To regenerate the golden after an intentional schema change (also bump the minor version):

node -e 'import("./compare-cli-mcp.mjs").then(m => process.stdout.write(JSON.stringify(JSON.parse(JSON.stringify(m.TOOLS)), null, 2)))' > tests/fixtures/tools-list.json

License

MIT. See ../LICENSE.