MCP server overview

sextant-mcp is the Model Context Protocol server that exposes Sextant’s grading engine to agents. It speaks JSON-RPC 2.0 over either stdio or HTTP and surfaces five tools.

Tools

Tool	Purpose
`grade_diff`	Grade just the changed lines. Inner-loop fast.
`grade_files`	Grade entire files. Slower, thorough.
`list_rules`	List every loaded rule.
`explain_rule`	Fetch a rule’s full markdown documentation.
`get_config`	Return the resolved Sextant config.

The schemas come straight from the engine’s Report / Verdict types — same data the CLI returns in --format json.

Transports

stdio (default)

sextant-mcp

The standard MCP transport: JSON-RPC over stdin/stdout, one message per line. Logging goes to stderr. This is what Claude Code, Claude Desktop, and most other MCP clients use.

HTTP

sextant-mcp --http 127.0.0.1:7878

Newline-delimited JSON-RPC over HTTP at the bound address. Each request is a single POST to /. Useful for clients that don’t speak stdio, debugging via curl, or running the server as a long-lived daemon.

curl -sS http://127.0.0.1:7878/ \
  -H 'content-type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

Environment

Variable	Purpose
`RUST_LOG`	tracing env-filter. Defaults to `warn`. Try `RUST_LOG=sextant_mcp=debug` while debugging.
`ANTHROPIC_API_KEY`	Used by LLM-evaluated rules when `[judge]` selects an Anthropic model.
`OPENAI_API_KEY`	Same, for OpenAI models.
`SEXTANT_NO_LLM`	If set to `1`, disables LLM evaluators globally — same as the CLI’s `--no-llm`.

The server reads .sextant/config.toml and .sextant/rules/**/*.md from the current working directory’s repository root, just like the CLI. Run it from the repo root.

Setup guides

Use with Claude Code — easy path is the bundled plugin.
Use with Claude Desktop — manual config.

Logging

All log output goes to stderr. The stdio transport reserves stdout for the JSON-RPC protocol, so any println! to stdout would corrupt the stream. In HTTP mode, stderr still gets the logs.

For verbose tracing during development:

RUST_LOG=sextant_mcp=debug,sextant_engine=debug sextant-mcp