gavel-mcp

The gavel acceptance oracle as an MCP server: one tool that turns an agent's "done" into a receipt. gavel_acceptance cold-runs a command and reports the exit code. Exit code 0 is the only passing verdict.

Setup

1. Build

Requires Node ≥ 20 and git.

cd gavel-mcp
npm install
npm run build        # → dist/index.js

dist/ is gitignored — every fresh clone needs this step before the server can start.

2. Wire it into ZCode

Two scopes; both auto-connect at session start.

Workspace scope — versioned with the repo, shared with the team. Create <repo>/.zcode/config.json:

{
  "mcp": {
    "servers": {
      "gavel": {
        "command": "node",
        "args": ["/ABS/PATH/TO/gavel-mcp/dist/index.js"]
      }
    }
  }
}

User scope — applies to every workspace. Put the same mcp.servers object in ~/.zcode/cli/config.json, and pair it with the acceptance rule (section 5) in ~/.zcode/AGENTS.md so every session knows when to call the tool, not just how.

A user-scope install pins every workspace to this machine's build:

• After changing src/, run npm run build — other sessions keep loading the old dist/ until you do.

• Moving or deleting the repo directory breaks every session at once.

Works from a git remote today — no registry needed. The prepare script builds dist/ on install, so npx handles the rest:

{
  "command": "npx",
  "args": ["-y", "github:newlix/gavel-mcp#v0.5.0"]
}

Pin a tag (#v0.5.0) to make the npx cache stable; without one you track the default branch and cache refresh is at npx's discretion. First start on a machine pays a one-time clone + install + build. Once published to npm, ["-y", "gavel-mcp"] is equivalent and skips the git requirement. Any other MCP host works too; only the config shape differs.

3. Restart the session

MCP servers connect at session start. An already-running session will not pick the server up.

4. Verify

• ZCode: Settings → MCP shows gavel connected.

• Or simply ask the agent to call gavel_acceptance with cmd: "test -d ." — expect verdict=pass exit=0.

5. The rule (AGENTS.md)

The tool is the structure; the rule tells the agent when to use it. Drop this into <repo>/AGENTS.md:

## Acceptance

- Done = `gavel_acceptance` returned exit 0. One self-contained
  command, cold from the repo root; report the verdict and the
  command itself — never a paraphrase of test results.
- The command asserts intent (what should happen), not the
  implementation.
- `refused` means it never ran. Report it verbatim.

For user-scope installs, the same block goes in ~/.zcode/AGENTS.md instead — user instructions load first, so a repo's own AGENTS.md can still narrow the rule per project.

Related MCP server: Spec Kit Acceptance Gate MCP

The contract

The oracle never trusts a paraphrased result — it runs the command itself, so a red acceptance cannot be narrated green. Two structural layers, cheapest first:

1. Lint (src/lint.ts): a command that cannot fail (true, exit 0, bare echo/printf, x && true with no real check in it) is refused before execution — passed: false, refused: <reason>, no receipt minted. The Go linter's syntax and destructive-pattern checks are deliberately dropped: syntax fails identically when executed, and policing dangerous commands is the host permission layer's job, not the verdict layer's.

2. Cold run (src/runner.ts): the command runs via the platform shell from the project root; exit code 0 is the only pass. Signal deaths report 128+signal, spawn failure -1, command-not-found 127.

Receipt semantics: refused = never executed. Report it verbatim.

Tools

gavel_acceptance(cmd, cwd?, timeout_sec?)

→ { passed, exit_code, duration_ms, refused, output }

• output: merged stdout+stderr, raw; head+tail with a marker when longer than ~20 KB.

• A timeout kills the whole process tree and fails the run.

Troubleshooting

• Server not connected (Settings → MCP shows an error): the dist path is wrong or npm run build was skipped. The path must be absolute and point at dist/index.js.

• exit_code: 127: the acceptance command itself was not found.

Dev

npm install
npm test       # node:test via tsx (24 tests)
npm run build  # tsc → dist/

Layout: src/index.ts is the thin stdio bootstrap; the MCP surface (buildServer) lives in src/server.ts so tests can drive it in-process over InMemoryTransport plus one cold stdio smoke via tsx. The manual smoke below is the same exchange the stdio test runs.

Manual smoke (MCP stdio is newline-delimited JSON):

printf '%s\n' \
  '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"smoke","version":"0"}}}' \
  '{"jsonrpc":"2.0","method":"notifications/initialized"}' \
  '{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"gavel_acceptance","arguments":{"cmd":"test -d ."}}}' \
  | node dist/index.js