litmus

The open behavioral litmus harness for MCP servers — grade A–F, reproducible.

Gate your CI on MCP grades — GitHub Action

Fail a build when an MCP server or an Agent Skill it ships grades D/F under the open behavioral litmus. For servers it is hybrid — a fast lookup of the published grade, then the harness when ungraded; for skills it is a fast static scan. Un-gradeable targets warn unless strict.

It's on the GitHub Marketplace as polygraphso/litmus@v1 — drop it into a workflow:

# .github/workflows/mcp-gate.yml
name: mcp-gate
on: [pull_request]
permissions:
  contents: read
jobs:
  gate:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - uses: polygraphso/litmus@v1
        with:
          # Auto-discovers MCP servers (.mcp.json / .vscode/mcp.json / .cursor/mcp.json)
          # and skills (SKILL.md dirs). Or name them explicitly:
          servers: |
            npm/@modelcontextprotocol/server-filesystem
          skills: |
            ./my-skill
          # min-grade: B      # stricter than the default D/F gate
          # strict: "true"    # also fail on targets that cannot be graded

Inputs: servers · skills · discover (default true) · min-grade · strict · working-directory · version · bearer. Outputs: result · failed · report.

Not on GitHub? The gate is a plain command — npx @polygraphso/litmus ci — so it runs in any CI or as a pre-commit hook. A grade is a measurement, not a guarantee: re-run the open harness to reproduce any result.

What litmus is

This is the source for @polygraphso/litmus, the open behavioral litmus harness for MCP servers from polygraph.so.

The harness connects to an MCP server the way an agent would, fingerprints its exact tool surface, and runs four probe categories — C-01 tool-output injection (static, dynamic, and second-order — one tool's output weaponized as another's input), C-02 permission/egress (in a hardened default-deny Docker sandbox, matched host and port), C-03 sensitive-data handling (planted canaries), C-04 adversarial-input handling (malformed/oversized and jailbreak inputs) — then grades the server A–F. A passing grade is a measurement, not a guarantee; the methodology and its disclosed limits are at polygraph.so (the open source here is the ground truth).

Alongside the grade, an npm target's dependency tree is checked against the osv.dev vulnerability database and any vulnerable dependencies are reported as dependency advisories. This is a separate, point-in-time signal — it is advisory only: it never affects the A–F grade and is not part of the reproducible evidence (vulnerability data changes over time, so folding it into the grade would break re-run reproducibility). It applies to npm targets only; other target kinds report it as skipped. Resolution runs npm install --package-lock-only --ignore-scripts, which resolves the tree without downloading tarballs or running any package code. Opt out with --no-deps-audit (or LITMUS_DEPS_AUDIT=0).

The same package also grades Claude Code / Agent Skills (a SKILL.md + bundle) under a separate static litmus (litmus-skill-v2): a deterministic byte-scan — S-01 prompt injection, S-03 data-exfiltration instructions, S-04 dangerous commands in bundled scripts — graded A/B/D/F and anchored by a whole-directory content hash, plus a separate advisory quality signal. It is static (no execution): an A is static-clean, not behavioral proof. See packages/litmus/README.md.

The hosted, operator-run grading service is not in this repo — it lives in a separate private repo and consumes this package from npm like any other client.

Layout

This is a pnpm monorepo. Only @polygraphso/litmus is published; the @polygraph/* packages are private building blocks that tsup bundles into it.

packages/
  litmus/          # @polygraphso/litmus — the only published package (lib + 3 bins: CLI, skill CLI, MCP)
  core/            # contract types, canonical JSON, identity helpers
  probes/          # the harness: connect, fingerprint, grade, probe runners, sandbox
  onchain/         # EAS attestation read + encode/decode (Base) — read-only, no minting
  agent/           # agent-gate decision logic + live-fingerprint recheck
  mcp/             # MCP server wrapper
  cli/             # CLI commands + target/auth resolution
  demo-*-mcp/      # demo MCP servers used as test fixtures

See packages/litmus/README.md for the npm-facing usage docs, and polygraph.so for the methodology and proof format.

Develop

pnpm install
pnpm -r typecheck
pnpm -r test
pnpm --filter @polygraphso/litmus build   # → packages/litmus/dist

Release

@polygraphso/litmus is versioned in packages/litmus/package.json. Tag to publish:

git tag litmus-v<x.y.z> && git push origin litmus-v<x.y.z>

The Publish @polygraphso/litmus workflow builds, typechecks, tests, and publishes with npm provenance. See CONTRIBUTING.md for the full process and the local-development workflow for downstream consumers.

License

Apache-2.0 — © polygraph.so.