assay-mcp-server

Agents got real tool access through MCP — and tool poisoning, rug pulls, and confused-deputy OAuth came with it. Most tools scan a server or filter a prompt. Assay sits at the tool-call boundary and does three things, in order.

One golden path: the release-pinned agent journey records the nine driven CLI/MCP steps and their exit/stdout contracts. Its protected-action fixture lives in examples/privileged-action-gate/.

Enforce, prove, stay honest

• Enforce. A deterministic, fail-closed gate decides every tools/call before it runs, with the precise reason for each allow or deny. On Linux it adds real kernel enforcement — an eBPF/LSM IPv4/TCP connect-egress block and a Landlock TCP-connect port allowlist, both opt-in and fail-closed. A policy it cannot express exactly is refused, never half-applied.

• Prove. Each decision and observed effect becomes an offline-verifiable, tamper-evident evidence bundle: the verdict, the pre-call establish journey, and declared-vs-observed conformance — all reviewable in CI, with no hosted backend.

• Stay honest. Every claim carries its basis (verified, self_reported, inferred, absent), and a gate refuses to let a claim exceed what was observed. A tool returning "success" is the provider's assertion, never proof. Assay ships no single safety score and never claims more than it can prove.

Quickstart

cargo install assay-cli --version 5.4.0 --locked

mkdir -p /tmp/assay-demo && echo "safe content" > /tmp/assay-demo/safe.txt
assay mcp wrap --policy examples/mcp-quickstart/policy.yaml \
  -- npx @modelcontextprotocol/server-filesystem /tmp/assay-demo

✅ ALLOW  read_file  path=/tmp/assay-demo/safe.txt  reason=policy_allow
❌ DENY   read_file  path=/tmp/outside-demo.txt      reason=path_constraint_violation
❌ DENY   exec       cmd=ls                          reason=tool_denied

Project manifests are shipped for Claude Code and Cursor; Codex uses the equivalent TOML entry documented in the editor MCP recipe. assay mcp config-path supports Claude and Cursor only. Python SDK: pip install assay-it. CI: GitHub Action. No hosted backend and no API keys for core flows. New to the threat model? The OWASP MCP Top 10 mapping lays out, per risk, what Assay covers and what it deliberately does not.

What ships

  Agent ──► Assay ──► MCP Server
              ├─ ✅ ALLOW / ❌ DENY  (policy, with reason)
              ├─► 📋 Evidence bundle (offline-verifiable)
              └─► 📊 Trust Basis → Trust Card → SARIF / CI

Current release: v5.4.0. CHANGELOG.md and release notes remain the authority for released behavior; merged changes after the tag are Unreleased, and crates.io publication is separate from merge state.

Related MCP server: governed-mcp

Is this for me?

Yes if you already have eval output, runtime decisions, inventory artifacts, or MCP tool-call tests, and you want a small reviewable CI artifact instead of a dashboard — bounded auditability, not a scalar trust badge.

Not yet if you need Assay to judge model correctness for you, want a hosted dashboard as the product, or want a compliance claim rather than a bounded evidence boundary. Assay is not a trust-score engine, a generic eval dashboard, or a hosted observability product — see what it is and is not.

See it work

An agent tries a privileged action — github.add_deploy_key — through the enforcing proxy, decided per call before it forwards, offline against a local mock (no real credentials):

cd examples/privileged-action-gate && ./run.sh

A deny is fail-closed caution, not a verdict on intent; an allow is the decision to forward, never proof the action happened. Declared-vs-observed conformance is recorded beside the verdict, never as a gate. Full walkthrough: privileged-action-gate.

Pick your path

The workflow stays small: import or record a bounded outcome, bundle and verify it, compile trust-basis.json, gate the Trust Basis diff. Assay doesn't make the upstream tool the source of truth; it makes the evidence boundary inspectable. For privileged tool actions, the MCP proxy records each tools/call as a structured tool-decision surface — keeping the asserted-versus-verified line honest.

Policy is simple

version: "2.0"
name: "my-policy"
tools:
  allow: ["read_file", "list_dir"]
  deny: ["exec", "shell", "write_file"]
schemas:
  read_file:
    type: object
    properties:
      path: { type: string, pattern: "^/app/.*" }
    required: ["path"]

assay init --from-trace trace.jsonl generates the runtime-observation policy used by the trace-generation flow (files, network, and processes); it is not an MCP authorization policy. Migrate a legacy MCP constraints: policy with assay policy migrate. See Policy Files.

Why Assay

Learn more

• MCP Quickstart · Editor MCP recipe — policy-enforcing MCP in Cursor / Claude Code / Codex

• MCP 2025/2026 protocol-era parity — pinned resultType and interim-result compatibility corpus

• Coding-agent governance · OpenTelemetry & Langfuse — observed runs → evidence

• Evidence Receipts in Action — Promptfoo / OpenFeature / CycloneDX receipt families

• CI Guide · Evidence Store (S3 / B2 / MinIO)

• OWASP MCP Top 10 mapping · Security experiments

• Positioning: ADR-033 · RFC-005

Trust claims use explicit epistemology, not a single safety score: verified (direct evidence or offline verification), self_reported (emitted without independent corroboration), inferred (bounded, documented rules), absent (no trustworthy evidence). Assay ships no aggregate trust score or safe/unsafe badge as the main output — see ADR-033.

Tool-decision path latency on an M1 Pro fragmented-IPI harness: main protection 0.771ms p50 / 1.913ms p95; fast-path 0.345ms p50 / 1.145ms p95. These are tool-decision timings, not end-to-end model latency.

Assay-Runner is an internal measured-run subsystem behind the delegated Linux/eBPF acceptance path — publish = false, not a standalone product, no release commitment.

Ecosystem

Repositories that compose with Assay's evidence layer:

• assay-action — GitHub Action: verify bundles, PR summaries, SARIF (Marketplace).

• Assay-Harness — recipe, gate, and report layer over canonical evidence artifacts.

• observed-effect-v0 — worked examples of the bounded observed-effect evidence record and its neutral carriers (in-toto, SCITT, MCP evidenceRef).

• gateway-evidence-replay — deterministic offline replay verifier for gateway-path evidence bundles.

• RGE-Bench — a conformance kit for evidence reviewability, maintained separately under its own machine-checked neutrality guard. Reproduction there is digest-scoped and does not carry forward: the v1 71-vector digest sha256:e769822bc6c9e31085da7b1a17b163b9747fe0d04314fbb8685d4e612087c7cb carries one reported independent implementation by a second author on a different stack (JM-Lab/rge-bench-java, written from the contract text importing nothing from the kit, recomputing every expected from inputs) against author-supplied vectors, while the current v2-candidate digest sha256:ba0e3795d75c788fa48313ab462493f22d78759851d1b3275d8117051bb22fd0 (95 vectors) has not been reproduced by anyone but the author. See its REPRODUCTIONS.md.

Open profile: privileged-mcp-action/v0

privileged-mcp-action/v0 is a composition and verification contract over evidence records that already exist: what a privileged MCP tool call decided, what was observed of its effect, and what stays unproven. It adds no new envelope and no aggregate verdict.

It ships with a 14-vector conformance corpus (5 accept, 9 reject) whose digest is a candidate: it is not called reproduced until a non-author implementation derives the expected outcomes from the specification text alone.

That reproduction is open, and the invitation is real: #1840. Any language, any stack. The invitation names the exact commit the current digest describes. The clean-room protocol provides an opaque, attested inputs pack, a one-command scoring action, and an implementation-report template without supplying verifier logic or expected outcomes. The corpus README states the authorship boundary and the claim ceiling.

Contributing

cargo test --workspace
cargo clippy --workspace --all-targets -- -D warnings

See CONTRIBUTING.md and GitHub Discussions.

License

MIT