Assay

The open, recomputable evidence profile for privileged MCP tool actions.
Assay records what a privileged tool call decided, what was observed, and what stays unproven, so a reviewer can replay the claim offline instead of trusting the agent's account of itself. Enforcement is deterministic and fail-closed, and the enforcing proxy is the reference producer rather than the contract itself. Kernel-level (eBPF/LSM) observation on Linux is an optional stronger vantage. CI-native, no backend, bounded by design.

Quickstart · How it works · See it work · MCP example · OWASP MCP Top 10 · Discussions

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

# Fast path: release installer for Linux and macOS.
curl -fsSL https://getassay.dev/install.sh | sh

# Confirm the command resolves; if setup fails, run `assay doctor`.
assay --version

# Source-build alternative (requires Rust):
cargo install assay-cli --version 6.2.1 --locked

python3 examples/mcp-quickstart/run.py

For v6.2.1, run the last command from a source checkout or an extracted published CLI archive. The installer is binary-only and does not carry the bounded quickstart assets. The live getassay.dev installer verifies the selected archive against its published SHA-256 sidecar before extraction. Set ASSAY_REQUIRE_PROVENANCE=1 to additionally require GitHub artifact provenance; the default reports provenance_not_requested and strict success reports provenance_verified. A checksum proves byte equality with the published sidecar, not producer identity. Provenance identifies the source and build, not runtime safety or semantic correctness.

Captured runner output (the bundled local mock performs no external action):

assay quickstart: PASS
mcp_requests=initialize,tools/list,tools/call
decision=allow tool=read_file
decision_artifact=.assay/quickstart/decisions.ndjson
non_claim=forwarded_to_local_mock_only

Released surfaces:

• Static project manifests are shipped for Claude Code and Cursor; Codex uses the equivalent TOML entry documented in the editor MCP recipe. Manifest presence is not host-discovery proof. assay mcp config-path supports Claude and Cursor only.
• Published v6.2.1 CLI archives cover Linux x86_64/arm64, macOS x86_64/arm64, and Windows x86_64. The Python wheels cover CPython 3.12 on macOS x86_64/arm64 and Linux x86_64; other interpreters and platforms are not claimed.
• Published assay-mcp-server archives cover Linux x86_64/arm64. MCPB and server.json package descriptors are also published; their presence is not host-discovery proof.
• CI: GitHub Action. Core flows need no hosted backend or API key. New to the threat model? The OWASP MCP Top 10 mapping states, per risk, what Assay covers and 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: v6.2.1. 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.

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

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 and the current v2 digest sha256:ba0e3795d75c788fa48313ab462493f22d78759851d1b3275d8117051bb22fd0 (95 vectors) each carry one reported independent implementation by a second author on a different stack. JM-Lab reported the v2 95/95 reproduction on 2026-08-24, from the contract text and author-supplied inputs without reading expected. 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