Assay MCP Server

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. In enforcement mode, the proxy gate for routed MCP tools/call requests is deterministic and fail-closed, and the enforcing proxy is the reference producer rather than the contract itself. Optional eBPF/LSM instrumentation on supported Linux hosts adds kernel-level observations. CI-native, no backend, bounded by design.

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

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Agents got real tool access through MCP — and tool poisoning, rug pulls, and confused-deputy OAuth came with it. 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. In enforcement mode, the gate decides tools/call requests routed through it before forwarding, with the precise reason for each allow or deny. Separate controls on supported Linux hosts include cgroup eBPF IPv4/TCP connect filtering (whose hook error path allows connections) and Landlock TCP-connect port allowlisting. assay sandbox --enforce refuses an unavailable backend or conflicting filesystem policy unless --allow-audit-fallback is explicit. --enforce-net requires --enforce; active Landlock network enforcement rejects policies it cannot express as TCP-port allowlists, even with audit fallback enabled.
• Prove. Configured producers can record decisions and bounded observations for export into offline-verifiable, tamper-evident evidence bundles. The privileged-action flow carries the verdict, pre-call establish journey, and declared-vs-observed conformance for CI review without a hosted backend. assay mcp wrap --decision-log records tool-call decisions; assay evidence prepare-incident-package turns that log into an incident package verify-incident-package accepts, with the surface grades the verifier derives. The package is not a count of all calls, not completeness of capture, not proof of what the server did, and an allow is not proof a human accepted.
• Stay honest. Trust Basis classifies supported claims as verified, self_reported, inferred, or absent; its gates check the declared claim boundaries. A tool returning "success" is the provider's assertion, never proof. Assay ships no single safety score; read each artifact’s source, coverage and non-claims before relying on it.

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 6.22.3 --locked

python3 examples/mcp-quickstart/run.py

For v6.22.3, 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 Desktop and Cursor only.
• Published v6.22.3 CLI archives cover Linux x86_64/arm64, macOS x86_64/arm64, and Windows x86_64. The Python wheels cover CPython 3.12, 3.13, and 3.14 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: v6.22.3. 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. Launch definition and support commitment: docs/LAUNCH.md.

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](https://docs.geta