Assay

The firewall for MCP tool calls — with a replayable audit trail.

See It Work · Quick Start · CI Guide · Discussions

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Your MCP agent calls read_file, exec, web_search — but should it?

Assay sits between your agent and its tools. It intercepts every MCP tool call, checks it against your policy, and blocks what shouldn't happen. Every decision produces an evidence trail you can audit, diff, and replay.

  Agent ──► Assay ──► MCP Server
              │
              ├─ ✅ ALLOW (policy match)
              ├─ ❌ DENY  (blocked, logged)
              └─ 📋 Evidence bundle

No hosted backend. No API keys. Deterministic — same input, same decision, every time.

The average MCP server scores 34/100 on security. Assay gives you the policy gate and audit trail to fix that. Covers 7 of 10 OWASP MCP Top 10 risks.

See It Work

cargo install assay-cli

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
✅ ALLOW  list_dir   path=/tmp/assay-demo/           reason=policy_allow
❌ DENY   read_file  path=/etc/passwd                reason=path_constraint_violation
❌ DENY   exec       cmd=ls                          reason=tool_denied

Is This For Me?

Yes, if you:

• Build with Claude Desktop, Cursor, Windsurf, or any MCP client
• Ship agents that call tools and you need to control which ones
• Want a CI gate that catches tool-call regressions before production
• Need a deterministic audit trail, not sampled observability

Not yet, if you:

• Don't use MCP (Assay is MCP-native; other protocols are on the roadmap)
• Need a hosted dashboard (Assay is CLI-first and offline)

Policy Is Simple

version: "1.0"
name: "my-policy"
allow: ["read_file", "list_dir"]
deny: ["exec", "shell", "write_file"]
constraints:
  - tool: "read_file"
    params:
      path:
        matches: "^/app/.*"

Or don't write one — generate it from what your agent actually does:

assay init --from-trace trace.jsonl

Add to CI

# .github/workflows/assay.yml
name: Assay Gate
on: [push, pull_request]
permissions:
  contents: read
  security-events: write
jobs:
  assay:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: Rul1an/assay-action@v2

PRs that violate policy get blocked. SARIF results show up in the Security tab.

Beyond MCP: Protocol Adapters

Assay already ships adapters for emerging agent protocols:

Protocol	Adapter	What it maps
ACP (OpenAI/Stripe)	assay-adapter-acp	Checkout events, payment intents, tool calls
A2A (Google)	assay-adapter-a2a	Agent capabilities, task delegation, artifacts
UCP (Google/Shopify)	assay-adapter-ucp	Discover/buy/post-purchase state transitions

Each adapter translates protocol-specific events into Assay's canonical evidence format. Same policy engine, same evidence trail — regardless of which protocol your agent speaks.

The agent protocol landscape is fragmenting (ACP, A2A, UCP, AP2, x402). Assay's bet: governance is protocol-agnostic. The evidence and policy layer stays the same even as protocols come and go.

Why Assay

Deterministic	Same input, same decision, every time. Not probabilistic.
MCP-native	Built for MCP tool calls. Adapters for ACP, A2A, UCP.
Evidence trail	Every decision is auditable, diffable, replayable.
Offline-first	No backend, no API keys. Runs on your machine.
Fast	< 5ms per tool call.
Tested	3 security experiments, 12 attack vectors, 0 false positives.

Install

cargo install assay-cli

In CI: use the GitHub Action directly.

Python SDK: pip install assay-it

Learn More

• MCP Quickstart — full walkthrough with a filesystem server
• CI Guide — GitHub Action setup
• OWASP MCP Top 10 Mapping — how Assay addresses each risk
• Evidence Store — push bundles to S3, B2, or MinIO
• Security Experiments — 12 vectors, 0 false positives

Contributing

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

See CONTRIBUTING.md. Join the discussion.

License

MIT