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Assay

Wrap any stdio MCP server in a policy gate — blocked tool calls never reach it, and every decision is written to an audit trail.

by Rul1an ↗Agent Guardrails, Content Safety & AuditCommunity

LABSCO SUMMARY

Two ways in: a gate the agent cannot bypass, or a check it can call itself.

The wrap is the enforcement — the server behind it never sees a denied call. The three assay_* tools are the cooperative half, letting an agent validate arguments or a sequence and get back a suggested fix rather than a failure. Generating a first policy from a real trace is the practical way to start, since writing an allowlist from scratch for an unfamiliar server usually means blocking something you needed.

What it is

A policy layer for MCP. assay mcp wrap puts the Assay engine in front of a server so every tool call is checked before it is forwarded, and a second set of tools lets an agent check its own call before making it.

What you get

  • assay mcp wrap --policy policy.yaml -- your-mcp-server gates a server without changing its code; blocked calls never reach it
  • A policy is YAML: tools.allow and tools.deny by name, plus JSON Schema constraints per tool — a path pattern, a minimum length, additionalProperties: false
  • Decisions come back readable: ALLOW with policy_allow, DENY with path_constraint_violation or tool_denied
  • assay_check_args validates arguments against the schema and returns violations with suggestions and a suggested fix
  • assay_check_sequence validates a proposed next tool against the history that came before it
  • assay_policy_decide does an exact-name check against a compatibility blocklist — argument-aware evaluation belongs to assay_check_args
  • assay init --from-trace trace.jsonl writes a starting policy from what your agent actually did
  • --audit-log records mandate lifecycle events and --decision-log records every ALLOW/DENY as NDJSON; assay evidence export and assay evidence verify package and check them, and assay evidence lint --pack eu-ai-act-baseline checks against a compliance pack

Requirements

The Assay CLI, from cargo install assay-cli, plus a policy file. Any stdio-based MCP server works behind it. assay mcp config-path cursor prints the detected Cursor config location and a ready-to-paste entry; Windsurf and Zed take the same wrapped command under mcpServers and context_servers respectively.

Setup effort

One command — cargo install assay-cli

See installation options