Introduction

The audit and governance layer for MCP.

What mcpose is

mcpose is a transparent proxy between an LLM client and an upstream MCP server. It mirrors the upstream MCP surface and routes supported calls through composable middleware. The client sees a normal MCP server. The upstream sees a normal MCP client. Nothing about the upstream changes.

Origin

mcpose was extracted from financial-elastic-mcp-server, an Elasticsearch MCP server built for financial institutions that needed PII redaction and audit logging on every tool call. Those cross-cutting concerns were originally hardcoded into a single server. mcpose lifts that pattern into a reusable, composable middleware layer that can wrap any upstream MCP server.

Problems it solves

• Composable middleware with a predictable onion model: each layer runs before and after the inner pipeline.
• Hide or gate tools and resources per caller, with a structured RejectionReason in every blocked call.
• Per-session identity resolution: resolve a caller once, then stamp the Identity on every request in the session.
• Tamper-evident audit trails (DORA Art. 17, SR 11-7) via @mcpose/audit: HMAC-chained events, a Merkle ReplayManifest, and AES-256-GCM for high-sensitivity tiers.
• HTTP/SSE transport with mTLS, session limits, and SSE reconnect replay.
• Production-minded: ships ESM with first-class TypeScript types and runs on Node.js 20+.

When to use mcpose

• You operate MCP servers in a regulated environment (finance, healthcare) and need PII redaction, caller identity, and a tamper-evident audit trail on every tool call.
• You want to add cross-cutting behavior (logging, redaction, rate limiting, governance) to an MCP server you do not control.
• You need to hide or gate specific tools and resources per caller, or stamp a resolved identity onto every request.

When not to use mcpose

• You are wrapping a single server with no cross-cutting concerns: a plain proxy is simpler.
• Your environment runs Node.js older than 20: mcpose requires Node.js 20+.

The ecosystem

mcpose is a monorepo. Each package publishes independently and has its own README on npm.

Next steps