MCP Hangar

The policy enforcement plane for MCP -- deterministic admission and egress policy, attributable audit, and SIEM export for your MCP server fleet. MIT, self-hosted, no SaaS.

Why

In MCP, the tool list is a hint the client caches; the call path is the only surface a provider mediates in real time. Every governance primitive worth having -- revocation, per-tenant scoping, audit -- attaches there, or attaches to nothing. Hangar puts a policy enforcement plane on that seam: one mediated path for lifecycle, policy, and telemetry across your whole MCP server fleet.

Background: The Advisory List -- Why MCP Governance Lives at the Call Path

Install

pip install mcp-hangar
# or: uv pip install mcp-hangar

This resolves to 2.0.0. Coming from 1.6.x, read the upgrade guide first — two of the changes need a decision before you upgrade, not after: Slack approval delivery now needs an adapter you run yourself, and approval resolution is authorized. Your upstream MCP servers do not have to move; a connection that negotiates the 2025-11-25 protocol keeps working. To stay on the old line while you plan, pin "mcp-hangar>=1.6,<2" — note that it is closed and receives no fixes.

Quickstart

Point Hangar at an MCP server in config.yaml:

mcp_servers:
  github:
    mode: subprocess
    command: [uvx, mcp-server-github]
    env:
      GITHUB_TOKEN: ${GITHUB_TOKEN}

Then serve it:

mcp-hangar serve --config config.yaml                     # stdio (Claude Desktop)
mcp-hangar serve --config config.yaml --http --port 8000  # HTTP + REST API at /api/

Hangar refuses to bind a non-loopback interface without auth. For a quick/insecure demo, pass --unsafe-no-auth; for anything real, configure the auth block.

Or skip the config entirely -- get filesystem, fetch, and memory servers wired into Claude Desktop in one line:

curl -sSL https://mcp-hangar.io/install.sh | bash && mcp-hangar init -y && mcp-hangar serve

What you get

The enforcement plane — what the call path actually decides:

• L7 egress policy -- allow/deny in MCP semantics: which upstream, which tool, which arguments. Deterministic, with no anomaly scores and no learned baselines, so every verdict is reproducible from the policy that produced it.
• Tool-schema digest pinning -- an upstream that changes a pinned tool's schema fails closed instead of quietly serving a different tool.
• Auth & RBAC -- API-key and OIDC/JWT identity with role-based access and RFC 8707 audience binding; bootstrap the first administrator with mcp-hangar auth bootstrap-admin, and every call carries a verified principal into the audit trail.
• Per-tenant tool projection -- front-door mode presents a different executable surface per caller, fail-closed on unknown identity.
• Human-in-the-loop approvals -- gate a call on an explicit decision, authorized and attributed to a real principal. Delivery channels are pluggable; core ships no vendor integration.
• Governed task relay -- Hangar interposes on the SEP-2663 task lifecycle and never becomes an executor: no scheduler, no job runner, no result store.
• Attributable audit -- an event-sourced trail exported to SIEM as CEF, LEEF 2.0, RFC 5424 syslog or JSON-lines, and to OTLP.

Everything else it takes to run a fleet:

• Parallel tool calls -- one hangar_call fans out to many MCP servers concurrently; all results returned together.
• Lifecycle management -- lazy start, health checks, single-flight cold starts, idle shutdown, and per-server circuit breaking.
• Hot config reload -- add or withdraw servers and tools via file watch, no restart.
• OAuth ingress -- advertise as an RFC 9728 protected resource and challenge external agents for verified tokens.
• Observability built in -- OpenTelemetry traces, Prometheus metrics, and structured logs.

One config gotcha: tools: is overloaded

The per-server tools: key accepts two forms that look similar and mean opposite things:

tools:                        # LIST -- pre-start visibility projection
  - name: add
    inputSchema: { type: object, properties: { a: { type: number } } }

tools:                        # DICT -- access policy
  allow: [create_issue, list_issues]
  deny: [delete_repository]

The list form only lets a tool be listed before its provider has started. It is not an access policy, and it does not survive startup: the provider's dynamic tools/list is authoritative and replaces it entirely, so a statically-listed tool the provider does not return becomes uncallable and fails with Tool not found: <name> at invocation.

The dict form is the access policy — glob patterns, three-level merge. Reach for it when you mean to restrict something. Full semantics in the configuration reference.

Documentation

• Getting Started · Configuration · Python API
• Governance & Front Door · Authentication & RBAC · Observability
• Kubernetes operator · Helm charts · All docs
• Release compatibility matrix · which core, operator, and chart versions are released and tested together

License

MIT