What is an MCP Aggregator?

MCP's architecture gives each client a 1:1 session with one server, so a host using ten servers maintains ten connections, each separately configured and authenticated. An aggregator collapses this: it presents itself as one server to the client, connects upstream to the rest, and forwards tools/list and tools/call traffic appropriately. Client configuration shrinks to a single entry, and adding or removing an upstream no longer touches every host config.

The hard problem is naming. Two upstreams can both expose a tool called search or create_issue, and the merged catalogue must keep them distinct. Aggregators typically namespace tool names with a server prefix (for example github__create_issue and linear__create_issue) or otherwise rewrite names, then strip the prefix when routing the call upstream. Without disciplined namespacing, collisions invite tool shadowing, where one server's tool masks or impersonates another's — so collision handling is a security feature, not just a convenience.

Aggregation also concentrates control. Because every tool call from the client crosses one chokepoint, the aggregator is a natural single point of policy: it can filter which upstream tools are advertised, deny calls per rule, and log everything uniformly. The same property makes it a single point of failure and a high-value target, which is why production aggregators are usually deployed as managed gateways with authentication, scoped access and audit rather than as bare multiplexers.

PolicyLayer's gateway aggregates a team's registered upstream MCP servers behind one endpoint with namespaced tools, then applies deterministic policy at that chokepoint: each person's scoped token determines which servers and tools they see, and every tools/call is evaluated before routing upstream.