# MCP Architecture Visualised

**AI Engineering Visualised** · Agent infrastructure · Interactive explainer · Updated 2026-09-09

The Model Context Protocol, MCP, is a standard for connecting a language model application to tools and data through servers. A host application discovers what a server offers, the model asks for a tool, and the host runs the call and returns the result.

> Follow numbered MCP steps from sender to receiver across the Host Application and MCP Server boundaries. Preview the next operation before advancing the 3D diagram.

Choose a scenario and click Next step. Each numbered step names its sender, receiver and operation. The preview explains the next action before you advance. A fictional Northbank colleague wants the Manchester office travel rule. The host manages conversation and permissions; its MCP client exchanges messages with the policy server. The lookup tool belongs to that server. A separate payroll client and server remain unused. Previous revisits a step, and Replay starts the same scenario again.

The blue Host Application and purple MCP Server show logical ownership, not physical deployment. Servers and models may run remotely. Requests cross towards the server; responses return to the client. Method names and IDs connect each response to its request. Local host decisions and internal tool work are labelled separately. The current message sits with the explanation of that step.

This trace follows MCP 2026-07-28, with per-request protocol metadata and server discovery. Earlier versions use a different connection lifecycle. Permission denial ends locally, an unknown tool returns a protocol error, and a failed tool returns isError. Every example is synthetic and performs no external action.

## Sources

- [MCP 2026-07-28: Architecture](https://modelcontextprotocol.io/specification/2026-07-28/architecture)
- [MCP 2026-07-28: Request metadata](https://modelcontextprotocol.io/specification/2026-07-28/basic/index)
- [MCP 2026-07-28: Tools and error handling](https://modelcontextprotocol.io/specification/2026-07-28/server/tools)
- [MCP 2026-07-28: Server discovery](https://modelcontextprotocol.io/specification/2026-07-28/server/discover)

## Related terms

- [MCP (Model Context Protocol)](https://understandingdata.com/ai-coding-dictionary/mcp/)
- [Tool call](https://understandingdata.com/ai-coding-dictionary/tool-call/)
- [Tool use](https://understandingdata.com/context-engineering-dictionary/tool-use/)

## More visualisations

- [Prompt Caching Visualised](https://understandingdata.com/ai-engineering-visualised/prompt-caching/)
- [Hybrid Retrieval Visualised](https://understandingdata.com/ai-engineering-visualised/hybrid-retrieval/)
- [GraphRAG Visualised](https://understandingdata.com/ai-engineering-visualised/graphrag/)

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Source: https://understandingdata.com/ai-engineering-visualised/mcp-architecture/
From AI Engineering Visualised by James Phoenix, Understanding Data. Published 2026-09-09.