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MCP Host / Client / Server — The Three Layers

Domain 2 · 20% ~12 min English narration

Scenario anchor

Aap Aaranya IT BFSI ke liye ek AI-powered trade-reconciliation system design kar rahe hain — ASX-India ke real-time feeds ko Claude ke saath integrate karna hai. Aapke team ne pucha: "Host kaun hai, Client kaun hai, Server kaun hai — aur inke beech responsibility boundary kahan draw karein?" Yahi confusion production mein tool-call failures aur context leakage ka root cause banti hai. Is lesson ke baad, aap MCP ke three-layer separation ko waise hi explain kar paayenge jaise aap apne API-gateway-microservices split explain karte hain.

Key Takeaways

MCP Host is the trust anchor and context-window owner — it enforces user-consent policies and decides which tool results are injected into the LLM prompt; never bypass it for latency optimisation.
MCP Client is a scoped, per-server JSON-RPC session manager — one Client instance per MCP Server, giving you the same isolation guarantees as a service-mesh sidecar per upstream dependency.
MCP Server exposes a versioned tool manifest with JSON Schema–validated inputs and outputs; treat this contract with the same rigour as an OpenAPI spec in a governed API gateway.
Memory anchor: Host = API Gateway (policy + routing), Client = service-mesh sidecar (scoped upstream session), Server = microservice with OpenAPI contract — same separation of concerns, new protocol.

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