How do URI templates work in MCP Resources, and how do subscriptions differ from a one-shot read?

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Explain how URI templates are used in MCP Resources and describe the difference between a `resources/read` call and a `resources/subscribe` subscription.

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Quick reference

TL;DR

URI templates let one MCP resource declaration cover a whole family of addresses; read is a one-shot pull, subscribe is a server push on change.

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Easy to grasp

Think of a parking garage. Instead of printing a separate sign for every single spot, the garage posts one rule: spot B-{number}. That one pattern covers hundreds of spots without listing each. An MCP URI template works the same way: the server says file://{path} and the host fills in the blank to address any file. Now imagine two ways to check if your car is still there. You could walk down every hour and look yourself, that is a read, one fetch, done. Or you could ask the attendant to text you the moment anything changes, that is a subscribe. The attendant pushes you a message instead of you walking back over and over. Same data, but one is you pulling and one is the server pushing.

Key concepts

MCP resources RFC 6570 URI templates Resource subscriptions

Quick scan · 2–4 min

Everything important, quickly.

5 min: define resources and URIs, explain RFC 6570 templates with a concrete example, contrast read pull versus subscribe push, then cover capability negotiation and when subscribe earns its state cost.

Side-by-side

Aspect	resources/read	resources/subscribe
Initiator	Host pulls	Server pushes after registration
Timing	Synchronous, one-shot	Ongoing until unsubscribe or disconnect
Returns content?	Yes, immediately	No, later via notifications/resources/updated
Server state	Stateless per call	Tracks subscribers per URI
Best for	On-demand snapshots	Live or frequently changing data

Real-world examples

Real products, models, and research that use this idea.

The official MCP filesystem server exposes a file://{path} template so a host can read any file under a root without enumerating the tree.
A Postgres MCP server advertises a postgres://{schema}/{table} template, letting the model address tables it discovers at runtime.
Claude Desktop subscribes to a watched config or log resource so an edit on disk pushes an update instead of forcing the client to poll.

Interviewer follow-ups

What an interviewer would ask next. Try answering before peeking at the approach.

QHow does a host know which resources support subscription versus only one-shot reads?

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QWhat goes wrong if a subscribed server crashes or the connection drops mid-session?

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QWhy expose data as a resource at all instead of wrapping every fetch in a tool call?

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60-second cheat sheet

The night-before-the-interview bullets. Scan these on the way to the call.

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References

Primary sources. Skim if you want the original framing.