How does the MCP Prompts primitive differ from the Tools primitive in terms of who controls invocation?

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Quick reference

TL;DR

Prompts are user-initiated templates the host surfaces in its UI, often as slash-commands; Tools are model-initiated functions the LLM decides to call on its own.

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Easy to grasp

Picture a kitchen. Tools are the appliances the chef reaches for whenever a recipe needs them; the chef decides, in the moment, to grab the blender. Prompts are the printed recipe cards pinned to the wall: a person walks up, picks the card they want, fills in the blanks like how many servings, and hands it to the chef to start. With MCP, the LLM is the chef and the user is the person at the wall. Tools get grabbed by the model when it judges they're useful. Prompts get chosen by the user from a menu the app shows them, usually as a slash-command they type. Both can take parameters, but who reaches first is completely different.

Key concepts

Prompts primitive Tools primitive Slash-command surfacing

Quick scan · 2–4 min

Everything important, quickly.

3 min: state the initiator axis, lay out the three control rails, connect prompts to slash-commands, then knock down each distractor.

Side-by-side

Aspect	Tools	Prompts
Initiator	Model-controlled, LLM decides	User-controlled, person selects
Where it surfaces	Model's tool list in context	Host UI, often a slash-command
Discovery method	tools/list	prompts/list
Invocation method	tools/call	prompts/get
Parameters	Yes, model supplies arguments	Yes, user supplies arguments
Side effects	Often side-effectful actions	Returns messages to seed the chat

Real-world examples

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

Claude Code surfaces MCP server prompts as slash-commands the user types to launch a parameterized workflow on demand.
Claude Desktop renders a connected server's prompts in a template picker, distinct from the tools the model calls itself.
The official MCP spec defines prompts/list and prompts/get with arguments, confirming prompts are parameterized user-driven templates.

Interviewer follow-ups

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

QWhy does MCP deliberately keep prompts on the user rail instead of letting the model select them?

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QWalk through the JSON-RPC flow when a user fires a slash-command backed by an MCP prompt.

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QHow do resources fit this initiator taxonomy, and who controls them?

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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.