Requests, Responses & Notifications
The three message shapes MCP exchanges all day.
Requests, Responses & Notifications is a free MCP Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the MCP Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Three Shapes of Talk
Under the hood, MCP speaks JSON-RPC. Every exchange is just one of three message shapes, and once you know them the wire stops feeling like magic.
What Is a Request?
A request asks the other side to do something and expects an answer. It carries an id so the reply can be matched back to it later.
{"jsonrpc": "2.0", "id": 1, "method": "tools/list"}The Reply Comes Back
A response answers exactly one request. It echoes the same id, so the client always knows which question it is replying to.
{"jsonrpc": "2.0", "id": 1, "result": {"tools": []}}Fire and Forget
A notification is a one-way heads-up. It looks like a request but has no id, so nobody sends a reply back.
{"jsonrpc": "2.0", "method": "notifications/initialized"}The id Is the Tell
The presence of an id is the whole difference: with an id you owe a reply, without one you do not. That single field separates requests from notifications.
Both Sides Can Ask
JSON-RPC is symmetric, so MCP is too. The server can send requests to the client, not just the other way around. Both parties play both roles.
Result or Error, Never Both
Every response carries either a result or an error, never both. Success fills result; failure fills error. You always know which one to read.
Notifications Power Updates
MCP uses notifications for live signals like progress, logs, and list changes. No reply is needed, so they keep the conversation flowing freely.
Matching by id
Because each request carries a unique id, many calls can be in flight at once. Replies need not arrive in order; the id keeps everything paired.
A Tiny Round Trip
Picture a round trip: the client requests tools/list, the server answers with the same id, and then the client knows which tools exist.
Why Three Is Enough
These three shapes cover everything MCP needs: ask and wait, answer, or announce. That simplicity is why JSON-RPC was chosen for the protocol.
Quick Check
You spot a JSON-RPC message with no id field. What kind is it?
Recap: Three Shapes
You learned MCP's whole vocabulary: requests ask with an id, responses answer with that id, and notifications announce with no id at all. Three shapes, endless conversations.
Frequently asked questions
Is the “Requests, Responses & Notifications” lesson free?
Yes — the full text of “Requests, Responses & Notifications” is free to read here on the web, and the MCP Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the MCP Academy course, upgrade to CoddyKit PRO.
What will I learn in “Requests, Responses & Notifications”?
The three message shapes MCP exchanges all day. You practise MCP Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start MCP Academy?
No prior experience is required. MCP Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Requests, Responses & Notifications” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this MCP Academy lesson?
Yes. Every MCP Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Requests, Responses & Notifications
- Anatomy of a JSON-RPC Call
- The Initialize Handshake
- Reading Errors in the Wire