Function calling is the LLM's tool-call vocabulary; MCP is the host to server protocol that delivers tools, resources, and prompts to the LLM in the first place.
Imagine a TV remote and the HDMI cable behind your TV. The remote (function calling) is how you tell the TV which input to switch to. The HDMI cable (MCP) is the standard wire that lets any device, a Blu-ray, a console, a streaming stick, plug into the TV without redesigning the back panel. Function calling is the model saying 'play movie' in a structured shape the host knows how to read. MCP is the standardized port behind the host that lets it talk to any tool server, file server, or prompt server without writing a custom integration per vendor. They live at different layers and play well together.
Detailed answer & concept explanation~4 min readEverything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example. Click to expand.
Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example. Click to expand.
Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example.
Everything important, quickly.
4 min: two-layer stack + three MCP primitives + JSON-RPC transport + discovery flow + when to skip MCP and hand-roll.
| Concern | Function calling | MCP |
|---|---|---|
| Layer | Model output contract | Host-to-server protocol |
| Defined by | Each model vendor (OpenAI, Anthropic, etc.) | Open spec, vendor-neutral |
| Transport | Inside the prompt and completion | JSON-RPC over stdio or HTTP+SSE |
| Primitives | Tool schemas, tool calls | Tools, resources, prompts |
| Reusable across apps | No, model-specific shape | Yes, one server runs in any host |
Real products, models, and research that use this idea.
- Claude Code ships a built-in MCP client and discovers tools from any registered server at session start.
- Anthropic's MCP server registry exposes filesystem, GitHub, Postgres, and Slack servers as drop-in plug-ins.
- Cursor and Zed both adopted MCP in 2025 so any MCP server written for Claude Code works in them too.
What an interviewer would ask next. Try answering before peeking at the approach.
QHow would you handle authentication when an MCP server talks to a private API?
QWhen would you NOT use MCP and prefer direct function calling?
Don't say thisRed flags and common mistakes that signal junior thinking. Click to expand.
Red flags and common mistakes that signal junior thinking. Click to expand.
Treating MCP and function calling as competing standards. They sit at different layers, the host can speak both at once.
The night-before-the-interview bullets. Scan these on the way to the call.
Primary sources. Skim if you want the original framing.
Same topic, related formats. Practice these next.