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MCP vs function calling

Both let a model call tools. One lives inside the model's runtime; the other is a protocol that reaches across systems.

Short version

Function calling is a model capability. You declare a set of functions in the request, the model may return a structured call, and your own code executes it. Everything happens inside one process that you control.

MCP is a protocol between a client and a server. The tools do not have to live in your process, they can be written by someone else, and the client discovers them at runtime instead of having them declared up front.

The distinction that matters

Function calling answers "how does the model express a call?" MCP answers "where do the callable capabilities come from, and how does anything find them?"

Side by side

AspectFunction callingMCP
What it isA model API featureA client–server protocol
Where tools liveIn your process, declared in the requestBehind a server, possibly written by someone else
DiscoveryYou pass the full list every callThe client discovers capabilities from the server
Scaling the listMore tools means a bigger request every timeCapabilities stay on the server side
ExecutionYour code runs the functionThe server runs the capability
Reuse across appsCopy the declarations and handlersPoint another client at the same server
CredentialsIn the calling applicationAt the server boundary, once

Where they overlap

Despite the table, they are not competitors. A client that supports MCP typically turns discovered capabilities into the same shape the model already understands. From the model's point of view, the call often looks identical whether it came from a declared function or a discovered MCP tool.

The difference sits outside the model: in who owns the implementation, how it is discovered, and where credentials and permissions live.

When to use which

Function calling is enoughFew tools, all implemented in the same service, no reuse across products, and the tool list is stable.
Reach for MCPTools are shared across clients, live in another system, or should be installable by users who never touch your code.
Both, deliberatelyCall local helpers directly and expose shared or third-party capabilities through MCP, so you do not pay protocol overhead for private code.
Watch the costDeclaring every tool inline grows the request on every turn. Past a certain size, discovery starts to win on token budget alone.

How they combine

In practice you rarely pick one. A typical client declares a couple of local functions and also connects to one or more MCP servers. The model sees a single list of callable things, and your client routes each call to the right place. 

local functions   β†’ executed in-process
mcp servers       β†’ capability discovered, call forwarded over the protocol

model sees:       one merged tool list
client decides:   route to the local handler or to the server

The design question is therefore not "function calling or MCP" but "which capabilities deserve to be local, and which deserve to be shared?"