What is the actual difference?

Plain function calling wires tools into one app you control. MCP is a standard that lets many clients, Claude and ChatGPT and Cursor, discover and call the same server without bespoke glue per client. If you own the app and the model, you may not need MCP at all.

When is plain tool calling the right answer?

When there is exactly one client, it is yours, and it will stay that way. Wiring functions directly is simpler, has no protocol to track, and skips the 2026-07-28 migration entirely because there is no MCP server to migrate. For a single owned app, that simplicity is a feature.

When does MCP earn its keep?

When more than one client needs the same tools, or when the client is one you do not control, like a customer using Claude. That is the case where writing per-client glue does not scale and a standard does.

Could the answer be neither?

Yes. If your users would never invoke your product from an assistant, neither MCP nor tool calling is worth building yet. The honest answer to "should we do this" is sometimes "not this year", and we would rather say that than sell you a server you do not need.

Compare

MCP vs plain API tool calling

If you own the one app and the one model, plain function calling is simpler and skips the 2026-07-28 migration entirely. MCP earns its keep when more than one client needs the same tools, or when the client is one you do not control, such as a customer inside Claude. If you have a single owned app and no plan to expose it elsewhere, this is not Portkeel, and direct function calling is the right tool.

Side by side

MCP versus plain API tool calling across four axes
Axis	Plain tool calling	MCP
Clients	One, yours	Many, including ones you don't own
Glue per client	n/a	None; that is the point
Protocol to track	None	Yes, e.g. 2026-07-28
Best for	One owned app	A product, or shared internal tools

Where we lose

Decide

Will more than one client call these tools?
Is any client one you do not control?
Would your users ever invoke this from an assistant at all?

A "yes" to the first two points to MCP. A "no" to the third means build neither, yet.

MCP vs tool-calling questions

What is the actual difference?: Plain function calling wires tools into one app you control. MCP is a standard that lets many clients, Claude and ChatGPT and Cursor, discover and call the same server without bespoke glue per client. If you own the app and the model, you may not need MCP at all.
When is plain tool calling the right answer?: When there is exactly one client, it is yours, and it will stay that way. Wiring functions directly is simpler, has no protocol to track, and skips the 2026-07-28 migration entirely because there is no MCP server to migrate. For a single owned app, that simplicity is a feature.
When does MCP earn its keep?: When more than one client needs the same tools, or when the client is one you do not control, like a customer using Claude. That is the case where writing per-client glue does not scale and a standard does.
Could the answer be neither?: Yes. If your users would never invoke your product from an assistant, neither MCP nor tool calling is worth building yet. The honest answer to "should we do this" is sometimes "not this year", and we would rather say that than sell you a server you do not need.

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