LLM decision guide
Tool Calling vs MCP
Tool calling is the model-facing action pattern; MCP standardizes how clients discover and communicate with capability providers.
Choose Tool calling when
- One application owns all tools
- A direct provider API is sufficient
Choose MCP when
- Many clients share capabilities
- Discovery and interoperability matter
Decision table
| Dimension | Tool calling | MCP |
|---|---|---|
| Scope | Invocation behavior | Client-server protocol |
| Discovery | Application-defined | Capability negotiation |
| Security need | Validate every call | Validate calls plus transport and server trust |
Example
A realistic choice
A single chat app may define a weather function directly; several IDEs can reuse a reviewed weather MCP server.
Hands-on
Test the decision
Draw the trust boundaries for one local tool and one remote MCP server.