Practical MCP with FastMCP & LangChain

Calling MCP Tools vs. Langchain Functions

The results from MCP tools are indistinguishable from the agent's perspective compared to local functions. The agent doesn't know (and doesn't need to know) whether a tool is implemented as a local function or an MCP tool — it just calls it and gets back results.

So from a purely functional perspective, there's no differences, we will get the results from the tools whether they are local or remote. The differences come in development experience, deployment flexibility, and architectural decoupling:

• Development Experience: Local functions are faster to iterate on because you can edit the code and see changes immediately. MCP tools usually require editing another external project and possibly dealing with serialization issues. However, MCP tools can be developed and tested independently of the agent, which can speed up development in larger teams or when the tool logic is complex.

• Deployment Flexibility