Practical MCP with FastMCP & LangChain

Completions (Server Side)

Completion in MCP is a narrow but important feature. It exists to help users supply valid inputs to structured server features such as prompts and resource templates. It does neither generate answers, nor reason about user intent or call models. It assists in filling parameters.

To understand it clearly, separate two layers:

1. The language model generates conversational responses.
2. The MCP server defines structured interfaces that require specific arguments.

Completion lives entirely in the second layer.

Imagine a server that exposes a prompt called "Deploy Service". That prompt requires three arguments:

• environment
• region
• version

When a user begins typing "pro" in the environment field, the server can return suggestions like:

• production
• preview

This is not the model interpreting meaning. It is the server offering valid options from its own domain knowledge.