Function calling versus MCP: separate model tool selection from system connection
Explain function calling within model requests and MCP in external connections, using the same task to check tool discovery, arguments and execution results.
Choose an approach
Function calling lets a model propose a defined tool and arguments. MCP standardizes connections between AI applications and services offering tools or resources. MCP can supply a tool subsequently used through tool calling, while an application can also expose only internal functions. Identify whether you need model calls or a reusable integration interface; these are not simple substitutes.
Tool definitions and service connections are different stages
OpenAI separates tool definitions, model-proposed calls and application-returned results. Proposed arguments do not make business actions happen. The executor handles parameters and errors and returns actual results, unlike a model simply writing operational advice in its answer.
MCP specifies client-server interactions, of which tool discovery is one part. After connecting, check the account’s visible resources and required fields. Protocol compatibility helps integration without guaranteeing successful business requests in the external service.
Check integration and calling within the same task
For a public project-information query, first check whether the service exposes the required tool, then ask the same question to assess model selection. If discovery succeeds but execution fails, inspect the actual error. If the model never selects the tool, check its definition and the task rather than repeatedly reconnecting.
Keep call identifiers, arguments and corresponding results to detect mismatches. Test inputs missing required fields and verify clarification or failure handling rather than guessed parameters. No MCP service was built here; these are inspection designs, not evidence that a connection passed.
Choose integration complexity around reuse needs
A simple function used by one application may only need a direct tool definition. Consider an MCP service when several compatible assistants need to share it. Maintenance includes connections, versions and permission scope as well as function logic, so code-line counts alone are inadequate.
Whichever approach is chosen, tool descriptions should specify inputs and actual returned fields. After interface changes, repeat the same question and record model selection separately from business execution. This identifies whether the definition, connection or service needs adjustment rather than endlessly expanding the prompt.
Checked sources
These sources support the concepts and methods described here. Examples and test scope are labeled separately; check the version of the tools you use.