SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/ai-tool-call-app-executes

With function calling, who actually runs a tool that the developer defined?

The developer's application. The model only asks for the call.

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  1. The developer's application ✔
  2. The model, inside its own weights
  3. The user, by hand

Why: The model returns a structured request naming the tool and its arguments. Your code runs it and sends the result back, and the model uses that result to write its answer.

Huh, didn't know that: Anthropic's docs split tools into client tools your app runs and server tools Anthropic runs, like web search.

The dad joke
Function calling is like my boss. Great at asking for things, never actually does them.
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References

Answer: The developer's application. The model returns a structured request naming the tool and its arguments. Your code runs it and sends the result back, and the model uses that result to write its answer.

  1. Claude API Docs: Tool use with Claude platform.claude.com/docs/en/agents-and-tools/tool-use/overview
    It then returns a structured call that your application executes (client tools) or that Anthropic executes (server tools).
    Checked 2026-10-10.

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