1# Tool — Function Calling (client-side)
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3Define custom functions the agent can invoke. Your app executes the function and returns results. Runs expire 10 minutes after creation — return tool outputs promptly.
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5> **Security:** Treat tool arguments as untrusted input. Don't pass secrets in tool output. Use `strict=True` for schema validation.
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7> **Not available via toolbox** — function calling executes in the client process, so it's declared on the prompt agent, not in a toolbox version.
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9## Prompt-agent SDK class
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11`FunctionTool` — wraps a Python callable; the SDK introspects its signature and docstring to build the schema sent to the model.
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13## References
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15- [Function Calling tool documentation](https://learn.microsoft.com/azure/foundry/agents/how-to/tools/function-calling)
16- [agent-tools.md](agent-tools.md) — tool index
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