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Give the model a tool of your own

For: Python developers who want the model to call their code. After this page you can turn a function into a tool, run it with a local model, and know what the model sees.

Tether exposes exactly one tool schema to the model, act(cmd, stdin). Your function becomes a command behind it, so adding ten tools does not add ten schemas to the prompt.

The function

import asyncio

from tether import Harness, tool

PRICES = {"apple": 0.5, "pear": 0.75, "melon": 3.2}


@tool(name="price", description="Look up the unit price of a fruit")
def price(fruit: str) -> str:
    """Return the price of one `fruit` in euros. Known fruit: apple, pear, melon."""
    if fruit not in PRICES:
        return f"Unknown fruit `{fruit}`. Known: {', '.join(PRICES)}."
    return f"{fruit}: {PRICES[fruit]:.2f} EUR"


async def main() -> None:
    harness = Harness(model="ollama/qwen3.5:9b", tools=[price])
    session = await harness.run("What do 3 pears and 1 melon cost together? Use the price tool.")
    print(session.status, "-", session.result)


asyncio.run(main())

Type hints and the docstring become the command's usage text. The model only sees the one-line description until it runs price --help or calls it wrongly, so a tool costs about a dozen prompt tokens until it is used.

Run it

python shop.py

COMPLETED - 5.45 EUR

Ten seconds, three steps: the model asked for the pear price, then the melon price, then did the sum (3 × 0.75 + 3.20 = 5.45) and finished. Note what the tool does on bad input: it returns the list of valid fruit and does not raise. A message that says what to do next is how a small model recovers on its own.

Make the error message do the work

Write tool errors for a reader that cannot ask a follow-up question: say what was wrong, list the valid options and show the next call. A tool that raises a stack trace teaches the model nothing.

Other ways to add things

  • In a skill. Put the function in skills/<name>/tools.py and list its name under tools: in SKILL.md; tools are opt-in, so an unlisted tool stays unavailable.
  • A model backend, not a tool. Declare it in ~/.tether/providers.yaml or register a class: Model providers.

When it goes wrong

  • unknown command for your tool. It was not passed in tools=[...] (or not listed in the skill's tools:).
  • The model passes the wrong argument. Tether answers with the tool's usage line; if it keeps happening, tighten the docstring and the error message.

More: Python SDK and custom tools, Extending tether.