Tool calling: letting a model call your functions
By the end of today you can give a model a set of functions, understand exactly what it does and does not do when it picks one, and explain why the model never executes anything itself.
YesterdayOn Day 71 you made the model return data matching a schema. A tool call is that, plus a name.
TomorrowTomorrow you harden the boundary: validating what the model asked for before you run it.
Why this matters
Tool calling is the mechanism behind every agent, every MCP server, and every AI feature that does something rather than saying something. It is much simpler than it sounds, and understanding it plainly demystifies the rest of the phase.
- Tool calling
- Letting a model choose one of your functions and supply its arguments.
- Tool definitions
- Name, description and schema; the description is the entire basis for selection.
- The tool exchange
- Request, execute, append result, respond. Two messages added every time.
- Who executes what
- The model requests; your code decides and runs. It executes nothing itself.
Learn it
70 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
3 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
45 minBuild a working tool-calling example with two tools of your own. Print the complete message array after every step so you can see exactly what the model returned and what you appended. Then rewrite one tool's description to be vague and confirm the model stops selecting it correctly.
Recall it
25 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
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Rate it
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Mastery tracking
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Recap
- 01The model requests; your code executes. It never runs anything itself
- 02The description is the entire basis for the model's choice
- 03Every tool call adds messages that are re-sent on every later turn
- 04One exchange is tool calling; a loop around it is an agent
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.