Tool-belt series · Microsoft 365 Copilot · Excel

Stop re-prompting.
Write the skill once, then share it.

A skill is a folder with a SKILL.md in it that teaches Copilot in Excel how your variance analysis, DCF, or month-end refresh is supposed to be done — so the tenth run looks like the first. Short page on purpose: what a skill is made of, how to write one that actually fires, how to share it, and three you can copy today.

How to use this page

Part A is what a skill actually is and where it lives. Part B is how to write one that fires when it should and does what you meant. Part C is sharing and keeping it trustworthy. Then three copy-paste examples. Check items off as you go — progress and collapsed sections save in this browser on this device.

Assumes a paid Microsoft 365 Copilot licence and Copilot in Excel with skills enabled. Custom skills are new and moving fast — the underlying SKILL.md format is the open Agent Skills spec, so what you learn here carries to Copilot Cowork and Claude alike.

Part A · Anatomy

What a skill is, and where it lives

A prompt is a request. A skill is a procedure Copilot can look up. Get the folder, the file and the frontmatter right and everything else is writing.

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The test is repeatability. If you will do it once, prompt it. If you will do it every month and you care that it looks the same every month, that is a skill — Copilot loads it, follows your steps, and applies your structure and formatting instead of improvising a fresh answer each time.

Use	When	Lives
Prompt	One-off question, exploratory analysis, "what's driving this?"	Nowhere — the chat
Agent mode	Outcome-based ask; Copilot plans steps, writes formulas, builds the output in the workbook	The session
Skill	A process you own that must land identically every time, for you or the team	`SKILL.md` in OneDrive

Name the task you re-prompt most often each close — that's your first skill.
Run it once by hand in agent mode first, and note every correction you had to make.
Those corrections are the skill body — write them down before you forget them.

Field noteA skill built from a transcript of corrections beats one written from scratch. You are encoding the fixes, not the ideal.

Every skill is a directory whose name matches the name in its frontmatter, containing at minimum a SKILL.md. Extra material goes in conventional subfolders that Copilot only opens when it needs them — that is the whole point of the layout.

skills/
└── variance-analysis/
    ├── SKILL.md          # required: frontmatter + instructions
    ├── references/       # optional: detail loaded on demand
    │   └── account-map.md
    ├── scripts/          # optional: Office.js run to completion
    │   └── build-bridge.js
    └── assets/           # optional: templates, lookup tables
        └── variance-template.xlsx

The frontmatter is fenced by --- and only two fields are required. name is lowercase letters, numbers and hyphens, max 64 characters, no leading/trailing or doubled hyphens, and must match the folder name. For Excel, tag it so it is recognised as workbook-facing:

skills/variance-analysis/SKILL.md

---
name: variance-analysis
description: Builds a monthly actual-vs-budget variance bridge with commentary
  thresholds. Use when the user asks for variance analysis, budget vs actual,
  a bridge, or explains-the-gap reporting on a P&L extract.
metadata:
  version: "1.0.0"
  tags: excel
---

# Variance analysis
...instructions...

Folder name === name field. A mismatch is the single most common reason a skill never shows up.
Add metadata.tags: excel so it's identified as an Excel skill.
Version it in metadata from day one — you will be editing this file for months.
Anything long (account mappings, entity lists, formatting rules) goes in references/, not the body.

Custom skills live in a skills folder in your own OneDrive. Copilot creates it for you: Copilot pane → Settings → Manage skills → Custom skills → Create OneDrive folder. Drop skill folders in there, hit Refresh, and they appear under Add work content → All Skills.

Invoke either way: pick it from the All Skills list, or @skill-name inline in your prompt — e.g. @variance-analysis on the Actuals sheet for June. Microsoft ships its own too, like @brandkit and @theme-design for workbook styling, plus a pre-built finance library worth reading before you write your own.

Create the OneDrive skills folder from Manage skills — don't hand-make one somewhere else.
Hit Refresh after every add or rename; Copilot won't notice otherwise.
Test the @mention path and the auto-fire path — they behave differently (see B1).
To retire a skill, rename its folder rather than deleting — instant rollback if you were wrong.

GotchaDisabling a skill only stops Copilot choosing it automatically. Anyone who @mentions it directly still gets it. "Disabled" is not "off".

Part B · Craft

Writing a skill that actually fires — and behaves

Two failure modes: it never triggers, or it triggers and does something you didn't ask for. The description fixes the first; the body and the guardrails fix the second.

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Skills load progressively. Only name and description sit in context all the time (~100 tokens); the body is read after Copilot decides to use the skill. So the description is not a summary — it is the routing logic. Say what it does and when to use it, in the user's words, with the keywords they'd actually type. Up to 1024 characters; use a couple of hundred of them.

# weak — never fires
description: Helps with variance.

# strong — does what and when, with trigger words
description: Builds a monthly actual-vs-budget variance bridge with
  commentary above a materiality threshold. Use when the user asks for variance
  analysis, budget vs actual, plan vs actual, a bridge, or "explain the gap"
  on a P&L or cost-centre extract.

Write the description last, after the body — you'll know what it really does.
Include the synonyms your team uses, not just the official term.
Name the boundary too — "not for forecast rebuilds" — so it doesn't grab neighbouring work.
Two skills with overlapping descriptions will fight. Sharpen one or merge them.

There are no format rules for the body, but Excel skills converge on the same shape. Write numbered, unambiguous steps — the reader is following instructions, not being inspired. Then two sections that are specific to Excel: Workbook output (because the deliverable is a sheet, not a chat reply) and Common pitfalls to avoid (because the failure modes are predictable and worth naming).

## Steps
1. Locate the actuals and budget columns on the active sheet by header,
   not by position. If either is missing, ask — do not guess.
2. Compute variance = actual − budget, and variance % = variance / budget.
3. Flag any line where |variance| > 5% and |variance| > 50,000.

## Workbook output
- Create a sheet named "Variance Bridge"; never overwrite an existing one.
- Columns: Account | Actual | Budget | Var | Var % | Commentary.
- Currency as #,##0;(#,##0); negatives red; % to one decimal.
- Leave Commentary blank for the preparer — do not write commentary.

## Common pitfalls to avoid
- Do not search unrelated sheets for data.
- Do not invent missing months or accounts.
- Do not hard-code values a formula should produce.
- Do not claim the workbook was updated if it wasn't.

Keep SKILL.md under ~500 lines; push detail to references/ one level deep.
Replace the generic "output format" section with Workbook output — sheet name, columns, number formats.
Write the Common pitfalls list from real mistakes you watched it make.
Say explicitly what to do when data is missing: ask, don't invent.
Keep worked examples small — a five-row sample teaches as well as five hundred.

Field note"Ask instead of guessing" is the highest-value line in most skills. Invented data that looks plausible is far more expensive than a clarifying question.

Two reference files earn their keep in almost every Excel skill. The first is a data guardrail: how the workbook should be cleaned or normalised before the skill writes anything, using the exact sheet and column names your scenario expects — scenario-specific, not generic Excel advice. The second covers in-Excel vs outside-Excel execution: what to do if someone invokes the skill from Copilot chat where there is no workbook to touch (answer: explain the steps, never claim a sheet was created).

If the skill needs to do something the model can't reliably do by hand, add an Office.js script under scripts/ and say in the body exactly when to run it — and when not to. Each script must run to completion unattended, and must not call Office.onReady or define Office.initialize; Copilot creates the runtime and loads Office.js for you.

// scripts/build-bridge.js — no Office.onReady, no Office.initialize
await Excel.run(async (context) => {
  const sheet = context.workbook.worksheets.add("Variance Bridge");
  // ...write headers, formulas, formats...
  await context.sync();
});

Add a data-guardrail reference naming the exact sheets/columns the skill needs.
Add an in-Excel vs out-of-Excel reference so it degrades honestly in chat.
Spell out run-this-script-when / don't-run-it-when for every script.
Test on a messy real workbook, not your tidy sample — that's where the pitfalls list grows.
Use the plan step: read Copilot's proposed actions before it edits, and fix the skill when the plan is wrong.

Preview cautionSkills that ship Office.js scripts are still preview territory — don't put one on a production close process until you've watched it run a full cycle on a copy.

Part C · Distribution

Sharing skills without losing control of them

Skills are user-level artefacts living in personal OneDrive. That makes sharing easy and governance a thing you have to do deliberately.

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There is no admin console that drops a SKILL.md into everyone's folder. Each person has their own skills folder, so distribution is: publish the canonical copy somewhere the team can see it, and each teammate copies the folder into their own OneDrive and hits Refresh. Sharing the file from OneDrive or Teams, or sharing the skill to specific people in your organisation from its detail page, are both just ways of handing over that copy.

Which means the real problem is drift: five copies, three of them stale. Solve it the way you'd solve it for code — one source of truth, a version number in metadata, and a note in the body saying where the canonical copy lives.

Pick one canonical home (team SharePoint/Teams folder, or a Git repo) and say so inside the SKILL.md itself.
Bump metadata.version on every change so people can tell what they're running.
Ship it with a two-line "how to install": copy folder into OneDrive skills, then Refresh.
Strip anything confidential from examples before sharing — skill files travel further than you expect.

Field noteKeep team skills in Git and treat a change like a PR. Plain-text Markdown diffs beautifully, and review is the whole point — you're editing a procedure the finance team will execute.

A shared skill is a shared procedure: when it's wrong, it's wrong for everybody, quietly. Two Copilot features do the heavy lifting. Plan makes Copilot draft its proposed actions first so you can approve, edit, or answer questions before anything in the workbook moves. Show Changes lists what it did alongside human edits, so a Copilot-made change is never anonymous.

Require the plan step for any skill that writes to a workbook others depend on.
Review Show Changes before saving a close-cycle workbook — treat it as your diff.
Re-read shared skills once a quarter; stale thresholds and renamed accounts are the usual rot.
Never let a skill be the only record of a control — the procedure still belongs in your documentation.

Copy-paste · three starters

Three skills worth stealing

Trimmed to the bones so they're readable. Each is a folder named for its name field, containing one SKILL.md.

skills/variance-analysis/SKILL.md

---
name: variance-analysis
description: Builds a monthly actual-vs-budget variance bridge with a
  materiality flag and a blank commentary column. Use when the user asks for
  variance analysis, budget vs actual, plan vs actual, a bridge, or to explain
  the gap on a P&L or cost-centre extract. Not for forecast rebuilds.
metadata:
  version: "1.2.0"
  tags: excel
---

# Variance analysis

## Steps
1. Find Account, Actual and Budget columns by header on the active sheet.
   If any is missing, ask for the exact header — do not guess or search
   other sheets.
2. Confirm the period with the user before computing anything.
3. Variance = Actual − Budget. Variance % = Variance / Budget
   (blank, not #DIV/0!, when Budget is zero).
4. Flag rows where |Variance| > 50,000 AND |Variance %| > 5%.

## Workbook output
- New sheet "Variance Bridge <period>". Never overwrite an existing sheet.
- Account | Actual | Budget | Variance | Var % | Flag | Commentary
- #,##0;[Red](#,##0) for currency; 0.0% for percentages; freeze the header row.
- Leave Commentary empty — the preparer writes it, not Copilot.

## Common pitfalls to avoid
- Do not write commentary or explanations into the sheet.
- Do not invent accounts or periods that aren't in the source.
- Do not reorder or re-map accounts; keep source order.

skills/data-hygiene-check/SKILL.md

---
name: data-hygiene-check
description: Audits an extract for the problems that break downstream
  models — text-formatted numbers, trailing spaces, merged cells, duplicate
  keys, inconsistent dates — and reports them without changing anything. Use
  before analysing a new export, or when totals look wrong.
metadata:
  version: "1.0.0"
  tags: excel
---

# Data hygiene check

## Steps
1. Work only on the active sheet unless told otherwise.
2. Check, in order: numbers stored as text; leading/trailing whitespace;
   merged cells; blank rows inside the table; duplicate values in the
   key column; mixed date formats; inconsistent account codes.
3. Count occurrences and capture up to 5 example cell references each.

## Workbook output
- New sheet "Hygiene Report": Issue | Count | Example cells | Suggested fix.
- Sort by Count descending. Do not modify the source sheet — report only.

## Common pitfalls to avoid
- Do not fix anything. This skill reports; the user decides.
- Do not flag intentional blanks in a subtotal block as errors.
- If the sheet has no recognisable table, say so and stop.

Why this one firstIt's read-only, so it's safe to hand round on day one — and it teaches the team what a skill feels like before you let one write to a workbook.

skills/house-format/SKILL.md

---
name: house-format
description: Applies our reporting format to a finished table — number
  formats, header style, column widths, print setup. Use when the user asks to
  format, tidy, or make a sheet presentable, or mentions house style.
metadata:
  version: "1.1.0"
  tags: excel
---

# House format

## Workbook output
- Header row: bold, white on #217346, frozen, autofilter on.
- Currency #,##0;[Red](#,##0) · percentages 0.0% · dates dd-mmm-yy.
- Inputs blue font, formulas black, links green. Never change a value.
- Column widths autofit, capped at 40. Gridlines off. Fit to one page wide.

## Common pitfalls to avoid
- Do not alter values, formulas, or column order — formatting only.
- Do not apply this to a raw data extract; it is for finished output.

Field noteFormatting skills pay for themselves fastest because the rules are objective and the reviewer notices immediately when they're missed.

← back to the tool belt

Stop re-prompting.
Write the skill once, then share it.

How to use this page

What a skill is, and where it lives

Prompt, agent mode, or skill?

The folder and the file

Getting it in front of Copilot

Writing a skill that actually fires — and behaves

The description is the trigger

The body: steps, workbook output, pitfalls

Guardrails, scripts and iteration

Sharing skills without losing control of them

Getting a skill to the rest of the team

Keeping shared skills trustworthy

Three skills worth stealing

Month-end variance bridge

Data hygiene pass

House-style formatting