excel-mcp - Claude MCP Skill

# Excel MCP Server Skill

Provides 227 Excel operations via Model Context Protocol. The MCP Server forwards all requests to the shared ExcelMCP Service, enabling session sharing with CLI. Tools are auto-discovered - this documents quirks, workflows, and gotchas.

## Workflow Checklist

| Step | Tool | Action | When |
|------|------|--------|------|
| 1. Open file | `file` | `open` or `create` | Always first |
| 2. Create sheets | `worksheet` | `create`, `rename` | If needed |
| 3. Write data | `range` | `set-values` | Always (2D arrays) |
| 4. Format | `range` | `set-number-format` | After writing |
| 5. Structure | `table` | `create` | Convert data to tables |
| 6. Save & close | `file` | `close` with `save: true` | Always last |

## Preconditions

- Windows host with Microsoft Excel installed (2016+)
- Use full Windows paths: `C:\Users\Name\Documents\Report.xlsx`
- Excel files must not be open in another Excel instance

## Calculation Mode Workflow (Batch Performance)

Use `calculation_mode` for **bulk write performance optimization**. When writing many values or formulas, disable auto-recalc to avoid recalculating after every cell:

```
1. calculation_mode(action: 'set-mode', mode: 'manual')  → Disable auto-recalc
2. Perform all writes (range set-values, set-formulas)
3. calculation_mode(action: 'calculate', scope: 'workbook')  → Recalculate once
4. calculation_mode(action: 'set-mode', mode: 'automatic')  → Restore default
```

**Note:** You do NOT need manual mode to read formulas - `range get-formulas` returns formula text regardless of calculation mode.

## CRITICAL: Execution Rules (MUST FOLLOW)

### Rule 1: NEVER Ask Clarifying Questions

**STOP.** If you're about to ask "Which file?", "What table?", "Where should I put this?" - DON'T.

| Bad (Asking) | Good (Discovering) |
|--------------|-------------------|
| "Which Excel file should I use?" | `file(list)` → use the open session |
| "What's the table name?" | `table(list)` → discover tables |
| "Which sheet has the data?" | `worksheet(list)` → check all sheets |
| "Should I create a PivotTable?" | YES - create it on a new sheet |

**You have tools to answer your own questions. USE THEM.**

### Rule 2: Always End With a Text Summary

**NEVER end your turn with only a tool call.** After completing all operations, always provide a brief text message confirming what was done. Silent tool-call-only responses are incomplete.

### Rule 3: Format Data Professionally

Always apply number formats after setting values:

| Data Type | Format Code | Result |
|-----------|-------------|--------|
| USD | `$#,##0.00` | $1,234.56 |
| EUR | `€#,##0.00` | €1,234.56 |
| Percent | `0.00%` | 15.00% |
| Date (ISO) | `yyyy-mm-dd` | 2025-01-22 |

**Workflow:**
```
1. range set-values (data is now in cells)
2. range set-number-format (apply format)
```

### Rule 4: Use Excel Tables (Not Plain Ranges)

Always convert tabular data to Excel Tables:

```
1. range set-values (write data including headers)
2. table create tableName="SalesData" rangeAddress="A1:D100"
```

**Why:** Structured references, auto-expand, required for Data Model/DAX.

### Rule 5: Session Lifecycle

```
1. file(action: 'open', path: '...')  → sessionId
2. All operations use sessionId
3. file(action: 'close', save: true)  → saves and closes
```

**Unclosed sessions leave Excel processes running, locking files.**

### Rule 6: Data Model Prerequisites

DAX operations require tables in the Data Model:

```
Step 1: Create table → Table exists
Step 2: table(action: 'add-to-datamodel') → Table in Data Model
Step 3: datamodel(action: 'create-measure') → NOW this works
```

### Rule 7: Power Query Development Lifecycle

**BEST PRACTICE: Test-First Workflow**

```
1. powerquery(action: 'evaluate', mCode: '...') → Test WITHOUT persisting
2. powerquery(action: 'create', ...) → Store validated query
3. powerquery(action: 'refresh', ...) → Load data
```

**Why evaluate first:**
- Catches syntax errors and missing sources BEFORE creating permanent queries
- Better error messages than COM exceptions from create/update
- See actual data preview (columns + sample rows)
- No cleanup needed - like a REPL for M code
- Skip only for trivial literal tables

**Common mistake:** Creating/updating without evaluate → pollutes workbook with broken queries

### Rule 8: Targeted Updates Over Delete-Rebuild

- **Prefer**: `set-values` on specific range (e.g., `A5:C5` for row 5)
- **Avoid**: Deleting and recreating entire structures

**Why:** Preserves formatting, formulas, and references.

### Rule 9: Follow suggestedNextActions

Error responses include actionable hints:
```json
{
  "success": false,
  "errorMessage": "Table 'Sales' not found in Data Model",
  "suggestedNextActions": ["table(action: 'add-to-data-model', tableName: 'Sales')"]
}
```

## Tool Selection Quick Reference

| Task | Tool | Key Action |
|------|------|------------|
| Create/open/save workbooks | `file` | open, create, close |
| Write/read cell data | `range` | set-values, get-values |
| Format cells | `range` | set-number-format |
| Create tables from data | `table` | create |
| Add table to Power Pivot | `table` | add-to-data-model |
| Create DAX formulas | `datamodel` | create-measure |
| Create PivotTables | `pivottable` | create, create-from-datamodel |
| Filter with slicers | `slicer` | set-slicer-selection |
| Create charts | `chart` | create-from-range |
| Control calculation mode | `calculation_mode` | get-mode, set-mode, calculate |
| Visual verification | `screenshot` | capture, capture-sheet |

## Reference Documentation

See `references/` for detailed guidance:

- [Core execution rules and LLM guidelines](./references/behavioral-rules.md)
- [Common mistakes to avoid](./references/anti-patterns.md)
- [Bulk write performance optimization](./references/calculation.md)
- [Data Model constraints and patterns](./references/workflows.md)
- [Charts and formatting](./references/chart.md)
- [Conditional formatting operations](./references/conditionalformat.md)
- [Dashboard and report best practices](./references/dashboard.md)
- [Data Model/DAX specifics](./references/datamodel.md)
- [DMV query reference for Data Model analysis](./references/dmv-reference.md)
- [Excel agent mode and advanced automation](./references/excel_agent_mode.md)
- [Gotchas and known limits](./references/gotchas.md)
- [Power Query M code syntax reference](./references/m-code-syntax.md)
- [PivotTable operations](./references/pivottable.md)
- [Power Query specifics](./references/powerquery.md)
- [Range operations and number formats](./references/range.md)
- [Screenshot and visual verification](./references/screenshot.md)
- [Slicer operations](./references/slicer.md)
- [Table operations](./references/table.md)
- [Window and visibility operations](./references/window.md)
- [Worksheet operations](./references/worksheet.md)