Excel MCP Server Skill

Provides 326 Excel operations via Model Context Protocol. The MCP Server hosts the ExcelMCP Service in-process and calls it directly for low-latency Excel automation. 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

Write format codes in US notation (, grouping, . decimal) regardless of the machine's locale — Excel translates them. The rendered separators follow the user's Windows regional settings, so $#,##0.00 shows $1.234,56 on a German system. Don't "fix" that by swapping the separators in the format code; it would break on every other locale.

Workflow:

1. range set-values (data is now in cells)
2. range set-number-format (apply format)
3. range_format auto-fit-columns (formatted values are wider than raw ones)

Step 3 is not optional. A column sized for 45678 is too narrow once that value renders as 2025-01-22 or $1,234.56, and Excel displays ##### instead of the number.

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(action: 'create', table_name: 'SalesData', range_address: 'A1:D100')

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

Rule 5: Session Lifecycle

1. file(action: 'open', path: '...')  → capture response.session_id as sessionId
2. workbook(action: 'get-info', session_id: sessionId)
3. file(action: 'close', session_id: sessionId, save: true)  → saves and closes

Pass that same value as session_id on every session-based follow-up call. sessionId above is a local variable, not an MCP argument name. When reusing a session from file(list), copy the matching entry's sessionId value into session_id. Never guess or substitute a session.

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-data-model') → Table in Data Model
Step 3: datamodel(action: 'create-measure') → NOW this works

Rule 7: Power Query Development Lifecycle

BEST PRACTICE: Test-First Workflow

**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', table_name: 'Sales')"]
}