Imported from InsightfulAnalytics/PBI_Agentic_Dev (plugins/pbip/skills/tmdl/SKILL.md). Install upstream with npx skills add InsightfulAnalytics/PBI_Agentic_Dev --skill tmdl. Copyright stays with the author.

TMDL Authoring

Expert guidance for authoring and editing TMDL (Tabular Model Definition Language) files directly in PBIP projects.

This skill is a last resort. Direct TMDL file editing lacks the validation, atomicity, and DAX query capabilities of the Tabular Editor CLI, Power BI MCP server, or the connect-pbid skill (TOM via PowerShell). Use those tools when available. TMDL editing is appropriate when:

• Working with PBIP files in a Git repo without Power BI Desktop open
• No Tabular Editor CLI or MCP server is installed
• Making quick text-level fixes (descriptions, format strings, display folders) where a full tool chain is overkill

Direct TMDL editing does not validate DAX syntax, check referential integrity, or verify that property values are valid. Errors will only surface when the model is next loaded in Power BI Desktop or deployed via XMLA. Use the pbip-validator agent to check TMDL files for syntax issues, indentation errors, and referential integrity before opening in PBI Desktop.

When to Use This Skill

Activate only when the Tabular Editor CLI, Power BI MCP server, or connect-pbid skill are not available, and tasks involve:

• Editing .tmdl files directly (measures, columns, tables, relationships)
• Adding or modifying measure definitions in TMDL
• Adding descriptions to columns, measures, or tables
• Fixing summarizeBy or formatString values
• Understanding TMDL syntax rules (indentation, quoting, property ordering)
• Writing multi-line DAX in TMDL format
• Understanding the difference between /// descriptions and // comments

Critical

• /// (triple-slash) sets the Description property on the object that immediately follows it. A /// line must be immediately followed by a declaration (measure, column, table, etc.); never by a blank line or another ///. Use // for regular comments.
• Indentation is semantic. TMDL uses whitespace indentation where depth equals nesting level (TMDL spec — Indentation). PBIP files use a single tab per level because Power BI Desktop and the TOM TmdlSerializer default to IndentationMode.Tabs. Spaces are also valid (IndentationMode.Spaces, default 4 per level), but be consistent within a file; mixed or incorrect indentation will break the model. Properties of a table are indented one level; properties of a column (which belongs to a table) are indented two levels.
• Name quoting rules: Only quote names that contain spaces, special characters, or start with a digit. Simple names and underscore-prefixed names are unquoted. See the Name Quoting section for details.
• M expressions and tables share a namespace. A name declared by expression <name> in expressions.tmdl and a name declared by table <name> in tables/*.tmdl collide; Power BI Desktop fails the load with 'duplicate member <name>'. Pick distinct names; the conventional fix is to suffix the M expression with Query or Source and have partitions reference it via source = #"<Name> Query". validate_pbip.py enforces this as an ERROR.

TMDL File Types

Object Nesting Rules

Objects must be nested inside their correct parent. The validator enforces these rules:

Root-level objects (indent 0 only): model, database, table, relationship, role, cultureInfo, perspective, dataSource, expression, queryGroup, function.

Syntax Rules

Indentation

TMDL uses whitespace indentation where depth equals nesting level. PBIP files use a single tab per level (the TOM TmdlSerializer default), so all examples below use tabs:

table Product                              // depth 0: top-level declaration
	lineageTag: abc-123                    // depth 1: table property

	measure '# Products' =                // depth 1: measure declaration
			COUNTROWS (                    // depth 3: DAX expression body (one deeper than properties)
			    VALUES ( Product[Name] )   // depth 3: continued
			)                              // depth 3: continued
		formatString: #,##0               // depth 2: measure property
		displayFolder: Measures            // depth 2: measure property
		lineageTag: def-456               // depth 2: measure property

	column 'Product Name'                  // depth 1: column declaration
		dataType: string                   // depth 2: column property
		lineageTag: ghi-789               // depth 2: column property
		summarizeBy: none                  // depth 2: column property
		sourceColumn: Product Name         // depth 2: column property

		annotation SummarizationSetBy = Automatic  // depth 2: column annotation

Key rules:

• Use tabs in PBIP files (Power BI Desktop's default); see the Critical section above for the spaces alternative
• Table-level objects (columns, measures, hierarchies, partitions) are at depth 1
• Properties of those objects are at depth 2
• Multi-line DAX expression bodies are always 2 levels deeper than the enclosing declaration (depth 3 for measures/columns inside a table; depth 2 for top-level functions; depth 4 for calculationItems)
• Annotations are at the same depth as properties of their parent object, separated by a blank line

Descriptions (///)

Triple-slash sets the Description property on the next declaration. This is native TMDL syntax (not a Tabular Editor extension); the TMDL spec treats /// as first-class description support.

/// Count of distinct products in the current filter context.
measure '# Products' =
		COUNTROWS ( VALUES ( Product[Product Name] ) )
	formatString: #,##0
	lineageTag: abc-123

Rules:

• /// must be immediately followed by a declaration on the next line
• No blank line between /// and the declaration
• Multiple /// lines concatenate into a single description
• /// applies to the next measure, column, table, hierarchy, or level

Common mistake:

// WRONG: blank line between /// and declaration
/// This is a description.

measure 'My Measure' = 1

// WRONG: /// used as a separator comment
///
measure 'My Measure' = 1

// RIGHT: /// immediately before declaration
/// This is a description.
measure 'My Measure' = 1

// RIGHT: // used for regular comments
// This is just a comment, not a description.
measure 'My Measure' = 1

Comments (//)

Double-slash is a regular comment with no semantic effect:

// This is a comment — it does not set any property
measure 'My Measure' = 1

Property Ordering

Properties should follow a consistent order, though TMDL is not strict about it. The conventional order is:

For columns: dataType, isHidden, isKey, displayFolder, lineageTag, summarizeBy, isNameInferred, sourceColumn, sortByColumn, then annotations.

For measures: DAX expression (on the = line or multi-line), formatString or formatStringDefinition, displayFolder, lineageTag, then annotations.

Name Quoting

When to Quote

Use single quotes around names that contain any of these characters:

• Spaces: 'Product Name'
• Dots: 'Sales.Amount'
• Equals: 'Price = Target'
• Colons: 'Date:Key'
• Single quotes (escape by doubling): 'Customer''s Name'
• Other special characters: 'Sales ($)', 'OTD % (Value)', '1) Selected Metric'
• Names starting with a digit: '4) Selected Period'

When NOT to Quote

Do not quote names that are simple identifiers:

• Product (simple word)
• _Measures (underscore prefix, no spaces)
• Date (simple word)
• CgMetricQuantity (PascalCase, no spaces)

Examples

table Product                    // unquoted: simple name
table _Measures                  // unquoted: underscore prefix
table 'Budget Rate'              // quoted: contains space
table 'Invoice Document Type'    // quoted: contains spaces
table '1) Selected Metric'       // quoted: starts with digit
table 'On-Time Delivery'         // quoted: contains space

Column Definitions

For complete column examples (basic, hidden, key, sortByColumn, description), see references/tmdl-file-examples.md. For full property reference, see references/column-properties.md.

Key column pattern:

column 'Product Name'
	dataType: string
	displayFolder: 1. Product Hierarchy
	lineageTag: abc-123
	summarizeBy: none
	sourceColumn: Product Name

	annotation SummarizationSetBy = Automatic

Measure Definitions

Single-Line DAX

measure '# Products' = COUNTROWS ( VALUES ( Product[Product Name] ) )
	formatString: #,##0
	displayFolder: Measures
	lineageTag: abc-123

Multi-Line DAX

Two syntaxes for multi-line DAX:

1. Indented block (most common) -- expression body indented two levels deeper than the declaration:

2. Triple-backtick block -- DAX enclosed in ``` fences, useful for expressions with complex indentation:

measure Percentage = ```
		VAR _Total = CALCULATE( SUM ( 'Table'[Quantitative] ), REMOVEFILTERS ( ) )
		RETURN
		DIVIDE ( SUM ( 'Table'[Quantitative] ), _Total )
		```
	formatString: 0.0%;-0.0%;0.0%
	lineageTag: abc-123

Indented block syntax (standard approach) -- indented two extra tabs from the measure's parent (table) level:

measure 'Actuals MTD' =
		CALCULATE (
		    [Actuals],
		    CALCULATETABLE (
		        DATESMTD ( 'Date'[Date] ),
		        'Date'[IsDateInScope]
		    )
		)
	formatString: #,##0
	displayFolder: 2. MTD\Actuals
	lineageTag: abc-123

Measure with Description

/// Number of workdays elapsed month-to-date, considering only dates in scope.
measure '# Workdays MTD' =
		CALCULATE(
		    MAX( 'Date'[Workdays MTD] ),
		    'Date'[IsDateInScope] = TRUE
		)
	formatString: #,##0
	displayFolder: 5. Weekday / Workday\Measures\# Workdays
	lineageTag: abc-123

Measure with formatStringDefinition (Dynamic Format)

measure 'Sales Target MTD vs. Actuals (%)' =
		Comparison.RelativeToTarget (
		    [Actuals MTD],
		    [Sales Target MTD]
		)
	displayFolder: 2. MTD\Sales Target
	lineageTag: abc-123

	formatStringDefinition =
			FormatString.Comparison.RelativeToTarget (
			    "SUFFIX",
			    1,
			    "ARROWS",
			    "",
			    ""
			)

Note: formatStringDefinition replaces formatString when the format is computed dynamically via a DAX expression (often a calculation group format function).

Other Object Types

For complete examples of calculated columns, roles (RLS/OLS), calculation groups, date table marking, hierarchies, partitions, relationships, shared expressions, and model configuration, see references/tmdl-file-examples.md.

Common Data Quality Patterns

summarizeBy Rules

Common fix pattern — changing summarizeBy: sum to summarizeBy: none for key columns:

// Before (wrong - key column should not sum)
column 'Customer Key'
	dataType: int64
	isHidden
	lineageTag: abc-123
	summarizeBy: sum
	sourceColumn: Customer Key

// After (correct)
column 'Customer Key'
	dataType: int64
	isHidden
	lineageTag: abc-123
	summarizeBy: none
	sourceColumn: Customer Key

formatString Patterns

PBI_FormatHint Annotation

Power BI Desktop may add a PBI_FormatHint annotation alongside formatString:

column Amount
	dataType: decimal
	formatString: #,##0.00
	lineageTag: abc-123
	summarizeBy: sum
	sourceColumn: Amount

	annotation SummarizationSetBy = Automatic

	annotation PBI_FormatHint = {"isGeneralNumber":true}

Do not fight this annotation. Power BI tooling re-adds it automatically. When setting a formatString, leave any existing PBI_FormatHint in place. If Power BI re-adds a removed PBI_FormatHint, accept it.

Quick Reference

Property Cheat Sheet

For the complete property reference for every object type, see references/object-properties.md.

Indentation Depth Summary

Rule: a multi-line DAX body is always 2 levels deeper than its enclosing object declaration.

Additional Resources

Reference Files

• references/object-properties.md - Complete property reference for all 30+ TMDL object types with valid enum values for every property type (dataType, summarizeBy, modeType, crossFilteringBehavior, etc.)
• references/column-properties.md - Column-specific property guide with summarizeBy rules, formatString patterns, PBI_FormatHint behavior
• references/naming-conventions.md - SQLBI naming conventions, display folder conventions, measure table conventions, and calculation group naming
• references/bim-to-tmdl.md - Converting between model.bim (TMSL) and definition/ (TMDL) via Tabular Editor CLI or TOM TmdlSerializer
• references/tmdl-file-examples.md - Complete examples for every TMDL file type (model, database, expressions, relationships, roles, perspectives, tables, cultures) including backtick-enclosed expressions, field parameters, calculation groups, and date tables

Fetching Docs

To retrieve current TMDL reference docs, use microsoft_docs_search + microsoft_docs_fetch (MCP) if available, otherwise mslearn search + mslearn fetch (CLI). Search based on the user's request and run multiple searches as needed to ensure sufficient context before proceeding.

Example Model

• examples/SpaceParts.SemanticModel/ -- Complete real-world TMDL model (SpaceParts) with 40 tables, 152 measures, 8 calculation groups, 8 RLS roles, 2 perspectives, DAX UDFs (functions.tmdl), shared M expressions, relationships, and cultures. Covers every TMDL file type. Key files to study:
  • definition/functions.tmdl -- DAX user-defined functions with parameters, types, and multi-line expressions
  • definition/tables/Z04CG1 - Time Intelligence.tmdl -- Calculation group with triple-backtick DAX
  • definition/tables/__Measures.tmdl -- Measures table with calculation group references
  • definition/tables/Invoices.tmdl -- Large fact table (51 measures, 18 columns)
  • definition/tables/Date.tmdl -- Calculated date table with 42 columns
  • definition/roles/Account Managers.tmdl -- RLS role with DAX filter expression
  • definition/relationships.tmdl -- 27 relationships including inactive
  • definition/expressions.tmdl -- Shared M/Power Query expressions and parameters
  • definition/perspectives/Measure Selection.tmdl -- Perspective definition

External References

• TMDL overview (Microsoft Learn)
• TMDL syntax reference (Microsoft Learn)
• SQLBI naming conventions