| name | tmdl |
| description | Direct TMDL file authoring and BIM-to-TMDL conversion for semantic models in PBIP projects. Automatically invoke when the user asks to "edit TMDL", "add a measure in TMDL", "TMDL syntax", "fix formatString", "fix summarizeBy", "TMDL indentation", "convert BIM to TMDL", "add a column description", "create a calculated column in TMDL", or mentions .tmdl file editing or BIM-to-TMDL migration. |
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
| File | Contents | Location |
|---|
model.tmdl | Model configuration, ref table entries, query groups, annotations | definition/ |
database.tmdl | Compatibility level, model ID | definition/ |
relationships.tmdl | All relationships between tables | definition/ |
expressions.tmdl | Shared M expressions and parameters | definition/ |
functions.tmdl | DAX user-defined functions (reusable parameterized DAX) | definition/ |
roles/<RoleName>.tmdl | One file per security role (RLS filters, role members, OLS) | definition/roles/ |
perspectives/<Name>.tmdl | One file per perspective (object membership) | definition/perspectives/ |
dataSources.tmdl | Legacy data source definitions (if present) | definition/ |
tables/<Name>.tmdl | Table definition with columns, measures, hierarchies, partitions | definition/tables/ |
cultures/<locale>.tmdl | Linguistic metadata and translations | definition/cultures/ |
Object Nesting Rules
Objects must be nested inside their correct parent. The validator enforces these rules:
| Object | Allowed Parent(s) |
|---|
column, measure, hierarchy, partition, calculationGroup | table |
level | hierarchy |
calculationItem | calculationGroup |
tablePermission | role |
columnPermission | tablePermission |
perspectiveTable | perspective |
perspectiveColumn, perspectiveMeasure, perspectiveHierarchy | perspectiveTable |
linguisticMetadata, translation | cultureInfo |
dataAccessOptions | model |
formatStringDefinition | measure, calculationItem |
detailRowsDefinition | measure, table |
alternateOf | column |
member | role |
annotation, extendedProperty | any object (including queryGroup, function, member) |
ref | model, table |
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
| Column Type | Correct summarizeBy | Reason |
|---|
| Keys (surrogate/natural) | none | Keys are never aggregated |
| Attributes (names, codes, types) | none | Text attributes are never summed |
| Dates | none | Dates are never summed |
| Boolean flags | none | Flags are never summed |
| Additive numeric facts (amounts, quantities) | sum | Default aggregation is SUM |
| Non-additive numeric facts (rates, percentages) | none | Cannot be meaningfully summed |
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
| Data Type | Pattern | Example |
|---|
| Integer | #,##0 | 1,234 |
| Decimal (2 places) | #,##0.00 | 1,234.56 |
| Percentage | #,##0% or 0.00% | 85% or 85.00% |
| Currency | $#,##0.00 | $1,234.56 |
| Date | mm/dd/yyyy or dd/mm/yyyy | 01/15/2024 |
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.
| Object | Property | Values | Notes |
|---|
| Column | dataType | string, int64, double, decimal, dateTime, boolean, binary, unknown, variant, automatic | Required for data columns |
| Column | summarizeBy | default, none, sum, min, max, count, average, distinctCount | Use none for keys/attributes |
| Column | type | data, calculated, rowNumber, calculatedTableColumn | Column type variant |
| Column | isHidden | (flag, no value) | Boolean flags: write the keyword alone on its own line |
| Column | isKey | (flag, no value) | Marks the column as the table's key |
| Column | isNullable | (flag, no value) | Column allows nulls |
| Column | isUnique | (flag, no value) | Column values are unique |
| Column | isNameInferred | (flag, no value) | Name inferred from source |
| Column | isDefaultLabel | (flag, no value) | Default label for the table |
| Column | isDefaultImage | (flag, no value) | Default image for the table |
| Column | isDataTypeInferred | (flag, no value) | Data type inferred from source |
| Column |
Indentation Depth Summary
Rule: a multi-line DAX body is always 2 levels deeper than its enclosing object declaration.
| Context | Depth | Tabs |
|---|
Top-level declaration (table, relationship, expression) | 0 | 0 |
| Table properties, column/measure/hierarchy declarations | 1 | 1 |
| Column/measure properties, hierarchy levels | 2 | 2 |
| DAX body for measure/column declared at depth 1 (inside table) | 3 | 3 |
| Level properties | 3 | 3 |
DAX body for top-level function declared at depth 0 | 2 | 2 |
calculationItem inside calculationGroup (depth 1) | 2 | 2 |
DAX body for calculationItem at depth 2 | 4 | 4 |
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 behaviorreferences/naming-conventions.md - SQLBI naming conventions, display folder conventions, measure table conventions, and calculation group namingreferences/bim-to-tmdl.md - Converting between model.bim (TMSL) and definition/ (TMDL) via Tabular Editor CLI or TOM TmdlSerializerreferences/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 expressionsdefinition/tables/Z04CG1 - Time Intelligence.tmdl -- Calculation group with triple-backtick DAXdefinition/tables/__Measures.tmdl -- Measures table with calculation group referencesdefinition/tables/Invoices.tmdl -- Large fact table (51 measures, 18 columns)definition/tables/Date.tmdl -- Calculated date table with 42 columnsdefinition/roles/Account Managers.tmdl -- RLS role with DAX filter expressiondefinition/relationships.tmdl -- 27 relationships including inactivedefinition/expressions.tmdl -- Shared M/Power Query expressions and parametersdefinition/perspectives/Measure Selection.tmdl -- Perspective definition
External References
