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TMDL Authoring · data-goblin | DecimalAI

databy data-goblin

TMDL Authoring

data-goblin/tmdl

Source data-goblin/power-bi-agentic-development ↗License GPL-3.0

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.

SkillScore95Maintained

Automatic

Data Analyst pack

Last verified 8/16/2026

What it changes

Passed 21 of 22 test cases with the skill, and 16 of 22 without it.

Without skill73%

With skill95%

Checks it passed that the model alone missed

Indents the lines of the multi-line DAX expression body to depth 3 (3 tabs).
Retains the PBI_FormatHint annotation in the updated column definition block.
Single-quotes the table name '1) Core Metrics' because it starts with a digit.

and 3 more in the full report

5 fixed16 passed either way1 still failing0 broke

Pass-rate lift: +23 pp
Tokens: +391%
Agent turns: 0%
Response time: -30%

Model Measured 8/16/2026v1 · latest is v221 of 22 cases passed with skill

Tokens, turns and time describe resource use, not a dollar price.

This benchmark measured an earlier version of the skill.

A newer attempt on gemini-3.8-flash on 10/20/2026 did not produce a publishable comparison (not comparable). The snapshot above is the previously measured run.

Use this skill

Save it into your project's skills folder. Claude Code picks it up in its next session.

curl -fsSL --create-dirs -o .claude/skills/data-goblin-tmdl/SKILL.md https://app.decimal.ai/s/data-goblin-tmdl/SKILL.md

Want it pinned and updatable? pip install "decimalai>=0.15", then decimalai skills add data-goblin-tmdl, and start Claude Code with claude --plugin-dir "$(git rev-parse --show-toplevel)/.agents"

1 · One-time — install the CLI (docs ↗)

pip install "decimalai>=0.15"

2 · Get the skill — writes .agents/skills/data-goblin-tmdl/ and pins it in .decimal/skills.lock

decimalai skills add data-goblin-tmdl

No CLI? Get the same file — curl · raw file

curl -fsSL --create-dirs -o .claude/skills/data-goblin-tmdl/SKILL.md https://app.decimal.ai/s/data-goblin-tmdl/SKILL.md

Raw file: /s/data-goblin-tmdl/SKILL.md ↗ · machine-readable summary at /s/data-goblin-tmdl.json ↗

Safety checks: 3 of 3 passed

Code scan CleanIntent PassedContent Clean

See it in action

One of 5 test cases the skill fixed. Same model, same prompt, with and without the skill.

1 of 5

Prompt

I am manually adding two table declarations to TMDL files in a PBIP project: one table is called 1) Core Metrics and another table is called _Dimensions. Write the TMDL header lines for both tables.

What was checked, without and with the skill
Checked	Without skill	With skill
Single-quotes the table name '1) Core Metrics' because it starts with a digit.	Missed	Passed
Leaves the table name _Dimensions unquoted because simple underscore-prefixed names do not require quotes.	Passed	Passed

case-03 · gemini-3.6-flash · 8/16/2026 · answers are capped and redacted

What's inside

The instructions your model reads when the skill loads.

SKILL.md450 lines6 files

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/`

Track record

Benchmark

21 / 22

test cases passed, vs 16 / 22 without

Adoption

Users: 0
Times used: 0

Safety and source details

SkillSafety

✓ 3 of 3 passed

Code scan15 of 15 checks clean

0 critical · 0 warning · 0 info — scanner v6 — 10/17/2026

View 15 scanner checks

live_secretremote_code_executionreverse_shelldata_exfiltrationssrf_metadataagent_snoopinganti_refusaldestructive_commandsobfuscated_unicodeunicode_homoglyphopaque_blobsuspicious_urltool_overreachtrigger_abuseprompt_injection_phrasing

IntentSafe

LLM judge · does the skill do what it claims

ContentClean

LLM judge · policy & harmful-content review

Source

Imported

data-goblin / power-bi-agentic-development

plugins/pbip/skills/tmdl/SKILL.md

View on GitHub ↗

License GPL-3.0Imported 10/17/2026Version v2Bundled 5 files

More skills from the same source and nearby in the registry, with their headline benchmark lift.

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See it in action

What's inside

TMDL Authoring

When to Use This Skill

Critical

TMDL File Types

Track record

Benchmark

Adoption

SkillSafety

Source

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