TSQL dialect formatting corrupts boolean true in MODEL header to (1=1)

[fresioAS]

Description

When formatting a TSQL model using sqlmesh format, boolean values (true) in the MODEL (...) header are transpiled to (1=1) — the T-SQL representation of boolean true. Once corrupted, SQLMesh's parser cannot re-parse (1=1) as a boolean in the MODEL DDL context, breaking the model file.

Steps to Reproduce

1. Create a TSQL model with a boolean property in the header, e.g.:

MODEL (
  name my_schema.my_model,
  kind FULL,
  standalone true
);

SELECT 1 AS col

2. Run sqlmesh format on the project.

3. The header is rewritten as:

MODEL (
  name my_schema.my_model,
  kind FULL,
  standalone (1=1)
);

4. SQLMesh can no longer parse the model file.

Root Cause

In sqlmesh/core/dialect.py, format_model_expressions renders all expressions — including the MODEL (...) / AUDIT (...) header — using the target dialect:

# sqlmesh/core/dialect.py ~line 781
return ";\n\n".join(
    expression.sql(pretty=True, dialect=dialect, **kwargs) for expression in expressions
).strip()

The MODEL (...) header is SQLMesh-specific DDL, not standard SQL. When dialect="tsql" is used, SQLGlot's TSQL generator converts exp.Boolean(this=True) → (1=1) because T-SQL has no native TRUE keyword.

The same issue exists in the early-return path for single-expression files (~line 750):

if len(expressions) == 1 and is_meta_expression(expressions[0]):
    return expressions[0].sql(pretty=True, dialect=dialect)

Affected Properties

Any boolean property set to true in a MODEL (...) or AUDIT (...) header, including:

• standalone true
• formatting true
• Any other boolean model/audit property

Suggested Fix

Meta expressions (the MODEL/AUDIT header) should be rendered with dialect=None since they are SQLMesh-specific syntax and must not be transpiled. Only the actual SQL query expressions should use the target dialect.

return ";\n\n".join(
    expression.sql(
        pretty=True,
        dialect=None if is_meta_expression(expression) else dialect,
        **kwargs
    )
    for expression in expressions
).strip()

And similarly for the early-return path:

if len(expressions) == 1 and is_meta_expression(expressions[0]):
    return expressions[0].sql(pretty=True, dialect=None)

Environment

• Dialect: tsql
• Command: sqlmesh format
• File: sqlmesh/core/dialect.py — format_model_expressions