@@ -0,0 +1,60 @@
+# src/rhosocial/activerecord/backend/impl/sqlserver/alter_table_modifier.py
+"""SQL Server ALTER TABLE column IF EXISTS handling.
+
+Kept as a sibling module of ``dialect.py`` (like ``collation.py``) to avoid
+the circular import triggered by the eager ``mixins/__init__.py`` chain
+(``mixins/backend_mixin`` -> ``dialect``).
+"""
+
+from rhosocial.activerecord.backend.dialect.exceptions import UnsupportedFeatureError
+
+
+class SQLServerAlterColumnModifierMixin:
+    """SQL Server ALTER TABLE column modifiers.
+
+    SQL Server supports ``DROP COLUMN IF EXISTS`` and
+    ``DROP CONSTRAINT IF EXISTS`` since 2016, but **not**
+    ``ADD COLUMN IF NOT EXISTS``. Applications opt in via
+    ``if_exists`` / ``if_not_exists``; ``None`` (default) renders the
+    plain form.
+    """
+
+    def supports_add_column_if_not_exists(self) -> bool:
+        """SQL Server does not support ``ADD COLUMN IF NOT EXISTS``."""
+        return False
+
+    def supports_drop_column_if_exists(self) -> bool:
+        """``DROP COLUMN IF EXISTS`` is supported since SQL Server 2016."""
+        return True
+
+    def supports_drop_constraint_if_exists(self) -> bool:
+        """``DROP CONSTRAINT IF EXISTS`` is supported since SQL Server 2016."""
+        return True
+
+    def format_add_column_action(self, action):
+        if getattr(action, "if_not_exists", None) is True:
+            raise UnsupportedFeatureError(
+                self.name,
+                "ADD COLUMN IF NOT EXISTS",
+                suggestion="SQL Server does not support ADD COLUMN IF NOT EXISTS. "
+                           "Pre-check sys.columns / information_schema.COLUMNS.",
+            )
+        column_sql, column_params = self.format_column_definition(action.column)
+        # SQL Server ALTER TABLE ... ADD takes the column without a COLUMN literal.
+        return f"ADD {column_sql}", column_params
+
+    def format_drop_column_action(self, action):
+        name = self.format_identifier(action.column_name)
+        if getattr(action, "if_exists", None) is True:
+            return f"DROP COLUMN IF EXISTS {name}", ()
+        return f"DROP COLUMN {name}", ()
+
+    def format_drop_table_constraint_action(self, action):
+        name = self.format_identifier(action.constraint_name)
+        if getattr(action, "if_exists", None) is True:
+            result = f"DROP CONSTRAINT IF EXISTS {name}"
+        else:
+            result = f"DROP CONSTRAINT {name}"
+        if getattr(action, "cascade", None):
+            result += " CASCADE"
+        return result, ()
@@ -95,6 +95,7 @@
 from rhosocial.activerecord.backend.dialect.protocols import PartitionSupport
 from rhosocial.activerecord.backend.dialect.exceptions import UnsupportedFeatureError
 from .collation import validate_sqlserver_collation_name
+from .alter_table_modifier import SQLServerAlterColumnModifierMixin
 # SQLServerPartitionMixin is registered lazily in _register_partition_formatters()
 
 if TYPE_CHECKING:
@@ -167,6 +168,7 @@ class SQLServerDialect(
     DateTimeMixin,
     DQLMixin,
     DMLMixin,
+    SQLServerAlterColumnModifierMixin,  # Before DDLColumnMixin to override format_*_action
     DDLColumnMixin,
     DDLTypeMixin,
     # Feature mixins
 
@@ -0,0 +1,81 @@
+# tests/rhosocial/activerecord_sqlserver_test/feature/backend/test_alter_table_if_exists.py
+"""Tests for SQL Server ALTER TABLE IF [NOT] EXISTS handling.
+
+SQL Server supports ``DROP COLUMN IF EXISTS`` and ``DROP CONSTRAINT
+IF EXISTS`` since 2016 but **not** ``ADD COLUMN IF NOT EXISTS``. The
+``ADD`` form omits the ``COLUMN`` literal (SQL Server syntax).
+"""
+
+import pytest
+
+from rhosocial.activerecord.backend.dialect.exceptions import UnsupportedFeatureError
+from rhosocial.activerecord.backend.expression.statements import ColumnDefinition
+from rhosocial.activerecord.backend.expression.statements.ddl_alter import (
+    AddColumn,
+    DropColumn,
+    DropTableConstraint,
+)
+from rhosocial.activerecord.backend.expression.types import TextType
+from rhosocial.activerecord.backend.impl.sqlserver.dialect import SQLServerDialect
+
+
+@pytest.fixture
+def dialect():
+    return SQLServerDialect(version=(15, 0, 0))
+
+
+class TestSQLServerAlterTableModifierCapabilities:
+    def test_supports_switches(self, dialect):
+        assert dialect.supports_add_column_if_not_exists() is False
+        assert dialect.supports_drop_column_if_exists() is True
+        assert dialect.supports_drop_constraint_if_exists() is True
+
+
+class TestSQLServerAddColumn:
+    def test_add_column_omits_column_literal(self, dialect):
+        action = AddColumn(dialect, ColumnDefinition("content", TextType()))
+        sql, params = action.to_sql()
+        assert sql.startswith("ADD [content]")
+        assert "ADD COLUMN" not in sql
+        assert params == ()
+
+    def test_add_column_if_not_exists_raises(self, dialect):
+        action = AddColumn(
+            dialect,
+            ColumnDefinition("content", TextType()),
+            if_not_exists=True,
+        )
+        with pytest.raises(UnsupportedFeatureError):
+            action.to_sql()
+
+
+class TestSQLServerDropColumnIfExists:
+    def test_if_exists_renders_qualifier(self, dialect):
+        action = DropColumn(dialect, column_name="x", if_exists=True)
+        sql, params = action.to_sql()
+        assert "DROP COLUMN IF EXISTS [x]" == sql
+        assert params == ()
+
+    def test_none_renders_plain_form(self, dialect):
+        action = DropColumn(dialect, column_name="x")
+        sql, params = action.to_sql()
+        assert "DROP COLUMN [x]" == sql
+        assert "IF EXISTS" not in sql
+        assert params == ()
+
+
+class TestSQLServerDropConstraintIfExists:
+    def test_if_exists_renders_qualifier(self, dialect):
+        action = DropTableConstraint(
+            dialect, constraint_name="fk", if_exists=True
+        )
+        sql, params = action.to_sql()
+        assert "DROP CONSTRAINT IF EXISTS [fk]" == sql
+        assert params == ()
+
+    def test_none_renders_plain_form(self, dialect):
+        action = DropTableConstraint(dialect, constraint_name="fk")
+        sql, params = action.to_sql()
+        assert "DROP CONSTRAINT [fk]" == sql
+        assert "IF EXISTS" not in sql
+        assert params == ()