diff --git a/datasette/views/table_create_alter.py b/datasette/views/table_create_alter.py
index f7775479..88bdafc4 100644
--- a/datasette/views/table_create_alter.py
+++ b/datasette/views/table_create_alter.py
@@ -22,7 +22,6 @@ from datasette.resources import DatabaseResource, TableResource
 from datasette.utils import (
     escape_sqlite,
     get_outbound_foreign_keys,
-    sqlite3,
     table_column_details,
 )
 from datasette.utils.asgi import NotFound, Response
@@ -602,7 +601,6 @@ AlterTableOperation = Annotated[
 
 class AlterTableRequest(_StrictPydanticModel):
     operations: list[AlterTableOperation] = Field(min_length=1)
-    dry_run: bool = False
 
 
 def _pydantic_errors(validation_error):
@@ -1143,14 +1141,6 @@ class TableAlterView(BaseView):
 
                 return _table_schema_from_conn(operation_conn, table_name)
 
-            if alter_request.dry_run:
-                memory_conn = sqlite3.connect(":memory:")
-                try:
-                    conn.backup(memory_conn)
-                    return before_schema, apply_operations(memory_conn)
-                finally:
-                    memory_conn.close()
-
             after_schema = apply_operations(conn)
             return before_schema, after_schema
 
@@ -1162,7 +1152,7 @@ class TableAlterView(BaseView):
             return _error([str(e)], 400)
 
         altered = before_schema != after_schema
-        if altered and not alter_request.dry_run:
+        if altered:
             await self.ds.track_event(
                 AlterTableEvent(
                     request.actor,
@@ -1189,10 +1179,7 @@ class TableAlterView(BaseView):
                 "altered": altered,
                 "schema": after_schema,
                 "before_schema": before_schema,
-                "operations_applied": (
-                    0 if alter_request.dry_run else len(alter_request.operations)
-                ),
-                "dry_run": alter_request.dry_run,
+                "operations_applied": len(alter_request.operations),
             },
             status=200,
         )
diff --git a/docs/json_api.rst b/docs/json_api.rst
index 1db46dd2..a42175f7 100644
--- a/docs/json_api.rst
+++ b/docs/json_api.rst
@@ -2285,8 +2285,6 @@ The request body should include an ``operations`` array. Each operation has the
         ]
     }
 
-Set ``"dry_run": true`` to validate the operations and return the schema that would be created without modifying the table.
-
 Supported operations:
 
 * ``add_column`` adds a new column. ``args`` accepts ``name``, optional ``type`` of ``text``, ``integer``, ``float`` or ``blob``, optional ``not_null``, optional literal ``default`` and optional ``default_expr``. If ``not_null`` is ``true`` either a non-null ``default`` or ``default_expr`` is required.
@@ -2316,8 +2314,7 @@ A successful response returns the new schema and the previous schema:
         "altered": true,
         "schema": "CREATE TABLE ...",
         "before_schema": "CREATE TABLE ...",
-        "operations_applied": 7,
-        "dry_run": false
+        "operations_applied": 7
     }
 
 Any errors will return ``{"errors": ["... descriptive message ..."], "ok": false}``, and a ``400`` status code for a bad input or a ``403`` status code for an authentication or permission error.
diff --git a/tests/test_api_write.py b/tests/test_api_write.py
index 1d16ad26..49f38cde 100644
--- a/tests/test_api_write.py
+++ b/tests/test_api_write.py
@@ -898,34 +898,6 @@ async def test_alter_table_operations(ds_write):
     assert event.after_schema == data["schema"]
 
 
-@pytest.mark.asyncio
-async def test_alter_table_dry_run(ds_write):
-    token = write_token(ds_write, permissions=["at"])
-    db = ds_write.get_database("data")
-    response = await ds_write.client.post(
-        "/data/docs/-/alter",
-        json={
-            "dry_run": True,
-            "operations": [
-                {"op": "add_column", "args": {"name": "slug", "type": "text"}}
-            ],
-        },
-        headers=_headers(token),
-    )
-    assert response.status_code == 200, response.text
-    data = response.json()
-    assert data["ok"] is True
-    assert data["dry_run"] is True
-    assert data["altered"] is True
-    assert data["operations_applied"] == 0
-    assert "slug" in data["schema"]
-    columns = (
-        await db.execute("select name from pragma_table_info('docs') order by cid")
-    ).dicts()
-    assert [column["name"] for column in columns] == ["id", "title", "score", "age"]
-    assert last_event(ds_write) is None
-
-
 @pytest.mark.asyncio
 async def test_alter_table_foreign_key_operations(ds_write):
     token = write_token(ds_write, permissions=["at"])
@@ -1246,6 +1218,15 @@ async def test_alter_table_permission_denied(ds_write):
 @pytest.mark.parametrize(
     "body,expected_error",
     (
+        (
+            {
+                "dry_run": True,
+                "operations": [
+                    {"op": "add_column", "args": {"name": "slug", "type": "text"}}
+                ],
+            },
+            "dry_run: Extra inputs are not permitted",
+        ),
         (
             {"operations": [{"op": "add_column", "args": {"type": "text"}}]},
             "operations.0.add_column.args.name: Field required",