ipython · tianrking · Jul 16, 2026 · Jul 17, 2026 · Jul 26, 2026 · Jul 26, 2026
diff --git a/IPython/core/inputtransformer2.py b/IPython/core/inputtransformer2.py
@@ -38,6 +38,22 @@ def leading_empty_lines(lines):
    return lines


+def leading_comment_lines(lines):
+    """Remove leading comments before a cell magic.
+
+    Cell magics must be the first meaningful line in a cell. Comments and
+    blank lines preceding a cell magic do not affect its execution, so remove
+    them before detecting the magic.
+    """
+    for i, line in enumerate(lines):
+        if not line.strip() or line.lstrip().startswith("#"):
+            continue
+        if line.startswith("%%"):
+            return lines[i:]
+        break
+    return lines
+
+
 def leading_indent(lines):
    """Remove leading indentation.

@@ -708,6 +724,7 @@ def __init__(self):
            leading_indent,
            classic_prompt,
            ipython_prompt,
+            leading_comment_lines,
        ]
        self.line_transforms = [
            cell_magic,

diff --git a/docs/source/whatsnew/pr/incompat-leading-comments-cell-magic.rst b/docs/source/whatsnew/pr/incompat-leading-comments-cell-magic.rst
@@ -0,0 +1,6 @@
+Leading comments before cell magics
+===================================
+
+Leading comments and blank lines are ignored before a cell magic, so a cell
+such as ``# setup`` followed by ``%%time`` is treated as a cell magic instead
+of an invalid line magic.
diff --git a/tests/test_inputtransformer2_line.py b/tests/test_inputtransformer2_line.py
@@ -25,6 +25,22 @@ def test_cell_magic():
    assert ipt2.cell_magic(sample.splitlines(keepends=True)) == expected.splitlines(keepends=True)


+def test_leading_comments_before_cell_magic():
+    cell = "# Set up timing\n\n# Measure this expression\n%%time\npass\n"
+
+    assert ipt2.TransformerManager().transform_cell(cell) == (
+        "get_ipython().run_cell_magic('time', '', 'pass\\n')\n"
+    )
+
+
+def test_leading_prompted_comments_before_cell_magic():
+    cell = "In [1]: # Set up timing\n   ...: %%time\n   ...: pass\n"
+
+    assert ipt2.TransformerManager().transform_cell(cell) == (
+        "get_ipython().run_cell_magic('time', '', 'pass\\n')\n"
+    )
+
+
 CLASSIC_PROMPT = (
    """\
 >>> for a in range(5):