From 47f84d5f2b15dfea21445fa8f78dc186668cc967 Mon Sep 17 00:00:00 2001
From: Myles Scolnick <myles@marimo.io>
Date: Tue, 28 Oct 2025 15:09:17 -0400
Subject: [PATCH 1/2] fix: fallback for chart.to_dict() and don't mutate charts

This change make it so `mo.ui.altair_chart` no longer mutates the given chart. This also provides a fallback for when our changes to the width affect the change in such a way that `chart.to_dict()` starts to fail.
---
 marimo/_plugins/ui/_impl/altair_chart.py     | 12 ++++-
 marimo/_smoke_tests/altair_examples/upset.py | 50 ++++++++++++++++++++
 tests/_plugins/ui/_impl/test_altair_chart.py | 17 +++++++
 3 files changed, 78 insertions(+), 1 deletion(-)
 create mode 100644 marimo/_smoke_tests/altair_examples/upset.py

diff --git a/marimo/_plugins/ui/_impl/altair_chart.py b/marimo/_plugins/ui/_impl/altair_chart.py
index 347f6e40b2b..15e233d5158 100644
--- a/marimo/_plugins/ui/_impl/altair_chart.py
+++ b/marimo/_plugins/ui/_impl/altair_chart.py
@@ -327,6 +327,9 @@ def __init__(
 
         register_transformers()
 
+        # Make a copy
+        original_chart = chart
+        chart = chart.copy()
         self._chart = chart
 
         if not isinstance(chart, (alt.TopLevelMixin)):
@@ -345,7 +348,14 @@ def __init__(
             if chart.autosize is alt.Undefined:
                 chart.autosize = "fit-x"
 
-        vega_spec = _parse_spec(chart)
+        try:
+            vega_spec = _parse_spec(chart)
+        except Exception:
+            # Sometimes the changes to width and autosize (above) can cause `.to_dict()` to throw an error
+            # similarly to the issue described in https://github.com/marimo-team/marimo/issues/6244
+            # so we fallback to the original chart.
+            LOGGER.info("Failed to parse spec, using original chart")
+            vega_spec = _parse_spec(original_chart)
 
         if label:
             vega_spec["title"] = label
diff --git a/marimo/_smoke_tests/altair_examples/upset.py b/marimo/_smoke_tests/altair_examples/upset.py
new file mode 100644
index 00000000000..309832040f7
--- /dev/null
+++ b/marimo/_smoke_tests/altair_examples/upset.py
@@ -0,0 +1,50 @@
+# /// script
+# requires-python = ">=3.13"
+# dependencies = [
+#     "altair-upset==0.4.0",
+#     "pandas==2.3.2",
+#     "pyarrow==22.0.0",
+# ]
+# ///
+
+import marimo
+
+__generated_with = "0.17.2"
+app = marimo.App(width="medium", sql_output="polars")
+
+
+@app.cell
+def _():
+    import marimo as mo
+    import pandas as pd
+    import pyarrow
+    from altair_upset import UpSetAltair
+    return UpSetAltair, mo, pd
+
+
+@app.cell
+def _(pd):
+    df = pd.DataFrame({"a": [1, 1, 1, 1, 1, 0], "b": [1, 1, 1, 0, 0, 0]})
+    return (df,)
+
+
+@app.cell
+def _(UpSetAltair, df):
+    chart = UpSetAltair(df, sorted(df.columns)).chart
+    return (chart,)
+
+
+@app.cell
+def _(chart):
+    chart
+    return
+
+
+@app.cell
+def _(chart, mo):
+    mo.ui.altair_chart(chart.copy())
+    return
+
+
+if __name__ == "__main__":
+    app.run()
diff --git a/tests/_plugins/ui/_impl/test_altair_chart.py b/tests/_plugins/ui/_impl/test_altair_chart.py
index e20759426a5..6d70f337287 100644
--- a/tests/_plugins/ui/_impl/test_altair_chart.py
+++ b/tests/_plugins/ui/_impl/test_altair_chart.py
@@ -628,6 +628,23 @@ def test_does_not_modify_original() -> None:
     assert combined1 == combined2._chart
 
 
+@pytest.mark.skipif(not HAS_DEPS, reason="optional dependencies not installed")
+def test_creating_altair_chart_does_not_mutate_original() -> None:
+    import altair as alt
+
+    data = {"values": [1, 2, 3]}
+    original_chart = alt.Chart(data).mark_point().encode(x="values:Q")
+
+    # Store the original spec
+    original_spec = original_chart.to_dict()
+
+    # Create marimo altair_chart wrapper
+    _ = altair_chart(original_chart)
+
+    # Verify the original chart hasn't been mutated
+    assert original_chart.to_dict() == original_spec
+
+
 @pytest.mark.skipif(not HAS_DEPS, reason="optional dependencies not installed")
 def test_get_dataframe() -> None:
     import altair as alt

From 623fa5a770a6f5d4daebf18347b0027ee6c180de Mon Sep 17 00:00:00 2001
From: Myles Scolnick <myles@marimo.io>
Date: Tue, 28 Oct 2025 21:26:10 -0400
Subject: [PATCH 2/2] fix test

---
 marimo/_plugins/ui/_impl/altair_chart.py     | 2 +-
 tests/_plugins/ui/_impl/test_altair_chart.py | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/marimo/_plugins/ui/_impl/altair_chart.py b/marimo/_plugins/ui/_impl/altair_chart.py
index 15e233d5158..7ca28c4fde7 100644
--- a/marimo/_plugins/ui/_impl/altair_chart.py
+++ b/marimo/_plugins/ui/_impl/altair_chart.py
@@ -330,7 +330,7 @@ def __init__(
         # Make a copy
         original_chart = chart
         chart = chart.copy()
-        self._chart = chart
+        self._chart = original_chart
 
         if not isinstance(chart, (alt.TopLevelMixin)):
             raise ValueError(
diff --git a/tests/_plugins/ui/_impl/test_altair_chart.py b/tests/_plugins/ui/_impl/test_altair_chart.py
index 6d70f337287..a6e5ba1e0ef 100644
--- a/tests/_plugins/ui/_impl/test_altair_chart.py
+++ b/tests/_plugins/ui/_impl/test_altair_chart.py
@@ -625,7 +625,7 @@ def test_does_not_modify_original() -> None:
     combined1 = alt1 | alt2
     combined2 = altair_chart(alt1) | altair_chart(alt2)
 
-    assert combined1 == combined2._chart
+    assert combined1.to_dict() == combined2._chart.to_dict()
 
 
 @pytest.mark.skipif(not HAS_DEPS, reason="optional dependencies not installed")