Regression in kaleido 1.3.0 when saving plot with markers

[StageTired]

Observe the simple reproducible example at the end of the post . It works just fine when running it with

uv run --with pandas==2.3.3 --with numpy==2.4.4 --with plotly==6.7.0 --with kaleido==1.2.0 generate_plot_png.py

When I run the same script but with kaleido==1.3.0, it raises an exception.

TypeError: Type is not JSON serializable: Timestamp

Command:

uv run --with pandas==2.3.3 --with numpy==2.4.4 --with plotly==6.7.0 --with kaleido==1.3.0 generate_plot_png.py

Code:

import numpy as np
import pandas as pd
import plotly.graph_objects as go


def generate_dataframe() -> pd.DataFrame:
    rng = np.random.default_rng(42)
    index = pd.bdate_range("2026-01-30", "2026-06-03")

    data = {
        "col_1": np.round(rng.uniform(10.0, 30.0, len(index))),
        "col_2": np.round(rng.uniform(1.0, 30.0, len(index))),
        "col_3": np.round(rng.uniform(20.0, 50.0, len(index))),
        "col_4": np.round(rng.uniform(4.0, 100.0, len(index))),
        "col_5": np.round(rng.uniform(30.0, 40.0, len(index))),
    }
    df = pd.DataFrame(data, index=index).astype("float64")
    df.index = df.index.astype("datetime64[ns]")
    return df


df = generate_dataframe()
fig = go.Figure()

for c in df.columns:
    fig.add_trace(
        go.Scatter(
            x=df.index,
            y=df[c],
            name=c,
            mode="lines",
        )
    )
    # Last point marker + label
    # SOMETHING HERE MAKES IT FAIL
    last_idx = df[c].last_valid_index()
    if last_idx is not None:
        last_val = df.loc[last_idx, c]
        fig.add_trace(
            go.Scatter(
                x=[last_idx],
                y=[last_val],
                mode="markers",
                showlegend=False,
                hoverinfo="skip",
            )
        )


fig.write_image("plot1.png")