MCP Server Part 6: Format callback results for LLM consumption

[KoolADE85]

Summary

• Add formatters that enrich callback responses with LLM-friendly content (images, markdown tables) for certain data types
• PlotlyFigureResult — renders Graph.figure outputs as PNG images via kaleido, returned as ImageContent
• DataFrameResult — renders DataTable.data and AgGrid.rowData outputs as markdown tables, returned as TextContent

Manual verification:

import plotly.express as px
from dash import Dash, html, dcc, Input, Output, dash_table

df = px.data.gapminder().query("year == 2007 and continent == 'Europe'")

app = Dash(__name__)
app.layout = html.Div([
    dcc.Dropdown(id="metric", options=["gdpPercap", "lifeExp", "pop"], value="gdpPercap"),
    dcc.Graph(id="chart"),
    dash_table.DataTable(id="table"),
])

@app.callback(Output("chart", "figure"), Input("metric", "value"))
def update_chart(metric):
    """Bar chart of the selected metric across European countries."""
    return px.bar(df, x="country", y=metric, title=f"{metric} in Europe (2007)")

@app.callback(Output("table", "data"), Input("metric", "value"))
def update_table(metric):
    """Top 5 countries for the selected metric."""
    return df.nlargest(5, metric)[["country", metric]].to_dict("records")

with app.server.app_context():
    from dash.mcp.primitives.tools.callback_adapter_collection import CallbackAdapterCollection
    from dash.mcp.primitives.tools.callback_utils import run_callback
    from dash.mcp.primitives.tools.results import format_callback_response

    collection = CallbackAdapterCollection(app)
    for adapter in collection:
        response = run_callback(adapter, {"metric": "lifeExp"})
        result = format_callback_response(response, adapter)
        for item in result.content:
            if item.type == "text":
                print(f"[text] {item.text[:200]}...")
            elif item.type == "image":
                print(f"[image] {item.mimeType}, {len(item.data)} chars base64")
        # update_chart: [text] JSON + [image] PNG
        # update_table: [text] JSON + [text] markdown table

[github-actions]

Thank you for your contribution to Dash! 🎉

This PR is exempt from requiring a linked issue due to its labels.

[camdecoster]

Looks good. I add a few minor suggestions.

[camdecoster]

Would using a dict instead of a list make more sense here? Then you wouldn't have to iterate through the list down below (assuming you could figure out which formatter to use before calling it).

[KoolADE85]

Thinking about this further, I'd like to keep the current structure for two reasons:

1. consistency: the pattern of "iterate providers, each decides if it applies" is already used for input schemas, tool descriptions, and resource providers; I think it's valuable to keep a consistent pattern here too.
2. flexibility: iterating lets each formatter decide dynamically whether it applies. While it's fairly static today, we are leaving room for multiple formatters to enrich the same output without restructuring later.

[camdecoster]

Why is this change necessary?

[KoolADE85]

just a style choice: I wanted it to read more clearly when it's subsequently used in the result formatter, that we are actually concerned with formatting Plotly figures, not more "generic" figures.