<esri:ArcGISTiledMapServiceLayer ID="AGOLayer"
Url="http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_ShadedRelief_World_2D/MapServer"/>
<esri:FeatureLayer ID="MyPointFeatureLayer"
Url="http://localhost/ArcGIS/rest/services/SilverTest/MapServer/0"
FeatureSymbol="{StaticResource YellowMarkerSymbol}"
OutFields="API,Operator">
<esri:FeatureLayer.Clusterer>
<esri:FlareClusterer
FlareBackground="#BBFF8C00"
MaximumFlareCount="4" />
</esri:FeatureLayer.Clusterer>
<esri:FeatureLayer.MapTip>
<Grid Background="LightYellow">
<StackPanel Margin="5">
<TextBlock Text="{Binding [API]}" FontWeight="Bold" />
<StackPanel Orientation="Horizontal">
<TextBlock Text="Web site: " />
<TextBlock Text="{Binding [Operator]}" />
</StackPanel>
</StackPanel>
<Border BorderBrush="Black" BorderThickness="1" />
</Grid>
</esri:FeatureLayer.MapTip>
</esri:FeatureLayer>
</esri:Map.Layers>
<TextBlock Text="{Binding [API]}" FontWeight="Bold" />
<StackPanel Orientation="Horizontal">
<TextBlock Text="Web site: " />
<TextBlock Text="{Binding [Operator]}" />
</StackPanel>
These Binding causes the BAD_PARSER_VALUE. I think you meant to put {Binding Attributes[API]} and {Binding Attributes[Operator]}.
Jennifer