Adjust analyzer diagnostic severities and improve MCP001 reporting (#1038)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Analyzers/Diagnostics.cs

@@ -25,7 +25,7 @@ internal static class Diagnostics
         title: "MCP method must be partial to generate [Description] attributes",
         messageFormat: "Method '{0}' has XML documentation that could be used to generate [Description] attributes, but the method is not declared as partial.",
         category: "mcp",
-        defaultSeverity: DiagnosticSeverity.Warning,
+        defaultSeverity: DiagnosticSeverity.Info,
         isEnabledByDefault: true,
         description: "Methods with MCP attributes should be declared as partial to allow the source generator to emit Description attributes from XML documentation comments.");
 }

src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs

@@ -72,7 +72,7 @@ private static void Execute(Compilation compilation, ImmutableArray<MethodToGene
         List<(IMethodSymbol MethodSymbol, MethodDeclarationSyntax MethodDeclaration, XmlDocumentation? XmlDocs)> methodsToGenerate = new(methods.Length);
         foreach (var methodModel in methods)
         {
-            var xmlDocs = ExtractXmlDocumentation(methodModel.MethodSymbol, context);
+            var xmlDocs = ExtractXmlDocumentation(methodModel.MethodSymbol, methodModel.MethodDeclaration, context);
 
             // Generate implementation for partial methods.
             if (methodModel.MethodDeclaration.Modifiers.Any(SyntaxKind.PartialKeyword))
@@ -97,7 +97,7 @@ private static void Execute(Compilation compilation, ImmutableArray<MethodToGene
         }
     }
 
-    private static XmlDocumentation? ExtractXmlDocumentation(IMethodSymbol methodSymbol, SourceProductionContext context)
+    private static XmlDocumentation? ExtractXmlDocumentation(IMethodSymbol methodSymbol, MethodDeclarationSyntax methodDeclaration, SourceProductionContext context)
     {
         string? xmlDoc = methodSymbol.GetDocumentationCommentXml();
         if (string.IsNullOrWhiteSpace(xmlDoc))
@@ -138,11 +138,14 @@ private static void Execute(Compilation compilation, ImmutableArray<MethodToGene
         }
         catch (System.Xml.XmlException)
         {
-            // Emit warning for invalid XML
-            context.ReportDiagnostic(Diagnostic.Create(
-                Diagnostics.InvalidXmlDocumentation,
-                methodSymbol.Locations.FirstOrDefault(),
-                methodSymbol.Name));
+            // Emit warning for invalid XML only if the method is partial (as that's when it would affect generated code)
+            if (methodDeclaration.Modifiers.Any(SyntaxKind.PartialKeyword))
+            {
+                context.ReportDiagnostic(Diagnostic.Create(
+                    Diagnostics.InvalidXmlDocumentation,
+                    methodSymbol.Locations.FirstOrDefault(),
+                    methodSymbol.Name));
+            }
             return null;
         }
     }

tests/ModelContextProtocol.Analyzers.Tests/XmlToDescriptionGeneratorTests.cs

@@ -381,7 +381,7 @@ public static string TestMethod(string input)
 
         // Should report MCP002 diagnostic
         var diagnostic = Assert.Single(result.Diagnostics, d => d.Id == "MCP002");
-        Assert.Equal(DiagnosticSeverity.Warning, diagnostic.Severity);
+        Assert.Equal(DiagnosticSeverity.Info, diagnostic.Severity);
         Assert.Contains("TestMethod", diagnostic.GetMessage());
         Assert.Contains("partial", diagnostic.GetMessage());
     }
@@ -412,7 +412,7 @@ public static string TestMethod(string input)
 
         // Should report MCP002 diagnostic because parameter has documentation
         var diagnostic = Assert.Single(result.Diagnostics, d => d.Id == "MCP002");
-        Assert.Equal(DiagnosticSeverity.Warning, diagnostic.Severity);
+        Assert.Equal(DiagnosticSeverity.Info, diagnostic.Severity);
     }
 
     [Fact]
@@ -441,7 +441,7 @@ public static string TestMethod(string input)
 
         // Should report MCP002 diagnostic because return has documentation
         var diagnostic = Assert.Single(result.Diagnostics, d => d.Id == "MCP002");
-        Assert.Equal(DiagnosticSeverity.Warning, diagnostic.Severity);
+        Assert.Equal(DiagnosticSeverity.Info, diagnostic.Severity);
     }
 
     [Fact]
@@ -559,7 +559,7 @@ public static string TestMethod(string input)
 
         // Should report MCP002 diagnostic because parameter description would be generated
         var diagnostic = Assert.Single(result.Diagnostics, d => d.Id == "MCP002");
-        Assert.Equal(DiagnosticSeverity.Warning, diagnostic.Severity);
+        Assert.Equal(DiagnosticSeverity.Info, diagnostic.Severity);
     }
 
     [Fact]
@@ -590,7 +590,7 @@ public static string TestPrompt(string input)
 
         // Should report MCP002 diagnostic for prompts too
         var diagnostic = Assert.Single(result.Diagnostics, d => d.Id == "MCP002");
-        Assert.Equal(DiagnosticSeverity.Warning, diagnostic.Severity);
+        Assert.Equal(DiagnosticSeverity.Info, diagnostic.Severity);
     }
 
     [Fact]
@@ -621,7 +621,7 @@ public static string TestResource(string input)
 
         // Should report MCP002 diagnostic for resources too
         var diagnostic = Assert.Single(result.Diagnostics, d => d.Id == "MCP002");
-        Assert.Equal(DiagnosticSeverity.Warning, diagnostic.Severity);
+        Assert.Equal(DiagnosticSeverity.Info, diagnostic.Severity);
     }
 
     [Fact]