Fix form mode elicitation schema check to apply for all form requests (#1151)

src/ModelContextProtocol.Core/Server/McpServer.Methods.cs

@@ -488,7 +488,7 @@ public abstract partial class McpServer : McpSession
             throw new InvalidOperationException("Client does not support elicitation requests.");
         }
 
-        if (string.Equals(request.Mode, "form", StringComparison.Ordinal) && elicitationCapability.Form is null)
+        if (string.Equals(request.Mode, "form", StringComparison.Ordinal))
         {
             if (request.RequestedSchema is null)
             {

tests/ModelContextProtocol.Tests/Protocol/UrlElicitationTests.cs

@@ -163,6 +163,26 @@ public partial class UrlElicitationTests(ITestOutputHelper testOutputHelper) : C
                     throw new McpException(ex.Message);
                 }
             }
+            else if (request.Params.Name == "TestFormElicitationMissingSchema")
+            {
+                try
+                {
+                    await request.Server.ElicitAsync(new()
+                        {
+                            Message = "Form elicitation without schema should fail.",
+                        },
+                        cancellationToken);
+                }
+                catch (ArgumentException ex)
+                {
+                    throw new McpException(ex.Message);
+                }
+
+                return new CallToolResult
+                {
+                    Content = [new TextContentBlock { Text = "missing-schema-succeeded" }],
+                };
+            }
 
             Assert.Fail($"Unexpected tool name: {request.Params.Name}");
             return new CallToolResult { Content = [] };
@@ -506,6 +526,35 @@ public partial class UrlElicitationTests(ITestOutputHelper testOutputHelper) : C
         Assert.Equal("Authorization is required to access Example Co.", elicitation.Message);
     }
 
+    [Fact]
+    public async Task FormElicitation_Requires_RequestedSchema()
+    {
+        var elicitationHandlerCalled = false;
+
+        await using McpClient client = await CreateMcpClientForServer(new McpClientOptions
+        {
+            Capabilities = new ClientCapabilities
+            {
+                Elicitation = new(),
+            },
+            Handlers = new McpClientHandlers()
+            {
+                ElicitationHandler = (request, cancellationToken) =>
+                {
+                    elicitationHandlerCalled = true;
+                    return new ValueTask<ElicitResult>(new ElicitResult());
+                },
+            }
+        });
+
+        var result = await client.CallToolAsync("TestFormElicitationMissingSchema", cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.True(result.IsError);
+        var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
+        Assert.Equal("An error occurred invoking 'TestFormElicitationMissingSchema': Form mode elicitation requests require a requested schema.", textContent.Text);
+        Assert.False(elicitationHandlerCalled);
+    }
+
     private ElicitationCapability AssertServerElicitationCapability()
     {
         var capabilities = Server.ClientCapabilities;