Emit root-relative /message endpoint when using SSE Server (#323)

Author: dogdie233

src/ModelContextProtocol.AspNetCore/SseHandler.cs

@@ -31,7 +31,9 @@ public async Task HandleSseRequestAsync(HttpContext context)
 
         StreamableHttpHandler.InitializeSseResponse(context);
 
-        await using var transport = new SseResponseStreamTransport(context.Response.Body, $"message?sessionId={sessionId}");
+        var requestPath = (context.Request.PathBase + context.Request.Path).ToString();
+        var endpointPattern = requestPath[..(requestPath.LastIndexOf('/') + 1)];
+        await using var transport = new SseResponseStreamTransport(context.Response.Body, $"{endpointPattern}message?sessionId={sessionId}");
         await using var httpMcpSession = new HttpMcpSession<SseResponseStreamTransport>(sessionId, transport, context.User, httpMcpServerOptions.Value.TimeProvider);
         if (!_sessions.TryAdd(sessionId, httpMcpSession))
         {

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs

@@ -32,23 +32,34 @@ public async Task MapMcp_ThrowsInvalidOperationException_IfWithHttpTransportIsNo
         Assert.StartsWith("You must call WithHttpTransport()", exception.Message);
     }
 
-    [Fact]
-    public async Task Allows_Customizing_Route()
+    [Theory]
+    [InlineData("/mcp")]
+    [InlineData("/mcp/secondary")]
+    public async Task Allows_Customizing_Route(string pattern)
     {
         Builder.Services.AddMcpServer().WithHttpTransport();
         await using var app = Builder.Build();
 
-        app.MapMcp("/mcp");
+        app.MapMcp(pattern);
 
         await app.StartAsync(TestContext.Current.CancellationToken);
 
-        using var response = await HttpClient.GetAsync("http://localhost/mcp/sse", HttpCompletionOption.ResponseHeadersRead, TestContext.Current.CancellationToken);
+        using var response = await HttpClient.GetAsync($"http://localhost{pattern}/sse", HttpCompletionOption.ResponseHeadersRead, TestContext.Current.CancellationToken);
         response.EnsureSuccessStatusCode();
+        using var sseStream = await response.Content.ReadAsStreamAsync(TestContext.Current.CancellationToken);
+        using var sseStreamReader = new StreamReader(sseStream, System.Text.Encoding.UTF8);
+        var eventLine = await sseStreamReader.ReadLineAsync(TestContext.Current.CancellationToken);
+        var dataLine = await sseStreamReader.ReadLineAsync(TestContext.Current.CancellationToken);
+        Assert.NotNull(eventLine);
+        Assert.Equal("event: endpoint", eventLine);
+        Assert.NotNull(dataLine);
+        Assert.Equal($"data: {pattern}/message", dataLine[..dataLine.IndexOf('?')]);
     }
 
     [Theory]
     [InlineData("/a", "/a/sse")]
     [InlineData("/a/", "/a/sse")]
+    [InlineData("/a/b", "/a/b/sse")]
     public async Task CanConnect_WithMcpClient_AfterCustomizingRoute(string routePattern, string requestPath)
     {
         Builder.Services.AddMcpServer(options =>