Surface JSON parse detail in streamable-HTTP 400 responses

When the POST body cannot be parsed as a JSON-RPC message, the 400
InvalidRequest response was a fixed one-liner that discarded the
JsonException's reason and position. A body truncated by an intermediary
(proxy, gateway, or transport) is therefore indistinguishable from any
other malformed body without server-side logs (#1842).

The catch now includes the parser message and line/byte position in the
error message, so the failure is diagnosable from the response alone.
The message prefix and the conformant id=null envelope are unchanged.

Author: ump45nose

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -81,13 +81,18 @@ await WriteJsonRpcErrorAsync(context,
         {
             message = await ReadJsonRpcMessageAsync(context);
         }
-        catch (JsonException)
+        catch (JsonException ex)
         {
             // The POST body was not a well-formed JSON-RPC message (malformed JSON, or a request whose
             // id was explicitly null, which MCP forbids). Surface a conformant JSON-RPC error response
-            // with a null id rather than letting the exception bubble up as an opaque 500.
+            // with a null id rather than letting the exception bubble up as an opaque 500. The parser's
+            // position detail is included so a truncated or corrupted body (e.g. one mangled by an
+            // intermediary) is diagnosable from the response alone (#1842).
+            var position = ex.BytePositionInLine is null
+                ? $"line {ex.LineNumber?.ToString() ?? "unknown"}"
+                : $"line {ex.LineNumber?.ToString() ?? "unknown"}, byte position {ex.BytePositionInLine}";
             await WriteJsonRpcErrorAsync(context,
-                "Bad Request: The POST body did not contain a valid JSON-RPC message.",
+                $"Bad Request: The POST body did not contain a valid JSON-RPC message: {ex.Message} ({position}).",
                 StatusCodes.Status400BadRequest, (int)McpErrorCode.InvalidRequest);
             return;
         }

tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs

@@ -316,6 +316,33 @@ public async Task PostMalformedJson_Returns400_InvalidRequest_WithNullId()
         using var doc = JsonDocument.Parse(await response.Content.ReadAsStringAsync(TestContext.Current.CancellationToken));
         Assert.Equal(JsonValueKind.Null, doc.RootElement.GetProperty("id").ValueKind);
         Assert.Equal((int)McpErrorCode.InvalidRequest, doc.RootElement.GetProperty("error").GetProperty("code").GetInt32());
+
+        // The parse failure itself must be diagnosable from the response alone: the message carries the
+        // parser's reason and position instead of an opaque one-liner (#1842).
+        var message = doc.RootElement.GetProperty("error").GetProperty("message").GetString();
+        Assert.Contains("did not contain a valid JSON-RPC message", message);
+        Assert.Contains("line 0, byte position", message);
+    }
+
+    [Theory]
+    [InlineData("""{"jsonrpc":"2.0","id":1,"method":"tools/list","para""")]
+    [InlineData("""{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{"a":1""")]
+    public async Task PostTruncatedJson_Returns400_WithParserDetail(string body)
+    {
+        await StartAsync();
+
+        // A body cut in half by an intermediary (proxy, gateway, or the transport itself) is the exact
+        // shape reported in #1842. The 400 response must say where parsing stopped so the truncation
+        // is identifiable from the response without server-side logs.
+        using var response = await HttpClient.PostAsync("", JsonContent(body), TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+
+        using var doc = JsonDocument.Parse(await response.Content.ReadAsStringAsync(TestContext.Current.CancellationToken));
+        Assert.Equal((int)McpErrorCode.InvalidRequest, doc.RootElement.GetProperty("error").GetProperty("code").GetInt32());
+
+        var message = doc.RootElement.GetProperty("error").GetProperty("message").GetString();
+        Assert.Contains("did not contain a valid JSON-RPC message", message);
+        Assert.Contains("line 0, byte position", message);
     }
 
     [Fact]