What causes "Expected property name." in microsoft/aspire?

Inspect the raw JSON stored in the telemetry entry and confirm each chat message is a JSON object like {"role":"user","parts":[...]}

microsoft/aspire · error · JsonException

Expected property name.

Error message

Expected property name.

What it means

GenAIMessageParsingHelper.ReadChatMessage manually walks a GenAI message JSON object with Utf8JsonReader. After reading '{' or consuming a property value it expects the next token to be a property name; anything else means the JSON is not shaped like a chat message object, so it throws JsonException. It is a guard against malformed or non-object payloads being rendered in the GenAI visualizer.

Solutions

Inspect the raw JSON stored in the telemetry entry and confirm each chat message is a JSON object like {"role":"user","parts":[...]}
Upgrade the emitting instrumentation (e.g. OpenTelemetry .NET GenAI exporters) so messages follow the documented GenAI content schema
Check for truncation of the recorded body (size limits when exporting) and re-export with larger limits
Wrap parsing in DeserializeWithErrorHandling so a malformed message is surfaced as a description instead of a raw JsonException

Example fix

// before
var message = JsonDocument.Parse(rawBody).RootElement[0]; // an array, not an object
// after
if (message.ValueKind == JsonValueKind.Object && message.TryGetProperty("role", out _))
{
    var (role, parts, truncated) = GenAIMessageParsingHelper.ReadChatMessage(message);
}

Defensive patterns

Strategy: try-catch

Validate before calling

using var doc = JsonDocument.Parse(json);
var ok = doc.RootElement.ValueKind == JsonValueKind.Object &&
         doc.RootElement.TryGetProperty("role", out _);
if (!ok) throw new FormatException("Chat message is not a JSON object with a role property.");

Type guard

static bool IsChatMessageObject(JsonElement e) =>
    e.ValueKind == JsonValueKind.Object && e.TryGetProperty("role", out _);

Try / catch

try
{
    var (role, parts, truncated) = GenAIMessageParsingHelper.ReadChatMessage(element);
}
catch (JsonException ex)
{
    logger.LogWarning(ex, "Malformed GenAI chat message; rendering raw content instead.");
}

Prevention

Validate telemetry bodies with JsonDocument.Parse before incremental Utf8JsonReader parsing
Pin instrumentation versions so emitters and the dashboard agree on the message schema
Avoid hand-editing exported telemetry JSON

When it happens

Trigger: Reading a telemetry GenAI chat message whose JSON at the current position is not an object with property names — e.g. the value of a message field is an array, string, or number instead of an object, or the JSON is truncated mid-structure so a non-property token appears where a name is expected.

Common situations: OTLP logs/traces where the GenAI message body was serialized by a different schema version or custom instrumentation; a user pasting/importing hand-edited telemetry JSON; truncated payloads saved from an export.

Understand the failure class

Background: JSON parse error: "Unexpected token" / "not valid JSON" / "failed to parse" — what JSON parsers are really complaining about — this error's family across 45 libraries.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/5818d6b5ad86fda6. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Dashboard/Model/GenAI/GenAIMessageParsingHelper.cs:107

        if (reader.TokenType != JsonTokenType.StartObject)
        {
            throw new JsonException("Expected start of chat message object.");
        }

        string? role = null;
        List<MessagePart>? parts = null;
        var partsTruncated = false;

        while (reader.Read())
        {
            if (reader.TokenType == JsonTokenType.EndObject)
            {
                break;
            }

            if (reader.TokenType != JsonTokenType.PropertyName)
            {
                throw new JsonException("Expected property name.");
            }

            var propertyName = reader.GetString();

            switch (propertyName)
            {
                case "role":
                    if (!reader.Read())
                    {
                        throw new JsonException("Unexpected end of JSON while reading role value.");
                    }
                    role = reader.GetString();
                    break;
                case "parts":
                    // DeserializeArrayIncrementally reads the StartArray token itself.
                    (parts, partsTruncated) = DeserializeArrayIncrementally<MessagePart>(ref reader, ReadMessagePart);
                    break;
                default:

View on GitHub (pinned to 25830f84bd)