What causes "Expected start of chat message object." in microsoft/aspire?

Fix the producer to emit chat messages as JSON objects with role and parts fields.

microsoft/aspire · error · JsonException

Expected start of chat message object.

Error message

Expected start of chat message object.

What it means

ReadChatMessage parses one chat message from a Utf8JsonReader and requires the current token to be StartObject. If the reader is positioned at any other token (array, string, number), a JsonException with this message is thrown. Each element of a chat-message array must be a JSON object with role and parts fields.

Solutions

Fix the producer to emit chat messages as JSON objects with role and parts fields.
Add a pre-check on the reader token type and skip/coerce non-object elements before calling ReadChatMessage.
Validate stored payloads against the expected GenAI message schema at write time.

Example fix

// before
var (role, parts, truncated) = GenAIMessageParsingHelper.ReadChatMessage(ref reader);
// after
if (reader.TokenType != JsonTokenType.StartObject)
{
    reader.Skip(); // tolerate malformed element
    return null;
}
var (role, parts, truncated) = GenAIMessageParsingHelper.ReadChatMessage(ref reader);

Defensive patterns

Strategy: try-catch

Validate before calling

if (reader.TokenType != JsonTokenType.StartObject)
{
    reader.Skip(); // element is not a message object
}

Try / catch

try
{
    var (role, parts, truncated) = GenAIMessageParsingHelper.ReadChatMessage(ref reader);
}
catch (JsonException ex)
{
    logger.LogWarning(ex, "Chat message element is not a JSON object; skipping");
}

Prevention

Ensure producers emit each chat message as an object with role and parts fields.
Validate messages arrays against the GenAI schema before storing.
Treat malformed array elements defensively (skip rather than abort the whole list).

When it happens

Trigger: An element inside the messages array is not an object — e.g. the array contains plain strings, numbers, or nulls instead of {"role":..., "parts":[...]} objects.

Common situations: Telemetry producers emitting non-conformant message arrays (legacy or custom exporters); hand-crafted test fixtures; schema drift where messages were stored as bare strings.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/c18d87f75da1fe10. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Dashboard/Model/GenAI/GenAIMessageParsingHelper.cs:91

            catch (InvalidOperationException)
            {
                return (items, true);
            }
        }

        return (items, false);
    }

    internal static MessagePart? ReadMessagePart(ref Utf8JsonReader reader)
    {
        return JsonSerializer.Deserialize(ref reader, GenAIMessagesContext.Default.MessagePart);
    }

    internal static (string role, List<MessagePart> parts, bool partsTruncated) ReadChatMessage(ref Utf8JsonReader reader)
    {
        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.");
            }

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