github/copilot-sdk · error · JsonException

Expected a non-empty string value when writing

Error message

Expected a non-empty string value when writing {typeToConvert.Name}.

What it means

GeneratedStringEnumJson.WriteValue validates on serialization that the string being written is non-empty. It throws this JsonException when serializing an object whose string-backed enum property is null, empty, or whitespace, because writing an empty string would produce an invalid enum representation.

Solutions

1. Initialize the property with a valid enum value before serializing.
2. Make the property nullable and skip it when null (JsonIgnoreCondition.WhenWritingNull).
3. Validate the value at construction time (guard in the wrapper type constructor).
4. Catch JsonException during serialization and log the offending member.

Example fix

// before
new MessageSource("")
// after
new MessageSource("user")

// validate before serializing
if (string.IsNullOrWhiteSpace(message.Source?.Value))
    throw new InvalidOperationException("Source must be a non-empty value before serialization");

static bool IsSerializable(MessageSource? s) => !string.IsNullOrWhiteSpace(s?.Value);

try { JsonSerializer.Serialize(payload); }
catch (JsonException ex) { log.LogError(ex, "payload contains empty string enum value"); }

When it happens

Trigger: Serializing an object where a GeneratedStringEnumJson-backed property is `""`, whitespace, or null (when the converter is invoked for null), e.g. `JsonSerializer.Serialize(new Message { Source = new MessageSource("") })`.

Common situations: Objects constructed programmatically with uninitialized string fields; models populated from databases with empty defaults; default-constructed wrappers holding "".

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• Expected a non-empty string token when reading
• Failed to serialize FFI JSON parameter.
• The value cannot be an empty string or composed entirely of…
• Failed to deserialize permission request
• Invalid tool name: must not be null or empty.

AI-assisted analysis of github/copilot-sdk@cd8cf15dc3 (2026-09-09). Data as JSON: /api/errors/0bf46bb541639239. Report an issue: GitHub.

Appendix: source

Thrown at dotnet/src/Types.cs:41

        if (reader.TokenType != JsonTokenType.String)
        {
            throw new JsonException($"Expected a string token when reading {typeToConvert.Name}, but found {reader.TokenType}.");
        }

        var value = reader.GetString();
        if (string.IsNullOrWhiteSpace(value))
        {
            throw new JsonException($"Expected a non-empty string token when reading {typeToConvert.Name}.");
        }

        return value!;
    }

    internal static void WriteValue(Utf8JsonWriter writer, string value, Type typeToConvert)
    {
        if (string.IsNullOrWhiteSpace(value))
        {
            throw new JsonException($"Expected a non-empty string value when writing {typeToConvert.Name}.");
        }

        writer.WriteStringValue(value);
    }
}

/// <summary>Diagnostic IDs for the Copilot SDK.</summary>
internal static class Diagnostics
{
    /// <summary>Indicates an experimental API that may change or be removed.</summary>
    internal const string Experimental = "GHCP001";
}

/// <summary>
/// Log level for the Copilot runtime. Use the well-known values exposed as
/// static members (<see cref="None"/>, <see cref="Error"/>, <see cref="Warning"/>,
/// <see cref="Info"/>, <see cref="Debug"/>, <see cref="All"/>), or construct
/// your own with <see cref="CopilotLogLevel(string)"/> if the runtime accepts

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