github/copilot-sdk · error · JsonException

SystemMessageSection value cannot be null.

Error message

SystemMessageSection value cannot be null.

What it means

After confirming a string token, the SystemMessageSection converter calls reader.GetString(); if it returns null the converter throws this JsonException, since a SystemMessageSection must wrap a concrete string value.

Solutions

1. Ensure the JSON is complete and valid before deserializing.
2. Provide a non-null string for the field.
3. Use a schema validator on the payload first.
4. Catch JsonException and wrap it with payload context.

Example fix

// before
{"section": null} // or truncated JSON
// after
{"section": "overview"}

using var doc = JsonDocument.Parse(json); // fail early on malformed input
_ = doc.RootElement.GetProperty("section").GetString() ?? throw new InvalidDataException("section missing");

try { sm = JsonSerializer.Deserialize<SystemMessage>(json); }
catch (JsonException ex) { throw new InvalidDataException("malformed SystemMessageSection", ex); }

When it happens

Trigger: reader.GetString() yields null during deserialization of a SystemMessageSection — typically from an unexpected reader state or a malformed document.

Common situations: Truncated JSON buffers; manually driven Utf8JsonReader misuse; documents where the string token cannot yield a value.

Understand the failure class

Background: "failed to unmarshal" / json.Unmarshal errors: why parsing a response into a Go struct fails and how to fix it — this error's family across 23 libraries.

Related errors

• ToolBinaryResultType value cannot be null.
• Failed to deserialize permission request
• Expected a string token when reading
• Expected a non-empty string token when reading
• Expected string for ToolBinaryResultType.

AI-assisted analysis of github/copilot-sdk@cd8cf15dc3 (2026-09-09). Data as JSON: /api/errors/0ba9455279f21f61. Report an issue: GitHub.

Appendix: source

Thrown at dotnet/src/Types.cs:2318

    /// <inheritdoc/>
    public override string ToString() => Value;

    /// <summary>Provides a <see cref="JsonConverter{SystemMessageSection}"/> for serializing <see cref="SystemMessageSection"/> instances.</summary>
    [EditorBrowsable(EditorBrowsableState.Never)]
    public sealed class Converter : JsonConverter<SystemMessageSection>
    {
        /// <inheritdoc/>
        public override SystemMessageSection Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            if (reader.TokenType != JsonTokenType.String)
            {
                throw new JsonException("Expected string for SystemMessageSection.");
            }

            var value = reader.GetString();
            if (value is null)
            {
                throw new JsonException("SystemMessageSection value cannot be null.");
            }

            return new SystemMessageSection(value);
        }

        /// <inheritdoc/>
        public override void Write(Utf8JsonWriter writer, SystemMessageSection value, JsonSerializerOptions options) =>
            writer.WriteStringValue(value.Value);

        /// <inheritdoc/>
        public override SystemMessageSection ReadAsPropertyName(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
            new(reader.GetString()!);

        /// <inheritdoc/>
        public override void WriteAsPropertyName(Utf8JsonWriter writer, SystemMessageSection value, JsonSerializerOptions options) =>
            writer.WritePropertyName(value.Value);
    }
}

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