microsoft/semantic-kernel · error · KernelException

Unexpected response from model

Error message

Unexpected response from model

What it means

Thrown by ClientBase.DeserializeResponse<T> when JsonSerializer.Deserialize throws a JsonException or returns null while parsing the model's HTTP response body. The original JsonException is wrapped in a KernelException with the raw response string stored in the exception's Data dictionary under the key 'ResponseData'. This is a catch-all for malformed or unexpected response payloads from the Google/Gemini API.

Source

Thrown at dotnet/src/Connectors/Connectors.Google/Core/ClientBase.cs:83

    protected async Task<HttpResponseMessage> SendRequestAndGetResponseImmediatelyAfterHeadersReadAsync(
        HttpRequestMessage httpRequestMessage,
        CancellationToken cancellationToken)
    {
        var response = await this.HttpClient.SendWithSuccessCheckAsync(httpRequestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken)
            .ConfigureAwait(false);
        return response;
    }

    protected static T DeserializeResponse<T>(string body)
    {
        try
        {
            return JsonSerializer.Deserialize<T>(body) ?? throw new JsonException("Response is null");
        }
        catch (JsonException exc)
        {
            throw new KernelException("Unexpected response from model", exc)
            {
                Data = { { "ResponseData", body } },
            };
        }
    }

    protected async Task<HttpRequestMessage> CreateHttpRequestAsync(object requestData, Uri endpoint)
    {
        var httpRequestMessage = HttpRequest.CreatePostRequest(endpoint, requestData);
        httpRequestMessage.Headers.Add("User-Agent", HttpHeaderConstant.Values.UserAgent);
        httpRequestMessage.Headers.Add(HttpHeaderConstant.Names.SemanticKernelVersion,
            HttpHeaderConstant.Values.GetAssemblyVersion(typeof(ClientBase)));

        if (this._bearerTokenProvider is not null && await this._bearerTokenProvider().ConfigureAwait(false) is { } bearerKey)
        {
            httpRequestMessage.Headers.Authorization =
                new AuthenticationHeaderValue("Bearer", bearerKey);
        }

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Catch KernelException and inspect exception.Data['ResponseData'] for the raw response body.
2. Upgrade the Connectors.Google NuGet package to match the API version you are targeting.
3. Verify the API version and model ID are compatible with the installed connector version.
4. Check for proxy/gateway interference that may alter the response body.

Example fix

// inspect the raw response when this fires
try { var result = await client.GetChatMessageContentsAsync(history); }
catch (KernelException ex)
{
    var rawBody = ex.Data.Contains("ResponseData") ? ex.Data["ResponseData"] : null;
    logger.LogError(ex, "Deserialization failed. Raw body: {Body}", rawBody);
}

null

null

try
{
    var result = await client.GetChatMessageContentsAsync(history, settings, ct);
}
catch (KernelException ex) when (ex.Message == "Unexpected response from model")
{
    var rawBody = ex.Data.Contains("ResponseData") ? ex.Data["ResponseData"]?.ToString() : "<unavailable>";
    logger.LogError(ex, "Failed to deserialize Gemini response. Raw body: {Body}", rawBody);
    throw;
}

When it happens

Trigger: The Gemini or Vertex AI API returns a response body that does not deserialize into the expected type T (e.g. GeminiResponse). This includes: JSON schema mismatch, truncated response body, HTML error page instead of JSON, or a response shape from a newer/older API version that the connector's model classes do not match.

Common situations: Google changes the API response format in a new version and the installed connector package is outdated. A network proxy or gateway injects HTML or non-JSON content. Rate limiting or auth failure returns an error page instead of the expected JSON. Mismatch between the API version configured and the connector's model classes.

Related errors

• GeminiPart is invalid. One and only one property among Text,
• Unexpected author role: {role}
• Gemini API doesn't support author role: {value}
• Prompt was blocked due to Gemini API safety reasons.
• Invalid response from model

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/bbabda2d73669648. Report an issue: GitHub.