Starting with Claude 4 models, streaming responses from Claude's API return stop_reason: "refusal" when streaming classifiers intervene to handle potential policy violations. This safety feature helps maintain content compliance during real-time streaming.

API response format

When streaming classifiers detect content that violates Anthropic's policies, the API returns this response:

{
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Hello.."
    }
  ],
  "stop_reason": "refusal",
  "stop_details": {
    "type": "refusal",
    "category": "cyber",
    "explanation": "This request was declined because it could enable cyber harm."
  }
}

In the event stream, stop_details arrives on the message_delta event alongside stop_reason.

On a refusal the stop_details object is always present, but its category and explanation fields can be null, for example when the refusal maps to no named category. Branch on stop_reason or stop_details.type rather than assuming category and explanation are populated, and provide your own user-facing messaging when they are null.

Reset context after refusal

When you receive stop_reason: refusal, you must reset the conversation context before continuing. You can remove or rephrase the turn that triggered the refusal, or clear the conversation history entirely. Attempting to continue without resetting will result in continued refusals.

When a refusal arrives before Claude generates any output, you are not billed for the request on the Claude API, and the usage counts in that response are informational only. When Claude generates output before the refusal, you are billed for that request.

Implementation guide

Here's how to detect and handle streaming refusals in your application:

Check for refusal in the stream

if echo "$response" | grep -q '"stop_reason":"refusal"'; then echo "Response refused - resetting conversation context" # Reset your conversation state here fi

```python Python
client = anthropic.Anthropic()
messages = []


def reset_conversation():
    """Reset conversation context after refusal"""
    global messages
    messages = []
    print("Conversation reset due to refusal")


try:
    with client.messages.stream(
        max_tokens=1024,
        messages=messages + [{"role": "user", "content": "Hello"}],
        model="claude-opus-5",
    ) as stream:
        for event in stream:
            # Check for refusal in message delta
            if event.type == "message_delta":
                if event.delta.stop_reason == "refusal":
                    reset_conversation()
                    break
except Exception as e:
    print(f"Error: {e}")

const client = new Anthropic();
let messages: Anthropic.MessageParam[] = [];

function resetConversation() {
  // Reset conversation context after refusal
  messages = [];
  console.log("Conversation reset due to refusal");
}

try {
  const stream = await client.messages.stream({
    messages: [...messages, { role: "user", content: "Hello" }],
    model: "claude-opus-5",
    max_tokens: 1024
  });

  for await (const event of stream) {
    // Check for refusal in message delta
    if (event.type === "message_delta" && event.delta.stop_reason === "refusal") {
      resetConversation();
      break;
    }
  }
} catch (error) {
  console.error("Error:", error);
}

List<Message> messages = new();
AnthropicClient client = new();

var parameters = new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 1024,
    Messages = [new() { Role = Role.User, Content = "Hello" }]
};

try
{
    await foreach (var streamEvent in client.Messages.CreateStreaming(parameters))
    {
        if (
            streamEvent.TryPickDelta(out var deltaEvent)
            && deltaEvent.Delta.StopReason == StopReason.Refusal
        )
        {
            ResetConversation();
            break;
        }
    }
}
catch (Exception e)
{
    Console.WriteLine($"Error: {e.Message}");
}

void ResetConversation()
{
    messages.Clear();
    Console.WriteLine("Conversation reset due to refusal");
}

var messages []anthropic.MessageParam

func resetConversation() {
	messages = []anthropic.MessageParam{}
	fmt.Println("Conversation reset due to refusal")
}
// ...
	client := anthropic.NewClient()

	stream := client.Messages.NewStreaming(context.TODO(), anthropic.MessageNewParams{
		Model:     anthropic.ModelClaudeOpus5,
		MaxTokens: 1024,
		Messages: []anthropic.MessageParam{
			anthropic.NewUserMessage(anthropic.NewTextBlock("Hello")),
		},
	})

streamLoop:
	for stream.Next() {
		event := stream.Current()
		switch eventVariant := event.AsAny().(type) {
		case anthropic.MessageDeltaEvent:
			if eventVariant.Delta.StopReason == anthropic.StopReasonRefusal {
				resetConversation()
				break streamLoop
			}
		}
	}

	if err := stream.Err(); err != nil {
		log.Fatal(err)
	}

import com.anthropic.core.http.StreamResponse;
import com.anthropic.models.messages.RawMessageStreamEvent;
import com.anthropic.models.messages.StopReason;
// ...

List<MessageParam> messages = new ArrayList<>();

void main() {
    AnthropicClient client = AnthropicOkHttpClient.fromEnv();

    MessageCreateParams params = MessageCreateParams.builder()
        .model(Model.CLAUDE_OPUS_5)
        .maxTokens(1024L)
        .addUserMessage("Hello")
        .build();

    try (StreamResponse<RawMessageStreamEvent> stream = client.messages().createStreaming(params)) {
        stream.stream().forEach(event -> {
            event.messageDelta().ifPresent(deltaEvent -> {
                deltaEvent.delta().stopReason().ifPresent(stopReason -> {
                    if (stopReason.equals(StopReason.REFUSAL)) {
                        resetConversation();
                    }
                });
            });
        });
    } catch (Exception e) {
        System.err.println("Error: " + e.getMessage());
    }
}

void resetConversation() {
    messages.clear();
    IO.println("Conversation reset due to refusal");
}

$client = new Client();
$messages = [];

function resetConversation(&$messages) {
    $messages = [];
    echo "Conversation reset due to refusal\n";
}

try {
    $stream = $client->messages->createStream(
        maxTokens: 1024,
        messages: [
            ['role' => 'user', 'content' => 'Hello']
        ],
        model: 'claude-opus-5',
    );

    foreach ($stream as $event) {
        if ($event->type === 'message_delta' && $event->delta->stopReason === 'refusal') {
            resetConversation($messages);
            break;
        }
    }
} catch (Exception $e) {
    echo "Error: " . $e->getMessage() . "\n";
}

client = Anthropic::Client.new
messages = []

def reset_conversation(messages)
  messages.clear
  puts "Conversation reset due to refusal"
end

begin
  stream = client.messages.stream(
    model: :"claude-opus-5",
    max_tokens: 1024,
    messages: [{ role: "user", content: "Hello" }]
  )

  stream.each do |event|
    if event.type == :message_delta && event.delta.stop_reason == :refusal
      reset_conversation(messages)
      break
    end
  end
rescue => e
  puts "Error: #{e.message}"
end

Current refusal types

The API currently handles refusals in three different ways:

Best practices

• Monitor for refusals: Include stop_reason: refusal checks in your error handling
• Reset automatically: Implement automatic context reset when refusals are detected
• Fall back to another model: Configure server-side fallback or the SDK middleware so refused requests are retried on another Claude model instead of surfacing a refusal to the user
• Redeem fallback credit on manual retries: If you build the retry yourself, pass the refusal's fallback credit token so the retry doesn't pay the prompt-cache cost twice
• Provide custom messaging: Create user-friendly messages for better UX when refusals occur
• Track refusal patterns: Monitor refusal frequency to identify potential issues with your prompts

Migration notes

If you built refusal handling when this feature first shipped, or you're adding it to an existing integration, check the following:

• Refusals are responses, not errors. A refusal arrives as a successful HTTP 200 response with stop_reason: "refusal", so monitoring built only on error rates won't surface it. Track refusals as their own signal.
• Refusals include structured detail. On every model, a refusal also includes a stop_details object that identifies the policy category behind the decline. See Refusals and fallback for the full response shape.
• Retry on a different model. Re-sending a refused request to the same model usually results in another refusal. Instead of only resetting context, retry on a fallback model with server-side fallback, the SDK middleware, or a manual retry, and redeem fallback credit when you build the retry yourself.
• Check batch results for refusals. A refused request in a Message Batch is returned as a succeeded result with stop_reason: "refusal", not as an errored result.
• Centralize handling on stop_reason. The API continues to consolidate refusal handling around stop_reason: "refusal", so branch on the stop reason rather than on model-specific behavior.

Next steps