Anthropic 平台文档（英文全量）

Stop reason values

end_turn

The most common stop reason. Indicates Claude finished its response naturally.

bash

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5",
    "max_tokens": 1024,
    "messages": [{"role": "user", "content": "Hello!"}]
  }' | jq 'if .stop_reason == "end_turn" then (.content[] | select(.type == "text") | .text) else . end'

bash

ant messages create \
  --model claude-opus-5 \
  --max-tokens 1024 \
  --message '{role: user, content: "Hello!"}' \
  --format json | jq 'if .stop_reason == "end_turn" then (.content[] | select(.type == "text") | .text) else . end'

python

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello!"}],
)
if response.stop_reason == "end_turn":
    # Process the complete response
    for block in response.content:
        if block.type == "text":
            print(block.text)

typescript

const client = new Anthropic();

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

if (response.stop_reason === "end_turn") {
  // Process the complete response
  const textBlock = response.content.find(
    (block): block is Anthropic.TextBlock => block.type === "text"
  );
  console.log(textBlock?.text);
}

csharp

AnthropicClient client = new();

var response = await client.Messages.Create(new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 1024,
    Messages = [new() { Role = Role.User, Content = "Hello!" }]
});

if (response.StopReason == "end_turn")
{
    // Process the complete response
    foreach (var block in response.Content)
    {
        if (block.TryPickText(out var textBlock))
        {
            Console.WriteLine(textBlock.Text);
        }
    }
}

client := anthropic.NewClient()

response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
	Model:     anthropic.ModelClaudeOpus5,
	MaxTokens: 1024,
	Messages: []anthropic.MessageParam{
		anthropic.NewUserMessage(anthropic.NewTextBlock("Hello!")),
	},
})
if err != nil {
	log.Fatal(err)
}

if response.StopReason == "end_turn" {
	// Process the complete response
	for _, block := range response.Content {
		if textBlock, ok := block.AsAny().(anthropic.TextBlock); ok {
			fmt.Println(textBlock.Text)
		}
	}
}

java

AnthropicClient client = AnthropicOkHttpClient.fromEnv();

Message response = client.messages().create(
    MessageCreateParams.builder()
        .model(Model.CLAUDE_OPUS_5)
        .maxTokens(1024L)
        .addUserMessage("Hello!")
        .build()
);

if (response.stopReason().map(StopReason.END_TURN::equals).orElse(false)) {
    // Process the complete response
    response.content().stream()
        .flatMap(block -> block.text().stream())
        .forEach(textBlock -> IO.println(textBlock.text()));
}

php

$client = new Client();

$response = $client->messages->create(
    maxTokens: 1024,
    messages: [['role' => 'user', 'content' => 'Hello!']],
    model: 'claude-opus-5',
);

if ($response->stopReason === 'end_turn') {
    // Process the complete response
    foreach ($response->content as $block) {
        if ($block->type === 'text') {
            echo $block->text, PHP_EOL;
        }
    }
}

ruby

client = Anthropic::Client.new

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

if response.stop_reason == :end_turn
  # Process the complete response
  response.content.each do |block|
    puts block.text if block.type == :text
  end
end

Sometimes Claude returns an empty response (exactly 2–3 tokens with no content) with stop_reason: "end_turn". This typically occurs when Claude interprets that the assistant turn is complete, particularly after tool results.

Common causes:

Adding text blocks immediately after tool results (Claude learns to expect the user to always insert text after tool results, so it ends its turn to follow the pattern)
Sending Claude's completed response back without adding anything (Claude already determined it's done, so it will remain done)

How to prevent empty responses:

```python Python
# INCORRECT: Adding text immediately after tool_result
messages = [
    \{"role": "user", "content": "Calculate the sum of 1234 and 5678"\},
    \{
        "role": "assistant",
        "content": [
            \{
                "type": "tool_use",
                "id": "toolu_123",
                "name": "calculator",
                "input": \{"operation": "add", "a": 1234, "b": 5678\},
            \}
        ],
    \},
    \{
        "role": "user",
        "content": [
            \{"type": "tool_result", "tool_use_id": "toolu_123", "content": "6912"\},
            \{
                "type": "text",
                "text": "Here's the result",  # Don't add text after tool_result
            \},
        ],
    \},
]

# CORRECT: Send tool results directly without additional text
messages = [
    \{"role": "user", "content": "Calculate the sum of 1234 and 5678"\},
    \{
        "role": "assistant",
        "content": [
            \{
                "type": "tool_use",
                "id": "toolu_123",
                "name": "calculator",
                "input": \{"operation": "add", "a": 1234, "b": 5678\},
            \}
        ],
    \},
    \{
        "role": "user",
        "content": [
            \{"type": "tool_result", "tool_use_id": "toolu_123", "content": "6912"\}
        ],
    \},  # Just the tool_result, no additional text
]
```

```typescript TypeScript
// INCORRECT: Adding text immediately after tool_result
let messages: Anthropic.MessageParam[] = [
  \{ role: "user", content: "Calculate the sum of 1234 and 5678" \},
  \{
    role: "assistant",
    content: [
      \{
        type: "tool_use",
        id: "toolu_123",
        name: "calculator",
        input: \{ operation: "add", a: 1234, b: 5678 \}
      \}
    ]
  \},
  \{
    role: "user",
    content: [
      \{ type: "tool_result", tool_use_id: "toolu_123", content: "6912" \},
      \{ type: "text", text: "Here's the result" \} // Don't add text after tool_result
    ]
  \}
];

// CORRECT: Send tool results directly without additional text
messages = [
  \{ role: "user", content: "Calculate the sum of 1234 and 5678" \},
  \{
    role: "assistant",
    content: [
      \{
        type: "tool_use",
        id: "toolu_123",
        name: "calculator",
        input: \{ operation: "add", a: 1234, b: 5678 \}
      \}
    ]
  \},
  \{
    role: "user",
    // Just the tool_result, no additional text
    content: [\{ type: "tool_result", tool_use_id: "toolu_123", content: "6912" \}]
  \}
];
```

```csharp C#
using System.Text.Json;
using Anthropic.Models.Messages;

var input = JsonSerializer.Deserialize&lt;Dictionary&lt;string, JsonElement>>(
    """\{"operation":"add","a":1234,"b":5678\}"""
)!;

// INCORRECT: Adding text immediately after tool_result
List&lt;MessageParam> messages =
[
    new() \{ Role = Role.User, Content = "Calculate the sum of 1234 and 5678" \},
    new()
    \{
        Role = Role.Assistant,
        Content = new List&lt;ContentBlockParam>
        \{
            new ToolUseBlockParam \{ ID = "toolu_123", Name = "calculator", Input = input \}
        \}
    \},
    new()
    \{
        Role = Role.User,
        Content = new List&lt;ContentBlockParam>
        \{
            new ToolResultBlockParam \{ ToolUseID = "toolu_123", Content = "6912" \},
            new TextBlockParam \{ Text = "Here's the result" \} // Don't add text after tool_result
        \}
    \}
];

// CORRECT: Send tool results directly without additional text
messages =
[
    new() \{ Role = Role.User, Content = "Calculate the sum of 1234 and 5678" \},
    new()
    \{
        Role = Role.Assistant,
        Content = new List&lt;ContentBlockParam>
        \{
            new ToolUseBlockParam \{ ID = "toolu_123", Name = "calculator", Input = input \}
        \}
    \},
    new()
    \{
        Role = Role.User,
        // Just the tool_result, no additional text
        Content = new List&lt;ContentBlockParam>
        \{
            new ToolResultBlockParam \{ ToolUseID = "toolu_123", Content = "6912" \}
        \}
    \}
];
```

```go Go
input := map[string]any\{"operation": "add", "a": 1234, "b": 5678\}

// INCORRECT: Adding text immediately after tool_result
messages := []anthropic.MessageParam\{
	anthropic.NewUserMessage(anthropic.NewTextBlock("Calculate the sum of 1234 and 5678")),
	anthropic.NewAssistantMessage(
		anthropic.NewToolUseBlock("toolu_123", input, "calculator"),
	),
	anthropic.NewUserMessage(
		anthropic.NewToolResultBlock("toolu_123", "6912", false),
		anthropic.NewTextBlock("Here's the result"), // Don't add text after tool_result
	),
\}

// CORRECT: Send tool results directly without additional text
messages = []anthropic.MessageParam\{
	anthropic.NewUserMessage(anthropic.NewTextBlock("Calculate the sum of 1234 and 5678")),
	anthropic.NewAssistantMessage(
		anthropic.NewToolUseBlock("toolu_123", input, "calculator"),
	),
	// Just the tool_result, no additional text
	anthropic.NewUserMessage(
		anthropic.NewToolResultBlock("toolu_123", "6912", false),
	),
\}
```

```java Java
ToolUseBlockParam toolUse = ToolUseBlockParam.builder()
    .id("toolu_123")
    .name("calculator")
    .input(ToolUseBlockParam.Input.builder()
        .putAdditionalProperty("operation", JsonValue.from("add"))
        .putAdditionalProperty("a", JsonValue.from(1234))
        .putAdditionalProperty("b", JsonValue.from(5678))
        .build())
    .build();

// INCORRECT: Adding text immediately after tool_result
List&lt;MessageParam> messages = List.of(
    MessageParam.builder().role(MessageParam.Role.USER)
        .content("Calculate the sum of 1234 and 5678").build(),
    MessageParam.builder().role(MessageParam.Role.ASSISTANT)
        .contentOfBlockParams(List.of(ContentBlockParam.ofToolUse(toolUse))).build(),
    MessageParam.builder().role(MessageParam.Role.USER)
        .contentOfBlockParams(List.of(
            ContentBlockParam.ofToolResult(
                ToolResultBlockParam.builder().toolUseId("toolu_123").content("6912").build()),
            // Don't add text after tool_result
            ContentBlockParam.ofText(TextBlockParam.builder().text("Here's the result").build())
        )).build()
);

// CORRECT: Send tool results directly without additional text
messages = List.of(
    MessageParam.builder().role(MessageParam.Role.USER)
        .content("Calculate the sum of 1234 and 5678").build(),
    MessageParam.builder().role(MessageParam.Role.ASSISTANT)
        .contentOfBlockParams(List.of(ContentBlockParam.ofToolUse(toolUse))).build(),
    // Just the tool_result, no additional text
    MessageParam.builder().role(MessageParam.Role.USER)
        .contentOfBlockParams(List.of(
            ContentBlockParam.ofToolResult(
                ToolResultBlockParam.builder().toolUseId("toolu_123").content("6912").build())
        )).build()
);
```

```php PHP
// INCORRECT: Adding text immediately after tool_result
$messages = [
    ['role' => 'user', 'content' => 'Calculate the sum of 1234 and 5678'],
    [
        'role' => 'assistant',
        'content' => [
            [
                'type' => 'tool_use',
                'id' => 'toolu_123',
                'name' => 'calculator',
                'input' => ['operation' => 'add', 'a' => 1234, 'b' => 5678],
            ],
        ],
    ],
    [
        'role' => 'user',
        'content' => [
            ['type' => 'tool_result', 'tool_use_id' => 'toolu_123', 'content' => '6912'],
            // Don't add text after tool_result
            ['type' => 'text', 'text' => "Here's the result"],
        ],
    ],
];

// CORRECT: Send tool results directly without additional text
$messages = [
    ['role' => 'user', 'content' => 'Calculate the sum of 1234 and 5678'],
    [
        'role' => 'assistant',
        'content' => [
            [
                'type' => 'tool_use',
                'id' => 'toolu_123',
                'name' => 'calculator',
                'input' => ['operation' => 'add', 'a' => 1234, 'b' => 5678],
            ],
        ],
    ],
    [
        'role' => 'user',
        // Just the tool_result, no additional text
        'content' => [
            ['type' => 'tool_result', 'tool_use_id' => 'toolu_123', 'content' => '6912'],
        ],
    ],
];
```

```ruby Ruby
# INCORRECT: Adding text immediately after tool_result
messages = [
  \{ role: "user", content: "Calculate the sum of 1234 and 5678" \},
  \{
    role: "assistant",
    content: [
      \{
        type: "tool_use",
        id: "toolu_123",
        name: "calculator",
        input: \{ operation: "add", a: 1234, b: 5678 \}
      \}
    ]
  \},
  \{
    role: "user",
    content: [
      \{ type: "tool_result", tool_use_id: "toolu_123", content: "6912" \},
      # Don't add text after tool_result
      \{ type: "text", text: "Here's the result" \}
    ]
  \}
]

# CORRECT: Send tool results directly without additional text
messages = [
  \{ role: "user", content: "Calculate the sum of 1234 and 5678" \},
  \{
    role: "assistant",
    content: [
      \{
        type: "tool_use",
        id: "toolu_123",
        name: "calculator",
        input: \{ operation: "add", a: 1234, b: 5678 \}
      \}
    ]
  \},
  \{
    role: "user",
    # Just the tool_result, no additional text
    content: [
      \{ type: "tool_result", tool_use_id: "toolu_123", content: "6912" \}
    ]
  \}
]
```

If you still get empty responses after fixing the message structure, add a continuation prompt in a new user message rather than retrying with the empty response:

```python Python
def handle_empty_response(client, messages):
    response = client.messages.create(
        model="claude-opus-5", max_tokens=1024, messages=messages
    )

    # Check if response is empty
    if response.stop_reason == "end_turn" and not response.content:
        # INCORRECT: Don't just retry with the empty response
        # This won't work because Claude already decided it's done

        # CORRECT: Add a continuation prompt in a NEW user message
        messages.append(\{"role": "user", "content": "Please continue"\})

        response = client.messages.create(
            model="claude-opus-5", max_tokens=1024, messages=messages
        )

    return response
```

```typescript TypeScript
async function handleEmptyResponse(
  client: Anthropic,
  messages: Anthropic.MessageParam[]
): Promise&lt;Anthropic.Message> \{
  let response = await client.messages.create(\{
    model: "claude-opus-5",
    max_tokens: 1024,
    messages
  \});

  // Check if response is empty
  if (response.stop_reason === "end_turn" && response.content.length === 0) \{
    // INCORRECT: Don't just retry with the empty response
    // This won't work because Claude already decided it's done

    // CORRECT: Add a continuation prompt in a NEW user message
    messages.push(\{ role: "user", content: "Please continue" \});

    response = await client.messages.create(\{
      model: "claude-opus-5",
      max_tokens: 1024,
      messages
    \});
  \}

  return response;
\}
```

```csharp C#
static async Task&lt;Message> HandleEmptyResponse(AnthropicClient client, List&lt;MessageParam> messages)
\{
    var response = await client.Messages.Create(new MessageCreateParams
    \{
        Model = Model.ClaudeOpus5,
        MaxTokens = 1024,
        Messages = messages
    \});

    // Check if response is empty
    if (response.StopReason == "end_turn" && response.Content.Count == 0)
    \{
        // CORRECT: Add a continuation prompt in a NEW user message
        messages.Add(new() \{ Role = Role.User, Content = "Please continue" \});

        response = await client.Messages.Create(new MessageCreateParams
        \{
            Model = Model.ClaudeOpus5,
            MaxTokens = 1024,
            Messages = messages
        \});
    \}

    return response;
\}
```

```go Go
func handleEmptyResponse(client anthropic.Client, messages []anthropic.MessageParam) (*anthropic.Message, error) \{
	response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams\{
		Model:     anthropic.ModelClaudeOpus5,
		MaxTokens: 1024,
		Messages:  messages,
	\})
	if err != nil \{
		return nil, err
	\}

	// Check if response is empty
	if response.StopReason == "end_turn" && len(response.Content) == 0 \{
		// CORRECT: Add a continuation prompt in a NEW user message
		messages = append(messages, anthropic.NewUserMessage(anthropic.NewTextBlock("Please continue")))

		response, err = client.Messages.New(context.TODO(), anthropic.MessageNewParams\{
			Model:     anthropic.ModelClaudeOpus5,
			MaxTokens: 1024,
			Messages:  messages,
		\})
		if err != nil \{
			return nil, err
		\}
	\}

	return response, nil
\}
```

```java Java
static Message handleEmptyResponse(AnthropicClient client, List&lt;MessageParam> messages) \{
    Message response = client.messages().create(
        MessageCreateParams.builder()
            .model(Model.CLAUDE_OPUS_5)
            .maxTokens(1024L)
            .messages(messages)
            .build()
    );

    // Check if response is empty
    boolean isEndTurn = response.stopReason().map(StopReason.END_TURN::equals).orElse(false);
    if (isEndTurn && response.content().isEmpty()) \{
        // CORRECT: Add a continuation prompt in a NEW user message
        List&lt;MessageParam> extended = new ArrayList<>(messages);
        extended.add(MessageParam.builder()
            .role(MessageParam.Role.USER)
            .content("Please continue")
            .build());

        response = client.messages().create(
            MessageCreateParams.builder()
                .model(Model.CLAUDE_OPUS_5)
                .maxTokens(1024L)
                .messages(extended)
                .build()
        );
    \}

    return response;
\}
```

```php PHP
function handle_empty_response(Client $client, array $messages)
\{
    $response = $client->messages->create(
        maxTokens: 1024,
        messages: $messages,
        model: 'claude-opus-5',
    );

    // Check if response is empty
    if ($response->stopReason === 'end_turn' && count($response->content) === 0) {
        // CORRECT: Add a continuation prompt in a NEW user message
        $messages[] = ['role' => 'user', 'content' => 'Please continue'];

        $response = $client->messages->create(
            maxTokens: 1024,
            messages: $messages,
            model: 'claude-opus-5',
        );
    }

    return $response;
\}
```

```ruby Ruby
def handle_empty_response(client, messages)
  response = client.messages.create(
    model: "claude-opus-5",
    max_tokens: 1024,
    messages: messages
  )

  # Check if response is empty
  if response.stop_reason == :end_turn && response.content.empty?
    # CORRECT: Add a continuation prompt in a NEW user message
    messages << \{ role: "user", content: "Please continue" \}

    response = client.messages.create(
      model: "claude-opus-5",
      max_tokens: 1024,
      messages: messages
    )
  end

  response
end
```

Best practices:

Never add text blocks immediately after tool results: This teaches Claude to expect user input after every tool use.
Don't retry empty responses without modification: Sending the empty response back won't help.
Use continuation prompts as a last resort: Only if these fixes don't resolve the issue.

max_tokens

Claude stopped because it reached the max_tokens limit specified in your request.

bash

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5",
    "max_tokens": 10,
    "messages": [{"role": "user", "content": "Explain quantum physics"}]
  }' | jq '.stop_reason'

bash

ant messages create \
  --model claude-opus-5 \
  --max-tokens 10 \
  --message '{role: user, content: "Explain quantum physics"}' \
  --format json | jq '.stop_reason'

python

client = anthropic.Anthropic()
# Request with limited tokens
response = client.messages.create(
    model="claude-opus-5",
    max_tokens=10,
    messages=[{"role": "user", "content": "Explain quantum physics"}],
)

if response.stop_reason == "max_tokens":
    # Response was truncated
    print("Response was cut off at token limit")
    # Consider making another request to continue

typescript

const client = new Anthropic();

// Request with limited tokens
const response = await client.messages.create({
  model: "claude-opus-5",
  max_tokens: 10,
  messages: [{ role: "user", content: "Explain quantum physics" }]
});

if (response.stop_reason === "max_tokens") {
  // Response was truncated
  console.log("Response was cut off at token limit");
  // Consider making another request to continue
}

csharp

AnthropicClient client = new();

// Request with limited tokens
var response = await client.Messages.Create(new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 10,
    Messages = [new() { Role = Role.User, Content = "Explain quantum physics" }]
});

if (response.StopReason == "max_tokens")
{
    // Response was truncated
    Console.WriteLine("Response was cut off at token limit");
    // Consider making another request to continue
}

client := anthropic.NewClient()

// Request with limited tokens
response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
	Model:     anthropic.ModelClaudeOpus5,
	MaxTokens: 10,
	Messages: []anthropic.MessageParam{
		anthropic.NewUserMessage(anthropic.NewTextBlock("Explain quantum physics")),
	},
})
if err != nil {
	log.Fatal(err)
}

if response.StopReason == "max_tokens" {
	// Response was truncated
	fmt.Println("Response was cut off at token limit")
	// Consider making another request to continue
}

java

AnthropicClient client = AnthropicOkHttpClient.fromEnv();

// Request with limited tokens
Message response = client.messages().create(
    MessageCreateParams.builder()
        .model(Model.CLAUDE_OPUS_5)
        .maxTokens(10L)
        .addUserMessage("Explain quantum physics")
        .build()
);

if (response.stopReason().map(StopReason.MAX_TOKENS::equals).orElse(false)) {
    // Response was truncated
    IO.println("Response was cut off at token limit");
    // Consider making another request to continue
}

php

$client = new Client();

// Request with limited tokens
$response = $client->messages->create(
    maxTokens: 10,
    messages: [['role' => 'user', 'content' => 'Explain quantum physics']],
    model: 'claude-opus-5',
);

if ($response->stopReason === 'max_tokens') {
    // Response was truncated
    echo 'Response was cut off at token limit', PHP_EOL;
    // Consider making another request to continue
}

ruby

client = Anthropic::Client.new

# Request with limited tokens
response = client.messages.create(
  model: "claude-opus-5",
  max_tokens: 10,
  messages: [{ role: "user", content: "Explain quantum physics" }]
)

if response.stop_reason == :max_tokens
  # Response was truncated
  puts "Response was cut off at token limit"
  # Consider making another request to continue
end

If Claude's response is cut off because it hit the max_tokens limit, and the truncated response contains an incomplete tool use block, you'll need to retry the request with a higher max_tokens value to get the full tool use.

```bash CLI
RESPONSE=$(ant messages create --max-tokens 1024 --format jsonl < request.yaml)

# Check if the response was truncated mid tool use
STOP_REASON=$(jq -r '.stop_reason' <<<"$RESPONSE")
LAST_TYPE=$(jq -r '.content[-1].type' <<<"$RESPONSE")
if [ "$STOP_REASON" = "max_tokens" ] && [ "$LAST_TYPE" = "tool_use" ]; then
  # Retry with a higher max_tokens
  ant messages create --max-tokens 4096 < request.yaml
fi
```

```python Python
# Check if response was truncated during tool use
if response.stop_reason == "max_tokens":
    # Check if the last content block is an incomplete tool_use
    last_block = response.content[-1]
    if last_block.type == "tool_use":
        # Send the request with higher max_tokens
        response = client.messages.create(
            model="claude-opus-5",
            max_tokens=4096,  # Increased limit
            messages=messages,
            tools=tools,
        )
```

```typescript TypeScript
// Check if response was truncated during tool use
if (response.stop_reason === "max_tokens") {
  // Check if the last content block is an incomplete tool_use
  const lastBlock = response.content[response.content.length - 1];
  if (lastBlock.type === "tool_use") {
    // Send the request with higher max_tokens
    response = await client.messages.create({
      model: "claude-opus-5",
      max_tokens: 4096, // Increased limit
      messages: messages,
      tools: tools
    });
  }
}
```

```csharp C#
using System.Linq;
using Anthropic;
using Anthropic.Models.Messages;

AnthropicClient client = new();

var parameters = new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 1024,
    Messages = messages,
    Tools = tools
};

var response = await client.Messages.Create(parameters);

if (response.StopReason == "max_tokens")
{
    var lastBlock = response.Content.Last();
    if (lastBlock.TryPickToolUse(out _))
    {
        response = await client.Messages.Create(parameters with { MaxTokens = 4096 });
    }
}
```

```go Go
response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
	Model:     anthropic.ModelClaudeOpus5,
	MaxTokens: 1024,
	Messages:  messages,
	Tools:     tools,
})
if err != nil {
	log.Fatal(err)
}

if response.StopReason == "max_tokens" {
	lastBlock := response.Content[len(response.Content)-1]
	switch lastBlock.AsAny().(type) {
	case anthropic.ToolUseBlock:
		response, err = client.Messages.New(context.TODO(), anthropic.MessageNewParams{
			Model:     anthropic.ModelClaudeOpus5,
			MaxTokens: 4096,
			Messages:  messages,
			Tools:     tools,
		})
		if err != nil {
			log.Fatal(err)
		}
	}
}
```

```java Java
// Check if response was truncated during tool use
if (response.stopReason().isPresent() && response.stopReason().get().equals(StopReason.MAX_TOKENS)) {
    ContentBlock lastBlock = response.content().get(response.content().size() - 1);
    if (lastBlock.toolUse().isPresent()) {
        // Send the request with higher max_tokens
        response = client.messages().create(
            MessageCreateParams.builder()
                .model(Model.CLAUDE_OPUS_5)
                .maxTokens(4096L) // Increased limit
                .messages(messages)
                .tools(tools)
                .build()
        );
    }
}
```

```php PHP
$response = $client->messages->create(
    maxTokens: 1024,
    messages: $messages,
    model: 'claude-opus-5',
    tools: $tools,
);

if ($response->stopReason === 'max_tokens') \{
    $lastBlock = end($response->content);
    if ($lastBlock->type === 'tool_use') {
        $response = $client->messages->create(
            maxTokens: 4096,
            messages: $messages,
            model: 'claude-opus-5',
            tools: $tools,
        );
    \}
\}
```

```ruby Ruby
response = client.messages.create(
  model: "claude-opus-5",
  max_tokens: 1024,
  messages: messages,
  tools: tools
)

if response.stop_reason == :max_tokens
  last_block = response.content.last
  if last_block.type == :tool_use
    response = client.messages.create(
      model: "claude-opus-5",
      max_tokens: 4096,
      messages: messages,
      tools: tools
    )
  end
end
```

stop_sequence

Claude encountered one of your custom stop sequences.

bash

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5",
    "max_tokens": 1024,
    "stop_sequences": ["END", "STOP"],
    "messages": [{"role": "user", "content": "Generate text until you say END"}]
  }' | jq '{stop_reason, stop_sequence}'

bash

ant messages create \
  --model claude-opus-5 \
  --max-tokens 1024 \
  --stop-sequence END --stop-sequence STOP \
  --message '{role: user, content: "Generate text until you say END"}' \
  --format json | jq '{stop_reason, stop_sequence}'

python

client = anthropic.Anthropic()
response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    stop_sequences=["END", "STOP"],
    messages=[{"role": "user", "content": "Generate text until you say END"}],
)

if response.stop_reason == "stop_sequence":
    print(f"Stopped at sequence: {response.stop_sequence}")

typescript

const client = new Anthropic();

const response = await client.messages.create({
  model: "claude-opus-5",
  max_tokens: 1024,
  stop_sequences: ["END", "STOP"],
  messages: [{ role: "user", content: "Generate text until you say END" }]
});

if (response.stop_reason === "stop_sequence") {
  console.log(`Stopped at sequence: ${response.stop_sequence}`);
}

csharp

AnthropicClient client = new();

var response = await client.Messages.Create(new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 1024,
    StopSequences = ["END", "STOP"],
    Messages = [new() { Role = Role.User, Content = "Generate text until you say END" }]
});

if (response.StopReason == "stop_sequence")
{
    Console.WriteLine($"Stopped at sequence: {response.StopSequence}");
}

client := anthropic.NewClient()

response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
	Model:         anthropic.ModelClaudeOpus5,
	MaxTokens:     1024,
	StopSequences: []string{"END", "STOP"},
	Messages: []anthropic.MessageParam{
		anthropic.NewUserMessage(anthropic.NewTextBlock("Generate text until you say END")),
	},
})
if err != nil {
	log.Fatal(err)
}

if response.StopReason == "stop_sequence" {
	fmt.Printf("Stopped at sequence: %s\n", response.StopSequence)
}

java

AnthropicClient client = AnthropicOkHttpClient.fromEnv();

Message response = client.messages().create(
    MessageCreateParams.builder()
        .model(Model.CLAUDE_OPUS_5)
        .maxTokens(1024L)
        .addStopSequence("END")
        .addStopSequence("STOP")
        .addUserMessage("Generate text until you say END")
        .build()
);

if (response.stopReason().map(StopReason.STOP_SEQUENCE::equals).orElse(false)) {
    IO.println("Stopped at sequence: " + response.stopSequence().orElse(""));
}

php

$client = new Client();

$response = $client->messages->create(
    maxTokens: 1024,
    messages: [['role' => 'user', 'content' => 'Generate text until you say END']],
    model: 'claude-opus-5',
    stopSequences: ['END', 'STOP'],
);

if ($response->stopReason === 'stop_sequence') {
    echo "Stopped at sequence: {$response->stopSequence}", PHP_EOL;
}

ruby

client = Anthropic::Client.new

response = client.messages.create(
  model: "claude-opus-5",
  max_tokens: 1024,
  stop_sequences: ["END", "STOP"],
  messages: [{ role: "user", content: "Generate text until you say END" }]
)

if response.stop_reason == :stop_sequence
  puts "Stopped at sequence: #{response.stop_sequence}"
end

tool_use

Claude is calling a tool and expects you to run it.

For most tool use implementations, use the tool runner, which automatically handles tool execution, result formatting, and conversation management.

bash

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5",
    "max_tokens": 1024,
    "tools": [{
      "name": "get_weather",
      "description": "Get the current weather in a given location",
      "input_schema": {
        "type": "object",
        "properties": {"location": {"type": "string", "description": "City and state"}},
        "required": ["location"]
      }
    }],
    "messages": [{"role": "user", "content": "What is the weather in San Francisco?"}]
  }' | jq '.stop_reason, (.content[] | select(.type == "tool_use"))'

bash

ant messages create --format json <<'YAML' | jq '.stop_reason, (.content[] | select(.type == "tool_use"))'
model: claude-opus-5
max_tokens: 1024
messages:
  - role: user
    content: What is the weather in San Francisco?
tools:
  - name: get_weather
    description: Get the current weather in a given location
    input_schema:
      type: object
      properties:
        location: {type: string, description: City and state}
      required: [location]
YAML

python

client = anthropic.Anthropic()
weather_tool = {
    "name": "get_weather",
    "description": "Get the current weather in a given location",
    "input_schema": {
        "type": "object",
        "properties": {
            "location": {"type": "string", "description": "City and state"},
        },
        "required": ["location"],
    },
}

def execute_tool(name, tool_input):
    """Execute a tool and return the result."""
    return f"Weather in {tool_input.get('location', 'unknown')}: 72°F"

response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    tools=[weather_tool],
    messages=[{"role": "user", "content": "What is the weather in San Francisco?"}],
)

if response.stop_reason == "tool_use":
    # Extract and execute the tool
    for block in response.content:
        if block.type == "tool_use":
            result = execute_tool(block.name, block.input)
            # Return result to Claude for final response

typescript

const client = new Anthropic();
const weatherTool: Anthropic.Tool = {
  name: "get_weather",
  description: "Get the current weather in a given location",
  input_schema: {
    type: "object",
    properties: {
      location: { type: "string", description: "City and state" }
    },
    required: ["location"]
  }
};

function executeTool(name: string, input: Record<string, string>): string {
  return `Weather in ${input.location ?? "unknown"}: 72°F`;
}

const response = await client.messages.create({
  model: "claude-opus-5",
  max_tokens: 1024,
  tools: [weatherTool],
  messages: [{ role: "user", content: "What is the weather in San Francisco?" }]
});

if (response.stop_reason === "tool_use") {
  // Extract and execute the tool
  for (const block of response.content) {
    if (block.type === "tool_use") {
      const result = executeTool(block.name, block.input as Record<string, string>);
      // Return result to Claude for final response
    }
  }
}

csharp

AnthropicClient client = new();

var weatherTool = new Tool
{
    Name = "get_weather",
    Description = "Get the current weather in a given location",
    InputSchema = new InputSchema
    {
        Properties = new Dictionary<string, JsonElement>
        {
            ["location"] = JsonSerializer.SerializeToElement(
                new { type = "string", description = "City and state" }
            ),
        },
        Required = ["location"]
    }
};

var response = await client.Messages.Create(new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 1024,
    Tools = [weatherTool],
    Messages = [new() { Role = Role.User, Content = "What is the weather in San Francisco?" }]
});

if (response.StopReason == "tool_use")
{
    // Extract and execute the tool
    foreach (var block in response.Content)
    {
        if (block.TryPickToolUse(out var toolUse))
        {
            // Execute toolUse.Name with toolUse.Input and return the result to Claude
        }
    }
}

client := anthropic.NewClient()

weatherTool := anthropic.ToolParam{
	Name:        "get_weather",
	Description: anthropic.String("Get the current weather in a given location"),
	InputSchema: anthropic.ToolInputSchemaParam{
		Properties: map[string]any{
			"location": map[string]string{"type": "string", "description": "City and state"},
		},
		Required: []string{"location"},
	},
}

response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
	Model:     anthropic.ModelClaudeOpus5,
	MaxTokens: 1024,
	Tools:     []anthropic.ToolUnionParam{{OfTool: &weatherTool}},
	Messages: []anthropic.MessageParam{
		anthropic.NewUserMessage(anthropic.NewTextBlock("What is the weather in San Francisco?")),
	},
})
if err != nil {
	log.Fatal(err)
}

if response.StopReason == "tool_use" {
	// Extract and execute the tool
	for _, block := range response.Content {
		if toolUse, ok := block.AsAny().(anthropic.ToolUseBlock); ok {
			fmt.Println(toolUse.Name, toolUse.Input)
			// Return result to Claude for final response
		}
	}
}

java

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

    Tool weatherTool = Tool.builder()
        .name("get_weather")
        .description("Get the current weather in a given location")
        .inputSchema(Tool.InputSchema.builder()
            .properties(JsonValue.from(Map.of(
                "location", Map.of("type", "string", "description", "City and state")
            )))
            .putAdditionalProperty("required", JsonValue.from(List.of("location")))
            .build())
        .build();

    Message response = client.messages().create(
        MessageCreateParams.builder()
            .model(Model.CLAUDE_OPUS_5)
            .maxTokens(1024L)
            .addTool(weatherTool)
            .addUserMessage("What is the weather in San Francisco?")
            .build()
    );

    if (response.stopReason().map(StopReason.TOOL_USE::equals).orElse(false)) {
        // Extract and execute the tool
        for (ContentBlock block : response.content()) {
            block.toolUse().ifPresent(toolUse -> {
                // Execute toolUse.name() with toolUse.input() and return the result to Claude
            });
        }
    }

php

$client = new Client();

$weatherTool = [
    'name' => 'get_weather',
    'description' => 'Get the current weather in a given location',
    'input_schema' => [
        'type' => 'object',
        'properties' => [
            'location' => ['type' => 'string', 'description' => 'City and state'],
        ],
        'required' => ['location'],
    ],
];

$response = $client->messages->create(
    maxTokens: 1024,
    messages: [['role' => 'user', 'content' => 'What is the weather in San Francisco?']],
    model: 'claude-opus-5',
    tools: [$weatherTool],
);

if ($response->stopReason === 'tool_use') {
    // Extract and execute the tool
    foreach ($response->content as $block) {
        if ($block->type === 'tool_use') {
            // Execute $block->name with $block->input and return the result to Claude
        }
    }
}

ruby

client = Anthropic::Client.new

weather_tool = {
  name: "get_weather",
  description: "Get the current weather in a given location",
  input_schema: {
    type: "object",
    properties: {
      location: { type: "string", description: "City and state" }
    },
    required: ["location"]
  }
}

response = client.messages.create(
  model: "claude-opus-5",
  max_tokens: 1024,
  tools: [weather_tool],
  messages: [{ role: "user", content: "What is the weather in San Francisco?" }]
)

if response.stop_reason == :tool_use
  # Extract and execute the tool
  response.content.each do |block|
    next unless block.type == :tool_use
    # Execute block.name with block.input and return the result to Claude
  end
end

A tool_use response can also contain a server_tool_use block whose id has no matching result block. That server tool call is not finished, and this response does not carry its result. In the common case, Claude calls a server tool and one of your client tools in the same group of parallel tool calls: the API returns without running the server tool so that you can run the client tools first. There is no other marker for the state; detect it by checking each server_tool_use or mcp_tool_use block's id for a matching result block.

With programmatic tool calling, the same response shape means something different. The client tool_use block comes from code that is running in the code_execution tool rather than from Claude directly, and its caller field names the code_execution block that called it. That code has already started: it is paused waiting for your tool_result blocks, and sending them resumes the execution instead of starting a deferred tool. The code_execution block's own result block arrives once the code finishes, which can take more than one round of tool results. The follow-up user message itself is the same in both cases; with programmatic tool calling, also pass back the id from the response's container field, as that page shows.

json

{
  "stop_reason": "tool_use",
  "content": [
    {
      "type": "server_tool_use",
      "id": "srvtoolu_01HxbWnMRmbWyMfUtJKC45rA",
      "name": "web_search",
      "input": { "query": "example article" }
    },
    {
      "type": "tool_use",
      "id": "toolu_01PjgRJLbXrXEMZwDNYLnBqk",
      "name": "run_command",
      "input": { "command": "uname -a" }
    }
  ]
}

The continuation is a user message of tool_result blocks, one for every tool_use block in the response (see Handle tool calls), with two extra rules: that message must contain nothing except the tool_result blocks, and the request must keep the same tools array. A resume request that no longer defines the waiting server tool fails with a 400 whose message ends but no `web_search` tool was provided. The API attaches your results to the still-open assistant turn, runs the deferred server tool (for paused code execution, resumes it), and continues the turn. For a server tool Claude called directly, the next response's content starts with the result block that answers the previous response's server_tool_use id.

json

{
  "role": "user",
  "content": [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_01PjgRJLbXrXEMZwDNYLnBqk",
      "content": "Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux"
    }
  ]
}

Adding anything after the tool_result blocks in that user message, such as text, ends the assistant turn; for a server tool Claude called directly, the request then fails with a 400 invalid_request_error that names the unresolved server tool:

text

`web_search` tool use with id `srvtoolu_01HxbWnMRmbWyMfUtJKC45rA` was found without a corresponding `web_search_tool_result` block

Leaving out a tool_result, or putting one after other content, fails earlier with the standard tool_use ids were found without tool_result blocks immediately after error instead. To give Claude more input, send it as a separate user message after the turn completes.

pause_turn

Returned when the server-side sampling loop reaches its iteration limit while executing server tools such as web search. The default limit is 10 iterations per request.

When this happens, the response may contain a server_tool_use block without a corresponding result block. To let Claude finish processing, continue the conversation by sending the response back as-is. A response that leaves a client tool_use block waiting on you never has a stop_reason of pause_turn: when Claude stops to call your tools, stop_reason is tool_use, and you continue it by sending the client tool_result blocks instead of the response itself.

bash

# The SDKs handle continuation directly. With cURL, inspect stop_reason
# on the response and re-POST with the assistant content appended.
curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5",
    "max_tokens": 4096,
    "tools": [{"type": "web_search_20250305", "name": "web_search"}],
    "messages": [{"role": "user", "content": "Search for latest AI news"}]
  }' | jq '{stop_reason, content}'

bash

# Inspect stop_reason; if it is pause_turn, re-run with the assistant
# response appended to --message.
ant messages create --format json <<'YAML' | jq '{stop_reason, content}'
model: claude-opus-5
max_tokens: 4096
tools:
  - {type: web_search_20250305, name: web_search}
messages:
  - {role: user, content: "Search for latest AI news"}
YAML

python

response = client.messages.create(
    model="claude-opus-5",
    max_tokens=4096,
    tools=[{"type": "web_search_20250305", "name": "web_search"}],
    messages=[{"role": "user", "content": "Search for latest AI news"}],
)

if response.stop_reason == "pause_turn":
    # Continue the conversation by sending the response back
    messages = [
        {"role": "user", "content": "Search for latest AI news"},
        {"role": "assistant", "content": response.content},
    ]
    continuation = client.messages.create(
        model="claude-opus-5",
        max_tokens=4096,
        messages=messages,
        tools=[{"type": "web_search_20250305", "name": "web_search"}],
    )

typescript

const response = await client.messages.create({
  model: "claude-opus-5",
  max_tokens: 4096,
  tools: [{ type: "web_search_20250305", name: "web_search" }],
  messages: [{ role: "user", content: "Search for latest AI news" }]
});

if (response.stop_reason === "pause_turn") {
  // Continue the conversation by sending the response back
  const continuation = await client.messages.create({
    model: "claude-opus-5",
    max_tokens: 4096,
    tools: [{ type: "web_search_20250305", name: "web_search" }],
    messages: [
      { role: "user", content: "Search for latest AI news" },
      { role: "assistant", content: response.content }
    ]
  });
}

csharp

List<ToolUnion> tools = [new ToolUnion(new WebSearchTool20250305())];
MessageParam userMessage = new() { Role = Role.User, Content = "Search for latest AI news" };

var response = await client.Messages.Create(new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 4096,
    Tools = tools,
    Messages = [userMessage]
});

if (response.StopReason == "pause_turn")
{
    // Continue the conversation by sending the response back
    var continuation = await client.Messages.Create(new MessageCreateParams
    {
        Model = Model.ClaudeOpus5,
        MaxTokens = 4096,
        Tools = tools,
        Messages =
        [
            userMessage,
            new()
            {
                Role = Role.Assistant,
                Content = response.Content.Select(block => new ContentBlockParam(block.Json)).ToList()
            }
        ]
    });
}

tools := []anthropic.ToolUnionParam{
	{OfWebSearchTool20250305: &anthropic.WebSearchTool20250305Param{}},
}
userMessage := anthropic.NewUserMessage(anthropic.NewTextBlock("Search for latest AI news"))

response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
	Model:     anthropic.ModelClaudeOpus5,
	MaxTokens: 4096,
	Tools:     tools,
	Messages:  []anthropic.MessageParam{userMessage},
})
if err != nil {
	log.Fatal(err)
}

if response.StopReason == "pause_turn" {
	// Continue the conversation by sending the response back
	var contentParams []anthropic.ContentBlockParamUnion
	for _, block := range response.Content {
		contentParams = append(contentParams, block.ToParam())
	}
	continuation, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
		Model:     anthropic.ModelClaudeOpus5,
		MaxTokens: 4096,
		Tools:     tools,
		Messages:  []anthropic.MessageParam{userMessage, anthropic.NewAssistantMessage(contentParams...)},
	})
	if err != nil {
		log.Fatal(err)
	}
	_ = continuation
}

java

Message response = client.messages().create(
    MessageCreateParams.builder()
        .model(Model.CLAUDE_OPUS_5)
        .maxTokens(4096L)
        .addTool(WebSearchTool20250305.builder().build())
        .addUserMessage("Search for latest AI news")
        .build()
);

if (response.stopReason().map(StopReason.PAUSE_TURN::equals).orElse(false)) {
    // Continue the conversation by sending the response back
    Message continuation = client.messages().create(
        MessageCreateParams.builder()
            .model(Model.CLAUDE_OPUS_5)
            .maxTokens(4096L)
            .addTool(WebSearchTool20250305.builder().build())
            .addUserMessage("Search for latest AI news")
            .addMessage(response)
            .build()
    );
}

php

$tools = [['type' => 'web_search_20250305', 'name' => 'web_search']];
$userMessage = ['role' => 'user', 'content' => 'Search for latest AI news'];

$response = $client->messages->create(
    maxTokens: 4096,
    messages: [$userMessage],
    model: 'claude-opus-5',
    tools: $tools,
);

if ($response->stopReason === 'pause_turn') {
    // Continue the conversation by sending the response back
    $continuation = $client->messages->create(
        maxTokens: 4096,
        messages: [
            $userMessage,
            ['role' => 'assistant', 'content' => $response->content],
        ],
        model: 'claude-opus-5',
        tools: $tools,
    );
}

ruby

tools = [{ type: "web_search_20250305", name: "web_search" }]
user_message = { role: "user", content: "Search for latest AI news" }

response = client.messages.create(
  model: "claude-opus-5",
  max_tokens: 4096,
  tools: tools,
  messages: [user_message]
)

if response.stop_reason == :pause_turn
  # Continue the conversation by sending the response back
  continuation = client.messages.create(
    model: "claude-opus-5",
    max_tokens: 4096,
    tools: tools,
    messages: [user_message, { role: "assistant", content: response.content }]
  )
end

Your application should handle pause_turn in any agent loop that uses server tools. Add the assistant's response to your messages array and make another API request to let Claude continue.

refusal

Claude declined to generate a response. Safety classifiers return this stop reason as a normal HTTP 200 response, not an error.

bash

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5",
    "max_tokens": 1024,
    "messages": [{"role": "user", "content": "[Unsafe request]"}]
  }' | jq '{stop_reason, stop_details}'

bash

ant messages create \
  --model claude-opus-5 \
  --max-tokens 1024 \
  --message '{role: user, content: "[Unsafe request]"}' \
  --format json | jq '{stop_reason, stop_details}'

python

client = anthropic.Anthropic()
response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "[Unsafe request]"}],
)

if response.stop_reason == "refusal":
    # Claude declined to respond
    print("Claude was unable to process this request")
    # Consider rephrasing or modifying the request

typescript

const client = new Anthropic();

const response = await client.messages.create({
  model: "claude-opus-5",
  max_tokens: 1024,
  messages: [{ role: "user", content: "[Unsafe request]" }]
});

if (response.stop_reason === "refusal") {
  // Claude declined to respond
  console.log("Claude was unable to process this request");
  // Consider rephrasing or modifying the request
}

csharp

AnthropicClient client = new();

var response = await client.Messages.Create(new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 1024,
    Messages = [new() { Role = Role.User, Content = "[Unsafe request]" }]
});

if (response.StopReason == "refusal")
{
    // Claude declined to respond
    Console.WriteLine("Claude was unable to process this request");
    // Consider rephrasing or modifying the request
}

client := anthropic.NewClient()

response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
	Model:     anthropic.ModelClaudeOpus5,
	MaxTokens: 1024,
	Messages: []anthropic.MessageParam{
		anthropic.NewUserMessage(anthropic.NewTextBlock("[Unsafe request]")),
	},
})
if err != nil {
	log.Fatal(err)
}

if response.StopReason == "refusal" {
	// Claude declined to respond
	fmt.Println("Claude was unable to process this request")
	// Consider rephrasing or modifying the request
}

java

AnthropicClient client = AnthropicOkHttpClient.fromEnv();

Message response = client.messages().create(
    MessageCreateParams.builder()
        .model(Model.CLAUDE_OPUS_5)
        .maxTokens(1024L)
        .addUserMessage("[Unsafe request]")
        .build()
);

if (response.stopReason().map(StopReason.REFUSAL::equals).orElse(false)) {
    // Claude declined to respond
    IO.println("Claude was unable to process this request");
    // Consider rephrasing or modifying the request
}

php

$client = new Client();

$response = $client->messages->create(
    maxTokens: 1024,
    messages: [['role' => 'user', 'content' => '[Unsafe request]']],
    model: 'claude-opus-5',
);

if ($response->stopReason === 'refusal') {
    // Claude declined to respond
    echo 'Claude was unable to process this request', PHP_EOL;
    // Consider rephrasing or modifying the request
}

ruby

client = Anthropic::Client.new

response = client.messages.create(
  model: "claude-opus-5",
  max_tokens: 1024,
  messages: [{ role: "user", content: "[Unsafe request]" }]
)

if response.stop_reason == :refusal
  # Claude declined to respond
  puts "Claude was unable to process this request"
  # Consider rephrasing or modifying the request
end

If you encounter refusal stop reasons frequently while using Claude Sonnet 4.5 or Claude Opus 4.1 (the latter retired, except on Bedrock and Google Cloud), you can try updating your API calls to use Haiku 4.5 (claude-haiku-4-5-20251001), which has different usage restrictions. Learn more about understanding Sonnet 4.5's API safety filters.

On a refusal, the stop_details object identifies the policy category that triggered it. The categories and the full refusal response shape are covered on Refusals and fallback. stop_details is null for all stop reasons other than refusal.

A refused request on Claude Fable 5.1, Claude Fable 5, or Claude Opus 5 can usually be served by retrying on another Claude model. Refusals and fallback shows how to set up that retry, server-side or in your client. If you build the retry yourself from Claude Fable 5.1, Claude Fable 5, or Claude Opus 5, fallback credit covers how to avoid paying the prompt-cache cost twice.

model_context_window_exceeded

Claude stopped because it reached the model's context window limit. This lets you request the maximum possible tokens without knowing the exact input size.

This stop reason is currently typed only in the SDKs' beta namespace, so the following examples call client.beta.messages and use the Beta-prefixed types. On Sonnet 4.5 and newer models the API returns this value without a beta header. For earlier models, add the model-context-window-exceeded-2025-08-26 beta header to enable it.

bash

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5",
    "max_tokens": 20000,
    "messages": [{"role": "user", "content": "Large input that uses most of context window..."}]
  }' | jq '.stop_reason'

bash

ant messages create \
  --model claude-opus-5 \
  --max-tokens 20000 \
  --message '{role: user, content: "Large input that uses most of context window..."}' \
  --format json | jq '.stop_reason'

python

# Request with maximum tokens to get as much as possible
response = client.beta.messages.create(
    model="claude-opus-5",
    max_tokens=20000,  # Python SDK requires streaming for max_tokens above ~21k
    messages=[
        {"role": "user", "content": "Large input that uses most of context window..."}
    ],
)

if response.stop_reason == "model_context_window_exceeded":
    # Response hit context window limit before max_tokens
    print("Response reached model's context window limit")
    # The response is still valid but was limited by context window

typescript

// Request with maximum tokens to get as much as possible
const response = await client.beta.messages.create({
  model: "claude-opus-5",
  max_tokens: 20000,
  messages: [{ role: "user", content: "Large input that uses most of context window..." }]
});

if (response.stop_reason === "model_context_window_exceeded") {
  // Response hit context window limit before max_tokens
  console.log("Response reached model's context window limit");
  // The response is still valid but was limited by context window
}

csharp

using Anthropic.Models.Beta.Messages;
using Model = Anthropic.Models.Messages.Model;

// Request with maximum tokens to get as much as possible
var response = await client.Beta.Messages.Create(new MessageCreateParams
{
    Model = Model.ClaudeOpus5,
    MaxTokens = 20000,
    Messages = [new() { Role = Role.User, Content = "Large input that uses most of context window..." }]
});

if (response.StopReason?.Value() == BetaStopReason.ModelContextWindowExceeded)
{
    // Response hit context window limit before max_tokens
    Console.WriteLine("Response reached model's context window limit");
    // The response is still valid but was limited by context window
}

// Request with maximum tokens to get as much as possible
response, err := client.Beta.Messages.New(context.TODO(), anthropic.BetaMessageNewParams{
	Model:     anthropic.ModelClaudeOpus5,
	MaxTokens: 20000,
	Messages: []anthropic.BetaMessageParam{
		anthropic.NewBetaUserMessage(anthropic.NewBetaTextBlock("Large input that uses most of context window...")),
	},
})
if err != nil {
	log.Fatal(err)
}

if response.StopReason == anthropic.BetaStopReasonModelContextWindowExceeded {
	// Response hit context window limit before max_tokens
	fmt.Println("Response reached model's context window limit")
	// The response is still valid but was limited by context window
}

java

import com.anthropic.models.beta.messages.BetaMessage;
import com.anthropic.models.beta.messages.BetaStopReason;
import com.anthropic.models.beta.messages.MessageCreateParams;

// Request with maximum tokens to get as much as possible
BetaMessage response = client.beta().messages().create(
    MessageCreateParams.builder()
        .model(Model.CLAUDE_OPUS_5)
        .maxTokens(20000L)
        .addUserMessage("Large input that uses most of context window...")
        .build()
);

if (response.stopReason().map(BetaStopReason.MODEL_CONTEXT_WINDOW_EXCEEDED::equals).orElse(false)) {
    // Response hit context window limit before max_tokens
    IO.println("Response reached model's context window limit");
    // The response is still valid but was limited by context window
}

php

// Request with maximum tokens to get as much as possible
$response = $client->beta->messages->create(
    maxTokens: 20000,
    messages: [['role' => 'user', 'content' => 'Large input that uses most of context window...']],
    model: 'claude-opus-5',
);

if ($response->stopReason === 'model_context_window_exceeded') {
    // Response hit context window limit before max_tokens
    echo 'Response reached model\'s context window limit', PHP_EOL;
    // The response is still valid but was limited by context window
}

ruby

# Request with maximum tokens to get as much as possible
response = client.beta.messages.create(
  model: "claude-opus-5",
  max_tokens: 20000,
  messages: [{ role: "user", content: "Large input that uses most of context window..." }]
)

if response.stop_reason == :model_context_window_exceeded
  # Response hit context window limit before max_tokens
  puts "Response reached model's context window limit"
  # The response is still valid but was limited by context window
end

messages

batches

organization

federation

sessions

events

threads

vaults

activities

batches

Stop reason values ​

end_turn ​

max_tokens ​

stop_sequence ​

tool_use ​

pause_turn ​

refusal ​

model_context_window_exceeded ​

Stop reason values

end_turn

max_tokens

stop_sequence

tool_use

pause_turn

refusal

model_context_window_exceeded