server-tools changed

The first capture of this source. The page was already there, and this is what it said.

---
title: Server tools
url: https://platform.claude.com/docs/en/agents-and-tools/tool-use/server-tools
description: "Work with Anthropic-executed tools: server_tool_use blocks, pause_turn continuation, mixed server and client tool turns, and domain filtering."
---

Server-executed tools share these mechanics: the `server_tool_use` block, `pause_turn` continuation, turns that mix server and client tools, Zero Data Retention (ZDR) eligibility, and domain filtering. For individual tools, see the [tool reference](https://platform.claude.com/docs/en/agents-and-tools/tool-use/tool-reference).

## The server\_tool\_use block

The `server_tool_use` block appears in Claude's response when a server-executed tool runs. Its `id` field uses the `srvtoolu_` prefix to distinguish it from client tool calls:

```json
{
  "type": "server_tool_use",
  "id": "srvtoolu_01A2B3C4D5E6F7G8H9",
  "name": "web_search",
  "input": { "query": "latest quantum computing breakthroughs" }
}
```

The API executes the tool internally. You see the call and its result in the response, but you don't handle execution. Unlike client `tool_use` blocks, you don't need to respond with a `tool_result`. The tool's result block (for example, `web_search_tool_result` for web search) follows the `server_tool_use` block in the same assistant turn, paired by `tool_use_id`. If Claude calls one of your client tools at the same time, the `server_tool_use` block appears without its result, and the response ends with `stop_reason: "tool_use"`. The API runs the tool when you return the client `tool_result` blocks in your next request.

## The server-side loop and pause\_turn

When using server tools such as web search, the API executes tool calls in a server-side agentic loop. On a long-running turn, the API might pause that loop and return a `pause_turn` stop reason.

Here's how to handle the `pause_turn` stop reason:

<CodeGroup>
  ```bash cURL
  # Initial request. If "stop_reason" in the response is "pause_turn", continue
  # the turn by re-sending the request with the assistant content appended to messages.
  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": "Search for comprehensive information about quantum computing breakthroughs in 2025"
        }
      ],
      "tools": [{"type": "web_search_20250305", "name": "web_search", "max_uses": 10}]
    }' | jq '{stop_reason, content}'
  ```

  ```bash CLI
  # Initial request. If "stop_reason" in the output is "pause_turn", re-run with
  # the assistant content appended to messages (see the SDK tabs).
  ant messages create --format json <<'YAML' | jq '{stop_reason, content}'
  model: claude-opus-5
  max_tokens: 1024
  tools:
    - {type: web_search_20250305, name: web_search, max_uses: 10}
  messages:
    - {role: user, content: "Search for comprehensive information about quantum computing breakthroughs in 2025"}
  YAML
  ```

  ```python Python
  client = anthropic.Anthropic()

  # Initial request with web search
  response = client.messages.create(
      model="claude-opus-5",
      max_tokens=1024,
      messages=[
          {
              "role": "user",
              "content": "Search for comprehensive information about quantum computing breakthroughs in 2025",
          }
      ],
      tools=[{"type": "web_search_20250305", "name": "web_search", "max_uses": 10}],
  )

  # Check if the response has pause_turn stop reason
  if response.stop_reason == "pause_turn":
      # Continue the conversation with the paused content
      messages = [
          {
              "role": "user",
              "content": "Search for comprehensive information about quantum computing breakthroughs in 2025",
          },
          {"role": "assistant", "content": response.content},
      ]

      # Send the continuation request
      continuation = client.messages.create(
          model="claude-opus-5",
          max_tokens=1024,
          messages=messages,
          tools=[{"type": "web_search_20250305", "name": "web_search", "max_uses": 10}],
      )

      print(continuation)
  else:
      print(response)
  ```

  ```typescript TypeScript
  const client = new Anthropic();

  // Initial request with web search
  const response = await client.messages.create({
    model: "claude-opus-5",
    max_tokens: 1024,
    messages: [
      {
        role: "user",
        content:
          "Search for comprehensive information about quantum computing breakthroughs in 2025"
      }
    ],
    tools: [
      {
        type: "web_search_20250305",
        name: "web_search",
        max_uses: 10
      }
    ]
  });

  // Check if the response has pause_turn stop reason
  if (response.stop_reason === "pause_turn") {
    // Continue the conversation with the paused content
    const messages: Anthropic.MessageParam[] = [
      {
        role: "user",
        content:
          "Search for comprehensive information about quantum computing breakthroughs in 2025"
      },
      { role: "assistant", content: response.content }
    ];

    // Send the continuation request
    const continuation = await client.messages.create({
      model: "claude-opus-5",
      max_tokens: 1024,
      messages,
      tools: [
        {
          type: "web_search_20250305",
          name: "web_search",
          max_uses: 10
        }
      ]
    });

    console.log(continuation);
  } else {
    console.log(response);
  }
  ```

  ```csharp C#
  AnthropicClient client = new();

  var parameters = new MessageCreateParams
  {
      Model = Model.ClaudeOpus5,
      MaxTokens = 1024,
      Messages = [
          new() {
              Role = Role.User,
              Content = "Search for comprehensive information about quantum computing breakthroughs in 2025"
          }
      ],
      Tools = [new ToolUnion(new WebSearchTool20250305 { MaxUses = 10 })]
  };

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

  if (response.StopReason?.Value() == StopReason.PauseTurn)
  {
      // Continue the conversation with the paused content
      var continuationParams = new MessageCreateParams
      {
          Model = Model.ClaudeOpus5,
          MaxTokens = 1024,
          Messages = [
              new() {
                  Role = Role.User,
                  Content = "Search for comprehensive information about quantum computing breakthroughs in 2025"
              },
              new() {
                  Role = Role.Assistant,
                  Content = response.Content.Select(block => new ContentBlockParam(block.Json)).ToList()
              }
          ],
          Tools = [new ToolUnion(new WebSearchTool20250305 { MaxUses = 10 })]
      };

      var continuation = await client.Messages.Create(continuationParams);
      Console.WriteLine(continuation);
  }
  else
  {
      Console.WriteLine(response);
  }
  ```

  ```go Go
  client := anthropic.NewClient()

  webSearchTool := []anthropic.ToolUnionParam{
  	{OfWebSearchTool20250305: &anthropic.WebSearchTool20250305Param{
  		MaxUses: anthropic.Int(10),
  	}},
  }

  response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
  	Model:     anthropic.ModelClaudeOpus5,
  	MaxTokens: 1024,
  	Messages: []anthropic.MessageParam{
  		anthropic.NewUserMessage(anthropic.NewTextBlock("Search for comprehensive information about quantum computing breakthroughs in 2025")),
  	},
  	Tools: webSearchTool,
  })
  if err != nil {
  	log.Fatal(err)
  }

  if response.StopReason == anthropic.StopReasonPauseTurn {
  	// Pass the paused response back as-is so Claude can continue the turn
  	continuation, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
  		Model:     anthropic.ModelClaudeOpus5,
  		MaxTokens: 1024,
  		Messages: []anthropic.MessageParam{
  			anthropic.NewUserMessage(anthropic.NewTextBlock("Search for comprehensive information about quantum computing breakthroughs in 2025")),
  			response.ToParam(),
  		},
  		Tools: webSearchTool,
  	})
  	if err != nil {
  		log.Fatal(err)
  	}
  	fmt.Println(continuation)
  } else {
  	fmt.Println(response)
  }
  ```

  ```java Java
  import com.anthropic.models.messages.StopReason;
  import com.anthropic.models.messages.WebSearchTool20250305;

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

      MessageCreateParams params = MessageCreateParams.builder()
          .model(Model.CLAUDE_OPUS_5)
          .maxTokens(1024L)
          .addUserMessage("Search for comprehensive information about quantum computing breakthroughs in 2025")
          .addTool(WebSearchTool20250305.builder()
              .maxUses(10L)
              .build())
          .build();

      Message response = client.messages().create(params);

      if (response.stopReason().isPresent()
              && response.stopReason().get().equals(StopReason.PAUSE_TURN)) {
          MessageCreateParams continuationParams = MessageCreateParams.builder()
              .model(Model.CLAUDE_OPUS_5)
              .maxTokens(1024L)
              .addUserMessage("Search for comprehensive information about quantum computing breakthroughs in 2025")
              .addMessage(response)
              .addTool(WebSearchTool20250305.builder()
                  .maxUses(10L)
                  .build())
              .build();

          Message continuation = client.messages().create(continuationParams);
          IO.println(continuation);
      } else {
          IO.println(response);
      }
  }
  ```

  ```php PHP
  $client = new Client();

  $response = $client->messages->create(
      maxTokens: 1024,
      messages: [
          [
              'role' => 'user',
              'content' => 'Search for comprehensive information about quantum computing breakthroughs in 2025'
          ]
      ],
      model: 'claude-opus-5',
      tools: [
          [
              'type' => 'web_search_20250305',
              'name' => 'web_search',

Cut at 300 lines. The page has the rest.