---
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-5-5",
      "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}]
    }'
  ```

  ```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 <<'YAML'
  model: claude-opus-5-5
  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-5-5",
      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-5-5",
          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-5-5",
    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-5-5",
      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_5,
      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_5,
          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_5,
  	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_5,
  		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_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_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-5-5',
      tools: [
          [
              'type' => 'web_search_20250305',
              'name' => 'web_search',
              'max_uses' => 10
          ]
      ],
  );

  if ($response->stopReason === 'pause_turn') {
      $messages = [
          [
              'role' => 'user',
              'content' => 'Search for comprehensive information about quantum computing breakthroughs in 2025'
          ],
          [
              'role' => 'assistant',
              'content' => $response->content
          ]
      ];

      $continuation = $client->messages->create(
          maxTokens: 1024,
          messages: $messages,
          model: 'claude-opus-5-5',
          tools: [
              [
                  'type' => 'web_search_20250305',
                  'name' => 'web_search',
                  'max_uses' => 10
              ]
          ],
      );

      echo $continuation;
  } else {
      echo $response;
  }
  ```

  ```ruby Ruby
  client = Anthropic::Client.new

  response = client.messages.create(
    model: "claude-opus-5-5",
    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
      }
    ]
  )

  if response.stop_reason == :pause_turn
    messages = [
      {
        role: "user",
        content: "Search for comprehensive information about quantum computing breakthroughs in 2025"
      },
      {
        role: "assistant",
        content: response.content
      }
    ]

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

    puts continuation
  else
    puts response
  end
  ```
</CodeGroup>

When handling `pause_turn`:

* **Continue the conversation:** Pass the paused response back as-is in a subsequent request to let Claude continue its turn.
* **Preserve tool state:** Include the same tools in the continuation request. A paused turn can end with a `server_tool_use` block whose tool has not run yet, and the API returns a validation error if that tool is missing from the continuation.
* **Repeat as needed:** A continued turn can pause again. Check `stop_reason` on each response and continue until you get a different stop reason, capping the number of continuations as you would any retry loop.

For the other `stop_reason` values and general handling patterns, see [Stop reasons and fallback](https://platform.claude.com/docs/en/build-with-claude/handling-stop-reasons).

## Mixing server tools and client tools in one turn

Claude can call a server tool and a client tool in the same group of parallel tool calls, for example, `web_fetch` together with a user-defined tool. A client tool is any tool that your code executes and that produces a `tool_use` block, whether it is user-defined or an Anthropic-schema client tool such as the [Bash tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/bash-tool). When that happens, the API does not run the server tool. It returns immediately so that you can run the client tool first:

* `stop_reason` is `"tool_use"`, not `"pause_turn"`.
* `content` contains the `server_tool_use` block and the client `tool_use` block, but no result block for the server tool: that call is not finished.
* There is no other marker. Detect the state by looking for a `server_tool_use` block whose `id` has no matching result block in the response. An `mcp_tool_use` block from the [MCP connector](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector) behaves the same way. Server tool calls that already have their result block in the same response are complete and need nothing from you.

<Note>
  With [programmatic tool calling](https://platform.claude.com/docs/en/agents-and-tools/tool-use/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.
</Note>

```json
{
  "stop_reason": "tool_use",
  "content": [
    {
      "type": "text",
      "text": "I'll fetch the article and check your system at the same time."
    },
    {
      "type": "server_tool_use",
      "id": "srvtoolu_01HxbWnMRmbWyMfUtJKC45rA",
      "name": "web_fetch",
      "input": { "url": "https://example.com/article" }
    },
    {
      "type": "tool_use",
      "id": "toolu_01PjgRJLbXrXEMZwDNYLnBqk",
      "name": "run_command",
      "input": { "command": "uname -a" }
    }
  ]
}
```

To continue the turn, run the client tools and send a user message whose content is only the `tool_result` blocks, one for each `tool_use` block in that response. 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_fetch` tool was provided``.

```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"
    }
  ]
}
```

The API attaches your results to the still-open assistant turn, runs the deferred server tool (for paused code execution, resumes it), and then lets Claude continue. For a server tool Claude called directly, the next response begins with the result block that answers the previous response's `server_tool_use` `id`, followed by the newly generated content and a fresh `stop_reason`:

```json
{
  "stop_reason": "end_turn",
  "content": [
    {
      "type": "web_fetch_tool_result",
      "tool_use_id": "srvtoolu_01HxbWnMRmbWyMfUtJKC45rA",
      "content": {
        "type": "web_fetch_result",
        "url": "https://example.com/article",
        "content": {
          "type": "document",
          "source": {
            "type": "text",
            "media_type": "text/plain",
            "data": "Full text content of the article..."
          }
        }
      }
    },
    {
      "type": "text",
      "text": "The article argues that... and your machine is running Linux..."
    }
  ]
}
```

A `server_tool_use` block and its result block pair up by `tool_use_id`, not by position: in this flow they arrive in two different responses, and the `server_tool_use` block is not repeated in the second one. On later requests, keep the whole exchange in your `messages` array in order: the first response as an `assistant` message, the `tool_result` user message, and then the next response as another `assistant` message, the same way you accumulate any other tool-use exchange.

<Warning>
  The follow-up user message must contain nothing except `tool_result` blocks. A block added after the results, such as text, tells the API that the assistant turn is over. For a server tool Claude called directly, that leaves the turn with an unresolved server tool call, and the request fails with a 400 `invalid_request_error`:

  ```text wrap
  `web_fetch` tool use with id `srvtoolu_01HxbWnMRmbWyMfUtJKC45rA` was found without a corresponding `web_fetch_tool_result` block
  ```

  A follow-up that puts content before the results, answers only some of the client `tool_use` IDs, or contains no `tool_result` blocks at all fails earlier, with the client tool error described in [Handle tool calls](https://platform.claude.com/docs/en/agents-and-tools/tool-use/handle-tool-calls):

  ```text wrap
  `tool_use` ids were found without `tool_result` blocks immediately after: toolu_01PjgRJLbXrXEMZwDNYLnBqk. Each `tool_use` block must have a corresponding `tool_result` block in the next message.
  ```

  To give Claude more input, send it as a separate user message after the turn completes.
</Warning>

**How this differs from `pause_turn`:** A [`pause_turn` response](https://platform.claude.com/docs/en/agents-and-tools/tool-use/server-tools#the-server-side-loop-and-pause-turn) can also end with a `server_tool_use` block that has not run, but it never leaves a client `tool_use` block waiting on you, so you continue it by re-sending the assistant content 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 rather than by re-sending the response. In both cases the API runs the pending server tool at the start of the next request.

The following example enables web fetch together with a user-defined `run_command` tool and handles the mixed response:

<CodeGroup>
  ```bash cURL
  # If "stop_reason" is "tool_use" and a server_tool_use block has no matching
  # result block, that call is not finished. Run the client tools, then POST
  # again with one more user message containing only their tool_result blocks
  # and the same tools array (see the SDK tabs).
  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-5-5",
      "max_tokens": 1024,
      "messages": [
        {
          "role": "user",
          "content": "Summarize https://example.com/article and run uname -a to tell me what system this is on."
        }
      ],
      "tools": [
        {"type": "web_fetch_20250910", "name": "web_fetch", "max_uses": 5},
        {
          "name": "run_command",
          "description": "Run a shell command on this computer and return its output.",
          "input_schema": {
            "type": "object",
            "properties": {"command": {"type": "string", "description": "The command to run"}},
            "required": ["command"]
          }
        }
      ]
    }'
  ```

  ```bash CLI
  # If "stop_reason" is "tool_use" and a server_tool_use block has no matching
  # result block, run the client tools and re-run with a user message of only
  # their tool_result blocks appended (see the SDK tabs).
  ant messages create <<'YAML'
  model: claude-opus-5-5
  max_tokens: 1024
  messages:
    - role: user
      content: "Summarize https://example.com/article and run uname -a to tell me what system this is on."
  tools:
    - {type: web_fetch_20250910, name: web_fetch, max_uses: 5}
    - name: run_command
      description: Run a shell command on this computer and return its output.
      input_schema:
        type: object
        properties:
          command: {type: string, description: The command to run}
        required: [command]
  YAML
  ```

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

  tools = [
      {"type": "web_fetch_20250910", "name": "web_fetch", "max_uses": 5},
      {
          "name": "run_command",
          "description": "Run a shell command on this computer and return its output.",
          "input_schema": {
              "type": "object",
              "properties": {
                  "command": {"type": "string", "description": "The command to run"}
              },
              "required": ["command"],
          },
      },
  ]
  messages = [
      {
          "role": "user",
          "content": "Summarize https://example.com/article and run uname -a to tell me what system this is on.",
      }
  ]

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

  tool_results = [
      {
          "type": "tool_result",
          "tool_use_id": block.id,
          # Run your tool here. This example returns a fixed string.
          "content": "Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux",
      }
      for block in response.content
      if block.type == "tool_use"
  ]

  if response.stop_reason == "tool_use" and tool_results:
      # A server_tool_use block with no result block in this response is not finished; its result arrives in a later response.
      # Send back only the client tool_result blocks, with the same tools.
      continuation = client.messages.create(
          model="claude-opus-5-5",
          max_tokens=1024,
          tools=tools,
          messages=[
              *messages,
              {"role": "assistant", "content": response.content},
              {"role": "user", "content": tool_results},
          ],
      )
      # If a web_fetch was deferred, it runs on this request and its
      # web_fetch_tool_result is the first block of continuation.content.
      print(continuation)
  else:
      print(response)
  ```

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

  const webFetchTool = {
    type: "web_fetch_20250910",
    name: "web_fetch",
    max_uses: 5
  } as const;
  const runCommandTool: Anthropic.Tool = {
    name: "run_command",
    description: "Run a shell command on this computer and return its output.",
    input_schema: {
      type: "object" as const,
      properties: {
        command: { type: "string", description: "The command to run" }
      },
      required: ["command"]
    }
  };
  const messages: Anthropic.MessageParam[] = [
    {
      role: "user",
      content:
        "Summarize https://example.com/article and run uname -a to tell me what system this is on."
    }
  ];

  const response = await client.messages.create({
    model: "claude-opus-5-5",
    max_tokens: 1024,
    tools: [webFetchTool, runCommandTool],
    messages
  });

  const toolResults: Anthropic.ToolResultBlockParam[] = [];
  for (const block of response.content) {
    if (block.type === "tool_use") {
      toolResults.push({
        type: "tool_result",
        tool_use_id: block.id,
        // Run your tool here. This example returns a fixed string.
        content: "Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux"
      });
    }
  }

  if (response.stop_reason === "tool_use" && toolResults.length > 0) {
    // A server_tool_use block with no result block in this response is not finished; its result arrives in a later response.
    // Send back only the client tool_result blocks, with the same tools.
    const continuation = await client.messages.create({
      model: "claude-opus-5-5",
      max_tokens: 1024,
      tools: [webFetchTool, runCommandTool],
      messages: [
        ...messages,
        { role: "assistant", content: response.content },
        { role: "user", content: toolResults }
      ]
    });
    // If a web_fetch was deferred, it runs on this request and its
    // web_fetch_tool_result is the first block of continuation.content.
    console.log(continuation);
  } else {
    console.log(response);
  }
  ```

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

  List<ToolUnion> tools =
  [
      new ToolUnion(new WebFetchTool20250910() { MaxUses = 5 }),
      new ToolUnion(new Tool()
      {
          Name = "run_command",
          Description = "Run a shell command on this computer and return its output.",
          InputSchema = new InputSchema()
          {
              Properties = new Dictionary<string, JsonElement>
              {
                  ["command"] = JsonSerializer.SerializeToElement(
                      new { type = "string", description = "The command to run" }
                  ),
              },
              Required = ["command"],
          },
      }),
  ];
  MessageParam userMessage = new()
  {
      Role = Role.User,
      Content = "Summarize https://example.com/article and run uname -a to tell me what system this is on."
  };

  var response = await client.Messages.Create(new MessageCreateParams
  {
      Model = Model.ClaudeOpus5_5,
      MaxTokens = 1024,
      Tools = tools,
      Messages = [userMessage]
  });

  var toolResults = new List<ContentBlockParam>();
  foreach (var block in response.Content)
  {
      if (block.TryPickToolUse(out var toolUse))
      {
          toolResults.Add(new ContentBlockParam(new ToolResultBlockParam()
          {
              ToolUseID = toolUse.ID,
              // Run your tool here. This example returns a fixed string.
              Content = "Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux",
          }));
      }
  }

  if (response.StopReason?.Value() == StopReason.ToolUse && toolResults.Count > 0)
  {
      // A server_tool_use block with no result block in this response is not finished; its result arrives in a later response.
      // Send back only the client tool_result blocks, with the same tools.
      var continuation = await client.Messages.Create(new MessageCreateParams
      {
          Model = Model.ClaudeOpus5_5,
          MaxTokens = 1024,
          Tools = tools,
          Messages =
          [
              userMessage,
              new()
              {
                  Role = Role.Assistant,
                  Content = response.Content.Select(block => new ContentBlockParam(block.Json)).ToList()
              },
              new() { Role = Role.User, Content = new MessageParamContent(toolResults) }
          ]
      });
      // If a web_fetch was deferred, it runs on this request and its
      // web_fetch_tool_result is the first block of continuation.Content.
      Console.WriteLine(continuation);
  }
  else
  {
      Console.WriteLine(response);
  }
  ```

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

  tools := []anthropic.ToolUnionParam{
  	{OfWebFetchTool20250910: &anthropic.WebFetchTool20250910Param{
  		MaxUses: anthropic.Int(5),
  	}},
  	{OfTool: &anthropic.ToolParam{
  		Name:        "run_command",
  		Description: anthropic.String("Run a shell command on this computer and return its output."),
  		InputSchema: anthropic.ToolInputSchemaParam{
  			Properties: map[string]any{
  				"command": map[string]any{
  					"type":        "string",
  					"description": "The command to run",
  				},
  			},
  			Required: []string{"command"},
  		},
  	}},
  }
  userMessage := anthropic.NewUserMessage(anthropic.NewTextBlock("Summarize https://example.com/article and run uname -a to tell me what system this is on."))

  response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
  	Model:     anthropic.ModelClaudeOpus5_5,
  	MaxTokens: 1024,
  	Tools:     tools,
  	Messages:  []anthropic.MessageParam{userMessage},
  })
  if err != nil {
  	log.Fatal(err)
  }

  var toolResults []anthropic.ContentBlockParamUnion
  for _, block := range response.Content {
  	if toolUse, ok := block.AsAny().(anthropic.ToolUseBlock); ok {
  		// Run your tool here. This example returns a fixed string.
  		output := "Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux"
  		toolResults = append(toolResults, anthropic.NewToolResultBlock(toolUse.ID, output, false))
  	}
  }

  if response.StopReason == anthropic.StopReasonToolUse && len(toolResults) > 0 {
  	// A server_tool_use block with no result block in this response is not finished; its result arrives in a later response.
  	// Send back only the client tool_result blocks, with the same tools.
  	continuation, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
  		Model:     anthropic.ModelClaudeOpus5_5,
  		MaxTokens: 1024,
  		Tools:     tools,
  		Messages: []anthropic.MessageParam{
  			userMessage,
  			response.ToParam(),
  			anthropic.NewUserMessage(toolResults...),
  		},
  	})
  	if err != nil {
  		log.Fatal(err)
  	}
  	// If a web_fetch was deferred, it runs on this request and its
  	// web_fetch_tool_result is the first block of continuation.Content.
  	fmt.Println(continuation)
  } else {
  	fmt.Println(response)
  }
  ```

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

      Tool runCommandTool = Tool.builder()
          .name("run_command")
          .description("Run a shell command on this computer and return its output.")
          .inputSchema(Tool.InputSchema.builder()
              .properties(JsonValue.from(Map.of(
                  "command", Map.of("type", "string", "description", "The command to run")
              )))
              .putAdditionalProperty("required", JsonValue.from(List.of("command")))
              .build())
          .build();
      String prompt = "Summarize https://example.com/article and run uname -a to tell me what system this is on.";

      Message response = client.messages().create(MessageCreateParams.builder()
          .model(Model.CLAUDE_OPUS_5_5)
          .maxTokens(1024L)
          .addTool(WebFetchTool20250910.builder().maxUses(5L).build())
          .addTool(runCommandTool)
          .addUserMessage(prompt)
          .build());

      List<ContentBlockParam> toolResults = new ArrayList<>();
      for (ContentBlock block : response.content()) {
          block.toolUse().ifPresent(toolUse -> toolResults.add(ContentBlockParam.ofToolResult(
              ToolResultBlockParam.builder()
                  .toolUseId(toolUse.id())
                  // Run your tool here. This example returns a fixed string.
                  .content("Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux")
                  .build()
          )));
      }

      boolean isToolUse = response.stopReason()
          .map(StopReason.TOOL_USE::equals)
          .orElse(false);
      if (isToolUse && !toolResults.isEmpty()) {
          // A server_tool_use block with no result block in this response is not finished; its result arrives in a later response.
          // Send back only the client tool_result blocks, with the same tools.
          Message continuation = client.messages().create(MessageCreateParams.builder()
              .model(Model.CLAUDE_OPUS_5_5)
              .maxTokens(1024L)
              .addTool(WebFetchTool20250910.builder().maxUses(5L).build())
              .addTool(runCommandTool)
              .addUserMessage(prompt)
              .addMessage(response)
              .addUserMessageOfBlockParams(toolResults)
              .build());
          // If a web_fetch was deferred, it runs on this request and its
          // web_fetch_tool_result is the first block of continuation.content().
          IO.println(continuation);
      } else {
          IO.println(response);
      }
  }
  ```

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

  $tools = [
      ['type' => 'web_fetch_20250910', 'name' => 'web_fetch', 'max_uses' => 5],
      [
          'name' => 'run_command',
          'description' => 'Run a shell command on this computer and return its output.',
          'input_schema' => [
              'type' => 'object',
              'properties' => [
                  'command' => ['type' => 'string', 'description' => 'The command to run']
              ],
              'required' => ['command']
          ]
      ]
  ];
  $userMessage = ['role' => 'user', 'content' => 'Summarize https://example.com/article and run uname -a to tell me what system this is on.'];

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

  $toolResults = [];
  foreach ($response->content as $block) {
      if ($block->type === 'tool_use') {
          $toolResults[] = [
              'type' => 'tool_result',
              'tool_use_id' => $block->id,
              // Run your tool here. This example returns a fixed string.
              'content' => 'Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux'
          ];
      }
  }

  if ($response->stopReason === 'tool_use' && count($toolResults) > 0) {
      // A server_tool_use block with no result block in this response is not finished; its result arrives in a later response.
      // Send back only the client tool_result blocks, with the same tools.
      $continuation = $client->messages->create(
          maxTokens: 1024,
          messages: [
              $userMessage,
              ['role' => 'assistant', 'content' => $response->content],
              ['role' => 'user', 'content' => $toolResults],
          ],
          model: 'claude-opus-5-5',
          tools: $tools,
      );
      // If a web_fetch was deferred, it runs on this request and its
      // web_fetch_tool_result is the first block of $continuation->content.
      echo $continuation;
  } else {
      echo $response;
  }
  ```

  ```ruby Ruby
  client = Anthropic::Client.new

  tools = [
    { type: "web_fetch_20250910", name: "web_fetch", max_uses: 5 },
    {
      name: "run_command",
      description: "Run a shell command on this computer and return its output.",
      input_schema: {
        type: "object",
        properties: {
          command: { type: "string", description: "The command to run" }
        },
        required: ["command"]
      }
    }
  ]
  user_message = {
    role: "user",
    content: "Summarize https://example.com/article and run uname -a to tell me what system this is on."
  }

  response = client.messages.create(
    model: "claude-opus-5-5",
    max_tokens: 1024,
    tools: tools,
    messages: [user_message]
  )

  tool_results = []
  response.content.each do |block|
    next unless block.type == :tool_use

    tool_results << {
      type: "tool_result",
      tool_use_id: block.id,
      # Run your tool here. This example returns a fixed string.
      content: "Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux"
    }
  end

  if response.stop_reason == :tool_use && !tool_results.empty?
    # A server_tool_use block with no result block in this response is not finished; its result arrives in a later response.
    # Send back only the client tool_result blocks, with the same tools.
    continuation = client.messages.create(
      model: "claude-opus-5-5",
      max_tokens: 1024,
      tools: tools,
      messages: [
        user_message,
        { role: "assistant", content: response.content },
        { role: "user", content: tool_results }
      ]
    )
    # If a web_fetch was deferred, it runs on this request and its
    # web_fetch_tool_result is the first block of continuation.content.
    puts continuation
  else
    puts response
  end
  ```
</CodeGroup>

This code is also correct when Claude does not mix the two kinds of call. A turn with only client `tool_use` blocks takes the same continuation path, and a turn with only server tool calls needs no client `tool_result` blocks from you: its result blocks are normally already present, and one that comes back suspended, such as a [`pause_turn` response](https://platform.claude.com/docs/en/agents-and-tools/tool-use/server-tools#the-server-side-loop-and-pause-turn), is re-sent as-is instead.

## ZDR and allowed\_callers

The basic versions of web search (`web_search_20250305`) and web fetch (`web_fetch_20250910`) are eligible for [Zero Data Retention (ZDR)](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention).

The `_20260209` and later versions with dynamic filtering are **not** ZDR-eligible by default because dynamic filtering relies on code execution internally.

To use a `_20260209` or later server tool with ZDR, disable dynamic filtering by setting `"allowed_callers": ["direct"]` on the tool:

```json
{
  "type": "web_search_20260209",
  "name": "web_search",
  "allowed_callers": ["direct"]
}
```

This restricts the tool to direct invocation only, bypassing the internal code execution step.

`allowed_callers` controls how a tool can be invoked: directly by Claude (`"direct"`), from inside a code execution container (for example, `"code_execution_20260120"`), or both. The `_20260209` versions of the web tools default to the code execution caller only; earlier versions default to `["direct"]`. On models that don't support programmatic tool calling, these versions require `allowed_callers: ["direct"]`; without it the API returns a validation error that says to set it.

<Note>
  Even when web fetch is used in a ZDR-eligible configuration, website publishers might retain any parameters passed to the URL if Claude fetches content from their site.
</Note>

## Domain filtering

Server tools that access the web accept `allowed_domains` and `blocked_domains` parameters to control which domains Claude can reach. Both are fields on the tool object:

```json
{
  "type": "web_search_20250305",
  "name": "web_search",
  "allowed_domains": ["example.com", "docs.python.org"]
}
```

When using domain filters:

* Domains should not include the HTTP/HTTPS scheme (use `example.com` instead of `https://example.com`).
* Subdomains are automatically included (`example.com` covers `docs.example.com`).
* Specific subdomains restrict results to only that subdomain (`docs.example.com` returns only results from that subdomain, not from `example.com` or `api.example.com`).
* Subpaths are supported for web search and match anything after the path (`example.com/blog` matches `example.com/blog/post-1`).
* Web fetch matches on the domain only: an entry that includes a path never matches a web fetch URL.
* You can use either `allowed_domains` or `blocked_domains`, but not both in the same request.

**Wildcard support:**

* Wildcards (`*`) are not allowed in the domain itself, only in the path after it.
* Valid: `example.com/*`, `example.com/*/articles`
* Invalid: `*.example.com`, `ex*.com`

Invalid domain formats are rejected at request time with a 400 `invalid_request_error`.

<Note>
  Request-level domain restrictions work together with any organization-level domain restrictions configured in Claude Console. Request-level `allowed_domains` must be a subset of the organization-level allowed list; entries outside it cause the API to return a validation error. A request-level allowed list that includes a domain your organization blocks is rejected with a `400` error that names the conflicting entries.
</Note>

<Warning>
  Unicode characters in domain names can bypass domain filters through homograph attacks: `аmazon.com` (with a Cyrillic `а`) looks identical to `amazon.com` but is a different domain. Use ASCII-only domain names in allow and block lists, and audit existing entries for non-ASCII characters.
</Warning>

[Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview) uses the same `allowed_domains` and `blocked_domains` fields on the `web_search` and `web_fetch` entries of the agent toolset. On Managed Agents, each list holds at most 64 entries, domains listed for `web_fetch` cannot include a path, and fields specific to the Messages API tools, such as `max_uses`, `citations`, and `cache_control`, are not available. See [Domain list rules](https://platform.claude.com/docs/en/managed-agents/tools-web-restrictions#domain-list-rules) for the full rules.

Organization-level web search and web fetch settings in the Claude Console apply to Messages API requests only. They do not apply to Managed Agents sessions, which use the per-tool lists on the agent toolset instead. For a session in a cloud environment with `limited` networking, the environment's `allowed_hosts` also applies to `web_search` and `web_fetch`; see [Networking](https://platform.claude.com/docs/en/managed-agents/environments#networking). The per-tool lists restrict these tools further, within the hosts that `allowed_hosts` allows.

## Dynamic filtering with code execution

The `_20260209` and later versions of web search and web fetch use code execution internally to apply dynamic filters against search results.

<Note>
  You don't need to add a `code_execution` tool for these versions: when dynamic filtering runs, the API provisions code execution for the request automatically, and both tools share a single execution container. If you do include one, use `code_execution_20260120` or later; the API rejects older code execution versions alongside these web tool versions.
</Note>

## Streaming server-tool events

Server-tool events stream as part of the normal server-sent events (SSE) flow. A `server_tool_use` block that Claude calls directly streams like a client `tool_use` block: a `content_block_start` event followed by `input_json_delta` events. The result block arrives complete in a single `content_block_start` event, with no deltas.

See [Streaming](https://platform.claude.com/docs/en/build-with-claude/streaming) for the full event reference. Individual tool pages document tool-specific event names where they differ.

## Batch requests

All server tools support batch processing. In a batch, the agentic loop runs just as it does for synchronous requests, with a higher per-turn iteration limit. If the loop reaches that limit, the response ends with `stop_reason: "pause_turn"`; you can continue it by submitting a follow-up request with the returned content. See [Server tools and the agentic loop](https://platform.claude.com/docs/en/build-with-claude/batch-processing#server-tools-and-the-agentic-loop) for details.

Common batch workloads include enriching a dataset with information from the web, checking a large set of documents against current sources, and running analysis code over many files.

## Next steps

<CardGroup cols={2}>
  <Card title="Troubleshooting tool use" icon="wrench" href="https://platform.claude.com/docs/en/agents-and-tools/tool-use/troubleshooting-tool-use">
    Fix the most common tool-use errors with symptom-to-fix diagnostic tables.
  </Card>

  <Card title="Web search tool" icon="magnifying-glass" href="https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-search-tool">
    Search the web and cite results.
  </Card>

  <Card title="Web fetch tool" icon="download" href="https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool">
    Fetch and read content from specific URLs to augment Claude's context with live web content.
  </Card>

  <Card title="Code execution tool" icon="terminal" href="https://platform.claude.com/docs/en/agents-and-tools/tool-use/code-execution-tool">
    Run Python and bash code in a sandboxed container to analyze data, generate files, and iterate on solutions.
  </Card>

  <Card title="Tool search tool" icon="compass" href="https://platform.claude.com/docs/en/agents-and-tools/tool-use/tool-search-tool">
    Discover and load tools on demand.
  </Card>
</CardGroup>